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Semiconductor Equipment Safety Standards
Working Paper · January 2017
DOI: 10.13140/RG.2.2.35008.74243
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Semiconductor Equipment Safety Standards
Nikola Zlatanov*
Introduction
Technical operations such as those performed in semiconductor and photovoltaic device fabrication have
inherent risks. For example, the equipment employed in these industries may operate at high
temperatures, under vacuum conditions and can employ high electrical voltage/ currents and/or
extremely hazardous, often pyrophoric and frequently corrosive chemicals. The failure of a critical system
component with any of these system characteristics can produce unsafe conditions that can lead to
severe injury or death of the operators, not to mention catastrophic damage to costly equipment. Other
industrial settings have similar or greater levels of risk. Operational safety is thus of paramount
importance in industrial semiconductor and other chemical processing systems. Because of this, a great
deal of effort has been expended over the past decades into efforts to establish reliable metrics for the
prediction of safe operational conditions for process equipment and to implement designs that meet the
acceptable risk level without compromising costs and programmability.
Operational safety is of paramount importance in OEM designs and the question of hard-wired safety
solutions vs. programmable hardware-based safety solutions must be considered for these designs.
In the past, safety engineers have typically resolved this question in favor of hard-wired interlocks in
which the logic was wired directly onto the PCB. As an example, consider the semiconductor industry.
Semi S2, the Safety Guideline for Semiconductor Manufacturing Equipment, is often cited as the “gold
standard” of interlock safety and it recommends the use of electro-mechanical components for safety
interlocks. However, it is commonly overlooked that the Semi S2 guidance also permits the use of solid
state devices in a safety system, as long as they can be proven to be safe (cf. S2-93 SEMATECH
Application Guide). The question confronting OEMs thus becomes: are operational safety solutions
achieved through the use of programmable hardware-based interlocks as safe, or indeed more safe than
hard-wired solutions. Since it is often the case that safety is best implemented using hard coded safety in
one instance while another is more suited to programmable hardware safety, solution providers must
offer safety interlocks in the form of both hardware based, and programmable hardware-based interlock
safety boards. Knowing that both options exist, with the required level of safety criteria, is beneficial to
the engineer who needs to make a decision about their safety solution.
Predicting System Safety
Mean Time Between Failures (MTBF) has been used as a metric for the prediction of a system’s ability to
operate reliably. MTBF is described analytically by the following relationships:
where λi = failure rate of i system component and n = number of components in the system.
If the failure rate is expressed as “per hour” then the MTBF is in hours. Unfortunately, while MTBF is an
excellent overall metric for assessing system reliability, it is not a metric that is specific for evaluating
system safety. The problem with using MTBF as a safety metric is characterized in the following
discussion: In any attempt to predict the safety characteristics of a given system, the most important
failure rate is that associated with dangerous failures rather than the overall rate of failure of the system,
1
as is the case for MTBF. Therefore, in evaluating safety, λ needs to be subdivided into the components
that reflect the impact of the type of failure on the system safety (i.e.):
λ = λS + λDD + λDU
λ = the rate of safe failures having no impact on safety (The system may cease function but no safety
issue will exist);
λDD = the rate of dangerous failures that are detected by the system safety design (Hence the system
safety design can avoid a safety issue);
λ DU = the rate of dangerous failures that are not detected by the system safety design (Such failures may
cause a safety issue).
Using these definitions, it can be seen that two systems having the same MTBF value can differ
significantly in terms of the λDU value. Thus, serious inequalities in the safety of such systems are not
predictable using just the MTBF metric. More reliable concepts for the prediction of system safety have
emerged in recent years. Specifically, the concept of Safety Integrity Levels (SILs) has evolved for the
design of safety critical systems such as those employed in the chemical and semiconductor process
industries. Developed over the last two decades, the SIL concept has emerged from efforts to improve
the safety of such systems. The concept has been developed with the purpose of moving away from the
consideration of safety as an either/or characteristic of a system. Rather than considering a system as
“safe” or “unsafe”, the SIL concept views safety as a continuous spectrum with a system’s position on the
spectrum directly related to the level of risk of entering an unsafe state. Risk definition and analysis
becomes a critical exercise in the development of safe systems.
SIL
4
On Demand Mode
of Operation
(Probability of failure to
perform
its
design
function on demand)
≥ 10-5 to < 10-4
Number of Treated
Demands
1-α = 0.99
1-α = 0.95
4.6 x 105
3 x 105
High Demand or
Continuous
Mode of
(Probability of a
dangerous failure per
hour)
-9
≥ 10 to < 10-8
Hours of Operation in
Total
1−α = 0.99
1-α = 0.95
4.6 x 109
3 x 109
3
≥ 10-4 to < 10-3
4.6 x 104
3 x 104
≥ 10-8 to < 10-7
4.6 x 108
3 x 108
2
≥ 10-3 to < 10-2
4.6 x 103
3 x 103
≥ 10-7 to < 10-6
4.6 x 107
3 x 107
1
≥ 10-2 to < 10-1
4.6 x 102
3 x 102
≥ 10-6 to < 10-5
4.6 x 106
3 x 106
The dominance of electronic control systems and integrated electronic circuitry in modern process
technology, coupled with the vastly improved reliability of electronic components, has prompted the
development of the international safety standard: IEC61508, “Functional Safety of Electrical/
Electronic/Programmable Electronic Safety-related Systems”. This standard codifies the requirements for
the use and design of electronic and programmable safety functional systems and is commonly used as
a method of proving that system safety is at a level sufficient for Semi S2 compliance and certification.
Developed using the SIL concept, IEC61508 requires that risk analysis for “each determined hazardous
event” be performed for each functional system within the Equipment Under Control (EUC) to establish
functional safety in the entire system. IEC61508 defines risk as a function of the likelihood of an event
and the severity of the consequences of that event. The system is considered safe if the risk can be
acceptably reduced by applying either electronic circuits and programmable components or the
traditional combination of traces and relays. IEC61508 defines four distinct Safety Integrity Levels; Safety
Integrity Level 1 (SIL1) is the lowest level and Safety Integrity Level 4 (SIL4) the highest. Under
IEC61508, safety systems are sorted into two categories: Continuous (high demand) Mode and On
(low) Demand Mode. For example, in a car the braking system constitutes a functional safety
system that works in Continuous Mode. The system must be ready to work at all times. On the other
hand, the air-bag system is an On Demand functional safety system. It is needed very rarely and
preferably never. But when it is needed, it must work. User configurable safety interlocks, such as
those being addressed in this paper, are On Demand functional safety systems. In On Demand systems,
2
the accident rate is defined as a combination of the frequency of demands and the probability that the
function will fail on demand (PFD). In Continuous Mode systems, the accident rate is the MTBF. Table 1
shows how the SIL levels are defined for On Demand and Continuous Mode safety functional systems.
The PFD or Probability of Failure on Demand metric determines the SIL category (1-4) of the
system. For example, in Table 1 above, a PFD value in the range 10-4 to 10-3 (e.g. SIL3 for On
Demand Mode of Operation) means that for one out of 1000 to 10,000 demands most of the
systems will fail. The lower the PFD number, the better the SIL. The relation between SIL and PFD
is taken from the IEC 61508 standard.
Electro-mechanical Interlocking devices
When a machine or plant has to be equipped with a movable guard, the question that arises for the
design engineer is: how is the position of this mobile safety device monitored? On the one hand the
regulations in standards must be observed here. Of prime relevance here is the Machinery Directive
2006/42/EC, Annex 1 (see Section 1.4 - Required characteristics of guards and protective devices) as
well as the A standard ISO 12100 (see Section 6.3.3) and - very important – the standard ISO 14119
"Safety of machinery - Interlocking devices associated with guards - Principles for design and selection".
At the same time the selection and design of a protective device and its interlocking device must be
considered under technical and economic aspects. The manipulation of protective devices must also be
considered, as this forms part of the man/machine interface.
The new ISO 14119:2013 was released at the end of 2013 and supersedes the previous version, ISO
14119 (1998-05) and the German DIN EN 1088. It is a “B2” standard in accordance with ISO 12100.
The standard deals with the selection of interlocking devices. But it also provides advice on their design
and assessment, and is therefore an aid to mechanical engineers wishing to design their own
interlocking device.
"Interlocking" and "guard locking"
Unfortunately, the much used term "interlocking device" is not self-explanatory. The definition can
already be found in the Type A standard ISO 12100 (Section 3.28.1), and has now been adopted
unchanged in ISO 14119 (Section 3.1). According to this definition, an "interlocking device" or "interlock"
is a "mechanical, electrical or other type of device, the purpose of which is to prevent the operation of
hazardous machine functions under specified conditions" (generally as long as the guard has not
closed). When this formal definition is translated into everyday language, an interlocking device is
therefore a position switch, proximity switch, guard locking etc. on a protective device, which has the
effect of enabling the machine controller to react to the position of a guard.
With some interlocking devices which use a position switch with separate actuator, the separate actuator
can be restrained so that the associated guard can only be opened under certain conditions, such as
when the machine stops. A device which enables such a function is called a guard locking device or
guard locking.
In everyday language, one would probably describe this locking of the guard as "interlocking" rather than
the link between the position of a guard and the machine controller. In cases of doubt, it is therefore
advisable to ask what is precisely meant by the term "interlocking": "interlocking" within the meaning of
the EC Machinery Directive and the corresponding European standards or "guard locking" as meant in
the sense described above? This confusing terminology of interlocking/guard locking is not only a
problem in English, but also in German (Verriegeln/Zuhalten) and in French (verrouillagel
interverroui/lage).
Fig 1: Example of an interlocking device
1) guard
2) Interlocking device
3) Actuator
4) Position switch
5) Actuating system
6) Output system
a) Direction of opening
3
Fig 2: Principles of Type 1, Type 2, Type 3 and Type 4 interlocking devices.
a) Type 1 interlocking device (actuated using uncoded cam, guard closed);
b) Type 2 interlocking device (actuated using encoded tongue, guard not closed);
c) Type 3 or 4 interlocking device (uncoded or encoded, non-contact actuated, guard closed).
1) movable guard
2) interlocking device
3) actuating element: a cam; b tongue; c RFID, reflector, surface
4) position switch
5) actuating system
6) output system
Four basic designs
For the first time "ISO 14119" (referred to below as standard) provides an illustration specifically
according to the definition in Section 3.1, and which makes it easier to understand the definition and the
various components of an interlocking device. The main component of an interlocking device is the
position switch, which itself is divided into an actuating system and output system. The actuating element
is the part connected to the movable guard. It can be provided by the user or be supplied by the
manufacturer of the position switch. The classification of interlocking devices into various types is
primarily made according to the actuation principle and then according to the encoding of the actuating
element.
Type 1
• Mechanically actuated through physical contact, i.e. using force;
• "Uncoded actuating element”.
This design is the classic position switch as shown in Figure 1. Type 1 switches have diverse uses, not
least because the actuating element can be configured by the user himself. Due to the many possible
designs of the actuating element, this is described as uncoded.
Type 2
• Mechanically actuated through physical contact, i.e. using force;
• "Encoded actuating element".
In DIN EN 1088 this model is called "Interlocking device with separate actuator". It has long been known
in Germany as a "category 2 switch" and more recently as a "type 2 switch". The latter name has now
also found its way into the international standard and has even been extended to include types 3 and 4.
Type 2 switches are characterized by very safe actuation whilst also being relatively easy to
4
"circumvent". This fact has already been addressed by A1 (Amendment 1) to DIN EN 1088, and the new
standard also devotes an entire section to reducing the circumvention potential.
Type 3
• Non-contact actuation, i.e. without physical contact;
• "Uncoded actuating element".
In this design, the actuating element and the actuating system are separated from each other. On
approaching (guard closed), they switch the enable to start the machine. A counterpart (actuator) is
required, but in the case of proximity switches with safety function, can be a metal flag for example.
Type 4
• Non-contact actuation, i.e. without physical contact;
• "Encoded actuating element".
In the case of Type 4 switches, the actuating element and the actuating system are separated from each
other. On approaching (guard closed), they switch the enable to tart the machine. The actuating element
is encoded; a counterpart to the sensor (actuator) is required.
Table 1 of the standard provides an overview of the design types and refers to the examples in Annexes
A to E.
Examples of actuating principle
Mechanical
Contact, force
Examples of actuating elements
Uncoded
Cam profile
Encoded
Abbreviation
Type 1
Linear cam
A.2, A.4
Hinge
A.3
Tongue (actuator key)
Type 2
Key transfer system
Non-Contact
Inductive
Uncoded
Suitable ferrous material
Magnetic
Magnet, electromagnet
Capacitive
Every suitable object
Ultrasound
Every suitable object
Optical
Every suitable object
Magnetic
Encoded
Examples*
A.1
Encoded magnet
B.1
B.2
Type 3
C
Type 4
D.1
RFID
Encoded RFID transponder
D.2
Optical
Optically encoded transponder
-
Table 1: Overview of the interlocking devices. * Examples of devices in Annex A through D of the standard
Encoded actuating elements
The term "uncoded actuating element" (Type 1 and 3) or "encoded actuating element" (Type 2 and 4) are
used in conjunction with the design types. Without a precise definition, misunderstandings frequently
arise about the meaning of "encoded" in this connection.
Sometimes a Type 2 position switch is described as "encoded" if its interlocking device can only be
actuated using the (always identical) supplied actuating element. However, there are also ranges of
actuating elements which have several thousand variations. The probability that an identically encoded
actuating element is available and that the interlocking device can be circumvented with it is extremely
small. The standard creates clarity here too by defining three encoding levels: low, medium and high.
Thus a "low level encoded actuator" is one which has between 1 and 9 encoding possibilities. The
number of possibilities for a medium encoding level is between 10 and 1,000; more than 1,000
possibilities correspond to a high encoding level.
A mechanical actuating element which, while having a specific shape, is always manufactured in its
thousands in the identical shape, is classified as being a low level encoded actuator.
5
Similarly, a magnetic actuator element is classed as encoded (at a low level) as soon as a specific rather
than a standard commercial magnet or simple metal flag is required for actuation. Reference is made to
a high encoding level for non-contact acting RFID-based interlocking devices, for which an almost infinite
number of encoding variations are available.
Interlocking devices with guard locking function
As can be seen from the heading, this product is an interlocking device which has been supplemented by
a guard locking mechanism so as to keep a movable guard closed during a hazardous machine function
(e.g. where dangerous stopping movements take longer). A separate status detection, which detects the
position of the guard locking device, and generates a corresponding output signal which is used for
control purposes, is a component of a guard locking device.
The guard locking device can be an integral part of the interlocking device or a separate unit. The link to
enable the machine (guard "closed" and "locked") must be guaranteed. As with electromechanical
interlocking devices with guard locking function, one way of achieving this is by means of series
connection or using the design measure of an integrated fail-safe locking mechanism. The design here
ensures that an enable to start the machine can only be given when the protective device is closed and
the guard locking is safely meshed.
Since there are now several interlocking devices with guard locking functions on the market which
contain a large number of potential-free contacts, the question for design engineers is, what are the right
contacts to integrate in the safety circuit. The familiar symbol for positively driven contacts from IEC
60947-5-1, Annex K is available here:
The standard has a new symbol for monitoring the guard locking of guard locking elements:
This symbol identifies the contacts that the design engineer must integrate in the safety circuit of the
controller in order to receive the message "protective device is closed". This does not apply in the case
of interlocking devices with guard locking and electronic evaluation. The safe enabling outputs are
realized by monitored outputs, usually transistor outputs (AOPD) here.
Modes of operation and functions of guard locking
An interlocking device with guard locking function, can be designed in a number of manifestations. For
example, it can be released or actuated using different types of actuation:
• Power-released
• Released by spring force
• Power-released, power-actuated
• Power-actuated.
However, the standard points out explicitly that when using guard locking that operates according to the
principle of spring force release or energy actuation, the guard locking will be released during a power
failure and that longer stopping times of machinery can arise and represent a hazard. Access to a danger
zone, e.g. before a movement stops, would then be possible. This should be taken into account during
the risk assessment, and additional measures may be required.
6
Fig 3: Actuation of a panic handle to leave a protected area
New in the standard: interlocking devices with electromagnetic guard locking
The interlocking device with electromagnetic guard locking has been newly adopted in ISO 14119. In this
case an electromagnet keeps the protective device closed. Mechanical locking mechanisms are not
present. The prerequisite for using such systems, however, is that the electromagnetic force is safely
monitored in order to ensure that the defined guard locking force of the magnet is always achieved. If this
is not the case, the interlocking device must not enable the machine start. As such a simple
electromagnet is not suitable for such tasks.
The advantage of these systems is that their smooth surfaces can be cleaned more easily. Compared to
conventional systems, there are no openings to support the locking bolt, and no components or modules
protrude on the moving part of the protective device. One disadvantage of the electromagnetic guard
locking is that high locking forces with correspondingly high continuous current must be purchased.
Release of interlocking devices with guard locking
There may be a number of different reasons why an interlocking device which is currently kept closed
should be released contrary to the command given by the machine controller. The standard defines three
new terms on this subject for the first time:
Emergency release
This is the manual release of the guard locking without tools from outside the protected area. It is
intended for freeing trapped persons or for fire-fighting, in other words in situations requiring fast action
and where other risks are present in addition to those risks presented by the machine.
Auxiliary release
As with the emergency re lease, the manual release is effected from outside the protected area, but with
the help of a tool or key. It is intended for interventions where speed is not of the essence, such as for a
repair, when the guard locking cannot be released by the controller as a result of a fault.
This is the manual release of the guard locking without tools from inside the protected area. Frequently, a
kind of "escape route actuator/panic handle" is used here for simple actuation in the case of an escape.
Selection of interlocking devices
The new standard bears the title "Principles for the design and selection of interlocking devices". It is
therefore directed at two, sometimes very different, target groups, namely the manufacturer of
interlocking devices and the design engineer who is deploying them on his products. This ambivalence
does not make the standard any easier to read. However, it does ensure that the specifications for
manufacturer and user are to some extent consistent.
7
Section 6 of the standard offers help in the selection of a suitable interlocking device. "The following
criteria must be taken into consideration when selecting or designing an interlocking device:
•
•
•
•
•
•
•
The conditions and the intended use of the machinery (see ISO 12100)
The dangers arising on the machinery (see ISO 12100)
The severity of the possible injury (see ISO 12100)
The stopping time of the entire system and the access time
The probability of a failure of the interlocking device
The required Performance Level PL (see ISO 13 849-1) or Safety Integrity Level SIL (see IEC
62061) of the safety function
Consideration of dynamic forces such as "bouncing back", especially to be considered for guard
locking
However, other criteria may also play a role depending on the application. One of the most important
factors for selection is the question of whether the stopping time of the overall system is greater than the
time required to reach the danger zone. If this question is answered in the affirmative, an interlocking
device with guard locking must be selected.
The flow diagram in the standard (see Figure 4) is designed to help come to a quick decision about
which type of interlocking device is needed. Irrespective of the technology, a sufficiently high locking
force on the guard locking device must be selected relative to the application. The informative Annex I in
the standard is helpful here.
The selection and specification of a suitable interlocking device with guard locking are the task of a Type
C standard or a mechanical engineer.
As a rule, the selection is made by a mechanical engineer. The device must, however, be selected in
such a way that it can resist the anticipated forces in the application. Dynamic effects, such as "bouncing
back", when a protective device is closed quickly and can rebound, must also be taken into
consideration.
Fig 4: Determination of the need for a locking interlock device
If the anticipated impact forces are greater than the forces that can be withstood by the selected device,
measures must be taken to reduce or prevent these forces. Attenuators such as shock absorbers
represent a solution here.
8
Non-contact interlocking devices
In addition to the familiar and proven electromechanical interlocking devices with positively driven
contacts, ever more non-contact interlocking devices are available and in use.
The advantages of such systems are clear to see. They are as follows:
• Specially suitable for removable protective devices
• Compact and have no external moving parts
• Little vulnerability to dust and liquids
• Easy to keep clean
• Encoded
• Tolerant to misalignment of the guard
• Free of wear and tear.
The new standard ISO 14119 has described these products and incorporated them in the informative
Annexes C and D (see Figure 6, Type 3 and Type 4). The requirements of a non-contact interlocking
device are described in the product standard IEC 60947-5-3. This standard deals with proximity switches
under fault conditions, but does not deal with the subject of manipulation in detail. For this reason, the
issue is dealt with thoroughly in ISO 14119 (see below).
NB: In the past, an interlocking device was also implemented using commercially available proximity
switches, and this is possible under ISO 14119. The EMC requirements are being made more stringent
with the revision of product standard IEC 60947-5-3, however. With the introduction of IEC 61326-3-1
IEC 60947-5-3, it will become difficult to comply with the requirements of this product standard, which is
also required under ISO 14119, using common inductive proximity switches.
Fig 5: Method to determine the possible incentive to tamper with interlocking devices and the requisite measures to be taken by
the manufacturer
9
Type 3 or Type 4 interlocking devices can be used to compensate for problems which can arise during
the use of Type 1 or Type 2 interlocking devices. When considering possible manipulation and the
resultant required measures, interlocking devices equipped with RFID technology are suitable, for
example.
Manipulation of interlocking devices
In addition to selecting interlocking devices that comply with standards, it is also necessary to consider
their design and/or integration in processes in terms of possible manipulation. There is a chapter in the
standard for this. It is entitled "The use of design to minimize the opportunities for circumvention of
interlocking devices". The background to this includes the BGIA manipulation study from 2006, which
established a strong accumulation of manipulation on machine tools.
Amendment 1/2007 of DIN EN 1088 described design requirements aimed at circumvention
opportunities; these have been incorporated in the latest ISO 14119. Section 7 of the standard deals with
this subject. The background is the requirement from the A standard ISO 12100, Section 1.4.
"Requirements on protective devices'', and in particular Section 1.4.1.2 "General requirements''.
Guards and protective devices other than a guard
• must be built strongly
• must not be capable of being easily circumvented or rendered ineffective
The basis for considerations to prevent opportunities for circumvention is a tiered concept based on the
tiered concept in ISO 12100:
• Basic measures, such as securely affixing the interlocking device, protection from external
influences and loosening of the fastening, compliance with tolerances etc., encoding of actuators
on interlocking devices (Type 2 and Type 4 devices), use of electromechanical switchgear with
positively driven contacts etc.
• Establishing whether a manipulation incentive exists. The informative annex in the standard offers
a procedure in the form of a tabular evaluation as suggestion, together with a further, completed
table as example. This enables the possible incentive to manipulate every protective device,
depending on the operating mode, to be checked and documented and for corresponding
countermeasures to be specified where applicable. This systematic approach is also suitable as a
component of the technical file.
Application of additional measures to minimize the opportunities for circumventing interlocking
devices (design check)
Additional measures must be taken if the risk of circumvention in a reasonably foreseeable manner still
exists after implementing the above mentioned measures. For such cases, the standard contains a
diagrammatic representation of the method to determine possible incentives and the measures that
machine manufacturers are required to take. It furthermore indicates additional measures which are
designed to at least make the circumvention of interlocking devices more difficult.
Additional measures
Essentially measures of a design nature are addressed here:
• Accessibility to the interlocking device
• Attaching out of reach
• Cordons or screening
• Attaching in a concealed position
• Avoidance of alternative actuation using available objects
• encoded actuators with low encoding level
• encoded actuators with high encoding level
• Avoidance of removal or changing the position
• Use of non-detachable fastenings. This primarily refers to Type 2 switch actuators.
• Integration of a circumvention monitoring in the Controller
• Status monitoring
• Periodic inspections
• Use of an additional position sensing and plausibility check.
10
Table 3 in the normative part of the standard lists the measures to be carried out when the measures
described in Figure 5 fail to produce the desired result. In some cases alternative measures are
proposed; in others extremely restrictive measures are demanded or recommended.
It is not always possible to implement the measures described above effectively or in a cost-neutral
manner. Therefore in many cases it makes sense to deploy products directly that have been produced
with a high encoding level.
These are now supplied as electromechanical interlocking devices (Type 2 switches - see Figure 7), as
interlocking device with guard locking (see Figure 8) or as electronic safety sensors with integrated RFID
technology.
Interface to the controller
Interlocking devices are safety-related parts of the control system of a machine (SRP/CS in accordance
with ISO 13849-1) or a subsystem of a safety-related part of an electrical control system (SRECS in
accordance with IEC 62061) and must correspond to the requirements of the above standards.
In their technical data and depending on the characteristics of their product, the manufacturers of
interlocking devices generally specify the requisite safety- related parameters (B10d, PFH etc.).
An interlocking device with a required PLr = e under ISO 13849-1 or SIL 3 under IEC 62061 demands a
minimum fault tolerance of 1 (e.g. two Type 1 interlocking devices), because faults cannot normally be
ruled out.
Fault exclusions are possible, however. ISO 13849-2 provides information on this in the informative
annexes (see article "Validation of control systems in accordance with ISO 13849-2" on Page 169). For
example, the following is supplemented in Annex D of ISO 13849-2 (Safety-related parts of control
systems: Validation) under Table D8 (Faults and fault exclusions – Switches - Electromechanical
Position Switches): "For PLe, no fault exclusion is permissible for mechanical (e.g. the mechanical
connection between switch and contact elements) and electrical aspects. In this case redundancy is
necessary." The conclusion from this is that a position switch or guard locking can be sufficient for
applications in PLr = d.
Monitoring the locked position
A further aspect which has been described in detail for the first time in the standard is the monitoring of
the locked position of guard locking. This additional safety function guarantees that a machine can only
be set in motion if:
• the safety device is closed; and
• the guard locking device keeps the protective device closed.
This safety function must be executed according to the risk analysis. All parts of the devices used to
unlock/lock the signal are counted as safety-related parts of the control system. The question here is:
does the guard locking also need to be performed redundantly in the case of a required PL= e or PL= d?
The standard answers this question in its final version: a fault exclusion in PL e and in PL d is possible
on the mechanical components of the guard locking, with the prerequisite that the requirements of guard
locking which are similarly described in the standard are complied with. It means it is sufficient for
applications in accordance with PL e to have just one guard locking and a further redundantly arranged
position switch.
ISO 14119 does not only result in an adjustment to technical progress through the consideration of new
principles of operation for interlocking devices, it also provides practical hints on the design of protective
devices and addresses the fact that protective devices are being repeatedly manipulated in practice. As
such, probability is also being increasingly incorporated in the world of machine safety standards in this
application area too. The standard was published at the end of 2013 and has transitional period of 18
months. Design engineers should be ready to be guided by the specifications and recommendations in
ISO 14119.
Standards Structure
ISO and IEC have already established an overwhelming number of standards. Constantly adding and
updating these standards to keep pace with daily product improvements is not possible.
11
However, classifying standards (ISO/IEC Guide 51) by type and using different types of standards in
combination allows them to be applied to most up-to-date machinery.
Three-tier Structure of International Safety Standards
Type A: Fundamental safety standards applicable to all machinery. Type A standards deal with basic
concepts, principles for design, and general aspects.
(Type A standards are positioned on the top of the international safety standard pyramid, or they deal
with one safety aspect.) Type B: Standards applicable to a wide range of machinery. Type B is divided
into two categories:
B1: Specific safety aspects (i.e., safety distances, surface temperature, and noise)
B2: Safety related devices (i.e., two-hand controls, interlocking devices, pressure-sensitive devices, and
guards)
Type C: Detailed standards applicable to a specific machine or a particular group of machines.
12
Type A
• ISO12100 along with ISO9000, the international standard for quality, and ISO14000 the
international standard for the environment, ISO12100 Series formally became the international
standard for safety of machinery in 2001.
• ISO 14121 (Safety of machinery - Principles of risk assessment) is a typical Type A standard
referred to by ISO12100.
Type B
• ISO13849-1: (Safety of machinery - Safety-related parts of control systems- part 1: General
principles for design). It was developed based on EN954-1, and defines safety-related
requirements for each category in accordance with ISO14121 (risk assessment).
• ISO13850: (Safety of Machinery - Emergency stop - Principles for design). It defines functional
requirements and design principles for stopping machinery in an emergency. It states that the
emergency stop function has the top priority over all other operation modes and that its
effectiveness cannot be overruled by any accidental operational command. It also states that
electrical devices such as emergency stop switches must conform to IEC60204-1 (Electrical
Equipment of Machines)
• ISO13852: (Safety of machinery - Safety distances to prevent danger zones being reached by the
upper limbs.) It defines safe distances to prevent the operator from reaching a dangerous zone.
• ISO14119: (Safety of machinery - Interlocking devices associated with guards - Principles for
design and selection) It defines the selection of safety switches to be used for guards.
• EN954-1: Same as ISO13849-1. However, EN954-1 refers to EN1050, instead of ISO14121.
• EN1088 : Same as ISO14119
• IEC60204-1: (Safety of Machinery - Electrical equipment of industrial machines - Part 1: General
requirements). It defines requirements for electrical systems of machines.
• IEC60079 series Explosion proof electrical devices. This Standard is comprised of many parts
including general requirements.
• IEC60947-5-1: (Low-voltage switch gear and control gear. Part 5: Control circuit devices and
switching elements - Section one: Electromechanical control circuit devices). IEC6024-1 requires
that low voltage switching gear conform to IEC60947-5-1.
• IEC60947-5-5: (Low-voltage switch gear and control gear - part 5-5: Control circuit devices and
switching elements - Electrical emergency stop device with mechanical latching function) It
defines mechanical requirements (latching function, button colors, shapes, etc.) and electrical
requirements (direct positive opening action function, etc.)
Type C
• ISO10218: (Manipulating industrial robots - Safety)
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Compressed Gas Safety
Compressed gas cylinders can present a variety of hazards due to their pressure and /or contents. All
compressed gases used at UGA must be ordered through the Purchasing Department. In addition to the
standard required work practices for inert gases, hazardous gases may require additional controls and
work practices including, but not limited to, the use of gas cabinets, gas monitors, emergency shutoffs,
proper equipment design, leak testing procedures, and the use of air supplying respirators for certain
highly toxic gases. The ESD can be contacted for assistance with these requirements and to provide
assistance with the safe design of equipment which involves the use of hazardous gases. It is the
objective if this policy to ensure that employees handling compressed gases properly handle, store, and
use them according to OSHA, DOT and other regulatory requirements. Students, regardless of status,
are not to handle compressed gases unless under the direct supervision of a faculty or staff member.
References: 29 CFR 1910.101, 102, 103, 104, 252, 253; 49 CFR 171 – 179; 14 CFR part 103;
Compressed Gas Association Pamphlets, C-6-1968 and C-8-1962
Compressed Gas Use Applications
Class 1 Application - Use of Inert Gases - Gases which are non-flammable and non-toxic, but which
may cause asphyxiation due to displacement of oxygen in poorly ventilated spaces
Class 2 Application - Use of Flammable, Low Toxicity - Gases which are flammable (at a concentration
in air of 13% by volume or have a flammable range wider than 13% by volume), but act as non-toxic,
simple asphyxiants (e.g. Hydrogen, methane)
Class 3 Application - Use of Pyrophoric Gases and Liquids - Gases or liquids which spontaneously
ignite on contact with air at a temperature of 130 F or below.
Class 4 Application - Use of Corrosive, Toxic, and Highly Toxic Gases - Gases which may cause acute
or chronic health effects at relatively low concentrations in air. (See Appendix A of Gas Monitoring
Program for additional details on this definition and examples)
Class 5 Application - Use of Compressed Gases in Fume Hoods
Use of Cryogenic Liquids - Use of liquids with a boiling point below -238 F (-150 C).
Use of Fuel Gases for Welding, Cutting, Brazing - Use of oxygen and fuel gases (e.g. propane,
acetylene) for gas welding and cutting applications.
Engineering Controls / Design Considerations
This includes a listing of typical engineering controls, referenced in the list above. In some cases, the
National Building Code may require additional controls. Additional controls or changes from the controls
listed below may be appropriate for a particular application or experiment. The controls appropriate for
each operation will be identified through the hazard review process.
1. Gas Cabinets -With the exception of cylinders containing a non-toxic, flammable gas, and cylinders
used in fume hood applications, hazardous gas cylinders must be housed in gas cylinder cabinets.
These cabinets must be equipped with sprinkler protection, and must be constructed and ventilated
according to applicable building code requirements. These requirements include, for instance, but are not
limited to, the need to provide 200 fpm air flow at the cabinet window.
2. Interlocks - In addition to automatic shutoff of gas flow due to loss of power or ventilation (described
below), it will often be appropriate for an automatic shutdown of gas flow due to conditions such as high
system pressure, high gas delivery pressure, loss of vacuum, loss of cooling, or other conditions
identified through the hazard review process.
3. Emergency Off - Where gases are used in gas cabinets, the emergency off buttons should be located
at the lab doorway. Activation of this button will cause pneumatic valves to shut, stopping gas flow. This
button should kill electrical power to hazardous lab equipment as well.
4. Equipment Enclosures and Ventilation - Experimental apparatus using hazardous gases should be
contained in an enclosed and exhausted enclosure (i.e., a laboratory hood). These enclosures must be
connected to the exhaust ventilation system. Ventilation rates must be sized to allow for 100 fpm of air
14
flow through the largest open enclosure door. Mass flow controllers carrying hazardous gases must be
housed in a separate ventilated enclosure (or in an enclosed compartment of a larger tool enclosure) so
that 100 fpm exhaust flow is available at the largest open door to the enclosure. All components should
be readily accessible for maintenance.
5. Smoke Detection - All labs using hazardous gases will have a smoke detector which is connected to
the building alarm system. In certain cases, it may be necessary to interlock smoke detector activation
with the shutdown of hazardous gas flow.
6. Sprinkler Protection - Where hazardous gases are contained in gas cabinets, sprinkler protection
should be provided to the interior of the gas cabinet. In some cases, this protection is required by code.
Sprinkler protection is recommended in all labs using hazardous materials.
7. Emergency Power - Emergency power is recommended to power exhaust fans connected to
hazardous gas enclosures. In certain cases, this protection is required.
8. Pneumatic Shutoff Valves - All corrosive, toxic, flammable, and pyrophoric gases will contain a
normally closed pneumatic shutoff valve, rated for at least full cylinder pressure, and located immediately
downstream of the cylinder valve. This valve will shut in the event of power failure, remote actuation of
an emergency off button (see this topic), or other appropriate conditions such as hazardous gas alarm
activation.
9. Scrubbers - When hazardous waste gases are generated, it is often advisable to treat/react these
gases prior to exhaust from the building. This may involve the use of bubblers in a fume hood or
sophisticated units for larger scale hazardous gas processes. Note that in some cases (e.g. minimal
volumes of hazardous gases produced) scrubbers may be not necessary or even unadvisable. Where
scrubbers are used, they need to be carefully reviewed as part of the hazard review. Maintenance
requirements and procedures need to be clearly understood and followed.
10. Vacuum Pumps - Vacuum pumps used for hazardous gases need to be carefully selected.
Depending on the gases being pumped, special precautions may be necessary. For processes where
pyrophoric gases are used, pumps need to be continuously purged with nitrogen, with loss of nitrogen
flow causing the pyrophoric gas supply valves to close. Pumps used for oxygen service will need to be
prepared for this services which includes the elimination of hydrocarbon oils for use due to flammability
concerns. In some cases, such as the use of highly toxic gases, vacuum pumps will need to be housed
in a ventilated enclosure.
11. Flow Restrictors - A means to limit hazardous gas flow rates to just over maximum flow needed
must be installed immediately downstream of each hazardous gas cylinder. For small scale experiments,
such as fume hood use, a needle valve is sufficient. For large cylinders a flow restricting orifice, installed
by the gas supplier in the cylinder valve or installed in the gas purge panel is required.
12. Ventilation Alarms - All ducts connected to enclosures used to exhaust hazardous compressed gas
cylinders or gas carrying components must be connected to a ventilation alarm. Typically, activation of
this alarm will cause pneumatic gas supply shutoff valves to close.
13. Eyewash and Showers - A safety shower or eyewash with a wand is required to be present in areas
where corrosive gases are used or stored.
14. Purge Panels - Where corrosive, pyrophoric, or toxic gases are in use, the gas installation must
include means to adequately purge the area between the cylinder valve and the regulator with an inert
gas prior to breaking these connections for maintenance or cylinder change. Inert gases used for this
purpose must be used solely for this purpose and not connected to other apparatus. Failure to
adequately purge cylinders can result in lack of ability to close the cylinder valve or "regulator creep"
which allows full cylinder pressure to be transferred to the low pressure side of the regulator. Review the
requirements listed in Purge Panels for your existing or planned installation.
15. Gas Monitors - See Gas Monitoring Program for a complete description of University gas monitoring
requirements.
15
16. Piping and Fittings - All gas piping must be compatible with the gases used and capable of
withstanding full cylinder pressure. For example, Tygon tubing should never be used with hazardous
gases or low hazard gases unless one end is open to atmosphere. Fittings should be selected based on
the service needs. Face seal or welding fittings should be used for hazardous gas service wherever
possible. All gauges and components subject to leakages which carry hazardous gases must be
contained in an exhausted enclosure.
17. Hardware - Never lubricate, modify, force, or tamper with a cylinder valve. Use the appropriate
regulator on each gas cylinder. Adaptors or homemade modifications can be dangerous. Assure all
components of the experimental apparatus that can handle full cylinder pressure or are otherwise
protected. Oil or grease on the high-pressure side of an oxygen, chlorine, or other cylinder of an oxidizing
agent can lead to an explosion. Whenever back siphoning of chemicals into the cylinder might be a
problem, use multiple traps or check valves.
Work Practices and Procedures - Hazardous Gases
1. Hazard Review - A hazard assessment is required for the following processes involving the use of
hazardous gases:
a. New or relocated equipment using a toxic, corrosive, or pyrophoric gas (contact 515-6860).
b. New or relocated equipment using a flammable gas in a non-standard application (Contact 515-6860).
Analytical equipment fuel gases, welding, cutting, brazing, and small scale use in fume hoods are
considered standard applications.
c. Existing gas installations should be self-inspected by the work area supervisor or Principal
Investigator. Please contact ESD for assistance with this review.
d. Existing installations using hazardous gases which are considered to present a significant risk or show
design deficiencies will have a hazard review conducted at the discretion of the ESD.
2. Training - All persons handling or using cylinders must have basic training. Review of the information
contained in this section, review of any additional information in the written safety plan for the work area,
and hands-on assistance by an experienced gas user will meet this minimum requirement. Additional
compressed gas safety training can be obtained through the ESD.
3. Hazard Information - The gas user must be thoroughly familiar with the properties of each gas they
are using. A review of a good quality SDS is necessary. (See the SDS section of the ESD Home Page).
4. Ordering - All gas cylinders used on main campus may only be ordered and received through Central
Stores. This allows for leak testing of highly toxic gases prior to delivery to your building. It also is the
mechanism for inventory of gases used at the University.
5. Receiving - Be sure the cylinder tag (don't rely on cylinder stenciling or color coding) indicates the gas
that has ordered. Hazardous gases (flammable, pyrophoric, toxic, corrosive) must be transported directly
from the shipper to the end use location. No staging of hazardous gases is permitted. Low hazard gases
(e.g. inert gases, Oxygen, Freon) may be stored temporarily in designated locations which provide
means for securing cylinders with chains or straps.
6. Leak Testing - Toxic, corrosive, and pyrophoric gases must be leak tested at the following intervals;
receiving, installation, disconnect/shipping. Highly toxic gases are leak tested by Environmental Health
and Safety personnel prior to delivery to the user. The end user is responsible for other leak test
intervals. It is important that toxic gases be leak tested prior to removal from their exhausted enclosures
and subsequent transport.
7. Storage -- For short term experiments using hazardous gases, always select the smallest returnable
cylinder available. Non-returnable cylinders are strongly discouraged. If non-returnable cylinders must be
used, you must have a way to treat the remaining contents of the cylinder so that the cylinder valve can
be removed prior to disposal. In cases where the gas will be used over an extended period of time
(several months to more than one year), you should order a gas quantity that will last for three to six
16
months. Corrosive gases should be returned to the gas supplier within one year to avoid regulator and
cylinder valve problems due to corrosion. In storage, restrain cylinders of all sizes by straps, chains, or
a suitable stand to prevent them from falling. Segregate full cylinders of low hazard gases from "empty"
cylinders awaiting return to the vendor. Assure hazardous gas cylinders are constantly stored in a
suitable exhausted enclosure as described in Engineering Controls. Do not expose cylinders to
temperatures higher than about 50 C. Some small cylinders, such as lecture bottles and cylinders of
highly toxic gases, are not fitted with rupture devices and may explode if exposed to high temperatures.
Never place cylinders where they may become part of an electric circuit. Avoid areas that are damp or
subject to other corrosive materials.
Do not store flammables and oxidizers together. Keep cylinders in storage upright, secure, and
interlocked into a compact group. Protect cylinders stored outside from standing water by providing
proper drainage. Where outdoors storage is necessary, an overhead cover is necessary to avoid sunlight
and rain.
8. Transporting Cylinders - Hazardous gas cylinders must be transported directly from the gas supplier
to the end user storage location, unless an exhausted and approved "staging" area has been
constructed. Cylinders must never be transported without valve protection caps in place. Never move a
cylinder with a regulator attached! . Cylinders larger than lecture bottle size should be chained or
strapped to a wheeled cart during transport to ensure stability. Transportation of cylinders must be done
only by trained personnel using approved trucks. Handle cylinders of compressed gases with the respect
that high-energy sources deserve.
9. Shipping - Promptly remove the regulators from empty cylinders, leak test hazardous gases, and
replace the protective caps at once. Mark the cylinder "MT". Never bleed a cylinder completely empty.
Leave a slight pressure to keep contaminants out. Toxic, corrosive, and pyrophoric gases must remain in
their exhausted enclosures until shipped back to the supplier. Cylinders used at UGa main campus may
only be shipped through arrangements with Central Stores.
10. Changing Cylinders - Special procedures are required for changing toxic, corrosive, and pyrophoric
gases and liquids. A proper cylinder purge panel is needed for high hazard gases, along with an
adequate purge procedure. Persons changing gas cylinders requiring SCBA must work with a partner
who is identically equipped.
11. Changing Pump Oil - Hazardous gases may be absorbed into vacuum pump oils. Personnel
performing vacuum pump oil changes on pumps used with highly toxic gases must use SCBA for pump
oil change. Hot pump oil should be allowed to cool prior to c hanging.
12. Other Equipment Maintenance Considerations - Consider equipment maintenance needs in
advance.
a. Consider reaction byproducts (e.g. use proper skin and eye protection when cleaning process
chambers or vacuum pumps). "Low hazard" gases such as Freon will generate chlorine and fluorine
decomposition products. Be sure to LOCK OUT upstream gas lines leading to equipment prepared for
maintenance. Compressed gases are a hazardous energy source requiring lockout procedure. Be sure
to adequately purge lines following lockout procedures and before beginning maintenance.
13. General Work Practices - Never use a cylinder that cannot be identified positively. Do not use
compressed gas or compressed air to blow away dust or dirt (unless specifically equipped with a 30 psi
or less diffuser for this application as used in machine shops). Flying dust and debris, as well as high
pressure air itself, can cause significant injury. When not in use, close cylinder valves. The main cylinder
valve should be tightly closed, but needle valves should only be finger tight to avoid ruining the valve
and/or valve stem.
14. Emergency Procedures - Leaking cylinders should not be removed from their exhausted
enclosures. Actuate remote emergency gas shutoff valve/button, if present. (Installed highly toxic gases,
if properly installed, will have flow limiting devices and/or automatic cylinder shutoff valves in place to
limit and shutoff the gas supply.)
Close the main cylinder valve if a leak is stopped or slow, hazardous gases are contained in their
enclosure, and it is clearly safe to approach. Do not extinguish a flame involving a highly combustible gas
17
until the source of gas has been shut off, otherwise, it can reignite, causing an explosion. Cylinders
leaking at the cylinder valve should be reported to Campus Police (this should be reported as a
"nonemergency" if the cylinder and gas are contained in an exhausted enclosure). If a hazardous gas is
released into an unexhausted enclosure and the gas supply cannot be promptly cutoff, actuate the
emergency evacuation procedure in your area and contact Campus Police.
This procedure will also be initiated automatically if gas monitors trigger the building evacuation alarm.
The Superfund Amendments and Re-authorization Act of 1986 (SARA Title III) states that releases of
extremely hazardous substances must be reported to EPA. Accidental discharge of cylinder contents will
be promptly reported to the ESD which will make reports to EPA. Cylinders found to be leaking upon gas
delivery should not be accepted from the gas supplier.
Fume Hood Use of Compressed Gases
Hazardous gas use in fume hoods is appropriate under the following conditions:
1. The experimental apparatus fed by the hazardous gas is located inside the same hood.
2. The experimental apparatus is appropriate to be stored in the hood.
3. The experiment involves low gas pressure and flow rates
4. The experiment will be attended.
5. The engineering controls used for the hazardous gas in the fume hood provide equivalent safety to a
gas cabinet
installation.
Since fume hood face velocities provide insufficient protection against pressurized gas leaks, special
care must be taken when hazardous gases are used in fume hoods.
The following is required for fume hood applications:
1. The smallest possible cylinder should be used for the experiment (a six-month bottle supply for routine
use gases is appropriate, while smaller cylinder supplies are suggested for short term use). Make an
effort to obtain gas cylinders in returnable bottles. Order bottles with the lowest cylinder pressure
possible.
2. Use a flow restricting orifice or needle valve to restrict flow to only that needed for the experiment.
3. Toxic and corrosive gases must be used with a normally-closed pneumatic shutoff valve, located
immediately downstream of the cylinder regulator, which closes with exhaust loss or power failure. This
valve should be fed from a pneumatic air (or nitrogen) supply valve, located at the entrance to the lab.
Activation of this quarter turn valve, causes the pneumatic cylinder valve to close in the event of an
emergency. The air supply to the pneumatic shutoff valve should also be connected to a three-way
electric solenoid that fails open on power loss, bleeding air from the pneumatic supply line, and closing
the valve. A static pressure alarm, monitoring the hood exhaust duct, will be interlocked to the pneumatic
shutoff valve.
4. Where determined necessary, run reactive or toxic gases through a suitable scrubbing media, then
directly into the exhaust hood plenum (scrubber output hose should be placed into the exhaust slot).
5. Place the cylinder in rear of the hood. High pressure leaks can readily escape the hood and capture is
best in the rear of the hood.
6. Assure all components in experiment can withstand full bottle pressure or incorporate pressure relief
(run relief line into a hood slot)
7. All gas lines connected to the hazardous gas source, including purge lines and gas supply lines, must
be completely contained inside of the hood.
18
Use of Fuel Gases for Welding and Cutting
OSHA lists requirements for oxygen-fuel gas welding and cutting in 29 CFR 1910 .253. Cylinder handling
precautions, materials of construction, and additional requirements are listed. This information should be
reviewed by persons who will be using acetylene, oxygen, and other fuel gases or those who are
designing facilities and equipment for this purpose. Please see the Personal Protective Equipment
section of this manual for information on eye protection for welding and cutting operations. Be sure that
all fuel gases are shut off at the cylinder valve after each use.
Cryogenic Liquids
Cryogenic liquids, as with all gases used at the University, must be ordered through Central Stores. Only
inert gases are permitted at the University in portable cryogenic containers. Liquid oxygen, liquid
hydrogen or other flammable or toxic cryogenic liquids are not permitted. Where appropriate, exterior
tanks of liquid oxygen, used as a source of gaseous oxygen, may be installed by qualified individuals.
All cryogenic liquids should be used with caution due to the potential for skin or eye damage due to the
low temperature, and the hazards associated with pressure buildups in enclosed piping or containers.
Portable containers should only be used where there is sufficient ventilation. Do not place containers in a
closet or other enclosed space where there is no ventilation supply to the area. The buildup of inert gas
in such an area could generate an oxygen deficient atmosphere.
A full-face shield, loose fitting cryogenic handling gloves, apron, and cuffless slacks are the
recommended equipment for transferring cryogenic fluids. Special vacuum jacket containers with loose
fitting lids should be used to handle small quantities. Vacuum jacketed containers provided by the gas
supplier will have overpressure relief devices in place. When plumbing cryogenic liquids, it is very
important to include a pressure relief valve between any two shutoff valves. Also, any space where
cryogenic fluids may accumulate (consider leakage into enclosed equipment as well) must be protected
by overpressure relief devices. Tremendous pressures can be obtained in enclosed spaces as the liquid
converts to gas. For example, one cubic centimeter of liquid nitrogen will expand to 700 times this
volume as it converts (warms) to its gaseous state. Lines carrying liquid should be well insulated.
Containers to be filled with cryogenic liquids should be filled slowly to avoid splashing. Cryogenic
containers showing evidence of loss of vacuum in their outer jacket (ice buildup on the outside of the
container) should not be accepted from the gas supplier. If already on a site, please contact Central
Stores to obtain a replacement. Contact with air (or gases with a higher boiling point) will can cause an
ice plug in a cryogenic container. Should ice plugs be noted, please contact Campus Police to obtain
assistance.
Constructing Equipment to be used with Hazardous Gases
The ESD has additional guidelines and resources available to assist in the design of equipment which
will be used with hazardous gases. Please contact ESD early in the planning stage of the project.
Flammable, Oxidizing and Other Pressurized Gases
Gas Cylinders
I. The following guidelines shall be followed by all personnel using or storing pressurized gases.
A. All personnel who will be working in areas where compressed gases are used or stored shall receive
instruction regarding the safe handling of cylinders, the use of appropriate personal protective
equipment, and the steps to be taken in the event of a leak or fire in an adjacent area.
B. Do not remove any labels or other form of identification from any gas cylinder.
C. Know how to detect the presence of leaks from any gas cylinder in your work area. Of particular
importance are flammable and toxic gases. Contact ESD at 2-5801 in the event of a cylinder or valve
leak.
19
Gas cylinder storage and labeling
A. When receiving a gas cylinder do not accept it until the following items are verified:
1. the contents are identified either by labels or stencils,
2. it contains the appropriate DOT label,
3. it contains a valve protection cap (if so designed).
B. Store gas cylinders in a well-ventilated area away from direct heat. All cylinders must be stored in an
upright position secured to a sturdy permanent structure to prevent the cylinder from falling or being
knocked over. Place protective caps on those cylinders which are not in use. All cylinders should be
anchored individually.
C. Gases should be stored in accordance with their physical and chemical properties. See individual
material safety data sheets (MSDS) for specifics with regards to this information.
D. Close valves on empty gas cylinders and mark them empty. Empty cylinders should be removed from
the lab as soon as possible. Store empty and full gas cylinders separately. Cylinders are considered
empty if their pressure is less than 25 psig. All cylinders will be considered full that are not properly
identified.
E. Store flammable gases away from oxidizing gases.
F. Do not store gas cylinders near elevators, ventilating systems, or other openings through which gas
may spread to other parts of the building should a leak occur.
Do not store cylinders where there is a risk of dropping them or having heavy objects fall on them.
G. Cylinders containing gases that are corrosive to cylinders or cylinder valves or that may become
unstable while stored in the cylinder shall have a maximum retention period of six months, unless a
shorter period is otherwise specified by the manufacturer.
H. Cylinders of all gases having a health hazard rating of 3 or 4 (refer to the MSDS for rating) must be
kept in a continuous mechanically ventilated storage hood or other continuous mechanically ventilated
enclosure. There must be no more than three cylinders within the hood or other ventilated enclosure.
Contact ESD for answers to questions regarding the storage of cylinders in continuous mechanically
ventilated enclosures/storage hoods.
I. The maximum volume size of a cylinder of a gas with a health hazard rating of three or four stored in a
laboratory work area shall be limited to 0.1 cubic feet. Cylinders of a gas with a health hazard rating of
three or four stored in a laboratory work area shall be limited to no more than three maximum size
cylinders or an equivalent volume (0.3 cubic feet) of smaller sized cylinders.
J. The maximum volume size of a flammable or oxidizing gas cylinder stored in a laboratory work area
shall be limited to two cubic feet. Cylinders of flammable and/or oxidizing gases stored in a laboratory
work area shall be limited to no more than six maximum sized cylinders or an equivalent volume (12
cubic feet) of smaller sized cylinders.
K. The maximum volume size of a liquefied flammable gas cylinder stored in a laboratory work area shall
be limited to 0.6 cubic feet. Cylinders of liquefied flammable gas stored in a laboratory work area shall be
limited to no more than three maximum sized cylinders or an equivalent volume (1.8 cubic feet) of
smaller size cylinders.
L. Gas cylinders shall not be retained for more than ten years. Small, disposable, empty, lecture
cylinders may be discarded in the lab trash after the valve stem has been removed. Small disposable
lecture cylinders that are not empty may either be returned to the supplier or disposed of by a licensed
gas cylinder disposal company. ESD shall be consulted prior to disposing of a cylinder using the
preceding methods. Non-disposable cylinders must be returned to the supplier.
M. Cylinders and other containers stored outdoors shall be stored off the ground on a raised concrete
pad and within a covered non-combustible rack. They shall not be stored where they are at risk of
dropping, having heavy objects dropped on them or being struck by a vehicle.
20
Proper handling of gas cylinders
A. Always open cylinder valves slowly. Never force the valve open. If the valve cannot be opened by the
wheel or small wrench provided, return the gas cylinder. To shut down a system, close the cylinder valve
and relieve the pressure from the entire system through a hose that is not being used.
B. Never interchange regulators and hose lines among different types of gases.
C. Always turn off cylinders from the main stem valve (not the regulator). Turn off cylinders slowly.
D. Suitable equipment must be available for moving cylinders and other portable containers. Hand trucks
must be equipped with a clamp or chain to secure the container in place or they must be specifically
designed for container handling. Never drag, roll, or slide a cylinder in an attempt to move it.
E. Never drop cylinders; never permit cylinders to strike each other; and never strike cylinders with a
metal instrument.
F. Inspect cylinders regularly for corrosion or leaks. In case of a leak, promptly remove the cylinder to the
outside (in accordance with manufacturer’s recommendations) and call ESD for assistance.
G. Do not use cylinders without a regulator.
H. Never attempt to refill a cylinder.
I. Never tamper with any part of a valve such as the safety nuts or packing nuts.
Equipment Safety for Radio Frequency and Microwave Processing
The use microwave technology is increasingly being investigated for existing and new materials
processes. Particular advantages and benefits have been identified for applications in ceramics. As
progress is made towards commercializing these applications, those who are unfamiliar with the
processing equipment will need to be aware of the associated safety issues. This paper reviews the
current literature on related health effects, regulatory safety standards, equipment designs practices and
guidelines for safe equipment use.
Introduction
Whenever microwave or radio frequency (RF) technology is considered for a particular industrial
process, one of the first concerns to be raised is “Is it safe?” Quite often the answer is “Yes, depending
on how you use it.” Admittedly, such a response is hardly comforting to a key decision maker who is new
to or unfamiliar with microwave processing. Without a thorough understanding of the issues regarding
the safe use of microwave technology, a cautious and slightly skeptical manager might well be reluctant
to invest in a new process having unforeseen and potentially costly consequences.
An understanding of a few basic topics relating to the health and safety issues involved in microwave and
RF (hereinafter the term “microwave” includes RF frequencies) processing provides confidence that it
can be as safe as most conventional heating technologies. The most important topics relate to a) known
health effects from working with microwave equipment, b) established regulatory standards and guidelines for safety equipment use, c) design practices of industrial equipment that ensure adherence to
safety standards, and d) recommended practices and procedures for safe equipment operation.
The following sections present an overview of these topics, highlighting certain key fundamentals that will
help provide a general understanding and working knowledge for ceramists involved in materials
research and process development. Certain topics, particularly equipment design, require a much
greater degree of study to fully understand all aspects of importance. As specific applications progress
towards commercialization, ceramists will prudently seek the advice and involvement of experts in these
fields to ensure a successful and safe implementation of their technology.
21
Health Effects related to Microwave Equipment
Public concern over the health and safety issues relating to human exposure to electromagnetic fields
has increased dramatically in recent years. Much of this concern is due to the rapid proliferation of cell
phone use worldwide, even though electromagnetic fields are emitted by numerous other natural and
man-made sources.1 As a result, numerous studies have been and are being conducted to determine
the health effects and risks. Findings covering a broad range of frequencies have been published.
Of particular interest are studies involving the “non-ionizing” microwave frequencies, generally de- fined
as ranging from 3 kHz to 300 GHz, whose energy levels are an extremely small fraction of that required
to ionize tissue and disrupt cellular DNA.2 This is in contrast to Gamma and X-ray frequencies having
energy levels sufficient to ionize (displace electrons within the atomic structure of) exposed materials.
The primary – and as yet, the only definite, proven - effect in biological materials of microwave radiation
exposure is thermal heating3 , which presents a potential health risk due to overheating. However, much
of the public concern has been focused on whether any thermal effects exist and pose a health risk.
Cancer Related Effects of Microwave Exposure
Numerous recent epidemiological studies on the relationships between various cancers and exposure to
electromagnetic fields have been reviewed by Elwood.4 Sources of emissions studied included radio and
television transmitters, cell phones, radar and occupational environments, while cancer types studied
included brain, lung, testicular, lymphatic and hematopoietic cancers, adult and childhood leukemia and
Hodgkin’s disease. Not all studies gave details of the level of exposure, but those that did indicate exposure levels were within established regulatory or otherwise recommended guidelines where such guidelines exist.
The results of these studies are inconsistent at best and show no statistically significant evidence of a
link between incidences of cancer and exposure to electromagnetic fields at microwave frequencies. In
most cases the incidence rate showed no significant increase in risk from casual exposure. For example,
some studies on broadcast transmitters indicated a marginally increased risk amongst residents near
one such emission source but no increased risk near other similar sources. One study on occupational
expo- sure actually showed a decrease in mortality rate that was attributed to a “healthy worker effect”.
Non-Cancer Related Effects
Clinical and epidemiological studies of effects on cataracts, sexual function and fertility, spontaneous
abortion and birth defects, neurological and cardiovascular disorders, and other non-cancer
epidemiological effects have been conducted.1 As in the cancer related studies, the results are largely
inconsistent and, in some cases, subject to scrutiny due to confounding by other causes. For example,
inconsistent findings between studies on cell phone use conducted in different countries were suspected
to be partly due to cultural influences.
Where conclusive evidence of an effect exists, it has mostly been shown to be due to a thermal response
of the tissue. An interesting example is the auditory response from high intensity pulse modulation of
electromagnetic fields, whereby thermal expansion in soft tissues in the head is conducted to the ear.5
This annoying effect, however, is not a health risk provided the heat absorption is not sufficient to cause
tissue damage. On the other hand, the eyes and testes are considered particularly vulnerable to
excessive heating due to their comparative lack of blood flow as a heat dissipation mechanism.
While the above mentioned studies have failed to provide convincing evidence of a strong relation- ship
between exposure and cancer, it should be noted that one cannot conclude from these results that other
possible hazards do not exist. Furthermore, most studies generally cover a relatively short time frame,
thus no conclusions can be made as to the likelihood or risks of any long term effects.6
Thermal Hazards to Microwave Exposure
Thermal injury is a time-temperature phenomenon whereby the rate of tissue cell protein destruction
exceeds its rate of self-repair for an amount of time sufficient to terminate cell metabolism.7 The rate of
protein destruction increases with temperature, thus decreasing the time required for thermal injury as
illustrated in Figure 1.
22
Figure 1. Threshold temperature vs. time at temperature for skin burns.
The rate at which a given volume of tissue is heated by electromagnetic energy varies according to
frequency and power density. Higher frequencies have a shorter penetration depth and thus will dissipate
heat in a more concentrated volume nearer the skin surface. Similarly, a focused or “contact” source of
power concentrates its energy within a smaller volume, resulting in a greater rate of local temperature
rise. Therefore, a lower power level is required to cause burns by high frequency and/or focused
exposures.8 As examples, a) a 20 Watt laser can cause a burn within a few seconds whereas a 150
Watt light bulb can make a person feel comfortably warm for hours, and b) a given power level at RF
frequencies will cause a lower rate of temperature rise at a given location in a human body than at
microwave frequencies due to the difference in penetration depth.
Cardiac Pacemakers
Many electronic devices can be subject to faulty operation if not properly shielded to prevent radio
frequency interference (RFI). While some very early pacemakers were designed without RFI shielding,
most or all pacemakers manufactured since the mid-1970’s include such shielding. A series of studies
conducted to determine the maximum threshold of interference for safe operation indicated that newer
models could withstand levels well above 1 mW/cm2.9 As a result, an editorial was published by the
American Medical Association stating that the pacemaker interference issue “does not at this time
constitute an important clinical problem.”10
High Voltage
Although the health effects of electromagnetic exposure seem to generate the most public interest and
concern, the hazards associated with high voltage are of equal importance to those working with
industrial microwave equipment. Almost all microwave generators contain high voltage circuitry which
can be lethal while the equipment is in operation and, in some cases, during non-operation depending on
the equipment design. Most microwave generators contain circuitry that stores a high voltage electric
charge in one or more capacitors. Although most do, not all designs provide for automatic safe discharge
of stored energy, thus creating a potential hazard to service personnel.
Injury or death may result when the human body becomes part of an active electrical circuit having a
current capable of overstimulating the nervous system or damaging internal organs.11 The extent of
injury due to exposure to high electrical energy depends on the type (AC or DC) and magnitude of
electrical current, the path in which the current flows through the body, and duration of current flow.
Direct contact with electrical energy, such as due to a high voltage arc to the body, often results in burns
to the skin and internal tissue.
A 50 or 60 Hertz (Hz) alternating current (AC) of 20 milliamps (mA) flowing through the chest area for an
extended period can cause death due to respiratory paralysis, whereas a current of 100 mA can cause
ventricular fibrillation.12 Under dry conditions, the body’s resistance is approximately 100,000 Ohms,
while wet or broken skin can reduce the resistance to 1,000 Ohms. Thus, exposure to common 120 Volts
AC mains voltage under dry conditions will produce a current of only 1.2 mA which is barely perceptible,
but under wet conditions the resulting 120 mA current can cause death due to ventricular fibrillation. High
voltage energy can further reduce the body’s overall resistance to 500 Ohms by breaking down the skin
23
layer. This results in extremely high current flow which can cause cardiac arrest and internal organ
damage. The human body can tolerate up to five times the level of direct current (DC) than AC.13
Typical power supplies used in microwave ovens generate approximately – 4,000 Volts which can result
in a body current of 8 Amps. Industrial microwave generators typically generate much higher voltages.
Thus, even with the higher tolerance threshold, exposure to these voltages can quickly cause death due
to cardiac arrest.
Regulatory Standards For Safety
Electromagnetic Exposure
An important parameter used in establishing guidelines for reducing the risk of injury due to exposure to
electromagnetic field is the “specific absorption rate” (SAR) which is usually expressed in units of Watts
per kilogram (W/kg).14 Numerous studies have determined the minimum SAR at which a risk of thermal
injury exists, and various international government agencies have adopted standards to limit exposure
such that the maximum safe SAR is not exceeded.
Most European countries have adopted guidelines established by the International Committee on NonIonizing Radiation Protection (ICNIRP) for maximum safe occupational and general public exposure
levels for whole -body average SAR and localized SAR.15 These guidelines give rise to reference levels
for plane wave power densities as given in Table 1 for frequency ranges of interest.
Frequency Range
10-400 MHz
400-2000 MHz
2-300 GHz
Occupation
1
f/400
5
General
0.2
f/2000
1
Table 1. ICNIRP recommended exposure reference levels for plane wave power densities (mW/cm2).
The International Electrotechnical Commission (IEC) has specified a standard for industrial microwave
heating equipment which defines maximum exposure levels for equipment operating under “normal
conditions” (as the equipment was designed or intended) and “abnormal conditions” (such as with an
empty cavity). Under this standard, which is applicable to equipment operating in the frequency range
from 300 MHz to 300 GHz, the power density is measured at least 5 cm from any accessible location on
the equipment and limited to 5 mW/cm2 during “normal” operation and 10 mW/cm2 during “abnormal”
operation.
Figure 2. ANSI/IEEE recommended maximum permissible exposure (MPE) for controlled environments
per standard C95.1-1999.
24
Figure 3. IEEE recommended maximum permissible exposure (MPE) for uncontrolled environments per
standard C95.1-1999.
In the USA, the Federal Communications Commission (FCC) has adopted standards based on
recommendations developed by the American National Standards Institute (ANSI) and Institute of
Electrical and Electronics Engineers (IEEE) as defined under IEEE standard C95.1.17 This standard
defines the maximum permissible exposure (MPE) averaged over a period of six minutes for “controlled
environments” (where persons exposed generally are either cognizant of the potential for exposure or
simply passing through the environment) and “uncontrolled environments” (where persons exposed have
no knowledge or control over their exposure) as summarized in Figures 2 and 3. Table 2 highlights MPE
values for specific industrial, scientific and medical (ISM) frequencies commonly used in materials
processing applications.
Frequency
915/896 MHz
2.45 GHz
5.8 GHz
Controlled
Environment
3
8.2
19
Uncontrolled
Environments
0.6
1.6
3.9
Table 2. IEEE recommended maximum permissible exposure (MPE) for plane wave power
densities (mW/cm2) at ISM microwave frequencies.
The Occupational Safety and Health Administration (OSHA) has established a regulation applicable to
frequencies from 10 MHz to 100 GHz whereby exposure is limited to no more than 10 mW/cm2
measured at 5 cm from the emission source and averaged over a 0.1 hour period.18 OSHA further
establishes regulations for hazard labeling of equipment.
High Voltage and Equipment Safety
Safety guidelines have been established for voltages used with and generated by microwave equipment.
Although the National Electrical Code (NEC) defines high voltage as greater than 600 Volts AC19,
various safety requirements for electrical wiring apply to all voltages. These requirements, applicable in
the United States, include circuit overload and ground fault protection devices, wire sizes and materials,
conduits and shielding, enclosures and workspace clearances. Specific requirements apply to industrial
machinery which includes most microwave processing equipment. Similar standards and requirements
25
have been established internationally by British Standards (BS), European Committee for
Electrotechnical Standardization (CENELEC), International Electrotechnical Commission (IEC), and
Japanese Industrial Standards (JIS).
Microwave generators, power supplies and other components that make up industrial processing
systems are governed under various standards established for product safety. Underwriters Laboratory
(UL) and American National Standards Institute (ANSI) standards generally apply to products used in the
United States, compliance to which is evaluated and certified by a Nationally Recognized Testing
Laboratory (NRTL). Similarly, products used internationally must also comply with safety standards
established within the countries where used, including those by Standards Council of Canada (SCC), the
European Union “Low Voltage” and Machinery directives (CE Mark) and, to a limited extent, the Chinese
Compulsory Certification (CCC Mark). Certain industry groups also have specific requirements for
equipment, such as the SEMI S2 Environmental, Health, and Safety Guideline for Semiconductor
Manufacturing Equipment.
Equipment Design For Safety
Microwave processing systems generally consist of three main elements: a microwave power generator, power delivery (waveguide) components and a process cavity (applicator). Systems can be as small
and simple as a typical residential microwave oven or as large and complex as a industrial food
processing conveyor. Many different types of generators and waveguide components are available and
can be configured in a variety of ways depending on the process characteristics. However, they all
perform essentially the same function: to safely and reliably generate and deliver microwave power to the
process cavity. The process cavity itself can also be configured in many ways, but its primary function is
to effectively “couple” the delivered microwave energy to the material being heated in such a manner as
to meet the requirements of the heating process. Process cavities are generally designed for either
“batch” or “continuous” material processing, while some are designed for semi-continuous flow.
Many common design practices for safety apply to all types of commercial and industrial equipment used
in materials processing as defined in the standards mentioned above. In addition to these commonalities,
microwave processing systems utilize several features that are uniquely necessary to ensure reliable and
safe operation. Most pertain to suppression of emissions, or “microwave leakage,” while others may be
specific to certain materials processes.
Microwave Leakage Suppression
Once high power microwave energy is generated, it must be contained almost entirely within the
processing system in order to comply with regulatory standards for safety and interference. The term
“Faraday cage” (British physicist Michael Faraday, 1791-1867) has often been used to describe a
microwave cavity in that it is an enclosure capable of blocking the entry or exit of electromagnetic waves.
The ideal Faraday cage is a metallic enclosure with no holes, broken seams or openings of any kind.
For all practical purposes, the term also applies to microwave generators and waveguide components.
But for obvious reasons, no practical microwave processing system can be a completely continuous
metallic enclosure as it must, as a minimum, allow access to its interior for entry and exit of the material
to be heated. In most cases it must also have seams or openings suitable for assembly, viewing,
ventilation, sensor insertion and other process related necessities.
Material Entry/Exit for Batch Processing: The most common type of opening in a microwave heating
system is the door of a household microwave oven. One can easily see that the door does not make
complete contact with the oven cavity, leaving the casual observer to wonder what is preventing the
micro- wave energy from radiating out of the oven. The door is designed with a nondissipative “reactive”
¼- wave choke along the opening perimeter that effectively blocks the transmission of energy at a
specific frequency. Most industrial systems having batch cavities utilize such door seals, although some
employ a direct contact seal using a woven metal mesh gasket or springy beryllium copper fingerstock.
In either case, the door seal design is robust and capable of withstanding repeated openings without
degradation in performance throughout the life of the equipment.
Material Entry/Exit for Continuous Processing: Continuous flow process systems require openings that
allow for material entry and exit on a continuous basis, such as for conveyorized and/or extruded
26
product. Depending on the size and geometry of these openings as required for the process material, the
means for suppressing microwave leakage is either reactive, as in the case described above for door
seals, or dissipative whereby microwave energy is actively absorbed within the structure of the opening,
or a combination of both. In either case, microwave energy is attenuated at some rate as it passes
through the opening. The rate of attenuation through these openings depends on the opening size and
attenuation method used. Thus, the opening typically has some length in order to attenuate the energy to
sufficiently safe regulatory levels, with larger openings generally having a lower attenuation rate and
requiring greater length. The term “tunnel” is often applied to openings for continuous flow processing.
Cavity Ventilation and Viewing Ports: Almost all microwave ovens have a window in the cavity door
through which the cooking process can be observed. Close inspection of this window reveals the
presence of a screen made of tiny perforations in thin metal. Each hole in the screen acts as a high-pass
filter that effectively blocks the transmission of electromagnetic energy at microwave frequencies yet
allows trans- mission of visible light. Other openings in the oven cavity, such as for ventilation and
illumination, are perforations in the cavity wall that function in a similar manner.
As microwave energy propagates through a waveguide or other restricted path, it is attenuated
exponentially as a function of wavelength and the size of the path according to
where P1 and P0 are the power densities at the path exit and entrance, x is the path length and α is the
rate of attenuation in dB/unit length as energy propagates along the path. Figure 4 illustrates the rates of
attenuation through a circular waveguide as functions of the inside diameter for various ISM frequencies.
The diameter at which the attenuation rate approaches zero is the “cut-off” diameter for the respective
frequency, and waveguides smaller than this dimension are said to be below or beyond cut-off. These
curves are used to determine the minimum screen thickness, or hole “length,” that provides the
necessary total attenuation for a given perforation hole diameter and power density.
For example, suppose the highest internal power density at a cavity opening is determined to be 1
kW/cm2 and the regulatory limit to be met outside the cavity is 1 mW/cm2. Using equation 1, the min i- mum
attenuation constant, α, required for a 0.020” thick screen is 345 dB/inch. Figure 4 then yields a
maximum perforation hole diameter of roughly 0.08”.
Figure 4. Attenuation curves for circular waveguide at three ISM frequencies.
27
Cut-off Tubes: The waveguide below cut-off principle is also commonly used for larger unobstructed
openings, referred to as “cut-off tubes,” such as for optical thermometry, fluid flow, etc. However, the
curves in Figure 4 apply only to unfilled tubes. When filled with a dielectric material, the effective inside
diameter is increased by a factor equal to the square root of the material’s dielectric constant. This
lowers the effective attenuation rate and increases the minimum tube length, although it will be
counteracted somewhat for high dielectric loss materials that absorb energy propagating through the
tube. The analysis is further complicated when the tube is filled only partially or with multiple materials of
various dielectric properties, such as when flowing water in Teflon tubing through the cut-off tube.
Feed-thru Openings: Inserting a metallic object, such as a thermocouple probe or wire, through a cavity
opening can be extremely dangerous by causing excessive microwave leakage. Unless it is in intimate
contact with the opening wall, the metal object in conjunction with the metallic opening wall will have a
coaxial transmission line effect, thus allowing microwave energy to propagate freely through the opening
and radiate from its end. Materials having a high dielectric constant can also create a similar effect when
inserted through a cavity opening, such as passing a small column of water passing through a
comparatively large cut-off tube. Electrical wires passing through a cavity opening require special
techniques such as capacitive filtering to prevent leakage and other forms electrical interference in the
external circuitry. Given the complexities involved with proper feed-thru design, professional advice and
assistance should be sought whenever installing feed-thru devices in microwave cavities.
Safety Interlock Devices
Interlock devices are typically defined as mechanical and/or electrical components or sub-systems that
prohibit the operation or functioning of other components or sub-systems if certain conditions are not
satisfied. In the case of microwave processing systems, this usually applies to the devices that prohibit
the operation of the microwave generator if a condition exists that might cause excessive microwave
emissions.
Cavity Access: Regulatory standards require interlock devices at all locations where the microwave
cavity can be opened without the use of tools. Of course, the most common access point is the cavity
door, although many industrial systems have other features that require periodic opening for
maintenance or other such purposes. Interlock devices prevent the microwave generator from operating
if the opening is not closed, and they are typically designed such that a failure of the interlock device
leaves the system in a safe non-operative mode. US government standards applicable to residential and
commercial micro- wave ovens require redundant interlock devices that are concealed and inaccessible,
at least one device of which having a means of fail-safe monitoring. IEC standards also require
redundant and concealed inter- lock devices at all cavity access locations, as well as interlocks for the
presence and/or flow of material through the cavity if necessary to maintain emissions within the guide
lines.
Waveguide Flanges: An often neglected location for microwave leakage due to inadvertent carelessness during equipment maintenance is a waveguide flange connection. Many systems are designed to
al- low easy disconnection of a waveguide component for routine maintenance, some even without the
use of tools. In such cases an interlocking device is required to ensure proper re-assembly before
bringing the system back on-line. In fact, literal interpretation of the IEC standards would require interlock
devices at all waveguide flange connections, although this practice is rarely found or deemed necessary
in industrial equipment.
High Voltage: As described earlier, microwave generators contain high voltage circuitry that can be lethal
when contacted inadvertently during operation. For this reason, interlock devices on enclosure access
covers are typical of most contemporary equipment designs and required by almost all regulatory
standards. These safeguards are generally sufficient to protect service personnel from exposure to
stored high voltage energy hazards. Most circuit designs provide for safe electrical discharge within the
time required to remove safety covers and gain access the internal circuitry.
Although not required by regulatory standards, many equipment designs also incorporate interlock wiring
into electrical connectors associated with inter-modular high voltage circuitry, thus disabling the high
28
voltage circuits whenever these connectors are not properly mated. However, even when the equipment
is designed with such protection, quickly disconnecting an electrical cable and exposing its contacts may
not allow sufficient time for stored energy to be discharged. Thus, caution must be exercised ANY time
an electrical cable is disconnected.
Warning Labels and Visual Indicators
Many regulatory standards require various labels and indicators alerting the equipment operator of a
potentially hazardous condition. Warning labels are required for both electrical shock and
electromagnetic radiation hazards and must indicate briefly a) the nature of the hazard, b) the potential
consequences of exposure to the hazard, and c) necessary actions to prevent exposure. Special labels
are required for cavity doors and other openings to warn against allowing foreign objects to interfere with
the door seals.
Visual indicators are required to alert nearby personnel that the equipment is in operation. These
indicators must be plainly visible during normal operating conditions or where personnel might typically
be present.
System Architecture and Process Safety
Fire and explosion prevention is particularly challenging as it combines the volatile characteristics of
certain materials processes with the unique propensity for ignition in a microwave system. Inert gas
purging is commonly used to remove oxygen from the cavity atmosphere, and regular cleaning of interior
surfaces helps to reduce the sources of fuel. But these practices may not be practical for or applicable to
all processes. The system developer must have a thorough understanding of both the materials process
and microwave technology. Recognizing that such hazards often are not completely avoidable, the
design must minimize the hazard potential and effectively deal with a hazard event.
There are many other aspects of the overall system design that are considered when minimizing the
risks and hazards associated with microwave processing, including walk-in cavity access, high
temperatures, automated mechanisms and moving parts. A thorough review of these considerations is
well be- yond the scope of this paper, so materials process developers are encouraged to seek the
advice of equipment experts having experience in similar processes.
Guidelines For safety Equipment Operation
Common sense plays an important role in the safe operation of any product regardless of the magnitude
of its associated hazards. Apart from that, a few basic practices and habits should be institutionalized
when operating microwave equipment.
Safety Training: Basic first aid treatment and emergency rescue procedures should be a part of a formal safety training program. Many hazards, particularly those associated with high voltage, can be prevented from becoming lethal if immediate rescue action is taken. For example, electrocutions resulting in
ventricular fibrillation do not necessarily cause immediate death, thus emergency cardiopulmonary
resuscitation (CPR) can be administered to potentially save the life of the victim.
Equipment Training: All personnel engaged in operating and/or maintaining industrial microwave
equipment should receive proper training to be aware of the hazards associated with their respective
tasks. As a minimum, all product user manuals and other documentation provided by the equipment
manufacturer should be read and thoroughly understood before attempting operation or service. Safety
training should emphasize awareness and recognition of both microwave radiation and high voltage
hazards and appropriate responses when identified. In addition, service personnel should be
knowledgeable and competent with regard to safe troubleshooting and repair of high voltage circuitry.
Buddy System: Never operate or perform maintenance on industrial microwave equipment while alone.
In the event of hazard exposure, the victim may be incapacitated and need immediate medical attention
for survival.
Leakage Detection: Every user of an industrial microwave heating system should have on hand or ready
access to a high quality microwave leakage detector, also referred to as a microwave survey meter.
29
Leakage should be checked on a periodic basis and always after any maintenance has been performed
on the equipment. Upon detecting excess emissions, the equipment should be immediately shut down
and the cause of emissions investigated and corrected before restarting. Also, the leakage detector
should be calibrated at least annually to ensure proper operation.
Periodic Maintenance: Many hazards result from inadequate or improper equipment maintenance
practices. A formal program of periodic inspection and maintenance should be implemented that addresses all potential hazards and hazard sources. The program should include not only the microwave
equipment itself but also any other tools or equipment required for the maintenance tasks. The most
basic tasks to be included are a) to inspect for microwave leakage, b) clean the cavity interior and all
internal waveguide surfaces, and c) replace damaged or broken electrical connectors even if still
functional.
While there are indeed many hazards associated with industrial microwave and RF processing
equipment, the risks associated with them are no greater than with conventional equipment when proper
safe- guards have been implemented. Microwave heating technology has been a vital component of
industrial production since the 1940’s and will continue to be applied as new materials processes are
developed. Newcomers to this technology can develop a sense of security with its use by learning and
understanding the basic concepts about health effects, regulatory standards, design safeguards and
operational guidelines presented here.
Laser Safety
Hazards of Laser Light
Laser beams can be hazardous, particularly for the eye (and sometimes also for the skin), mostly
because they can have high optical intensities even after propagation over relatively long distances.
Even when the intensity at the entrance of the eye is moderate, laser radiation can be focused by the
eye's lens to a small spot on the retina, where it can cause serious permanent damage within fractions of
a second – even when the power level is only of the order of a few milliwatts. Damage can result from
both thermal and photochemical effects. Laser damage of the eye is not always immediately noticed: it is
possible e.g. to burn peripheral regions of the retina, causing blind spots which may be noticed only
years later.
Ultraviolet lasers can cause corneal flash burns, a painful condition of the cornea. UV radiation can also
cause photokeratitis and cataracts in the eye's lens. (For these reason the XeCl excimer laser has
acquired the nickname “cataract machine”.) Mid-infrared lasers, particularly those operating at certain
wavelengths with very strong absorption in the cornea (e.g. 3 μm or 10 μm), can also cause painful
corneal injuries.
How much light an eye can tolerate depends on many circumstances: not only the intensity, but also
particularly the wavelength and the duration of irradiation (e.g. the pulse duration). There are detailed
sets of rules for calculating safe exposure limits (maximum permissible exposure, MPE) for a given
situation. Such rules are occasionally revised according to new scientific findings.
Eyes are particularly sensitive, but laser radiation can also cause skin injury. For infrared light, this
occurs mainly via thermal effects (thermal skin burns), similar to burning the skin with other means. The
penetration depth depends on the wavelength, and for such reasons a laser beam at 1.5 μm wavelength
causes more pain on the skin than a 1-μm beam. Whereas such burning should in most cases not have
serious long-term consequences, ultraviolet light can in addition induce photochemical reactions. These
can lead to changes in the pigmentation, erythema (sunburn), and (most importantly) skin cancer.
Some laser safety issues arise from indirect effects of laser radiation:
•
•
•
Intense laser beams can incinerate materials and thus possibly start severe fires.
Particularly in laser material processing, poisonous fumes, dust or hot droplets of molten material
can affect nearby workers. For example, fumes containing arsenic, chromium or nickel can occur
when metal pieces are cut, and plastic parts (polymers) can form dangerous organic substances.
Secondary radiation (e.g. ultraviolet light or even X-rays) can be generated when high-intensity
beams heat certain targets to high temperatures.
30
Non-Beam Hazards
LASER is an acronym which stands for Light Amplification by Stimulated Emission of Radiation. The
energy generated by the laser is in or near the optical portion of the electromagnetic spectrum (see
Figure 1). Energy is amplified to extremely high intensity by an atomic process called stimulated
emission. The term "radiation" is often misinterpreted because the term is also used to describe
radioactive materials or ionizing radiation. The use of the word in this context, however, refers to an
energy transfer. Energy moves from one location to another by conduction, convection, and radiation.
The color of laser light is normally expressed in terms of the laser's wavelength. The most common unit
used in expressing a laser's wavelength is a nanometer (nm). There are one billion nanometers in one
meter. In addition to the direct hazards to the eye and skin from the laser beam itself, it is also important
to address other hazards associated with the use of lasers. These non-beam hazards, in some cases,
can be life threatening, e.g. electrocution, fire, and asphyxiation. Table 1 indicates some of the potential
non-beam hazards associated with laser usage. Because of the diversity of these hazards, the
employment of safety and/or industrial hygiene personnel to effect the hazard evaluations may be
necessary.
The optical spectrum. Laser light is nonionizing and ranges from the ultra-violet (100 - 400nm),
visible (400 - 700nm), and infrared (700nm - 1mm).
Not Only Light is Dangerous
Further issues are not even related to laser beams:
• High electric voltages are used in laser power supplies (e.g. for discharge lamps), creating
hazards in maintenance operations or when cables are damaged.
• Some laser systems contain hazardous chemicals such as certain laser dyes.
• Other lasers contain potentially exploding or imploding glass tubes (e.g. arc lamps).
In fact, probably most victims of accidents with lasers have been hurt by such hazards (particularly by
electric shocks) rather than by laser radiation.
Particularly Hazardous Situations
The following list of important safety issues can never be complete, but is meant to improve awareness
of the multitude of possible hazards:
• High-voltage power supplies can be dangerous if workers can come into contact with the inner
parts or with a defective high-voltage cable.
• Some lasers require the handling of hazardous chemicals, e.g. carcinogenic dye solutions in dye
lasers. Some of these solutions can penetrate the skin, and therefore need to be handled with
special care.
• Near-infrared laser beams are much more hazardous than visible light with the same power level,
because their radiation is focused to the retina just in the same way as visible light, whereas the
blinking reflex of the human eye (normally closing the eye's lid quickly when the intensity is too
high) is not active. Also, no warning is possible e.g. through weak stray light: nothing can be seen
when a dangerous beam propagates in an unexpected direction.
• Ultraviolet lasers endanger not only eyes, but also the skin (see above).
• Pulsed laser sources, e.g. Q-switched lasers or regenerative amplifiers, generate pulses with a
peak power many orders of magnitude higher than the average output power even of a highpower laser. A single pulse from a hand-held miniature laser can totally destroy an eye.
• In open laser setups, parasitic specular reflections (caused either by parts of the setup or by
movable metallic tools, watchbands, rings, etc., but also by the residual reflectivity of anti-
31
•
•
reflection coatings) may allow hazardous beams to leave the setup, which might hit someone's
eye.
Optical fibers, e.g. transporting high optical powers between different rooms, may release
dangerous radiation when being damaged. They therefore need to be specially protected and
marked.
High-power lasers (e.g. with powers in the kilowatt region) can damage not only the eye but also
the skin within short exposure times, and can easily start a fire, e.g. when the beam hits materials
such as wood or plastics; toxic fumes may also be generated.
Often less dangerous are the following cases:
•
•
setups with low-power visible beams, where the blinking reflex of the human eye may provide
sufficient protection against occasional exposure of an eye with moderate power levels.
sources operating in certain eye-safe spectral regions (e.g. with wavelengths longer than
≈ 1.4 μm) where the light is absorbed in the eye's lens and therefore cannot reach the (more
sensitive) retina fully closed laser setups with an interlock, which automatically switches off the
radiation source as soon as the case is opened.
Laser Hazard Classification
To give some guidance on adequate handling and required precautions, laser devices are assigned to
different safety classes, with class 1 being the least dangerous (containing e.g. lasers with microwatt
power levels) and class 4 the most hazardous one. Note that the assignment to a laser safety class
depends not only on the laser power, beam quality and laser wavelength, but also on the accessibility of
hazardous areas: even a high-power laser may be in safety class 1 when there is no risk that dangerous
radiation can leave a fully encapsulated housing.
Details such as the large diameter or the divergence of involved laser beams are largely ignored in such
simplified classification schemes. The concept is solely to classify the laser product itself according to
some emission limits, rather than evaluating a particular setup containing a laser. The classification is
indirectly based on some exposure limits for the eye (see above), but also takes into account a number
of worst case assumptions concerning e.g. the distance of persons from the laser aperture, the exposure
duration and the possible use of optical instruments. Therefore, the classification tends to overestimate
certain risks, and a complete safety assessment has to consider the details of the whole setup and the
way it is used.
Safety Standards
There are a variety of laser safety standards including Federal and state regulations, and non-regulatory
standards. The most important and most often quoted is the American National Standards Institute's
Z136 series of laser safety standards. These standards are the foundation of laser safety programs in
industry, medicine, research, and government. The ANSI Z136 series of laser safety standards are
referenced by the Occupational Safety and Health Administration (OSHA) and many U.S. states as the
basis of evaluating laser-related occupational safety issues.
ANSI Z136.1 Safe Use of Lasers, the parent document in the Z136 series, provides information on how
to classify lasers for safety, laser safety calculations and measurements, laser hazard control measures,
and recommendations for Laser Safety Officers and Laser Safety Committees in all types of laser
facilities. It is designed to provide the laser user with the information needed to properly develop a
comprehensive laser safety program.
For manufacturers of laser products, the standard of principal importance is the regulation of the Center
for Devices and Radiological Health (CDRH), Food and Drug Administration (FDA) which regulates
product performance. All laser products sold in the USA since August 1976 must be certified by the
manufacturer as meeting certain product performance (safety) standards, and each laser must bear a
label indicating compliance with the standard and denoting the laser hazard classification.
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Table 1: International laser safety classes, with somewhat simplified and approximate descriptions. For details,
consult the applicable laser safety standard documents.
Safety
class
Simplified description
1
The accessible laser radiation is not dangerous under reasonable conditions of
use.
Examples: 0.2-mW laser diode, fully enclosed 10-W Nd:YAG laser
1M
The accessible laser radiation is not hazardous, provided that no optical
instruments are used, which may e.g. focus the radiation.
2
The accessible laser radiation is limited to the visible spectral range (400–
700 nm) and to 1 mW accessible power. Due to the blink reflex, it is not
dangerous for the eye in the case of limited exposure (up to 0.25 s).
Example: some (but not all) laser pointers
2M
Same as class 2, but with the additional restriction that no optical instruments
may be used. The power may be higher than 1 mW, but the beam diameter in
accessible areas is large enough to limit the intensity to levels which are safe
for short-time exposure.
3R
The accessible radiation may be dangerous for the eye, but can have at most
5 times the permissible optical power of class 2 (for visible radiation) or class 1
(for other wavelengths).
3B
The accessible radiation may be dangerous for the eye, and under special
conditions also for the skin. Diffuse radiation (as e.g. scattered from the some
diffuse target) should normally be harmless. Up to 500 mW is permitted in the
visible spectral region.
Example: 100-mW continuous-wave frequency-doubled Nd:YAG laser
4
The accessible radiation is very dangerous for the eye and for the skin. Even
light from diffuse reflections may be hazardous for the eye. The radiation may
cause fire or explosions.
Examples: 10-W argon ion laser, 4-kW thin-disk laser in a non-encapsulated
setup
Note that there are different classification schemes (e.g. international and American ones), using classes
such as 1 to 4 but with somewhat different definitions. (The American system uses classes I, IA, II, IIIA,
IIIB and IV similar to the classes 1 to 4 of the international system, but with significant deviations.)
Particularly important standards are the IEC 60825-1 international laser safety standard of the
International Electrotechnical Commission (IEC) those based on the US user standard ANSI Z-136 (with
various variations Z-136.X, in particular the Z-136.1, revised in 2007)
The IEC standard has been fully adopted by the European standardization organization as EN 60825-1
and is published in national versions such as DIN EN 60825-1 in Germany. Note that these standards
cover much more than only defining safety classes; they also determine the measures to be taken in
order to work safely with laser products in such classes. There are also government regulations such as
the relatively outdated 21 CFR 1040.10, which is still relevant for the US, although the IEC / EN standard
is now also accepted there with some additions.
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Generally, it is the duty of the manufacturer of a laser product to classify the product and to equip it
accordingly with warning labels. However, the classification may change when a laser product is
modified by a user, and the user is then responsible for reclassification.
The Nominal Hazard Zone
Originally, the required safety measures for a given laser setup where basically determined only by the
safety class of the laser. As mentioned above, this classification does not reflect details such as beam
divergence, which can be very relevant for safety issues: a strongly focused laser beam can be so
divergent that within a moderate distance after the focus the intensities fall below the allowable exposure
level for the eye. In such situations, one sometimes defines a “Nominal Hazard Zone” (NHZ) within which
safe exposure levels may be exceed, in order to apply certain restricting measures to this zone instead of
the whole room.
Technical Precautions
Examples of frequently used technical laser safety precautions are:
•
•
•
•
•
•
•
•
the use of protective goggles (→ eye protection), strongly absorbing radiation with wavelengths
near the laser wavelength
full or partial encapsulation of laser systems, ideally with absorbing housing materials, avoiding
specular reflections
protective housings around dangerous working areas, monitoring the presence of persons e.g.
with light curtains, laser scanners or people counters
interlocks that automatically switch off lasers or block laser beams e.g. when a protective box or a
door is opened
key-operated switches for power supplies, preventing unauthorized use
written warnings (indicating e.g. the types of lasers behind a door), warning lights (indicating that
hazardous laser sources are operated) and automatic door locks, preventing people from
entering dangerous areas
beam stoppers (not only for main beams, but also for parasitic reflections), preventing dangerous
beams from leaving the optical setup
low-power visible pilot beams and the like, marking the paths of dangerous invisible laser beams
Non-technical Measures
Technical measures alone are generally not sufficient for keeping safety hazards under control. A
number of non-technical measures are therefore very important:
•
•
•
•
The risks have to be carefully assessed before anything adverse can happen. They need to be
reassessed every time when important circumstances change, e.g. the devices used,
applications, staff, and details of the room.
On that basis, reasonable ways of dealing with these risks need to be developed. This involves
the implementation of technical measures (see above) and establishing suitable working
practices. The results need to be clearly described in written safety regulations.
Adequate safety education needs to be ensured, such that all people who may be at risk are
properly informed about both the risks and the proper ways to deal with them. Personal
instruction by a knowledgeable person is certainly very valuable, and should be supplemented
with additional training materials such as clearly written notes, a laser safety video or DVD.
All responsibilities need to be properly assigned and clearly defined.
It is also very important to establish a spirit which motivates all staff to take safety issues serious,
recognize responsibilities for themselves and for their colleagues, suggest practical solutions, etc.
Laser Safety Regulations
Making laser safety regulations for some production facility is a difficult task, and for a research
laboratory it is even harder. The reason is that there are partially conflicting goals:
•
Regulations must be clear and understandable for those reading them.
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•
•
The rules should be sensible, ideally under all conceivable circumstances: the implied restrictions
should be so that all risks are minimized without being in straight conflict with the actual goals of
the work.
The set of rules should be compact, so that people can be expected to read them carefully and to
memorize them.
It is clear that various trade-offs are inevitably involved, e.g. between compactness and suitability for
many different circumstances, or between safety and productivity. Giving absolute priority to maximum
safety while ignoring productivity and similar practical requirements will not even serve safety, because it
increases the risk of regulations being ignored or forgotten. It can be difficult task to analyze existing
hazards and to identify the most practical way of dealing with them.
Common Obstacles
Unfortunately, reasonable laser safety regulations are either not in place or (more frequently) routinely
ignored in many places such as research and development laboratories. Possible reasons (but not good
excuses) are:
•
•
•
•
•
•
•
•
•
•
•
•
a lack of general knowledge on laser safety issues
the lack of available information on specialized safety issues, e.g. related to potential hazards of
ultrashort laser pulses from mode-locked lasers (hazard potential determined only by average
power, or also by peak power and pulse duration?), or awareness of risks associated with fumes
unexpected effects such as accidentally misaligned beams, vaporization of poisonous
substances, defects or poor design of safety equipment, etc.
wrong interpretation of labels like “low-power laser”: a 10-mW near-infrared laser may have a low
power, but is still very dangerous to the eye!
irrational assessment of risks and inappropriate judgment on working routines (“We have always
done it like this!”)
the general human tendency to underestimate invisible risks, particularly when they occur over
long times without apparent effects
missing safety devices (e.g. insufficient numbers of laser goggles in situations with visitors)
highly inconvenient, uncomfortable or otherwise impractical safety devices, e.g. laser goggles
which cannot be used over longer times
excessive pressure to produce results quickly
nonsensical laser safety regulations which undermine the awareness that the adherence to the
rules is in the operator's own interest
very formal and abstract sets of rules, obviously made primarily for avoiding legal problems for
the bosses, rather than providing help in real life
the negligence and the bad example of irresponsible supervisors, who sometimes even ridicule
more responsible persons
Due to such factors, which are difficult (if not impossible) to eliminate altogether, perfect laser safety
(making accidents impossible) is probably impossible to reach. However, sensible regulations can greatly
diminish the risks without affecting the productivity too severely.
Laser Hazards & Beam Hazards
The laser produces an intense, highly directional beam of light. If directed, reflected, or focused upon an
object, laser light will be partially absorbed, raising the temperature of the surface and/or the interior of
the object, potentially causing an alteration or deformation of the material. These properties which have
been applied to laser surgery and materials processing can also cause tissue damage. In addition to
these obvious thermal effects upon tissue, there can also be photochemical effects when the wavelength
of the laser radiation is sufficiently short, i.e., in the ultraviolet or blue region of the spectrum. Today,
most high-power lasers are designed to minimize access to laser radiation during normal operation.
Lower-power lasers may emit levels of laser light that are not a hazard.
The human body is vulnerable to the output of certain lasers, and under certain circumstances, exposure
can result in damage to the eye and skin. Research relating to injury thresholds of the eye and skin has
been carried out in order to understand the biological hazards of laser radiation. It is now widely
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accepted that the human eye is almost always more vulnerable to injury than human skin. The cornea
(the clear, outer front surface of the eye's optics), unlike the skin, does not have an external layer of dead
cells to protect it from the environment. In the far-ultraviolet and far-infrared regions of the optical
spectrum, the cornea absorbs the laser energy and may be damaged. Figure 2 illustrates the absorption
characteristics of the eye for different laser wavelength regions. At certain wavelengths in the nearultraviolet region and in the near-infrared region, the lens of the eye may be vulnerable to injury. Of
greatest concern, however, is laser exposure in the retinal hazard region of the optical spectrum,
approximately 400 nm (violet light) to 1400 nm (near-infrared) and including the entire visible portion of
the optical spectrum. Within this spectral region collimated laser rays are brought to focus on a very tiny
spot on the retina. This is illustrated in Figure 3.
Absorption characteristics of the human eye (From Sliney &
Wolbarsht, Safety with Lasers and Other Optical Sources,
Plenum Press, 1980)
In order for the worst case exposure to occur, an individual's
eye must be focused at a distance and a direct beam or
specular (mirror-like) reflection must enter the eye. The light
entering the eye from a collimated beam in the retinal hazard
region is concentrated by a factor of 100,000 times when it
strikes the retina. Therefore, a visible, 10 milliwatt/cm2 laser
beam would result in a 1000 watt/cm2 exposure to the retina,
which is more than enough power density (irradiance) to cause
damage. If the eye is not focused at a distance or if the beam is
reflected from a diffuse surface (not mirror-like), much higher
levels of laser radiation would be necessary to cause injury.
Likewise, since this ocular focusing effect does not apply to the
skin, the skin is far less vulnerable to injury from these
wavelengths.
Focusing effects of the human eye(From Sliney & Wolbarsht,
Safety with Lasers and Other Optical Sources, Plenum Press,
1980)
If the eye is not focused at a distance or if the beam is reflected from a diffuse surface (not mirror-like),
much higher levels of laser radiation would be necessary to cause injury. Likewise, since this ocular
focusing effect does not apply to the skin, the skin is far less vulnerable to injury from these wavelengths.
The Laser Safety Officer
ANSI Z136.1 specifies that any facility using Class 3b or Class 4 lasers or laser systems should
designate a Laser Safety Officer to oversee safety for all operational, maintenance, and servicing
situations. This person should have the authority and responsibility to monitor and enforce the control of
laser hazards. This person is also responsible for the evaluation of laser hazards and the establishment
of appropriate control measures.
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The Laser Safety Officer (LSO) may be a full or part-time position depending on the demands of the laser
environment. This person may be someone from occupational health and safety, industrial hygiene, or
similar safety related departments. The LSO may also be part of the engineering or production
department. In any case, the LSO must be provided the appropriate training to properly establish and
administer a laser safety program.
Some of the duties the LSO may perform include hazard evaluation and establishment of hazard zones,
control measures and compliance issues, approval of Standard Operating Procedures and
maintenance/service procedures, approval of equipment and installations, safety training for laser
personnel, recommendation and approval of personal protective equipment, and other administrative
responsibilities.
Controlling Laser Hazards
Like any other potentially hazardous operation, lasers can be used safely through the use of suitable
facilities, equipment, and well trained personnel. The ANSI Z136 series of laser safety standards provide
a detailed description of control measures which can be put into place to protect against potential
accidents.
These control measures are divided into two distinctive categories, Engineering Controls and
Administrative/Procedural Controls. Examples of Engineering Controls include protective housings and
interlocks, protective filter installations, key-controls, and system interlocks. Administrative/Procedural
Controls include standard operating procedures and personal protective equipment.
Engineering Controls are generally more costly to develop but are considered far more reliable by
removing the dependence on humans to follow rigorous procedures and the possibility of personal
protective equipment failure or misuse.
Administrative/Procedural Controls are designed to supplement Engineering Controls to assure that laser
personnel are fully protected from potential laser hazards. The focus of these controls are to provide
adequate education and training, provisions for protective equipment, and procedures related to the
operation, maintenance and servicing of the laser.
Safety training is desired for those working with Class 3 lasers and systems. Operation within a marked,
controlled area is also recommended. For Class 4 lasers or systems, eye protectors are almost always
required and facility interlocks and further safeguards are used. Control measures for each laser
classification are defined fully in the ANSI Z136.1 laser safety standard. This document is the single most
important piece of information regarding the safe use of lasers and should be part of every laser safety
program. For more information on laser safety, please refer to this standard. ANSI Z136 laser safety
standards may be obtained by contacting Laser Institute of America.
A few guidelines to keep in mind:
• Thoughtful risk assessment and sensible regulations are required before accidents occur (i.e.
before the work begins).
• Regulations must be practical and convincing, because otherwise they are likely to be breached.
Staying on the “safe” side by imposing unrealistic and nonsensical rules on workers will
undermine the respect for the regulations, and can thus be very counterproductive.
• All responsibilities must be clarified for everyone involved. The assignment of a laser safety
officer alone (possibly as a scapegoat without sufficient time and powers for enforcement of rules)
is not sufficient.
• Stupid arguments for breaching rules, e.g. of the style “we have always done it like that” or “I
know others who also do that”, must be banned.
• Formal adherence to given rules is not sufficient – operators must stay risk-aware during routine
work.
* Mr. Nikola Zlatanov spent over 20 years working in the Capital Semiconductor Equipment Industry. His work at Gasonics,
Novellus, Lam and KLA-Tencor involved progressing electrical engineering and management roles in disruptive technologies.
Nikola received his Undergraduate degree in Electrical Engineering and Computer Systems from Technical University, Sofia,
Bulgaria and completed a Graduate Program in Engineering Management at Santa Clara University. He is currently
consulting for Fortune 500 companies as well as Startup ventures in Silicon Valley, California.
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