4.1.11 Electricity at Work
1. Introduction
The Health and Safety at Work Act 1974 to which the University is subject as
an employer imposes duties on the University to provide a safe working
environment. As far as electrical systems are concerned, the legal
requirements are contained in the Electricity at Work Regulations 1989. The
precautions contained in the Safety Manual meet these legal requirements but
are primarily for guidance in preventing electrical fires and injury to
individuals. Nevertheless, if anyone is in any doubt concerning the safety of
equipment on which they are working, they should consult their Safety
Coordinator or can directly contact the Health, Safety and Environment Unit.
Reference may also be made to the local safety rules and codes held in each
Department pertaining to electrical and electronic working in teaching and
research laboratories.
UNDER NO CIRCUMSTANCES SHOULD A PERSON USE ELECTRICAL
APPARATUS ABOUT WHICH THEY HAVE ANY DOUBTS AS TO ITS
SAFETY.
2. Responsibilities
The Bursar and the Estates Department have responsibility for:
a. The University sub-stations;
b. Distribution facilities eg isolators, circuit breakers, fuse boxes, socket
outlets etc up to the service itself;
c. All permanent and emergency lighting;
d. Fire alarm systems;
e. All plant concerned with building services, both that distributed
throughout local sites and that concentrated in plant rooms.
To conform to statute, only one person at a time may take charge of the 11 kV
mains and this responsibility is managed by Estates. No work of any kind is
performed on these mains without the knowledge and consent of that person
in charge, who ensures that a rigid ‘Permit to Work’ system is enforced.
Departments are themselves responsible for the electrical arrangements and
equipment fed from the sockets and for departmental equipment fed from
fixed outlets.
In student residences, the Bursar is responsible for the fixed installation while
the Senior Resident Tutor is responsible for electrical equipment belonging to
the University. For practical reasons the University cannot assume total
responsibility for the safety of students’ personal electrical equipment.
Nevertheless, students should be advised that all such equipment should be
in good condition and designed to a recognised specification, such as a
British Standard, when it is brought onto University property. If subsequently
any equipment is found not to meet these conditions, then it should be
removed from use until such time as adequate repairs are effected.
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4. Electrical Hazards
Injury to persons - there are several ways in which personal injury may be
caused:
1. Shock. Electric shock is the effect produced on the body, particularly its
nervous system, by an electrical current passing through it, and its effect
depends on the current strength which in turn depends on the voltage, the
path the current takes through the body, the surface resistance of the skin
(much reduced when wet) and several other factors. A voltage as low as
15 V can produce discernible shock effects and 70 V has been known to
cause death. Generally speaking, however, those fatalities which occur
from this cause involve normal domestic and industrial voltages of 240 V
ac and above, causing currents of greater than 30 milliamps to flow
through the body for longer than 40 milliseconds. The most common
cause of death from shock is suffocation and accordingly it is highly
desirable that those dealing with electricity should be trained in
resuscitation. Minor shocks in themselves may not be serious but they
can lead to serious consequences; for example, the associated muscle
contraction may lead to falls from working platform or ladders.
2. Burns. These are caused by the passage of heavy current through the
body or by direct contact with an electrically heated surface. They may
also be caused by the intense heat generated by arcing from a short
circuit. Electrical burns are a very unpleasant form of burn and require
immediate attention.
3. Explosion. The main causes of electrically-induced explosions are listed
below:
a. In situations where flammable gases or vapours are present so that a
spark could ignite an event. In such environments all electrical
equipment should be flame proof.
b. Where electric arcing takes place in a confined space causing intense
local heating with a consequent bursting of the enclosure by the
expansion of trapped air.
c. Rechargeable batteries emitting hydrogen when being charged,
giving rise to an explosive atmosphere. Such operations should
therefore be carried out in a well ventilated area, the temperature of
which should not exceed 18 degrees C.
4.
Eye injuries. These can be caused by exposure to the strong ultraviolet
rays of an electric arc, where the eyes become inflamed and painful after
a lapse of several hours, and there may indeed be a temporary loss of
sight. Although very painful, the condition usually passes off within 24
hours. Precautions to protect the eyes must always be taken by persons
working with or near electric arc welding. Also permanent injury to the
eyes can arise from the energy propagated in microwave apparatus: noone should look along the wave guide when it is in use or examine a
highly directional radiator at close quarters. Precautions also need to be
taken with lasers to prevent eye injury (see http://www.bath.ac.uk/hr/hseunit/manual/s4/2-23-lasers.pdf).
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5. Body injuries from radio-frequency (rf) energy and induction
heaters. The energy in microwave and rf apparatus can damage the
body, especially those parts with a low blood supply. Operators should
never be exposed to an energy level exceeding 10 milliwatts per sq cm. If
personnel have had bones pinned with metal at any time, they must be
careful not to expose themselves to these sorts of energy propagation as
they may cause substantial internal damage before the operator is aware
he is in danger. Similarly, induction heaters can cause rapid heating in
any circuit brought within say 1m and consequently personnel using such
equipment should remove any metallic objects from their person and
those with pinned bones etc should avoid them altogether.
6. Fires. A large percentage of fires are of an electrical origin, caused by
one or more of the following:
i. Sparks. A spark arises from a sudden discharge through the air
between two conductors, or from one conductor to earth. The
current produced is usually small so that serious fires are unlikely
unless explosive gases or vapours are present, or highly flammable
material is in contact with the conductor.
ii. Arcs. An arc is a much larger and brighter discharge where the
current flow may be hundreds of amps. It usually arises when a
circuit is broken or when a conductor melts or fractures leaving a
gap across which current continues to flow. When an arc is
established, the air in the vicinity becomes ionised and forms a
conductor which may allow current to flow to a nearby metal
framework. Any combustible material in the vicinity could therefore
lead to a fire.
iii. Short circuits. A short circuit is formed when the current finds a
path from the outward conductor wire to the return wire other than
through the equipment to which it is connected. The current flow
may be large because of the low resistance of the leads, and arcing
often occurs at the contact between the conductors. Insulation may
therefore be burned and set fire to adjacent flammable material.
Batteries have a low internal resistance and can give rise to very
large currents under short circuit conditions, causing a large arc
from which molten metal may be splashed.
iv. Overloading and old wiring. Wiring must not be overloaded,
otherwise it will overheat and the insulation will be damaged. This
can lead to a short circuit at some point in the length of the
conductor, or more likely at connection points. The insulation of
wiring which has been in use for a number of years tends to
become brittle and, where alterations and additions are required,
the cable must always be checked by a competent electrician and
replaced completely if there are indications of failure of the
insulation. Installations should be protected against overloading
and short circuits by fuses or circuit breakers.
4. Safety Measures
Cables. Cables must be of sufficient size to carry the current which flows
through them under normal conditions and must be adequately insulated to
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allow handling with safety. Under fault conditions, they must be able to
withstand excessive currents for the time taken for the supply to be
disconnected by a fuse or circuit breaker. Those cables which provide the
basic services within a building are normally housed in conduits or troughs but
where apparatus is wired up from socket outlets, no such permanent
protection is available and hence particular care is required. Such cable must
be sufficiently robust to withstand the wear and tear of laboratory use and fully
waterproof where water may come within the vicinity of the apparatus.
Fuses. These devices will open a circuit when an excessive current flows.
Circuit breakers. These are a form of switch which open automatically if the
current in the controlled circuit becomes excessive. It may operate on either
thermal or magnetic principle. It is essential to select the correct size of the
fuse or circuit breaker for any particular circuit, especially those in series, so
that the correct disconnection is made ie the faulty circuit and only the faulty
circuit is isolated and not a whole subgroup of services, whilst they must allow
normal operation of the equipment without random tripping.
Residual current device (RCD). These devices should be used in areas of
hazard, eg where water is being used near electrical equipment as a back-up
protective device. They are sensitive to earth currents and are designed to
isolate the supply before the user of an equipment is subject to serious harm.
(Plug-in RCD’s must be manufactured to BS7071.)
Isolation. Means must be provided of disconnecting cables or apparatus
from the source of supply in an emergency or when maintenance is to be
carried out and safeguards instituted to prevent the supply being remade
whilst the apparatus is being worked on.
Should it be necessary for an isolating switch to be remote from the
apparatus, fuses should be drawn when the switch is in the ‘off’ position or if
the switch is lockable, it should be locked off. The key or fuses should then
be retained by the operator directly concerned. Appropriate notices should be
displayed so that all persons may be aware of the situation.
Earthing. Any conducting part of a system which could conceivably become
live, say under fault conditions, and yet to be handled, eg the external metal
casing of electrical apparatus, must be earthed as a legal requirement. The
reasons for this are:
a. to prevent the accessible metal parts rising to a dangerous voltage under
fault conditions such as a short circuit between the live conductors and
casing;
b. to ensure that a faulty circuit is automatically disconnected from the
supply by drawing sufficient current to blow the fuse or operate the circuit
breaker or residual current device.
All new Class 1 equipment (ie all equipment designed with an earth) must be
tested to ensure that it is in fact properly earthed before putting it into use.
Bad joints, rust or paint can cause the resistance to increase to a dangerous
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level. Where the earth connection of a chassis is made with a nut and bolt, a
shakeproof washer should be used adjacent to the chassis to ensure good
contact is maintained in use.
Low voltage supplies (<1000V). Portable tools and hand inspection lamps
can be a source of danger because:
a. they are subjected to abnormal wear and tear.
b. they are liable to be used in conditions where dampness has reduced the
body’s resistance.
Where conditions are particularly dangerous, eg when working in interiors of
metal enclosures or where water is continuously present, mains voltage
equipment should not be used. A double-wound transformer with the
secondary centre tapped to earth to give 110 V should be used, thereby
ensuring that no part of the equipment will be at a voltage greater than 55 V
relative to earth.
The availability of cordless power tools has had a great impact on increasing
convenience and reducing the risk of injury. Operating at extra-low voltage
the potential for electric shock has been dramatically reduced, although care
must be taken with the charging unit. There are no power cords to tangle
with, trip over or cut through, and temporary supplies are not required for
electrical work. These tools are a highly recommended way of reducing the
risk of injury, and consideration should be given to their purchase in place of
standard 240 V tools.
Insulation. Double insulation apparatus and tools are now made which
require no earthing - such equipment has the ‘double square’ logo ( ) and is
categorised as Class 2 equipment.
‘Double Insulated’ implies the use of two layers of insulation on all live parts,
each layer of insulation being adequate to insulate the conductor but together
ensure an improbability of danger arising from insulation failure. This
arrangement avoids the need for any external metal work to be connected to
earth. Such apparatus and tools which are not earthed should be stored in a
dry environment and tested to ensure that the insulation is effective.
Space. The circulation space in laboratories and workshops must be kept
clear to prevent the risk of tripping and hence accidental contact with live
electrical conductors.
Plug connections. In all instances the connection of equipment to the mains
must be correctly made by a competent person. Should it be found that the
power leads on imported equipment do not conform with the current British
Standard (BS) of
brown = live,
blue = neutral
green with yellow = earth,
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it is wise to replace the original powerlead with the BS pattern. Remove only
the required amount of insulation so that no bare conductors are exposed
when the connections are made, and remove any ‘whiskers’ which may be
present. Ensure that the flex is firmly held by the cordgrip before the plug top
is screwed back into position.
Extension arrangements. If there is a shortage of sockets to supply the
increasing number of portable electrical appliances being used, it is
permissible to feed one four-way extension block from one power point
provided the block feeds only low power equipment (ie less than 500 W or 2
amps current rating). Kettles, microwaves, and heaters etc which consume
much greater power must be fed from an installed socket point. Flexible
cables should be run in such a way as not to present a tripping hazard. In
some instances four-way extensions have overheated and caused fires due to
poor connections by the fuse link. Four-way extensions are thus preferred
unswitched and unfused, saved for the 13 amp fuse in its plug which provides
adequate electrical protection. If a fused four-way extension becomes warm
around the fuse link, immediately take out of service and replace.
5. Electrical Testing
The law requires that all electrical equipment and systems are not only
designed to be safe to operate but should be maintained in as safe conditions
as is reasonably practicable. To ensure the latter a testing is required at
regular intervals and records kept of results.
Equipment fitted with 13 A plugs. There are four basic safety tests in
addition to regular visual tests by the user:
1. Visual inspection of mains plugs, cables and earthing arrangements.
Fuses in plugs and equipment should be checked to establish that they
are correctly rated. If a plug incorporates an RCD, the latter should be
functionally tested.
2. Measurement of insulation resistance between the live parts and earth,
using 500 V dc.
3. Measurement of the earth loop resistance (not applicable for double
insulated equipment), using a high current, usually 25 amperes
4. Flash test (a higher voltage insulation test) for double insulated
equipment, using 3kV ac. NB: Do not subject electronic equipment
(eg computers) to the flash test.
Portable Appliance Testers (PATs) are available to carry out the checks at 2,
3 and 4 quickly and reliably, and the Health, Safety and Environment Unit
holds instruments which can be borrowed for the purpose should
Departments have insufficient equipment to justify the purchase of their own.
The testing may be carried out in-house if a member of staff is competent to
do so, or the testing may be undertaken by Estates or external contractors.
Equipment permanently wired into the supply. The tests and standards
are in general the same as above (except no plug is involved) but the use of
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PATs is not possible and Meggers and earth loop testers will be required
instead. As it will be necessary to isolate supplies while the test is being
carried out, the involvement or co-operation of the Estates Department will be
necessary. Similarly, 415 V equipment, even if fitted with plugs, may require
Estates Department involvement as again PATs cannot be used.
Fixed Installations. There are three classes of safety tests, namely:
1. Earth loop measurements;
2. Insulation checks;
3. Operation times of circuit breakers, residual current devices etc.
The procedures for testing and the acceptable tolerances for fixed
installations are contained in the IEE Wiring Regulations for Electrical
Installations, Sixteenth Edition, copies of which can be obtained from the
Health, Safety and Environment Unit (HS&E Unit).
The periodicity of the tests is not definitively laid down by regulation and
should be determined according to circumstances. (Refer to the HSE
pamphlet ‘Maintaining Portable Electrical Equipment in Offices and other Low
Risk Environments’ at http://www.hse.gov.uk/pubns/indg236.pdf.) The period
between tests will vary depending on the use and the risks posed by the
equipment. A formal visual inspection every 2 years with a full electrical test
every 5 years will suffice for low-risk office equipment such as computers.
However, equipment such as kettles, used daily and posing a much higher
risk, would be visually inspected every 6 months and fully tested annually.
Appliances such as hand tools used outside in all weathers (eg on a
construction site) would be tested as often perhaps as fortnightly. The best
way to judge the frequency of testing would be to check the failure rate of
appliances to see if many were damaged in the interval between testing and
adjust the interval accordingly.
For fixed installations, a periodicity of 5 years is recommended with earth
loops and residual current devices being checked annually. Records of
results should be retained for a period covering at least three consecutive
inspections to enable signs of progressive deterioration to be detected. For
portable equipment, the use of stickers to indicate whether equipment is ‘in
date’ can be used for record purposes, provided departments have instructed
their staff not to use equipment which has lapsed. If practical, a central record
should be kept as well.
Statutory Testing. In addition, there are a number of installations which
have special statutory testing requirements, eg fire alarm systems and
electrical installations in flammable stores, lifts etc. These are the
responsibility of Estates and the requirements are not detailed here.
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6. Experimental Work in Laboratories requiring
Special Precautions
Live Working. Experiments may call for the use of electrical equipment
which has to be altered or improved whilst power is ‘on’. Such situations need
to be justified and authorised by the Safety Coordinator or supervisor as
appropriate and in any event the following precautions would need to be
taken:
a. Only insulated test prods and tools are to be used.
b. Where live working is likely to be a continuous requirement consideration
should be given to making the appropriate work bench and surrounding
area as ‘earth free’ as far as is reasonably practicable with the main
supply being fed through a 1:1 isolating transformer.
c. The power switch (ie on/off) must be within easy reach of the operator.
d. Another person must be within earshot who is cognisant of the potential
hazards and who is trained in artificial respiration.
e. Workspaces should be tidy and floors clear of debris to remove the
hazard of tripping.
f. Consideration should be given to the fitting of a residual current device
(RCD) in the supply circuit to the equipment.
High voltage (ie greater than 1kV). Normal electrical safety precautions of
earthing and isolation are inadequate to prevent injury to persons operating
equipment in the vicinity of high voltages and consequently experiments using
such voltages need additional specific safety measures to be adopted.
Precautions to be taken when undertaking experiments involving high voltage
are as follows:
a. No internal adjustments or modifications are to be made whilst the
equipment is live, ie no live working.
b. Measurements must be taken with permanently connected instruments.
c. Any part of the equipment which is at a voltage of or greater than 240 V is
to be made inaccessible to human contact when the equipment is live by
the provision of appropriate insulation, protective barriers or other means.
d. Sufficient electrode spacing must be provided to prevent flashovers.
e. The voltage source must be capable of rapid isolation by the person in
charge of the experiment in case of an emergency.
f. Equipment is only to be operated when there is at least one other person
within earshot, in order that assistance can be given should the operator
receive an injury from the equipment.
g. Capacitor banks should be discharged after the equipment has been
switched off and bleed resistors incorporated into the design so that any
remaining capacitor energy is dissipated.
h. Before making adjustments to the de-energised equipment,
measurements must be taken to ensure that there are no dangerous
voltages present, having first carried out the capacitor discharge routine,
as appropriate.
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i.
To prevent inadvertent contact by casual visitors, the apparatus must be
guarded by insulating screens or barriers placed at an adequate distance
from any exposed live part and warning notices must be displayed.
Adequate distances are considered to be:
up to 50 kV 3 metres
50 kV - 100 kV 4 metres
150 kV - 250 kV 5 metres
7. Unattended Operation of Electrical Equipment
Because of the danger of fire, electrical equipment should be switched off
when unattended, wherever possible. In cases where switching off is
impractical, precautions should be taken to reduce the possibility of a fire
occurring in the first place and should one start, to contain its subsequent
spread, namely:
a. Fuses, circuit breakers, residual current devices etc (ie devices which
will automatically disconnect the supply under fault conditions) are
correctly rated and in good order.
b. Flammable material in the neighbourhood of the equipment is
removed.
c. Fire detection and fire extinguishing equipment is working satisfactorily.
In some cases, the fitting of additional detection devices (either heat or
smoke) may be justifiable where high value equipment or buildings are
at risk.
d. For experimental equipment, authorisation should be obtained from a
supervisor or other suitable responsible person and the Security Office
informed.
e. A warning notice should be displayed near the equipment being run
giving salient details to enable safe action to be taken by a member of
the emergency services should a hazardous situation arise.
The above particularly applies to experimental equipment being run out of
normal working hours as opposed to proprietary equipment such as
refrigerators, drying ovens etc. Nevertheless, the HS&E Unit should be
contacted for advice where doubtful or especially difficult cases arise.
8. References
Electricity at Work Regulations, 1989 (HMSO)
Electrical Safety in High Voltage and Pulsed Power Laboratories, 1986
(Sowerby Research Centre)
BS EN60825: 1992 Radiation Safety of Laser Products, Equipment
Classification, Requirements and User’s Guide (BSI)
Safety in Universities, Notes of Guidance Part 2.1 Lasers, Revised 1992
(CVCP)
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