Arc Flash Mitigation
IE
ARC-FLASH
EVERY MILLISECOND COUNTS
Arc-Flash occurs when electric current flows uncontrolled from phase to earth, phase to neutral,
and/or phase to phase through air – during flash initiation the insulating property of the surrounding
air is compromised by rapid ionisation, resulting in a conducting plasma cloud. The intense light
and heat energy at the point of the arc is called an Arc-Flash. An Arc-Flash builds up in milliseconds
and releases a huge amount of energy.
By Merv Savostianik, Littelfuse
A
rc-Flash may be defined as an intense luminous discharge of
energy that occurs when electric current flows uncontrolled
through what is usually an insulating medium – air.
Arc-Flash Cause and Effect
The most common causes of Arc-Flash are insulation failure
(caused by defective or aging insulation material), poor or incorrect
maintenance, ingress of dust, moisture or vermin and human errors
(touching a test probe to the wrong surface or a tool slipping and
touching live conductors, etc.)
Arc-Flash events are particularly dangerous and potentially
fatal to personnel. During an Arc-Flash, an enormous amount of
concentrated energy is released in the form radiant energy (visible,
infrared and ultraviolet light), overpressure and flying debris (solid
particles and blobs of molten metal). When the switchgear, motor
control centres or other electrical compartments cannot effectively
contain the overpressure (the pressure exceeds the explosion limit
of the electrical gear), it can cause serious injury to personnel who
are in close vicinity. Injuries can include severe burns, ruptured
eardrums, collapsed lungs, damaged eyesight and death. Electricians
have been injured even at three or more meters away from the arc
point. An Arc-Flash can produce a large shock wave that can knock
personnel off their feet and against an adjacent wall or equipment,
and there is a risk of being struck by flying debris.
A plasma cloud develops
Temperatures at the arc point can reach as high as 19,000°C – a
few times the temperature of the sun’s surface (about 5,500°C).
The intense heat creates superheated toxic gasses and vaporises the
copper and other metal parts in the vicinity of the blast. Gaseous
copper expands to a volume over 40,000 times that of solid copper
within a few milliseconds. Molten metal is blasted and splattered
from the arc point. A plasma cloud develops as part of the electrical
arcing process. Plasma is often referred to as the fourth state of
matter, the other three being solid, liquid and gas. Heat emitted
from the plasma cloud is mostly infrared radiation. The volume of
hot ionised gas produced increases proportionally with energy. It is
thus crucial to remove the source of energy as soon as possible in
order to eliminate the avalanche effect of an Arc-Flash.
several thousand amps
Readers familiar with arc welding will have a good idea of the
intense light, heat and splattering metal associated with the arcwelding process – except that the arc drawn during welding involves
significantly less energy, with current normally between 80 and
160 A. During an arc fault the current is several thousand amps (it
could typically be 15 kA or higher).
Typical electrical enclosures do not have adequate mechanical
strength to withstand or contain the energy released during an ArcFlash fault for extended periods.
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Most electrical switchgear enclosures are designed to withstand
an internal arc for only 500 ms to one second. Without proper
pressure relief, Arc-Flash incidents could possibly collapse entire
substation buildings.
Arc-Flash Can Occur on Low-Voltage Switchboards
Most 480 V electrical systems have sufficient electrical energy
capacity to cause an Arc-Flash hazard. Medium-voltage equipment
(above 1,000 V) have higher energy levels and the higher voltage is
more likely to ionise the air and therefore a bigger potential for ArcFlash hazards exists within medium-voltage switch and control gear.
As an example of the energy released in an Arc-Flash incident,
consider a single phase-to-phase fault on a 480 V system with
25 kA of fault current. The resulting power is 12 MW. If the fault
lasts for 10 cycles (167 ms) at 60Hz, the resulting energy would
be 2.0 megajoules. For comparison, TNT releases 2175 J/g when
detonated. This fault energy is thus equivalent to 921 grams of TNT.
An Arc-Flash involves more heat and light and less mechanical
shock than a chemical explosion, but the resulting devastation
is comparable. Using the figures in Table 1, let’s increase the arc
duration time from 167 to 500 ms; the power is still 12 MW but the
resulting energy is now 6 MJ or 2.8 kg of TNT.
Table 1 Graphical representation of the relation between arc duration and
energy released
These elementary calculations clearly show the effect the duration
of the arc fault has on the energy released; the amount of energy
released is directly proportional to the duration of the Arc-Flash.
Conventional Electrical Protection Equipment Does
Not Sufficiently Limit the Effect of an Arc-Flash
The duration of an Arc-Flash is mainly determined by the time it
takes for overcurrent or earth-fault protective devices to detect the
fault, send a trip signal to the circuit breaker and for the circuit
breaker to subsequently disconnect the energy source.
Industrial Electrix
January-March 2012
IE
Fast acting fuses may disconnect the circuit from the energy source
in 8 ms or less when subjected to the high short-circuit currents
usually appearing in three-phase symmetrical bolted cases, while
other devices may take much longer to operate and remove the
source of energy. But unbalanced, single-phase and high-impedance
fault currents are lower than three-phase bolted-fault currents, so
protection devices may not necessarily detect and limit arc-fault
current and will require more time to clear the fault.
In many cases the protection devices at main distribution points
are coordinated with downstream protection systems. This means
that there may be a considerable time delay before the standard
overcurrent and earth-fault protection devices at the main
distribution point operate, to allow a downstream protection device
to clear the fault first.
Arc-Flash Protection Strategies
Although there are a few options available, most of the protection
strategies have disadvantages:
When users order new metal-enclosed switchgear they could
specify that the switchgear must be type tested to IEC 60298.
According to this standard it is possible to specify an internal arc
rating with fault times of 100, 500 or 1,000 ms. One should bear in
mind that the cost of the switchgear increases drastically for longer
arc ratings (the price could be double for a 1s rating when compared
with a 100 ms rating). The reasoning behind this: it is expected that
the standard protection will detect and clear the fault within the
designed internal withstand time (say 500 ms).
Although the switchgear is designed to remain intact in the event
of an internal arc (at least for the specified internal arc rating time)
it will not prevent internal damage which would definitely lead to
extended electrical outages. Arc-resistant switchgear loses its rating
when a cubical door is open, or not properly closed.
In most cases Arc-Flash faults occur at the cable crutches, inside
the cable compartment which means that this area falls outside the
busbar protection zone. The busbar protection will not operate for
Arc-Flash occurring inside the cable compartments.
This solution is also rather expensive, requiring complicated
modifications and additional equipment (e.g. CTs, relays, etc) if
intended to be installed on existing switchgear panels.
protective clothing
The third means of protection is protective clothing. Human errors
when performing work on live equipment are one of the most
common causes of Arc-Flash. It is possible to protect personnel from
some of the Arch-Flash hazards by ensuring that they wear proper
personal protective equipment (PPE). PPE includes clothing, gloves,
insulated tools, face protection, and glasses. However, workers may
not always wear cumbersome PPE, and if they do, it may not protect
them against the blast of an Arc-Flash event, which can break bones,
puncture organs, and propel workers into walls or equipment.
Figure 3 Arc-Flash point sensors or fiber-optic sensors are strategically
placed in all compartments of a switchgear cabinet.
dedicated Arc-Flash relay
Perhaps the most reliable solution is an Arc-Flash relay. A dedicated
Arc-Flash relay and optical sensors can be retrofitted to electrical
cabinets to ensure that an Arc-Flash in any compartment is detected
and that the source of electrical energy is removed in the shortest
possible time. This solution provides the best advantages in terms of
effectiveness, cost, and ease of installation.
This equipment is specifically designed to detect an Arc-Flash and
to issue a trip (almost instantaneously) to the main circuit breaker.
The duration of the Arc-Flash is reduced to the mechanical opening
time of the circuit breaker. The operating time of modern circuit
breakers is in the region of 35 to 60 ms.
Figure 2 An Arc-Flash sensor installed inside a switchgear cabinet.
high-impedance bus protection
Another protection strategy is a dedicated busbar protection
scheme, such as high-impedance bus protection (based on the
circulating current principle). This type of protection would typically
detect a fault inside the protected zone in 40 to 65 ms (plus the
breaker operation time). The disadvantage of this solution is that
it is regarded as a zone protection scheme, which means that
the protected zone is limited to the actual position of the current
transformers (mounted inside the switchgear).
www.powertrans.com.au
Comply with the Law
In Australia, the Occupational Health and Safety (Safety Standards)
Regulations 1994 clearly require the identification and elimination
or minimisation of risk. According to the Regulation, “An employer
must identify any risk of injury from (a) the existence or use of
electricity in the workplace; and (b) the existence, operation or use
of electrical installations at the workplace.” There is a duty to control
risk; “…the employer must…either: (i) take measures to eliminate
the risk; or (ii) if it is not reasonably practical to eliminate the risk –
take measures to minimise the risk as far as reasonably practicable.”
An Arc-Flash relay comprehensively protects the functional
reliability of the distribution board and at the same time helps to
eliminate workplace electrical risk.
The Benefits of Arc-Flash Relays
As mentioned above, the consequences of an Arc-Flash in a power
system can be disastrous. Within a very short time (milliseconds)
after a fault occurs, pressures and temperatures rise to levels that
can easily destroy the metal housing surrounding the equipment.
The energy released by such a fault is enough to cause sheet metal
to melt or evaporate and a serious fire could follow.
Industrial Electrix
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IE
By installing reliable and fast acting Arc-Flash detecting and
protection equipment in the electrical switchgear and distribution
systems, plant owners will enhance the safe operation of the
equipment. Arc-Flash detection equipment protects both personnel
and equipment and reduces repair and reconstruction costs.
System downtime is shortened due to the fact that the amount
of damage caused by the Arc-Flash is limited. Therefore the total
cost of ownership is drastically reduced. Installing and maintaining
Arc-Flash protection equipment may also help reduce insurance
premiums and possibly workers compensation and insurance claims.
How an Arc-Flash Relay Works
The Arc-Flash relay uses light-sensitive sensors to detect an arc flash.
These sensors consist of two silicon photo cells encapsulated in a
transparent polyester body, or consist of a “leaky” optical fibre that
captures light and transmits it to a photo cell. The sensors are strategically
placed in the various cubicles or drawers inside the switchboard.
An Arc-Flash relay system should have an integrated three-phase
current-measurement capability. Although it is not a requirement for
the system to operate, this capability increases the system reliability
(minimise spurious tripping). The theory behind this is that when
an Arc-Flash occurs the electrical current feeding into the fault will
increase drastically. Two conditions need to be fulfilled before the
relay trip signal is sent to the circuit breaker: a current flow that
exceeds the normal operating current of the system and a signal
from an optical Arc-Flash sensor that has reacted to a high-intensity
light source.
Some systems also offer a circuit-breaker fail function which
makes it possible to trip the upstream supply breaker after a time
delay of 50 to 150 ms if the overcurrent and arc signal remains after
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Figure 4
The PGR-8800 from Littelfuse is an example of an Arc-Flash relay used in
new equipment or to retrofit existing switchboards. It is simple to install and
straightforward to commission. This model can be supplied from an external
power supply (or fed from station batteries) at 110 to 250 VAC/DC or 12 to
48 VDC. The system provides continuous surveillance of the complete system
including the connected sensors. Any system faults, e.g. a faulty sensor cable,
is indicated by a flashing LED, an alarm contact, and is logged and reported.
A USB connection provides easy access to configuration settings and a realtime event log with detailed diagnostic information about events such as
configuration changes, test, reset and power cycling.
the circuit breaker was supposed to open (meaning that the circuit
breaker has failed to trip).
There are two types of Arc-Flash sensors available: point sensors
and optical fibre sensors. In some systems, the condition of the
sensors (optical fibre sensors included) is continuously monitored.
Industrial Electrix
January-March 2012