Internal Arc Fault

We will discuss about the internal arc fault in switchgears which has the most severe effect in the power system and is directly related to the safety of the operators and the installed base. We will try to understand the reasons which causes the generation of an internal arc fault within the electrical equipment. We will also cover the effects of an arc flash on the surrounding environment.

What is an internal arc fault? In the simplest form an internal arc fault in the switchgear is the result of rapid release of energy due to arc travel through ionized air between phases or a phase and ground.

In power systems an internal arc fault is considered as the most severe type of fault, as its results are quite devastating. If the switchgear installation is non-internal arc proof, this fault can cause severe injuries to the operators present in the close vicinity of the effected installation and can also cause damage to other nearby installations.

By large it would be right to say that the occurrence of an arc fault can never be completely prevented. From the switchgear’s perspective an arc fault is usually considered to be initiated by external factors.

The most common cause of an arc fault is insulation failure. This can be any insulated material used in a switchgear to achieve the di-electric clearance between the live parts and ground. The most common cause of insulation failure is natural deterioration due to the ageing effect. Insulation material going through excessive heat due to any reason significantly reduces the life of the insulation.

Other factors include contaminated environment which causes insulation degradation. Chemicals if present in the environment encounter the insulation material permanently reduce the insulation resistance. The presence of moisture, which is conductive in nature and contains impurities, when it comes in contact with insulation material it penetrates through the cracks and pores of the insulation. It provides a low resistance path and causes failure. Dust in combination with moisture can become conductive and causes creepage which results in an arc fault.

In experience the tendency for an arc fault increases if the manufacturer recommended practices for product installation are not completely followed during the installation and commissioning phase or these activities are performed by non-qualified operators.

For instance, during the cable termination process if the certain metallic parts such as cable plate or auxiliary cables duct covers are removed and are not installed again, or installation is not according to product guidelines it opens a breach for the foreign objects to come inside the switchgear and causes the initiation on an arc fault.

During fastening of cables to the copper pads, if the required guidelines for the fasteners or required tightening torque are not followed it will cause sparking, which may create heat and eventually may result in the initiation of an arc fault. Further, the arc faults are also initiated due to the damaged or corroded wires and wire terminals.

During the commissioning process or during maintenance, it is recommended that switchgear should be cleaned thoroughly and all contamination resulting from the installation or maintenance work should be removed. Any kind of leftover fasteners or forgotten tools will result in the initiation of an arc fault. All removed partitions and covers in the busbar and circuit breaker compartment must be re-mounted.

When the switchgear is energized the integrated mechanical interlocking system present in switchgear prevents the operator access to different switchgear compartments. Interlocking systems are designed to ensure the safety of operator and the installation. Any attempt to operate the switchgear without understanding the interlocking mechanism or any attempt to by-pass the interlocking system may damage it and the consequence can be severe in result of an arc fault.

It is common to use air as an insulation medium in medium voltage and low voltage switchgears. There are various reasons to use air as an insulating medium such as it is not harmful to the environment, after break-down it restores itself, and above all it is free. However, in the presence of impurities air becomes conductive at higher temperature ranges above 1,000°C.

Once the arc is ignited within the switchgear the surrounding air will get ionized and arc will continue to burn at very high temperature until it is interrupted. After the initiation of an arc fault the electrical energy is first converted into radiation and thermal energy.

The bright flash resulting from an arc fault is as high as 1 million lux which is 2,000 times higher than the lux level defined for normal office lighting. This bright flash can result in temporary or permanent blindness to the operator working near the installation.

During an arc fault the temperature can rise to a level of 20,000°C which is four times the temperature on the surface of the sun. This heat emerges from this high temperature can burn the human skin and clothing within the fraction of seconds. It can also cause severe damage to the nearby installations.

The high temperature resulting from an arc fault liquifies and vaporizes the copper, aluminum and steel parts used in the installation. Since the metal parts change state from solid to vapor therefore causes volumetric expansion and results in sound and pressure waves.

The sound blast result due to an arc fault can be as high as 140 dB and can cause eardrum rupture which result in temporary or permanent hearing loss.

The high-pressure wave generated due to an arc fault can throw the operators working nearby across the room against the wall or any other installation. The molten metal sprayed and the flying metal parts resulting from the explosion can cause severe injuries and equipment damage.

The solution to the problem of an internal arc fault is the internal arc classified switchgears as per IEC 62271-200, which prescribed the criteria for the protection of operators in the event of an arc fault.

It is important to mention that the initiation of an arc fault cannot be completely disregarded, however, if the switchgears are installed, operated and maintained according to manufacturer recommendation there is a little probability that internal arc fault would occur during the entire lifetime of the installation. The effectiveness of the design to provide prescribed level of protection to the operators against the internal arc fault is defined under IEC 62271-200.

An enclosure to qualify as internal arc classified the acceptance criteria as defined by IEC 62271-200 should be met after the initiation of an arc fault within the switchgear assembly. After the test all doors should remain closed, and covers do not open. No fragmentation of enclosure during the specified time of test. Arcing should not cause holes in the accessible sides of switchgear. Indicators do not ignite because of hot gasses. The indicators are pieces of cotton cloth placed at designated locations of enclosure to simulate the burning effect due to the emission of hot gases. The enclosure remains connected to its earthing point.

You would notice that all these acceptance criteria are for the safety of the operators working in the vicinity of the installation.

IEC 62271-200 defines the accessibility class for internal arc classified switchgears. Accessibility class A is restricted to authorized persons only. Class B, which is not the part of our discussion, is for unrestricted areas including public & Class C which is for pole mounted equipment.

F, L and R represent the sides of the switchgear where the protection is provided in case of an internal arc fault. F represents the front side of switchgear; R represents the rear side and L represents the lateral sides. If the switchgear installation is against the wall normally internal arc classification AFL is considered. AFLR classification provides protection in all sides of the switchgear. It is important to note that, as per IEC 62271-200 the recommended duration of tests are 1s, 0.5s or 0.1s.

The arc ducts provided within the switchgear play an important role in reducing the pressure within the enclosure in case of the arc fault. These ducts direct the hot gases to the upward side of the switchgear where the gases are exhausted by the opening of flaps on the switchgear roof. These ducts are made in all three compartments of the switchgear containing power components, the cable compartment, the circuit breaker compartment, and the busbar compartment.

In addition to the ducts inside the switchgear there are three different arrangements to further safely channel the hot gases coming out of the switchgear. The first method is the introduction of a deflector system on top of the switchgear. Deflectors direct gases away from front, lateral and rear side of the switchgear. The second method is the introduction of tunnel arrangement which directs the hot gases out of the switchgear room. The third arrangement is the absorber system which is also installed on top of the switchgear enclosure. Absorber arrangement due to its construction reduces the pressure and temperature coming out of the switchgear and allows safe vent within the switchgear room.

In addition to the channel for venting out of hot gases, switchgear enclosure is equipped with reinforced door arrangement which is equipped with fail safe integrated interlocking system which guarantees that switchgear doors do not open in case of an arc fault. The door of non-internal arc classified switchgear are simple hinged doors and will open during an arc fault. The purpose of the mechanical reinforcements is to ensure the safety of operators and nearby installation in case of an arc fault.