Internal Arc Fault in MV Switchgear

Background

Internal Arc Faults can occur in low-voltage as well as medium-voltage switchgear panels. However, we have chosen to discuss the internal arc fault phenomenon in medium-voltage switchgear and its impact on human safety because of its severity in the latter case.

What is an Internal Arc Fault?

Very important to understand what this phenomenon is. An Arc Fault is a High-Power Discharge in the insulating air between two or more conductors. As the name suggests it happens inside the switchboard. This insulation breakdown could be because of the weakening of insulation due to high humidity inside the switchboard enclosure or tracking between the phases or between phases and the earth. It can happen in any functional chamber viz VCB, Bus Bar or Cable Chamber.

Once the arc is ignited the surrounding air gets ionized and continues to burn at high temperature until gets interrupted. The ionized air becomes plasma and offers almost no resistance to the arc fault current. The temperature could be around 800 deg C at the arcing point depending on the let-through energy discharged during the arcing.

We know the let-through energy is (V²/r)*t

Where V is the system voltage

              r is resistance and

              t is the time duration of arcing

It is evident that if the operating voltage is higher, energy discharge will also be higher. This discharged energy is called let-through energy which gets converted to heat.

Impact of Internal Arc Fault on MV Switchgear

Let us refer to our school days learning of Gay-Lussac Law which states that pressure is directly proportional to temperature at constant volume. If pressure is represented by P and temperature by T then the said law can be represented by below simple equation.

P ∝ T where T is in Kelvin

The volume of MV Switchgear Enclosure remains constant, which means the pressure will increase if the temperature increases. This increased pressure can tear the enclosure or open the joints and hot gas leaks out of the enclosure. At that high temperature of more than 800 degrees C, metal parts also melt, and small breakaway parts could come out of a tear hole. Because of high pressure, the breakaway part comes out at a very high speed and may cause an accident if anyone comes in line with this flying part.

The same hot gas at a high temperature can lead to a fire and cause burn to the operating personnel if it does not get contained and extinguished inside the MV Switchgear Enclosure. Internal Arc Faults can be dangerous for human and installation safety. You can visualize the power of internal arc faults. Please watch the below video.

How to Eliminate the Danger of Internal Arc Faults

From the discussion above in the impact section, we can conclude that

• The mechanical strength of the MV Switchgear Enclosure must be enough to sustain the pressure generated inside during internal arc faults,
• There must be a protective device that isolates the power source feeding the fault,
• Design a safe passage and discharge of hot exhaust gases out of the enclosure,
• Design the enclosure that does not tear or open joints to prevent hot exhaust gas leaks and
• Get the design type tested to validate it as per IEC 62271-200.

What IEC Standard Says

The relevant standard for MV Switchgear Panels is IEC 62271-200. As per the standard Internal Arc Fault is a mandatory type test. That means all the MV Switchgear Panels must comply with Internal Arc Fault requirements. Internal arc classification is given as AFLR kA/s where

 A is Accessibility type A, access to the switchgear panel is restricted to authorized personnel.

FLR- Front, Rear and Lateral means accessibility on all sides

kA-   Designated Internal Arc Fault Current

s-      test duration in seconds

Accessibility types are also classified as B and C. In the case of type A access is up to 300mm whereas B is for unauthorized personnel which means the general public can also have access and it is up to 100mm. in case of type C restricted by installation out of reach. The access distance is defined by the manufacturer in this case.

Internal Arc Fault Compliant MV Switchgear

The test is carried out on all three compartments/chambers of the MV Switchgear Panel, with cotton indicators placed at the specified distance according to accessibility class from the enclosure. A short circuit is created within the switchgear in one of the chambers at a time. For other chambers, it has to be repeated on separate prototypes with the same design. The short circuit current is then injected for the specified duration of the test. The following criteria are considered for a successful type test:

• Correctly secured doors and covers do not open. Deformations are accepted, provided that no part comes out as far as the position of the indicators on every side.
• No fragmentation of the enclosure occurs within the specified test duration. Projections of small parts, up to an individual mass of 60 grams, are accepted.
• Arcing does not cause holes in the accessible sides up to a height of 2 meters.
• Indicators do not burn because of hot gases. If indicators start to burn during the test and proof is established that the ignition was caused by glowing particles rather than hot gases, the assessment criterion may be regarded as having been met. The test laboratory can use pictures taken by high-speed cameras, video, or any other suitable means to establish evidence. Indicators ignited because of paint or stickers burning are also excluded.
• The enclosure remains connected to its earthing point. Visual inspection is sufficient to assess the compliance.

Refer to the below picture of an Internal Arc Compliant MV Switchgear panel. At the top, we can see a duct that is extended on both sides for exhaust gases. The internal passage is also created from individual chambers connected to the duct through openable flaps. The extension can be further extended out of the electrical room in a secluded place for discharge of the exhaust or can be buried in the earth inside a pit.

Conclusion

A lot of people in the industry say that this is overkill to make an Internal Arc Fault compliant MV Switchgear Panel as it makes it costly. Another argument they give is how many times this kind of fault occurs.

My point is, that it is not the question of how many times, as even once in its life cycle period is enough to inflict an irreparable loss. On the part of making the switchgear panel costlier, here again, my point is no cost is higher than the cost of a life or the cost of completely losing a building or an industry because of fire due to an Internal Arc fault. Just give a thought on “Can we afford to lose a precious life” just to save a few bucks?

Even if we go by absolute terms, the cost of designing and manufacturing an Internal Arc Fault compliant MV Switchgear Panel is nominal.

So, we can conclude that all the MV Switchgear Panels whether 11kV or 33kV must be an Internal Arc Compliant for the safety of equipment, personnel and assets. This compliance also adds a lot of reliability to the product.

External Reference

Please refer to the below link for further insights into the Internal Arc phenomenon in MV Switchgear Panels.

https://www.eaton.com/ecm/groups/public/@pub/@eaton/@corp/documents/content/pct_3031849.pdf