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Participant

Join Date: Mar 2018
Posts: 2

Air Circuit Breaker Compartment Burned and Damaged

03/13/2018 12:12 AM

A problem in LV room switchgear panel. The scenario was such that:

The ACB compartment showed that it has been exposed to severe heat caused damage. Furthermore the Current transformer connected on red phase bus bar had sustained severe damage compared to the blue and yellow phases.

The ACB did not sustained any physical damage. However, from rear view it can be noted that yellow phase copper conductor had melted due to arching activity. The red and blue phases are partially melted.

The damage to the transfer switch had showed severely at upper section compared to lower section. The conductor had partially melted due to high current from the incident.

Please explain what is the possible cause of this problem?

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Pathfinder Tags: ACB engineering power
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Guru
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#1

Re: Air circuit breaker compartment burned and damaged

03/13/2018 1:00 AM

Please explain the prefault and postfault conditions that lead to the transfer switch operating, including such things as: size, type, and quantity of motors/loads; phase currents; auxiliary equipment such as Power Factor Correction capacitors/controllers, transformers, local generation, lightning/surge arresters, capacity of main/auxiliary power sources, any lightning or switching elsewhere in the system, overall age and condition, any recent changes, etc.

Professional tools such as SLDs (Single Line Diagrams), fault recorder data, and pictures of the actual damage, would also help to reduce guesswork. Without corroborating data the answer is obvious; either too much current, too much voltage, or a combination of both, plus under-rated/designed equipment, and/or operator error.

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Participant

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#2
In reply to #1

Re: Air circuit breaker compartment burned and damaged

03/13/2018 1:47 AM

Hello RAMConsult,

That for a valuable reply.
Actually the detailed information isnot available.. but this scenario is provided and you can think of it as an exam paper. There is no Pre or post fault condition, neither ratting, type,size are given. just need to make a case report by using only the given scenario. which can explain what can the possible cause be...

I have attached some screen shots which gives a complete scenario..

Moderator's note: images removed at request of OP.

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Guru
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#4
In reply to #2

Re: Air circuit breaker compartment burned and damaged

03/13/2018 4:13 AM

It is disappointing that these images were not available in the original post, thereby introducing delays to responses to it.

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#10
In reply to #2

Re: Air circuit breaker compartment burned and damaged

03/13/2018 11:45 PM

Without being able to fully investigate the situation all I can provide is anecdotal.

It reminds me of a LV variable speed drive section that "burnt down". In that situation it appears the problem started with a loose connection that was followed with an arcing fault to ground. The fault current was well below the CB rating. The ground fault detection system had been bypassed due to false tripping. The end result was a massive arcing fault burn-down that went from the secondary connections and involved the primary CB connections - and eventually when the lights started to flicker someone saw the flames and shutdown the main PDC. The whole MCC section was lost - holes burnt right through the door.

I have also seen animals (rodents) ignite arcing faults, and also MOV's exploding and creating arcing faults to ground and between phases.

There are lots of causes and results.

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#3

Re: Air circuit breaker compartment burned and damaged

03/13/2018 4:11 AM

Among the possibilities:

  • Electrical connections not as "clean, tight and bright", as they needed to have been, suggesting a lack of care on the maintenance agenda.
  • Incorrect setting of a circuit protective device simultaneously with an overload condition, suggesting a lack of care on the commissioning agenda.
  • Incorrect sizing of the conductors for the expected load, suggesting a lack of care on the design agenda.
  • Some external heat source: sun/fire/military or terrorist activity/etc., suggesting a lack of care on the security agenda.
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#5

Re: Air circuit breaker compartment burned and damaged

03/13/2018 7:54 AM
  • Ingress of an extraneous foreign object: (rodent/bird/tool/etc.), suggesting a further lack of care on the maintenance agenda.
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#7
In reply to #5

Re: Air circuit breaker compartment burned and damaged

03/13/2018 9:19 AM

...or unwanted fluid...

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#6

Re: Air circuit breaker compartment burned and damaged

03/13/2018 9:09 AM

Just asking because I don't work with LV switchgear:

Since arcing is expected, is there no provision for magnetic arc suppression or am I just not recognizing it?

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#8

Re: Air circuit breaker compartment burned and damaged

03/13/2018 10:24 AM

Seeing that this is a LV circuit breaker compartment, there are a number of conditions that could have led to this failure. Some have already been mentioned, but without the details that were asked for and a thorough inspection of the equipment, there is no way to isolate it to any one of the possible failure modes. Here are some:

1. Breaker bus connector did not fully or properly seat on the bus stab in the cell, causing overheating and failure.

2. Contamination in the cell (liquid or accumulated solids such as dust/dirt, etc., provided a path for fault to ground or to adjacent phase).

3. CT on the breaker stab failed, due to any of a number of reasons: pinched wire, loose or open connection, insulation failure, etc.

4. Overload on phase caused overheating, which then escalated as components heated, burned, loosened, or oxidized over time, eventually leading to failure.

5. Operator closed breaker onto a faulted phase, which causing failure at the "weakest point," in this case a breaker stab with some other weakness such as contamination, looseness, etc.

6. Insulation barrier board allows fault to ground due to accumulated contamination causing tracking and burning.

7. Circuit breaker trip unit failed, thus preventing trip for a condition that it should have tripped, leading to breaker & cell damage.

8. Moisture accumulation in breaker insulation, leading to insulation failure.

I could go on, but a thorough investigation should help narrow down the source of the fault and the conditions that allowed it to escalate to this level of damage.

Good luck!

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#9

Re: Air circuit breaker compartment burned and damaged

03/13/2018 2:24 PM

Ok, a theoretical exam question.

A little hard to see from the photos but my money is on one of the following as the primary cause of the problem (excessive current flow and arc damage are likely NOT the primary cause, only an end result).

1) loose connections with the power cables to terminals causing overheating. Seriously, this is one of THE most common reasons for this type of fault in the industry.

2) Incorrect seating or dirty/damaged ACB contacts causing poor contact and localised overheating.

and 3) Lack of maintenance which would likely have prevented the above issue.

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#11

Re: Air circuit breaker compartment burned and damaged

03/14/2018 8:33 AM

From the photos and description the most likely candidate due to the fact one phase was more heavily damaged would be loose/corroded contacts between the breaker frame and the buss stabs.

This can usually be prevented with regular maintenance of the switch gear with intervals dictated by ambient conditions, gear loading, frequency of breaker cycling and power quality in system.

We use a combination of bi-annual thermal scans of entire power distribution system (IR cameras built in gear for continuous monitoring are now affordable), live switch gear power quality monitoring with alarming feature (very inexpensive now), Quarterly ultrasonic check (we use ultrasonic tester for air leak checks primarily) and 8 year switch gear maintenance interval.

Our plant has 8 LV switch gears averaging 3000 amps each, 2 reverse feed switch gears attached to half a mega watt of solar power generation and one sub station with no distribution system faults to date.

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