unexplained flashover/arcing inside panel?

[phillyboy]

My client is a large international bank. For the past year, they have experienced strange and unexplained occurrences with their power distribution boards (sub-boards) in their office.

Let me describe a brief architecture arrangement of the distribution system in question. In the office, power supply to desks, sockets and other equipment is distributed by various sub-boards. The sub-boards are 3 phase panel but are normally wired for 10 to 16 single phase outgoing circuits. There is a main isolator (not a circuit breaker) in each of the sub-board.

Upstream of each sub-board, a circuit breaker of an appropriated rating is installed in a main distribution panel (main-boards) to protect the down-stream sub-board.

What happened for the past year?

There were over 10 incidences occurred over 6 months? period where different sub-boards with no ?out-going? circuits experienced internal flashover or arcing. There are no loads connected to the sub-boards.

The flashover occurs where the 20A single phase circuit breakers plugged into the main bus bar of the sub-board. The bus bar of the particular bland of sub-boards is made of male type of connectors. The female type sockets circuit breakers plugged into the bus bar and it is at the point of connections were the flashover or arcing occurred. There are damages at the point of connections especially at the tip of made plugs of the sub-boards bus bar. A small flashover burnt mark is also left on the surface of the circuit breaker.

When this occurred, the 20A circuit breaker did not trip. The tripping occurred at the upstream main-board circuit breaker feeding the sub-board.

Full investigations were conducted on the affected sub-boards for workmanship which included lab tests and insulation tests. No apparent defeats were found and the problem remains unresolved.

What happen inside the sub-boards and why are there flashovers when there are no loads connected? Please help out.

 

[davidr43229]

The sub-boards are 3 phase panel but are normally wired for 10 to 16 single phase outgoing circuits

Phillyboy,
I see you took great care in explaining however, what voltages are we talking with the 3 phase then wired for single phase?

 

[brian john]

1. From what you described the 20 amp CB should not trip as the "flash over" occured at the line bus connection of the 20 amp CB.

2. If there was no load on the CB at the time, that excludes a high resistance connection.

3. What was the test voltage utilized to verify the resistance of the insulation.

4. What test were performed on the CB.

5. As noted above what is the distribution system voltage, it sounds like a 120 system being a commercial office I would assume 120/208 3-phase 4-wire wye. It is very unusual for 120/208 to flash over.

What I would do.

1. Disconnect power to the affected panel and open the "main isolator" (I assume you mean a switch, similar to a CB with no overcurrent protection).

2. With the defective componets in place megger the panel. If there has been a flash over the megger readings should be low in comparision to a new or or non-damaged panel. I would question the test lab and ask to see their reports.

3. Repeat item 2 with defective componets removed.

4. I would test a non damged panel with 20 amp CBs loading them to 20 amps, utilizing (Fall of Potential) FOP test check the resistance of several CBs. I know you said they were not loaded at the time, but the damaged may have been in the past and I always question if the end user is telling the truth or knows the truth or may have confused the issue.

5. I would be looking for defective insulators, heat damage insulators, improperly installed equipment.

6. I would IR all the panels ASAP.

7. I would contact the manufacture for possible warranty and recall issues that may exist. Be aware they may not know the truth or tell the truth.

8. That's a start I am sure others will have additional ideas.

 

[ghostbuster]

After you have gone through Brian's list ,and then if their still doesn't seem to be an apparent problem with these boards,I would do the following:
1.Install a high speed (1 microsecond response time) voltage transient monitor
at 1 of the "good" boards.If the budget permits,I would also simultaneously install another at the main board.
2.There maybe an intermittent arcing ground generator in this system which will then flashover at the weakest point of insulation in this system(your boards).Using the 2 monitor system allows you to zero in on this arcing source generator

Good Luck.

 

[brian john]

Durn it GB

.Install a high speed

I knew I overlooked at least one thing.

I posted this for another thread but felt I better add it here.

I did not add and after a recent phone call from a contractor, I MUST SAY THIS, when meggering distribution equipment in the 120/208-277/480 DO NOT USE over 1000 VDC.

 

[dbuckley]

Just to be clear, in addition to these unloaded panels, there are other similar distribution panels that do have loads on them, which do not suffer this problem?

The circuit breakers involved in the flashover; they sound like the isobar type suitable for live working; were the CBs plugged in and connected, or isolated? It sounds like you are describing them as being plugged in, and the flash occurred from where the male / female bits meet.

 

[loren]

It is odd that it is a three phase panel and it has 10 and 16 spaces. Could be difficult to balance load. So I am thinking that maybe the system is a single phase three wire system. In that case I would look up stream at the neutral in the distribution panel.
It also possible that there is a line side connection that is making and breaking sending a higher voltage surge down stream.
The connection clearances may not be able to handle any higher voltage than the normal 120 volts.
Loren

 

[Rockyd]

It may sound simple, but remind the crew to make sure everything downstream from where they are meggering is unplugged! Lots of things (besides apprentices) don't handle the hot side of a megger very well. Murphy's rule says that you will smoke the most expensive piece of equipment they have, if your not careful.

 

[brian john]

Rocky:

We always start meggering branch circuits in occupied buildings at 50 or 100 VDC, to minimize the issue you mentioned.

 

[dbuckley]

Wish the OP would get back and give us more info; this is a really curious case.

It reminds me of this thread in the sci.engr.electrical.sys-protection newsgroup (via Google) back in 2000. The usenet thread ends without a resolution, so I emailed (back in 2004!) the chap investigating ('Chico'); there is a little more to the story, but bottom line, the mystery was never solved, the dentist moved out...

 

[brian john]

One thing I have learned there is a reason for everything, no magic only unsolved problems, with perseverance all electrical anomalies can be resolved.

 

[dbuckley]

I'm with you on this, which is why I hate these unsolved mysteries. It just means we haven't figured it out - yet.

 

[brian john]

I hate unresolved mysterys. I have often stayed on my own time to figure out a problem, just to satisfy this drive to know.