Discuss A first for everything, never expected this to cause a short. in the Commercial Electrical Advice area at ElectriciansForums.net

darkwood

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dorman3.gifDorman1.gifDorman4.gif
Check out the pics above; this was a callout to lost power in a factory, found 2 phases out blown 200amp fuses and a quick Meggar confirmed a short circuit between Y and B phases and unusual low readings to earth. id already isolated all outgoings and main switch was off to busbars but still dead short.

After totally stripping the dorman smith mccb board of incomming and outgoing mccb's only leaving the shell and empty busbars it yet again still showed dead S/C... at this point the only thing connecting between the bus-links was the paxolin insulation supports so as you can guess by now they had become conductive..... traced the reason to water leak settling on Paxolin and slowly cooking it till it charred up and result was a conductor shorting across the phases.

As a temp i filed down all the charring and raised the insulation value from a short to an acceptable 15Mohms but fitting back the mccb's reduced this as water must still be within the mccb's.... so until it rains heavy again they have business up and running and landlord has been made aware of what will happen if he dosn't get repairs done asap.

Im just a bit stunned that Paxolin has this trait that it becomes a conductor when charred, also yes its not a great concern anymore as it got banned years ago.
 

darkwood

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I had been informed that the previous sparky had come along changed the fuses and switched on and all the panel was glowing through every crack before it blew.... ermmm replacing fuses without at least checking it was safe to do so, think he was more comfortable with house bashing as he told them its beyond his field and he didn't know what had caused it, and he didn't feel comfortable working on the old Dorman Smith board.
 

snowhead

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Carbon is a good conductor, apparently.

Reminds me of a disaster we had with a main switch.
We'd found broken bits of paxolin under a 40 year old MEM 1000amp switch.
Went in on a planned shut down for transformer maintenance one evening, turned off the main switch to investigate and half of it fell into the bottom of the chamber,,oops, major supermarket full of frozen food.
 
E

Engineer54

Im just a bit stunned that Paxolin has this trait that it becomes a conductor when charred, also yes its not a great concern anymore as it got banned years ago.

The paxolin doesn't become conductive in itself, it's the charring, or more correctly the ''Carbon'' that is the conductive component. It wouldn't matter what type of insulative material was used, when a carbon (or other conductive mateial) compound is created, by the presence of another element, that electricity can pass through, you will have the same situation.

Even the very best solid insulation materials can be breeched in this way. Just have to look at those MV/HV videos of arch faults in switchboards to see it...
 
G

Guest111


The paxolin doesn't become conductive in itself, it's the charring, or more correctly the ''Carbon'' that is the conductive component. It wouldn't matter what type of insulative material was used, when a carbon (or other conductive mateial) compound is created, by the presence of another element, that electricity can pass through, you will have the same situation.

Even the very best solid insulation materials can be breeched in this way. Just have to look at those MV/HV videos of arch faults in switchboards to see it...
the place my cousin works at got in lumber with the supply people as they were burning rubbish in the yard and the carbon in the smoke was causing problems with the insulators on a pylon in the yard on the 400kv network.
 

darkwood

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Yes can understand why it done it now as its paper and linen based (hence carbonisation) but reading up on it i was wondering why its used when other better resistant products were available at the time but it turns out to be a cheap end option, just a market tactic to get costs down and was only on a LV system, E54 and i do have much appreciation for HV/MV systems and the breakdown of their insulators but thats a different level completely with comparrison to 400v. Think i'll stick to LV less need for the blast wear ;)
 
K

Knobhead

You’ll get exactly the same with most insulators. Once tracking starts the carbon builds up until you’ve got a short.
Working in quarries I’ve had it happen countless times. The worse was on 4000A bus bars, they went with one hell of a bang!
 

darkwood

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Must be just the nature of my jobs that i dont see this often then, usually dry factories and with new design better IP rated dist' boards and designed with non exposed internals i doubt il come across it too often in future, and as i said it was an old Dorman system that fell foul of water dripping through it and collecting and the insulator shelfs. Glad though cos was a pain to fully strip down to release the insulator mounts.
 
E

Engineer54

One reason why i always insist that any Switchboard/MDB that is installed on my projects gets a through internal clean, after installation and termination work has been completed (especially where other work is going on around it) before any testing and being put into service. On one project, one of the main switchboards produced about 15kg of dust and debris. The Civils were conducting remedial works in the area at the same time the electricians were terminating several parallel feeders at both ends of the switchboard.
 

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