Testing Wire from Fire

[brian john]

Re: Testing Wire from Fire

Bennie:


I beg to differ with you on this issue, If one knows what they are doing, this is the test. You think an electrician or insurance adjuster can look at cable and tell if it good or bad.? I have done countless fire jobs from apartment fires to building fires and switchgear meltdowns common sense, expierence and testing are they way to do this. Random rip, tear and leave show a lack of education, fore sight and a poor approach to completing electrical work in a WORKMANSHIP LIKE MANNER.

 

[brian john]

Re: Testing Wire from Fire

Please do not instruct others to use a Megger to test wire after a fire, as the only deciding factor.

Please read my post. At no time did I say the Megger was an end to all problems. One must understand what they are doing, have COMMON SENSE and understand the test they are performing (as previously stated). But to not utilize all options available in a fire situation (or any other electrical situation) is a BIGGER MISTAKE. If the extent of the fire is such that there is wiring that may be good (looks good and is thought to be good) not to use a Megger is a mistake.

I have never had anything I’ve tested blow up/ fault, I have been to countless sites for blow ups and or faults of new and old equipment that was not tested prior to energizing.
I’ve seen numerous electricians run as they energize gear, or try to figure out how to close a main so they can be away from the gear. When I test a job I have no problem turning on any switch, cause I know it is free from shorts/faults.

 

[brian john]

Re: Testing Wire from Fire

Bennie:

Without wanting to seem anal about a subject, this post and your statement about my responses hit a nerve for a variety of reasons.

I contacted two of my competitors regarding your false negatives; they like me shrugged their shoulders and questioned the capabilities of the person using the meter. Additionally wire lube will remain in a liquid form for some time, like months and in some cases years.

So I will instruct ALL electricians to utilize a Megger a one of many test procedures coupled with COMMON SENSE and to do other wise is negligent.


I have enclosed a few links for those not familiar with insulation testing.
thttp://www.avointl.com/common/documents/Diagnostic%20insulation%20testing%20by%20S%20Drennan.pdf


http://www.itweb.co.za/office/spescom/0009201142.htm

http://www.transcat.com/TechReference/PDFs/Application%20Notes/BasicInsulationTesting.pdf

http://www.ulb.ac.be/polytech/laborulb/pdmgen/theorymegger.pdf

http://www.aemc.com/techinfo/html/Tech-Megohmmeters.asp

 

[don_resqcapt19]

Re: Testing Wire from Fire

Brian,
I've seen what Bennie is talking about happen. The wire was damaged when it was installed in the underground conduit, but because it was not touching another bare conductor or the metallic conduit the megger said the wire was fine and it was energized. A few weeks later, after a rain storm the conductor failed. The water provided the conductive path that resulted in the failure. The wire had, in fact, been damaged when it was installed, but it was not in a "faulted" condition when it was tested.
Don

 

[brian john]

Re: Testing Wire from Fire

Don: As I said above one has to understand the use of the test equipment there are limits to all test proceedures, conductors in PVC can be a problem.

 

[bennie]

Re: Testing Wire from Fire

Why are conductors in PVC a problem?

 

[sjaniga]

Re: Testing Wire from Fire

Brian, I understand your opinion on the megger, it is a good test instrument, but to say I am doing my customers an injustice by not using one is obscene, there is no way with 100% confidence you can tell a customer that a wire will not fail because you tested it with a megger, If you read my previous reply you would know I dont use it for fire repairs, I use other experts, and I to have never had a call back to fix a bad wire. I will use it to troubleshoot, and test service wires before the switch is thrown, and untill there is a procedure for testing fire damaged wires I will continue doing what I do, because it works for me and I can sleep at night.

 

[sjaniga]

Re: Testing Wire from Fire

I would also like to say that a 24 unit apartment is different than a single house, we've had fires in one apartment but had to rewire 6 units, and repair all wires from the others that had penetrated the attic, I couldnt imagine the time to meg roughly 300 wires.

 

[brian john]

Re: Testing Wire from Fire

Bennie:

To address the PVC issue it is possible for conductors to not be in contact with a ground or other conductor that may have questionable insulation. But I think you knew that.

sjaniga:

IMHO your may be wasting time and as bennie has said on more than one occasion PRECIOUS resources. I guarantee you if you have questionable cables (UNDERSTAND THIS) your not sure if they need to be replaced (QUESTIONABLE) the Megger is the way to go. And I would guarantee the cables if I test them, have done it and will continue to do it and this would be faster than replacing.

Either there is a misunderstanding of my post or a lack of understanding the testing procedures involved

 

[don_resqcapt19]

Re: Testing Wire from Fire

Brian,
So you are telling us that the megger will always catch all damage to the cable that resulted from the fire? I don't believe it. There is always the possibility that some of the conductor insulation has been damaged or destroyed, but these conductors are not in contact with other conductors or grounded surfaces at the time of the testing. Later movement cause by natural building vibration or heating and cooling may cause the conductor to fault to another conductor or a grounded surface. Yes, this would be rare, but very possible, especially in case where the insulation has melted away but had not been burned. If the insulation has been burned, then most likely there will be enough carbon residue for the megger to detect a problem. If the insulation has melted and run from the surface of the conductor you will still get a clear megger reading as long as the conductors are not touching.

Don

 

[pierre]

Re: Testing Wire from Fire

What is the difference between a hipot and megger test?

Pierre

 

[brian john]

Re: Testing Wire from Fire

Don:

Nothing is full proof, please read all the past post. Common sense, proper use of the equipment, knowledge of what one is doing and then YES I will assure you the cables I test are acceptable. If there is fire damage evident on cable to leave it in place is wrong, BUT AS STATED questionable cables, not sure if they should be replaced. Then Testing is the way IMHO.

Were talking BX armored cable and type NM cable. YES YES YES

 

[brian john]

Re: Testing Wire from Fire

Don:

Insualtion that is fire damaged in no longer an insulator it is a semi-conductor or conductor, perfect for this type of testing. But cables with this level of damage would be replaced under my common sense clause.

 

[brian john]

Re: Testing Wire from Fire

Megger is a trade name or AVO (and I believe Megger was the firms name in England) and has become a standard term for hi potential testers generally in the 100-1000 VDC and 500-5000 VDC range, though some firms are now making 10,000 VDC Meggers. Hi pots are generally above this test range 60 KV VDC, 110kv VDC, ect. Generally any test that involves applying higher voltage that is normally experienced is considered hi potential testing. Additionally meggers give results in megaohms while hi pots results are in leakage current.

One has to be careful not to perform destructive testing by applying excessive voltage levels to equipment under test. Several organizations have test specs for completing this work in a safe approved manner.

The above statement is m understanding not industry gospel.

 

[don_resqcapt19]

Re: Testing Wire from Fire

Brian,
I'm just trying to make it very clear that testing does not always prove that the cables are ok. I would not accept any cable that passes anywhere near the fire area unless I could both physically inspect the full length of the cable and electrically test it. I would accept electrical testing of cables that were only exposed to smoke and water, but want a full visual inspection as well as electrical testing of all cable that was exposed to heat. While you may be comfortable in staking the lives of other people on the results of testing alone, I'm not.

Insualtion that is fire damaged in no longer an insulator it is a semi-conductor or conductor, perfect for this type of testing.

I agree that if the insulation is burnt, this will be the case, but I have seen cases where the heat melted the insulation away from the conductor without burning it. It this case the megger will read clear.
Don

 

[flightline]

Re: Testing Wire from Fire

I've been watching this thread go along since shortly after it originated. I would agree that as a general practice, insulation resistance testing is highly beneficial in determining the acceptance or suitability of wiring and equipment. But I must agree with both Don & Bennie, in that it is not foolproof. I don't care how good you are. And before you start formulating a response, please know that I own 4 meggers myself, from a small go/ no-go unit up to my 15kV Biddle. You should also know that I and the crew that I work with are responsible for miles of cable, and numerous types of rotating machinery. Our crew was trained by IBEW/NETA instructors. for the 600 volt cable, we test using the NETA 7.3.2 standard.

You seem to be belaboring either your point or abilities, however qualified or experienced you may be. Statements like:

I may be the lone voice on this but I shocked at the lack of understanding of this issue.

So I will instruct ALL electricians to utilize a Megger a one of many test procedures coupled with COMMON SENSE and to do other wise is negligent

come off as didactic and somewhat arrogant, and you do yourself a dis-service. Yes we all understand that tests must be set up and executed properly, and the results must be interpreted properly for a test to be of value. We have all read the qualifiers that you put in your posts.

However in the end, we come back to a statement that people here, including yourself have made. "Nothing is foolproof". If you feel comfortable with your positon that's nice. But please don't force it at us as gospel. Most of us here are learned professionals, and while we can always learn something new or be incorrect on a particulat subject, this has come about as far as it can on this unless you know of some different standard.

[ July 18, 2003, 01:11 PM: Message edited by: flightline ]

 

[bennie]

Re: Testing Wire from Fire

Brian: I am just learning how to test cables.

Please review the proper procedure and principal of operation of a megger and a high pot.

I am afraid you and your competition have trashed a lot of wire, if you are using the methods you describe.

Only on the ionization test is a voltage higher than 150% of operating voltage applied to a cable.

The voltage applied on a high pot test is never more than the working voltage.

A high pot test is the best for testing wire that may be coated with smoke residue. A high pot test takes a minimum of 10 minutes per conductor. The conductor must be clear, then grounded and discharged after the test.

Good luck on your future projects.

 

[tom baker]

Re: Testing Wire from Fire

I agree with don. the megger will only show poor insulation if the bad spot is close enough to ground. Air is an insulator, of course it depends on the voltage.
Brian when you test with a megger, do you perform a di-electric absorbtion ratio (DAR)test? How long do you apply the megger voltage? A one time test for 15 seconds does not tell you much.

 

[brian john]

Re: Testing Wire from Fire

Tom:

Obviously I’m not going to convince anyone once they, have made their mind up.

As for types of test it depends on the equipment to be tested and what happens when one performs the test.

Short pieces of BX and NM if in good condition will go full scale of the meter, (Our meters are 10 gigaohm at full scale 1000VDC) in less than one minute. Typically small cables are spot checked at 250 VDC then 1000 VDC for a minimum of one minute. With smaller lengths of cable the reading changes little after one minute.

NETA specs are a minimum of 1 megaohms; we like to see a minimum of 5 megaohms Temperature and humidity are taken into account..

I won’t try to convince anyone of what is considered the norm around electrical testers.
But I will add we test with meggers everyday. We have rebuilt switchboards that have had terrible arcing faults that meggered 0 ohms when we arrived. When we are done we energized these switchboards with no fear of further arcing faults, because we are confident of our work and the test we perform. Believe me NM and BX after a fire is a cakewalk.

 

[bennie]

Re: Testing Wire from Fire

I know I am a dummy. but you are scary.