Testing wire for damaged insulation

Fred B

Senior Member
Location
Upstate, NY
Occupation
Electrician
Is there a way to easily test existing wiring for damaged insulation that won't further damage it? I know meggering would generally find wire insulation faults but also heard that it might further damage a wire that might be only marginally suspect (and it is 40+ yrs old).
Dealing with a house full of old NM that as work began to isolate and change out lighting fixtures, I found multiple wires that original "electrician" (using title loosely with all the shoddy workmanship) had severely sliced the conductors when removing the NM sheathing, some of these slices extend the entire length of the conductors (house is undergoing an extensive renovation). About minimally 75% of all wires inside the breaker panel and relay switching distribution trough has this damage. I now am wondering of the rest of the system and damage that I haven't seen, (hidden J boxes have found a few, and outlets not opened as not changing). Yesterday had to go into a subpanel that had kitchen breakers and found the same. AFA I can determine its originally wireing from house build date of early 70's, no new NM B.

A little extra background, old Westinghouse 200A main panel, old Murry 100A subpanel, old GE Remote Control relay switching panel. Murray panel doesn't have a seperate ground bar (they terminated a bunch of the grounds under a single screw lug normally used for connecting your system ground).

My gut says a whole rewire, but need some substantiation or proof as "the wireing has worked for 40+ years", breakers aren't tripping (but not even sure if they can), as GC on job doesn't want to have that large of a project for wiring. How would you proceed to get substantiation or validity of leaving it in place?
Anyone know of a "sleeving" product for repair of insulation damage?
Another question what would be the cause, several of the breakers in both panels have an oily substance oozing up around breaker handles, what is it, and do they need to be replaced? Is that from breakers heating?
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
I find this sort of material is great for slipping over damaged insulation:

There are various grades, which will all function well as insulation and have sufficient temperature rating. The most expensive 'expandable' sleeving is fiberglass and silicone rubber and it easily stretches over larger diameter bits. The least expensive has to be carefully sized but would probably work great on THHN with a slot cut in the insulation.

Experience caution: I am a lab guy, not an electrician. The only times I've used the above materials in building wiring, it was DIY work.

-Jon
 

flashlight

Senior Member
Location
NY, NY
Have used heat shrink tubing in the past. With a heat gun or hair dryer it did not get hot enough to damage anything.
But that was a long time ago, supervisor at the time showed me the trick. Not sure of its insulation value now.

But what Winnie posted sounds interesting.

Also: It sounds like this project could be a can of worms, IMO.
 

Rock86

Senior Member
Location
new york
Occupation
Electrical Engineer / Electrician
Also: It sounds like this project could be a can of worms, IMO.
Yeah, I would agree. You start testing insulation on an old residential job, sure you may be able to sell a job rewiring a house, but if you cause damage during the test, now you own it.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
An insulation tester is like a normal ohm-meter: apply a test voltage, detect current flow, and calculate resistance based on Ohm's Law.

The difference is the voltage used for the test. As noted above, a common test voltage is 2x the insulation rating. You will also see '2x insulation rating + 1000V'. The point is that the voltage is high enough that it can age the insulation, or punch through marginal insulation and render a working system broken.

Say you have a 120V circuit with marginal '600V' insulation. It is working at 120V, but when you apply the 1200V test voltage you get a small arc which burns through a weak part of the insulation. You've now seemingly broken a working system. But the whole issue is that there was a weak point in the insulation, the system was on its way out. You've just caught it in a controlled situation with strictly limited current.

As also noted: air is a rather good insulator. If you have these breaks in the insulation, but nothing is touching them, then there won't be current flow and the circuit will pass the insulation test. Think old knob and tube wiring with the insulation completely flaked off exposing bare conductor.

-Jon
 
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