Request recommendations on testing efficacy of electrical power cable insulation

If you are concerned about the insulation, then conduct a hipot/meggar test ( http://en.wikipedia.org/wiki/Hipot ). It may prove "prove extremely challenging", to schedule it, but the test should not take much downtime. Alternately, if you have dual feeds, then you can run off of one while the other is shutdown, isolated and tested.

I would get a couple of estimates for complete replacement of the cables. If the cost of replacement is anywhere near the cost of testing... (or the cost of testing PLUS replacement, should the test results recommend so.)

From what you have described it may outlive us. You may never have seen knob and tube wiring, etc. There is no practical testing method for this low voltage wiring, the insulation may be good for 5,000 volts on a 120 volt circuit.

But if there is one short the whole bundle of wires will be burnt up and useless.

I would budget, engineer and install over years a new cable tray with isolators for each room / area / process fed. Then as each is available for outage tie in the new wiring. You may want to replace some breaker panels and have a redundant or switch-over to alternate power feed also.

As has been suggested the way to test the integrity of cable insulation is to apply a high voltage to the wires and using a meter read for leakage. That is what is comprised in the "megger" mentioned above.

Nonetheless, I find that situation very interesting. If I read you right the outer most layer of the insulation on those wires, having been exposed to the Sun's UV for twenty years has deteriorated and peeled.

I gather you are the facilities maintenance technician. Megohm testing is really a simple affair which you could learn to do with a few minutes reading providing you already have the skills of an electrician.

Nonetheless, with that long exposure to the elements and the resultant peeling of the outer layer, my own response and keeping cost in mind as if I were doing the job, would simply be to start replacing the wiring, one unit at a time.

The problem with that as far as cost is you are talking about three thousand feet of wire, copper at that. You could of course switch to aluminum but that introduces its own problems, i.e. oxidation and electrolysis, which you can protect against, but also a tendency for connections to loosen over time because of dissimilar coefficients of expansion of dissimilar metals at the connection points.

I would opt for copper.

If you need to keep the plant running 24/7, and providing there is enough space at the connecting points, and providing you have enough understanding and technique, you ought to be able to do the job, one cable at a time, hot, i.e., without disconnecting from the operations requiring power or from the power source.

I would caution, and this is something you would have to decide for yourself, you can get seriously injured if you don't have the experience to do that and in that case I would call in outside electricians with that level of skill.

I have myself had on one occasion a need to replace an entire three phase distribution breaker panel and replace it with two such and along with the usual manufacturing equipment, keep a bank of main-frame computers on line. It can be done but be sure you know what you are doing and have the confidence to do it. No insult intended; just concern for your safety.

One issue I have not raised here is voltage drop over those long runs and the operating voltages. I am assuming that having run as long as they have that has not been a problem.

I would make one suggestion or really ask one question that might make a large difference in cost. Can you not get the power company to site poles and transformers closer to the points of use. After all you are talking about cable runs of almost a half mile.

Except for the fact that you mentioned roofs I would think those long runs were going down into some sort of mine or quarry.

And, because I am curious and I imagine others here are too, would you mind telling us just what this operation is?

j.

We here in India normaly do not lay cables externaly. We either lay them overhead in cable trays or in trenches which have removeable covers. If you have laid externaly then you should have selected cables suitable for external use like armoured cables or running through conduit pipes which protect the cables. Present installation is fire hazzard as any time if cable loose insulation and get exposed it can lead to fire. So giving consideration to Safety First kindly change the present system of laying cables.