Insulation Resistance Test Procedure 1

[LRB45]

We are currently working on a project which requires that we perform a 'Insulation Resistance Test' on a 50-pair direct bury cable which was installed. The spec simply reads " Make and record end-to end test of each conductor to all other conductors and all conductors to ground (shield)"
As of yet I have been unable to locate the correct procedure for this test to be performed on a telephone cable. I can find procedures for high voltage cable but this doesn't seem to correspond to what I need.

Any help or direction to material covering this would be greatly appreciated.

 

[SYQUEST]

Who is the Authority Having Jurisdiction (AHJ) and what code are they referencing for this requirement?

....JIM....

 

[LRB45]

The AHJ is NASA and as such there is no reference to code requirements simply an internal mandate. I currently have RFIs in asking for clarification but am trying to be as prepared as I can when it comes up in our next meeting.

 

[kwtaylor]

If their looking for continuity any tester that records test results will do that. But if their looking for continuity, attenuation, x-talk, your gonna need a tester of WireScope quality.

But if the test is Insulation Resistance refering to your title a bucket of water and match is all your gonna need to determine if the jacket is water time or Plenum..

Breath deep now....

 

[LRB45]

I'm familiar with the level III testers as we have several DTX 1800s but we don't have a Megohmeter which it's looking like is the only way to perform this test.
If anyone has any experience with this test feedback would be greatly welcomed. Otherwise I'll just keep plugging away at searches thru the ASTM and BICSI sites and hopefully find something. I think it may be D466 from the ASTM but haven't verified that yet for sure.

Thanks

 

[wires]

While more commonly used in motors and other higher voltage applications insulation resistance testing would work the same way for a telephone cable. The main bit of information you need is the test voltage and duration of the test.

Here are some articles:

findarticles.com/p/articles/mi_qa3726/is_199705/ai_n8760464
ecmweb.com/mag/electric_insulation_resistance_testing_3/

http://www.coleparmer.com/techinfo/techinfo.asp?htmlfile=AEMC_insulation.htm&ID=5

It looks like a Fluke 1507 might be the a decent choice for the instrument to use. It will be time consuming since there ill be a LOT of combinations to test and record.

Depending on how you want to interpret "all conductors to ground" that is either 1 test or 100 tests. My math challenged mind figures that for the "end-to end test of each conductor to all other conductors" there will be 249 tests (or 99). My guess is 350 tests, 100 tests or someone used the wrong spec sheet when bidding the job.

If some math smart person can test and confirm or come up with the correct formula for this problem I would give them a star. For 50 pair cable you have 100 wires. The formula I came up with was:

w = 100

w + (w-1) + (w/2) = 249 pair combinations

It seems to work with even wire counts above 4. But what do I know...

 

[wires]

I was WAY off.

The math concept is permutations see en.wikipedia.org/wiki/Permutation.

The permutation P for wires (elements) n=100 and pairs (elements selected) r=2 works like this:

P(n,r) = n!/(n-r)!

P(100,2) = 100!/(100-2)! = 9.3326E+157/9.4269E+153 = 9900

In Excel it can be done with =PERMUT(n,r)

Since for insulation testing wire1->2wire is the same as wire2->wire1 then the result can be divided by 2. This still leaves 4,950 tests to be done if the spec is to test each wire to each other wire.

If instead the spec calls for connecting all wires together except one and testing the insulation between wire1 and wires2-100 then 100 tests should do it for the first part. Then testing wires 1-100 to ground is the second part. So that would be 101 tests.

I am starting to lean in the direction that the spec for insulation resistance testing was misapplied to this particular job.




Another example, 4 wires, 2 pairs:

P(4,2) = 4!/(4-2)! = 4!/2! = 1x2x3x4/1x2 = 24/2 = 12

Then divide the 12 by 2 to eliminate duplicates and you have 6 distinct pair combinations. 1-2, 2-3, 3-4, 4-1, 1-3, 2-4.

 

[wires]

Here is the spec:

http://specsintact.ksc.nasa.gov/masters/NASA masters/NASAKSC/pdf/27 15 00.23 98.PDF

Looks like they really want insulation resistance testing on a 50 pair telephone cable. At one point there is a reference to 600 pair audio cable. That could be over 179,700 tests!

An Air Force specification, TO 31W3-10-15, is also referenced but I cannot find it online. This document details Outside Plant Cable Testing and may have the answers to the big questions:

1. Test voltage. There is a reference to 500 volt test sets on page 11. Both the Fluke 1507 and 1503 have that test voltage.

2. Test duration.

3. Between which wires to test the insulation.

4. Desired resistance. For 500 volts it should be at least 1.5 megohm.



Another interesting bit from the above document:

"The KSC duct system does not contain pulling lines and can contain orangeburg material. Some sections could require mechanical rodding equipment with cutting tools and water pressure equipment to clean and align the defective or blocked orangeburg duct as necessary."

So the Kennedy Space Center used ducts made of material resembling newspaper and tar.

http://www.sewerhistory.org/grfx/components/pipe-orng1.htm

 

[SYQUEST]

That seems familiar to the 500 volt breakdown tests we used to do at Western Electric in the service center (repair shop) on telsets and other apparatus. But I don't remember who made the equipment. The Methods & QC people took care of that stuff. WECO wanted to make sure the phone equipment would not shock anybody.

....JIM....

 

[LRB45]

So I was ale to receive clarification as to what is expected.
We will be required to perform a Megger test at 500V as typical for this test. The good news is that we only have to perform the tests as follows:
1)All conductors as a single group to the shield
2)Each individual conductor (wire 1) to all remaining conductors (wires 2-100) as a single group.
For a grand total of 101 tests performed. Much better than the 4,950 possible as proved wires calculation.

 

[jet042]

Wires, you are actually looking for a combination not a permutation. Permutations are order-sensitive in that {1,2,3} is different than {2,1,3} and {3,2,1} etc. Combinations are order-insensitive and the formula already takes care of duplicates, so you don't need this part:

Then divide the 12 by 2 to eliminate duplicates and you have 6 distinct pair combinations. 1-2, 2-3, 3-4, 4-1, 1-3, 2-4.

 

[wires]

LRB45:
101 tests sound like something a human could actually do. "Megger" is a trade name for one company that manufactures insulation resistance testers. Many other manufacturers make equivalent equipment. But Megger is the Kleenex or Q-tip of the market.

jet042:
Very cool, The divide by 2 thing was a bit of a hack. I must of been daydreaming when combinations came in in class.