insulation test

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john reese

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we are conducting insulation test on 600v cable we have purchased a fluke 1587 as recommended by the people in this forum. someone noted that at 1000v the meter should read 50 megohms min. i found some other info for new installation testing it should be 100 megohms min. with less than 20% difference between the conductors. since i am new at this type of testing i need to better understand my testing results. i have found plenty of information on how to use the meter and am confident of my readings. but have had a hard time finding information to help me better understand my test results. can anyone send me in the right direction to find this information
thanks for your help john
 
The formula for insulation resistance is: IR = K*log(D/d) where D is the diameter over the insulation and d is the diameter under the insulation. K is the insulation resistance constant which in this case is 10,000 minimum per ICEA S-95-658-1999.

In your case:
K = 10,000
D = 1.011
d = 0.821
IR = (10,000)*log(1.001/0.821) = 904 megaohms for 1000 feet.

Notice that it says megaohms FOR 1000 feet, not megaohms PER 1000 feet. That means in order to adjust the value to your cable length you will need to multiply the above number by a ratio of (1000 / # feet in your run). For example, if your run is 350 feet, you need to take (1000 / 350)*(904) = 2580 megaohms.

Unless your run is very long, I would say your readings are quite low.
 
NETA spec 100M for 600V cable.

You are confused about the 20% between conductors, where did you get that from? So if Phase A and B are 40G and phase C is 50G that is bad???

Are you testing this cable on the reel?

How long did you apply the test voltage?
 
i got the 20% from an electrical acceptance test out of a spec. bookfor a university it reads [the insulation resistance between conductors, and from each conductor to ground, shall be 100 megohms minimum in one minute or less. in addition, the lowest resistance value shall not differ from the highest value by more than 20%. if all megger readings for a given circuit or feeder are above 1000 megohms, the 20% balance may be waived. ]
 
i have not posted any readings yet. but now i am really confused. the existing cables [circuits] are 480v we want to bump them to 600v. i got the following readings: ckt#1 brn to org 185m, brn to yel 230m, brn to grn 160m, org to yel 150m, org to grn 100m, yel to grn 105m. ckt length 300' ckt#2 only checked to ground at this point brn 80m, org <550m, yel <550m. ckt. length 350'. ckt.#3 to ground only at this point brn 45m, org 46m, yel 48m. ckt. length 325'. if these cables need to be at 2580m to function properly for 600v then why isnt ckt #3 blowing to pieces at 480v right now.
 
1. Is this new cable?
2. On the reel or in conduit?
3. Terminated on both ends or free in air.

I assume the colors you are using are to indicate phase and ground? Utilizing L1, l2, L3 and ground or A, B, C ground is easier to discern (at least for me)

A-B=
B-C=
C-A=
A-GND=
B-GND=
C-GND=

Do you have a set spec (it appears you do) if so this is the standard you follow. If not usually there is some standard specified.


Typically NEW cable, (not including MI cable) of this length one would expect/HOPE for readings of 4,000 megohms (infinity/full scale of this meter [I thin 4,000 is full scale of the meter in use]).

I have seen the The 20% above 1,000 meg specified before, If I am getting 1,000 megohms I would not worry about the 20% difference, 1,000 meg is as good (ALMOST AT 1000VDC) as 1,200 meg. And I would say with digital meters not at all unexpected or unusual.
 
john reese said:
i have not posted any readings yet. but now i am really confused. the existing cables [circuits] are 480v we want to bump them to 600v. i got the following readings: ckt#1 brn to org 185m, brn to yel 230m, brn to grn 160m, org to yel 150m, org to grn 100m, yel to grn 105m. ckt length 300' ckt#2 only checked to ground at this point brn 80m, org <550m, yel <550m. ckt. length 350'. ckt.#3 to ground only at this point brn 45m, org 46m, yel 48m. ckt. length 325'. if these cables need to be at 2580m to function properly for 600v then why isnt ckt #3 blowing to pieces at 480v right now.
Click to expand...

To give any feedback on those reading you have to answer Brian's 3 questions, basically what are the cable conditions as you are testing?

Also, an IR reading below any recognized spec will not blow up right away, Insulation Failure is a progressive event that accelerates towards a failure. It wont blow up until the IR is 0Meg.
 
brian john said:
I have seen the The 20% above 1,000 meg specified before, If I am getting 1,000 megohms I would not worry about the 20% difference, 1,000 meg is as good (ALMOST AT 1000VDC) as 1,200 meg. And I would say with digital meters not at all unexpected or unusual.
Click to expand...

You remmember where? I am curious to see where this comes from. I know you compare readings between phases for fuses, contacts, winding resistances, etc but never heard that before for IR.
 
brian john said:
I have seen the The 20% above 1,000 meg specified before, If I am getting 1,000 megohms I would not worry about the 20% difference, 1,000 meg is as good (ALMOST AT 1000VDC) as 1,200 meg. And I would say with digital meters not at all unexpected or unusual.
Click to expand...

You remmember where? I am curious to see where this comes from. I know you compare readings between phases for fuses, contacts, winding resistances, etc but never heard that before for IR.
 
this is an existing direct durial cable. it was landed in the switchgear. on one end. the other end was disconnected from a transformer before testing. the meter was put on the disconnected side. more info:this is an apartment compex built in 1970. there is a 480v main switchgear in a central building. the feeders run from the switchgear into the ground [direct burial] to other buildings where the voltage is stepped down to 208v to feed the apartments. the owner wants to upgrade the apt. heat/ac. each feeder is going to gain about 40 amps. the feeder is not big enough to handle the amperage gain unless we bump the voltage. at 600v the feeder will actually have less load than it does now at 480v.so there isn't any spec i just need to confirm that the insulation will hold up to the 600v. thanks again for your help john
 
The switchgear is figuring into this reading should be isolated if you really only want the cables value.

1970 cables EXCELLENT readings. But deteriorating and will continue. If this was mine I would recommended new cables for long term use. But if the continue to utilize at the higher voltage I would recommend regular testing to monitor cables.
 
I second all of that, except maybe the EXCELLENT part, post your readings after you disconnect the cables from the switchgear, then we will know.
 
zog said:
I second all of that, except maybe the EXCELLENT part, post your readings after you disconnect the cables from the switchgear, then we will know.
Click to expand...
Curious about one thing. Since the readings are otherwise acceptable, would you really recommend that they be disconnected from the switchboard to narrow it down? I generally don't do this if the readings I get are otherwise acceptable. Maybe if it gets worse in coming years, then isolate the cables at that point? Or check it on a day where the weather is different?
 
I would but then I am a bit (OK a whole lot) anal.

Zog the 20% was in on an airport job not sure if it was an FAA spec or the engineering firms requirement.
 
mdshunk said:
Curious about one thing. Since the readings are otherwise acceptable, would you really recommend that they be disconnected from the switchboard to narrow it down? I generally don't do this if the readings I get are otherwise acceptable. Maybe if it gets worse in coming years, then isolate the cables at that point? Or check it on a day where the weather is different?
Click to expand...

I am required to follow NETA specs, 45M is not acceptable per the NETA MTS, it needs to be at aleast 100m.
 
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