Cable Testing

[triniboy]

Guys

I a nice question for all you knowledgable guys out there, I am installing cables on an LNG project in Peru, now we have come across some suspect cable these are all General Cable TC 600v cables for electrical controls and are various from 3c 12awg to 19c 14awg, now the problem is that some are falling their megger checks we have a benchmark on the project of 100 megohm and above to be accepted, some of these cables are failing but what is strange is that we now find out that if the cables are meggered in the morning they all were above the benchmark but when they are left and checked later in the day they are all not meeting the benchmark, now we have a nice sunny climate here in Peru so dampness is not an option but when they have been in the sun they seem to fail, now this is both cables that are in a trench and also cables still on the drum, anybody any ideas what the problem is.

Triniboy

 

[Falcon007]

Are there certain colors that tend to fail? Sometimes red,black,etc have too high a level of color concentrate in the insulated conductors and can have adverse affects. Seems odd though that it's limited to a certain time of day or temp. woithout the presence of moisture.

 

[zog]

Guys

I a nice question for all you knowledgable guys out there, I am installing cables on an LNG project in Peru, now we have come across some suspect cable these are all General Cable TC 600v cables for electrical controls and are various from 3c 12awg to 19c 14awg, now the problem is that some are falling their megger checks we have a benchmark on the project of 100 megohm and above to be accepted, some of these cables are failing but what is strange is that we now find out that if the cables are meggered in the morning they all were above the benchmark but when they are left and checked later in the day they are all not meeting the benchmark, now we have a nice sunny climate here in Peru so dampness is not an option but when they have been in the sun they seem to fail, now this is both cables that are in a trench and also cables still on the drum, anybody any ideas what the problem is.

Triniboy

Are you temperature correcting your results? All megger readings should be corrected to 20 degrees C, if your cable heats up in the sun and is 35 degrees C there is a correction factor of 2.00, so that 80 Meg reading you got is actually 160M. At 40 degrees C the correction factor is 2.5 and you 80M reading is realy 200M.

 

[don_resqcapt19]

Are you temperature correcting your results? All megger readings should be corrected to 20 degrees C, if your cable heats up in the sun and is 35 degrees C there is a correction factor of 2.00, so that 80 Meg reading you got is actually 160M. At 40 degrees C the correction factor is 2.5 and you 80M reading is realy 200M.

That is something I really don't understand. I assume that is just to make comparisons between readings taken under different conductions.

 

[ramsy]

LNG project in Peru, ..TC 600v cables (fail) 100 megohm ..when they have been in the sun ..both in trench and on the drum

Did you forget to discharge between tests and fry a toy Megger?
Liquid Natural Gas contracts should require procured-equipment specs, and verified field tests. What if you are actually using a hi-pot test, or damage cable from over voltage.

 

[zog]

That is something I really don't understand. I assume that is just to make comparisons between readings taken under different conductions.

Right, and the 100M spec is based on a temp of 20 degrees C. Temp correction is vital for trend analysis over time, the values recorded are worthless in not corrected.

 

[wptski]

If you use a PI test or ten minute test, you can ignore temperatures.

 

[zog]

If you use a PI test or ten minute test, you can ignore temperatures.

True, but a PI test does not apply to LV cables, your PI will be close to 1. PI is for inductive equipment.

 

[drbond24]

According to A Stitch In Time, which is Megger's publication on insulation testing, they use this rule of thumb:

For every 10 C increase in temperature, halve the resistance;
or, for every 10 C decrease, double the temperature.

 

[zog]

According to A Stitch In Time, which is Megger's publication on insulation testing, they use this rule of thumb:

For every 10 C increase in temperature, halve the resistance;
or, for every 10 C decrease, double the temperature.

Better to do it right, that rule of thumb is close for insulation systems containing oil, not so close for solid insulations.

 

[drbond24]

Better to do it right, that rule of thumb is close for insulation systems containing oil, not so close for solid insulations.

Thanks for the document, and you are right of course.

There is a similar table provided in A Stitch in Time on the page after the quote I posted earlier. It includes Class A and B rotating equipment, oil-filled transformers, and several different types of cable with different correction factors for all. However, they correct the cable to 15.6 C instead of 20 C. Do you know what the reason is for that? :-? Seems like an oddly specific number...

 

[acrwc10]

Guys

I a nice question for all you knowledgable guys out there, I am installing cables on an LNG project in Peru, now we have come across some suspect cable these are all General Cable TC 600v cables for electrical controls and are various from 3c 12awg to 19c 14awg, now the problem is that some are falling their megger checks we have a benchmark on the project of 100 megohm and above to be accepted, some of these cables are failing but what is strange is that we now find out that if the cables are meggered in the morning they all were above the benchmark but when they are left and checked later in the day they are all not meeting the benchmark, now we have a nice sunny climate here in Peru so dampness is not an option but when they have been in the sun they seem to fail, now this is both cables that are in a trench and also cables still on the drum, anybody any ideas what the problem is.

Triniboy

If it were me I would be in contact with General Cable. They will be able to give you a much better understanding of "what and why" with the cable. Those seem like very low numbers for a brand new cable on the spool, I would like to hear what they have to say about it. If you don't have better contact information for them, http://www.generalcable.com/GeneralCable/en-US/Company/ContactUs/

 

[GeorgeB]

However, they correct the cable to 15.6 C instead of 20 C. Do you know what the reason is for that? :-? Seems like an oddly specific number...

It is 60F, but I don't know why they would use that temperature.

 

[drbond24]

It is 60F, but I don't know why they would use that temperature.

Ha! I should slap myself for not seeing that.

60 F is a standard temperature for cable ampacity ratings. That is probably why they used it, although 75 F is more common.