An unusuall case on Insulation Resistance Test on MI Cable 1

[EEAOC]

I have the following reading on 800 feet MI cable for insulation resistance. The reading on the Phase to ground [A-G] was too small in the range of 2.7 to 2.9 Meg-Ohms. Where as the manufacturer’s specification for MI cable is 25 Meg-Ohms and larger. The reading on Phase B and are unusually too high in the range of 20 Gig-Ohms range. Have any one seen such a huge deviation between phase A and B and C? According to manufacturer’s specification, phase A feeder fails. The price for MI cable now a day is about $120 per feet for 800 feet of cable. I have a tough decision to make without knowing why this vast difference between each phases. All readings take were tested with more than one good Megger and found to be accurate and consistent.

I wonder if anyone had a similar experience.



Test performed A-G B-G C-G

30 sec Spot Reading Test 2.7 20,000 20,000
60 sec Spot Reading Test 2.8 20,000 20,000
10 min Spot Reading Test 2.9 20,000 20,000
Step Voltage 500V/1KV 2.7/2.8 20000/20000 20000/20000
Polarization index 10/1 min 1.04 1.00 1.00
Dielectric absorption ratio 1.04 1.00 1.00

 

[busbar]

Is the circuit three single conductors or one 3-conductor cable?

Depending on urgency, consider baking the Aø cable by shorting center conductor to jacket and injecting rated(?) current for a week, using a transformer and powerstat.

Long shot — reterminate the cable if possible.

 

[edison123]

Insulation cracks near the terminations leading to creepage paths ?

 

[EEAOC]

Is failure of insulation resistance test on one phase[Phase A] is it good enough to replace an MI cable $100,000+ worthjust to replace one phase?. This thing feeds a Fire Pump.

 

[Alwaysexciter]

Depending on what the cable is installed in you may be able to locate the bad section of "A" Phase cable and repair it with a splice. It could have been damaged during pulling in. "B" and "C" phase megger readings are about what you would expect for good cable. How old is this cable? That should factor into whether to repair or replace.

 

[busbar]

Price sounds steep, but there must be conditions that contribute to that cost.

A second opinion after an onsite inspection and independent measurements may be warranted.

"Baking" or retermination might be considered before such an investement.

 

[corrosivo]

EEAOC

These readings are not unusual, they are telling you that the ph. A have very low insulation resistance to ground and the other ph. are ok. What are the readings between phases? You must look for the cause of this low reading, probably the cause of the low reading is in a very localized spot. Look for cracks as edison123 says or dirt, are you sure both ends are completely disconnected? Remove any type of termination and clean the insulation of the individual wires in the ends with a apropiate cleaner, and try again. Maybe the cable insulation is broken somewere along the length and is causing the low reading the good readings in the other phases suggest a healty cable. This cable feeds a fire pump so it is a critical equipment, probably it will be standby for years but you do not want to descover that when you need your fire pump it trips due to a grounded phase so if you do not find the problem, replace it.