Another Megger question

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Pierre C Belarge

Pierre C Belarge

Senior Member
Location
Westchester County, New York
was called and asked about a meggering situation. Here is the info as best as I can pass it on to you. Maybe someone has had this experience and can help.

There is a 200ft conductor run, 6 sets of 600kcmil Al, Type XHHW conductors. Those conductors are terminated to lugs on a 52ft SQD-Iline bus duct.
The system is 480V-2000amps.

The electrician meggered out each phase while the XHHW conductors were terminated to the bus duct. He meggered all 6 parallel conductors for each phase at one time.


His setting for the megger was at 1000V.
His readings for A-B and B-C were 88M (the voltage of the megger actually read at 1080V).


His reading for C-A were very low (I believe under 10M), with the voltage of the megger not reaching greater than 147V


He tested the megger again on the other phases and recorded the same readings as earlier.

SQD-Iline literature states the megger results should be at approximately 52M for the 52ft run.


Does the coupling of the bus duct to the XHHW conductors cause the (what I think is a low reading for new work) 88M reading?


What do you think could be the reason for the unusual readings of A-C?


The tester is the Fluke 1507, brand new...with the batteries that come with the unit. Could it possibly be the batteries?

He is going to take a reading of the XHHW conductor, isolated from the bus duct. I asked him to at both 500V and 1000V for my own curiosity.
 
This happened at a construction site once when they dug up the
cable they ran their hands along the length of the cable. They
felt a lump or bulge and opened the cable to find the manufacturer
had butt spliced two seperate ends with out crimping them. The
cable was then finished out and rolled up to sell! The contractor
was a bit ticked off. The wholsaler mounted the nasty piece of
cable on the sales room wall to make a point! It was Aluminum and
the megger issues were simular to yours. Good luck!
 
Pierre C Belarge said:
There is a 200ft conductor run, 6 sets of 600kcmil Al, Type XHHW conductors. Those conductors are terminated to lugs on a 52ft SQD-Iline bus duct. The system is 480V-2000amps.
The electrician meggered out each phase while the XHHW conductors were terminated to the bus duct. He meggered all 6 parallel conductors for each phase at one time.
His setting for the megger was at 1000V.
His readings for A-B and B-C were 88M (the voltage of the megger actually read at 1080V).
Click to expand...

The cables should be isolated and tested individually.

His reading for C-A were very low (I believe under 10M), with the voltage of the megger not reaching greater than 147V
Does the coupling of the bus duct to the XHHW conductors cause the (what I think is a low reading for new work) 88M reading?
Click to expand...
Using the Okonite formula IR = K x log D/d where K = 50000 for this cable,
D = diameter over insulation and d = conductor diameter
IR = 50000 x Log 1.053/0.60 = 50000 x 0.244 = 12200 meg ohms per 1000ft.
For 200 ft IR = 12200/.200 ft = 61000 meg ohms. A typical value from
a MFG catalog was 75000 meg ohms.
What do you think could be the reason for the unusual readings of A-C?
Click to expand...
Do not know. I question why all of the reading appear to be low. Probably because the cables were terminated on the buss duct.
 
Yep, he is not meggering the cables, he is meggering the whole system. Disconnect the cables and test individually. Doing them alll at once is a shortcut novices like to do, but can cause poor readings and more work in the long run.
 
Be careful not to megger through instrumentation Like ground fault relays or factory metering equipment. It sounds like there is a problem going on to megger in the first place so further isolation is in order here if in fact you have a phase to phase high impedence short. You need to find it and clear it or further dammage will certainly occur.
 
I agree isolate the cables. Then Meg the Bus Duct. We had a CH 480 2500A duct with low reading phase to phase. Found condensation in the couplings. This was outdoor rated duct with weep holes, but we have high humidity in the south.
 
This happened at a construction site once when they dug up the
cable they ran their hands along the length of the cable. They
felt a lump or bulge and opened the cable to find the manufacturer
had butt spliced two seperate ends with out crimping them. The
cable was then finished out and rolled up to sell! The contractor
was a bit ticked off. The wholsaler mounted the nasty piece of
cable on the sales room wall to make a point! It was Aluminum and
the megger issues were simular to yours. Good luck!
Click to expand...

Why would this affect the megger readings, if the insulation is intact?

As for the OP divide and conquer (as noted by others)
 
This is all good information...
The person who contacted me I am sure has read these posts. He is going back to megger in the fashion that has been suggested on this thread. Either he or I will let you know the results.

The individual who contacted me is one of the brightest men I have had the pleasure of being associated with in this industry. His concern led to him purchasing his own meter and testing the installation. If not for his concern and dedication to doing the right thing (even at his own cost), I believe this situation could have been much worse if it had been engergized first without the testing he performed.
We need more people like him in our industry!!!
 
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