Megger Testing

[BLK]

Our company performs roadway lighting installations. We are looking for a standard for megger testing 600 volt cableS such as a IEEE standard. We are having a disagreement with an engineer whom wants us to test cables with surge arrestors, for luminaires, connected during testing. These contain MOV's or Metal Oxide Varistors, that leak during megger testing. When you have 8 to 30 of these in parallel it is impossible to get a accurate reading. Does anyone know of a standard, expert, or firm that could help us resolve this issue. Please respond. Thank you for the time and input.


BLK

 

[ron]

The job of a surge arrestor or TVSS is to shunt overvoltages. Most megger testing that I've been involved with utilize a voltage higher than the nominal, so the surge arrestor or TVSS will attempt to shunt the over voltage condition and give weird results.

 

[tom baker]

There is not a standard expert or firm needed, only that your engineer understand how the insulation test works and how a surge arrestor works.
However there is a manual from Biddle called Insulation Testing for the Practical Man. Also check the instructions for your insulation tester. It must state to disconnect all ballasts, electronics etc...

 

[petersonra]

generally the megger testing specs I have seen require such devices to be disconnected prior to testing.

 

[brian john]

NETA has specifications set by members, and is often specified by some engineers and on Fed Gov. projects.

http://www.netaworld.org/


As mentioned Megger AVO has a nice publication.

http://www.megger.com/eu/index.php

This site talks about high voltage DC testing of cable (cable over 1000 VAC rating) but does touch on some "Megger" testing.

http://www.okonite.com/engineering/high-voltage-testing.html

The surge arrestors will cause problems with your readings. Assuming you are there to test only the cables the cables ends should be isolated and insulation around the exposed ends cleaned with an approved cable cleaner.

It is mandatory at my company that all feeder conductors be meggered prior to energizing. Every truck carries a megger and I train all new employees on their use. I also furnish them with literature (which I bet few read).

If I was king I would have all neutrals meggered prior to energizing panels to avoid neutral to ground shorts, and while you are doing the neutral hit the phase conductors.

I would bet that 95% of all buildings have multiple neutral to ground shorts downstream from the service or separately derived system neutral ground bond.

Busway should be meggered piece by piece as it is installed and the complete length should be meggered as a piece is added.