O.T. Megger Training

I am trying to learn how to use a megger and I am having trouble finding some step by step guides.
We currently have one of the old crank type and I want to learn to check motor windings and wire pulls through conduit,etc.
How do these old crank type compare to the new fluke digital models like this http://us.fluke.com/usen/Products/Fluke+1587+1577.htm ?
Anyone have any good links on megger tutorials?
Are there any tables for acceptable resistance for a specific motor size? Wire Size?

You can start here:

http://www.patchn.com/index.php?option=com_content&task=view&id=24&Itemid=74

Kid
The first thing you need to do is proof test a megger. This is done by putting one lead in bosses left hand and other in right hand. One good sharp twist should get a reaction. If no reaction you know one of two things
A megger broke
B boss dead

OK humor aside.
These are not a toy -- supposedly the small ones will not electrocute but have NO doubt they will ring your chimes.

To check it short leads you should get 0 and with leads in open air you should get infinite. I suppose you could try a 1 meg resistor - never saw it done and do not recall meggers being calibrated.

The Doran article is good to understand the instrument. The next one is real good and very detailed. Well worth the time to read.

NEVER MEGGER SOLID STATE ANYTHING
disconnect or short out if you like to gamble - me I disconnect or check with a VOM. VOM is not as good a test as megger but it will not blow up electronics.

In general
use a thumbrule of 1 megohm per thousand volt of equpment rated voltage. In the case of AC use the RMS value ie 460 for 460 etc. Boil this down to a simple number 1 meg for any AC equipment up to 1,000 and 50 K for DC equipment (these were numbers we used in Navy our DC was 250).

While 1 meg is acceptable for operation I always felt better with 5 and better yet 10 or more on AC motors. Distribution gear should be 10 or better in my mind.


Less than 1 meg on motors I would check for moisture. Sometimes a hair dryer 12" away and pointed at the intake and left for a shift will dry it out. The best is to clean with soap and water then distilled water then alcohol then dry with heat. Royal PITA I know disassembly is best. I have seen people dry them by turning them on and running
- I stand back, and let them shut the disconnect and push the start button.

You can run em less than one meg but I would be ordering a replacement or calling a rewind shop.

Dan Bentler

I took a 4 day course at AVO Multi-amp on using a megger. There are more tests to do than you can shake a stick at. There are PI, DAR, IR, etc. The book already described "A Stitch in Time" from omnicontrols is a great place to start. After understanding the megger, then you can get into DC hipot testing, AC hipot testing, and Doble powerfactor testing. All of these will provide results on equipment that is over the standard 600 volt insulation.

One general rule: Use a 250V megger for 120V devices, a 500V for 230/240V devices, and 1000V for 460V devices.

DickDV said:

One general rule: Use a 250V megger for 120V devices, a 500V for 230/240V devices, and 1000V for 460V devices.

Click to expand...

And beware of instruments designed for HV (MV in the USA) systems. These typically start at 5000V and are potentially lethal. They should not be used without specialised training.

A little aside - we had an guy at Tech College in the 60's - yes that old - who was a pain in the 'A' and keep playing pranks on people - dangerous at that. We hooked up a crank winder Megger (2500V) to a 2500V capacitor. Charged it up and hooked one terminal to the frame of the steel framed chair and the othet terminal was connected via a wire soldered to a drawing pin. No need to guess where the drawing pin was placed!
Nearly killed him I think - as he had done to others. The HV meggers can be quite dangerous.

Meggers can be fun

Used to take my Dad's hand crak megger fishing. Put the probes in the water, a few cranks and the fish came to me. Dad didn't agree it was proper way to fish but it was fun.

I used to hang on to the 1000V megger while the other apprentices cranked for all they were worth then dare them to try. The secret is to hold tight and you hardly feel a thing, hold it loose and it hurts.
I made the misstake of shocking one of the journeymen and recieved a thick ear.
I wouldn't try that with one of the battery powered variety