Re: Megger reading
Samz, I want to say there is no real standard, but actually there are several standards, and guess what. Each one is different! I will just tell you what I have always gone with. And so you know I have been what is commonly referred to as a motor head for about 20 years.
Anyway, Anything at 1 meg or higher is generally not a problem. Again there are formulas some have for figuring out the minimum for different voltage motors and runs of cable. If you meg it right at the motor again 1 meg and higher usually means OK. Anything other it's not even necessarily time to do anything. In the Navy under 1 meg you put heat on it till it came above 1 meg then start it.
There are many contributing factors that drive the ins. resistance down. Humidity, Temperature of the motor at shutdown, Temperature of the air and other things. I will admit we had a 12,000 KV motor that we used to start and it would read around 250 K ohms before we started it. This was in Syracuse NY, always freezing cold when we did the shutdowns, and with 8 years there and at least 4 or 5 starts a year, we never burned it up. Megger readings are something you need to trend to know if they are bad. You can put in a brand new motor and have it read 1 meg but it's still good.
Since I've rambled like crazy. 1 meg is OK to start, keep checking it to see if it goes down.
The other thing that will indicate a real problem is what is called a polarization test. You hold the megger for about 5 minutes and take a reading every minute then agoin there's formula's to use. I don't know the formula. I would have to look it up, but if you do a check on polarization index it will show up.
