We did a phase to ground megger test in between phases of a 2500A, 277/480V, 3-phase, 4W service (with Gnd fault prot.) and we are getting 424MOhms. According NETA table 100.1 we are within acceptable range. Our client is concerned that the difference would create a potential which will affect the Arc Flash of the system and would eventually trip the Main CB. Is this really a possibility and what precautions can we take. Thanks.
Phase to Ground Acceptable Megger Value
- Thread starter BearState
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iMuse97
Senior Member
- Location
- Chicagoland
Is that each phase individually to ground?
zog
Senior Member
- Location
- Charlotte, NC
Huh? Not following you at all.
renosteinke
Senior Member
- Location
- NE Arkansas
"Megging" isn't really an exact science, with 'cast in stone' values.
The simple facts are that insulation deteriorates over time, and no installation is perfect.
If you had a new installation, I wouldn't worry about the reading. Yet, the real value of the megger test does not come from just one reading.
Rather, over time, the values (whatever they are) will hold .... hold ... hold ... dip a little ... then really start to drop. This drop warns you that the insulation is failing, and lets you fix it before something really bad happens. "Predictive" maintenance.
If this is an older installation, your reading might be a suggestion that you need to repeat the test every month or so, and see if you've started on that downward trend.
The simple facts are that insulation deteriorates over time, and no installation is perfect.
If you had a new installation, I wouldn't worry about the reading. Yet, the real value of the megger test does not come from just one reading.
Rather, over time, the values (whatever they are) will hold .... hold ... hold ... dip a little ... then really start to drop. This drop warns you that the insulation is failing, and lets you fix it before something really bad happens. "Predictive" maintenance.
If this is an older installation, your reading might be a suggestion that you need to repeat the test every month or so, and see if you've started on that downward trend.
Cow
Senior Member
- Location
- Eastern Oregon
- Occupation
- Electrician
Who's doing the testing?
What voltage?
Are you sure the test leads were connected properly?
Reason I ask is 99999MOhms is very high, I didn't even know meters went that high? My fluke 1507 stops at 11GOhms or 11000MOhms.
zog
Senior Member
- Location
- Charlotte, NC
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
I would be nervous about one phase being so much lower than the others. But it seems well within acceptable limits.
My guess is that the 99999 reading just means the reading is over range. Which in this case would be good.
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