jameselectric
Senior Member
What is the rule of thumb on what type of resistance you should have on standard insulation, thhn-thhwn 2 wire ?? is it 100 ohms reistance per 100 volts ? (ie 600 v would have 600,000)??
Megger rule of thumb
- Thread starter jameselectric
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The "rule of thumb" on equipment that runs less then 1000 volts is one megohm.
check out page 46 on this link http://www.biddlemegger.com/biddle/A%20Stitch%20In%20Time.pdf
check out page 46 on this link http://www.biddlemegger.com/biddle/A%20Stitch%20In%20Time.pdf
quogueelectric
Senior Member
- Location
- new york
megger question
megger question
Where is Marc when we need him?
megger question
Where is Marc when we need him?
jameselectric
Senior Member
acrwc10 said:
thanks for the link
so if a 'test' is lets say 0.2 meg ohms, at 1000 volts, is this really a cause of concern? seeing this is only 277/480 volts?
mdshunk
Senior Member
- Location
- Right here.
If you're reading THHN around 1 megohm, something serious has happened to it already, even it it's not less than one meg. I know what the book says, but measure some new stuff and see for yourself. If I'm measuring anything less than 20 megohm, that's my point at which I'll condemn the wire.
- Location
- Massachusetts
mdshunk said:
Marc, not busting your chops but do you have any explanation for the large difference between you and the book? :-?
brian john
Senior Member
- Location
- Kilmarnock, Va
- Occupation
- Retired after 52 years in the trade.
Yeah the book was written by engineers sitting in a lab, Marc has field expierence.
My rule of thumb for 480 VAC distribution.
I like 50 megohms or higher.
At 10 megohms I get concerned. And want to investigate.
Never like to energize below 5, I ALWAYS leave this in the customers hands to make a decision.
I have had contractors energize WITHOUT MY APPROVAL at 1-5 megohms and to my knowledge all 4 case blew up in short time. One on Christmas eve (underground feeders blew in about a month), one in less than 24 hours (4000 amp busway) and one within a month (switchboard) and one instantly (switchboard).
My rule of thumb for 480 VAC distribution.
I like 50 megohms or higher.
At 10 megohms I get concerned. And want to investigate.
Never like to energize below 5, I ALWAYS leave this in the customers hands to make a decision.
I have had contractors energize WITHOUT MY APPROVAL at 1-5 megohms and to my knowledge all 4 case blew up in short time. One on Christmas eve (underground feeders blew in about a month), one in less than 24 hours (4000 amp busway) and one within a month (switchboard) and one instantly (switchboard).
- Location
- Massachusetts
brian john said:
Yes, and I know you do to, that is why I want to ask you these questions. :smile:
electricalperson
Senior Member
- Location
- massachusetts
the 1 megaohm rule is for equipment like motors, transformers etc. not wire. anything under 50 megaohms should be replaced. i done an experiment with romex and i had arcing at 100 volts and it was above the 1 megaohm rule.
also id like to thank brian john and mdshunk for helping me understand the megger more. i use it a lot to check work before energizing
also id like to thank brian john and mdshunk for helping me understand the megger more. i use it a lot to check work before energizing
brian john
Senior Member
- Location
- Kilmarnock, Va
- Occupation
- Retired after 52 years in the trade.
Bob my post was not to met to be sarcastic, it is just I have dealt with SOOOO many manufactures engineers, that have little or no clue what really happens in the field.
The things I have been told when inquiring about test equipment with manufactures.
WHY ONE PIECE OF EQUIPMENT NEVER WORKED
"That equipment was not really designed for field use." UM READ YOUR SPEC.s
WHY ONE PIECE OF EQUIPMENT SELDOMED WORKED after years of frustration.
"It is a conflict with other software"
"You are improperly utilizing the equipment."
Finally they admitted it was a defective piece of equipment.
WHY THE READINGS WERE ALWAYS OFF.
"We really do not know we get the same results"
WHY THE READINGS WERE OFF SOMETIMES.
"yeah that happens sometimes"
And there is the megger by a major manufacture
I bought 5 of them all were inoperable within two years, major cal issues.
No major test equipment manufacture is immune from this. Except there is no lemon law fro test equipment.
The things I have been told when inquiring about test equipment with manufactures.
WHY ONE PIECE OF EQUIPMENT NEVER WORKED
"That equipment was not really designed for field use." UM READ YOUR SPEC.s
WHY ONE PIECE OF EQUIPMENT SELDOMED WORKED after years of frustration.
"It is a conflict with other software"
"You are improperly utilizing the equipment."
Finally they admitted it was a defective piece of equipment.
WHY THE READINGS WERE ALWAYS OFF.
"We really do not know we get the same results"
WHY THE READINGS WERE OFF SOMETIMES.
"yeah that happens sometimes"
And there is the megger by a major manufacture
I bought 5 of them all were inoperable within two years, major cal issues.
No major test equipment manufacture is immune from this. Except there is no lemon law fro test equipment.
Pierre C Belarge
Senior Member
- Location
- Westchester County, New York
In the last few years, I have performed many megger tests. For THHN/THWN, the readings are generally very high - 550 megs at 500v test (or "pegging" the meter). If I am in a particular buildiing, I try to pay particular attention to the readings I am getting in that buildiing. Remember too, that the somewhat lower reading (maybe 160-200 megs at 500v) could also be long runs of cable.
When I am consistantly getting readings for a particular wire type in one building and suddenly I get a much lower reading, I make a note of it, there are several reasons I am aware of that may create those "different" readings.
Lets not forget that sometimes the hardest part of meggering is opening all of the loads - I do not perform that part, usually the contractor does. You can get a low reading through loads (if not continuity) that have been missed in the prep.
I am like Marc and Brian. If I see a reading that trends lower than 20-30 megs for standard testing, I wonder why it is that low.
If I get readings as low as 20 megs and the conductor type is old, in my report, I suggest replacing the run in the near future. I may even have the contractor leave those conductors off of the breaker and investigate the reason for such low readings.
When I am consistantly getting readings for a particular wire type in one building and suddenly I get a much lower reading, I make a note of it, there are several reasons I am aware of that may create those "different" readings.
Lets not forget that sometimes the hardest part of meggering is opening all of the loads - I do not perform that part, usually the contractor does. You can get a low reading through loads (if not continuity) that have been missed in the prep.
I am like Marc and Brian. If I see a reading that trends lower than 20-30 megs for standard testing, I wonder why it is that low.
If I get readings as low as 20 megs and the conductor type is old, in my report, I suggest replacing the run in the near future. I may even have the contractor leave those conductors off of the breaker and investigate the reason for such low readings.
brian john
Senior Member
- Location
- Kilmarnock, Va
- Occupation
- Retired after 52 years in the trade.
Oh forgot to add there are standards for meggering, published by NETA and manufactures also have standards.
The Flukes are decent. Megger AEMC...All manufactures have had some issuse with some equipment I forget the current model Flukes I recent purchased.
The Flukes are decent. Megger AEMC...All manufactures have had some issuse with some equipment I forget the current model Flukes I recent purchased.
JohnJ0906
Senior Member
- Location
- Baltimore, MD
220/221 said:
http://www.professionalequipment.com/extech-autoranging-digital-megohmmeter-380260/megohmmeter/
I got this one. Seems to be a good "starter" model.
electricalperson
Senior Member
- Location
- massachusetts
get the MJ159220/221 said:
220/221 said:
I don't happen to be a lab engineer. Couldn't stand it. Can you imagine what it would be like after the 10,000th call by some moron that can't find the on-off switch nor can be bothered to read the book. All that aside, I do have a question.
Oh Wise Wire Benders, please tell us: How many of you would have a job without those clueless lab engineers?
carl
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
Pardon my ignorance but how is this sort of wire tested with a megger? Is it done inside a metal conduit, with one megger lead on the wire and one megger lead on the conduit? Or is it done with 2 wires together in conduit, with the two leads on the two separate wires? Seems like in the second situation, the 2 wires might not be in contact very much inside the conduit which would limit the usefulness of the reading. Unless I misunderstand how the megger works, which is quite possible.jameselectric said:
Cheers, Wayne
zog
Senior Member
- Location
- Charlotte, NC
acrwc10 said:
No, thats for rotating equipment, 1 Meg is horrible and unsafe for a cable. Pg 48-49 of your link has the ICEa recommended values and formulas. NETA specs requires >100M for most conductors.
zog
Senior Member
- Location
- Charlotte, NC
brian john said:
All good manufactures, but I like the new AEMC's the best, they have these new cases that are much tougher than thier old ones and have some great new features, not that there is anything wrong with those other 2 guys, I just noticed that AEMC really improved last year.
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