Megger rule of thumb

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electricguy61 said:
Can someone give me an explanation of why the wide difference in prices of the meggers listed on the website above?

I bought an Extech model 380360 a few years ago, and thought that it was fine for the 4 times a year I need a megger. Am I delusional?
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If you only use it 4 times a year, you dont need anything better, For us guys that use it every day for different types of equipment and tests, we need something that goes up to 5kV, calculates DAR and PI, stores data from hundreds of tests that we can download to or test reporting software and is durable.
 
What voltage are you testing at.

For 208 volts?

For 480 volts?


And do you use the same criteria for both voltages? I see Brian stated his 480 volt cirteria. What would it be for 208 volt system or would it be the same since the wire is rated for 600v?
 
SmithBuilt said:
What voltage are you testing at.

For 208 volts?

For 480 volts?
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I test at whatever setting is closest to twice the regular operating voltage, without regard to any higher rating the wire might have. Reason being, wire used on a 208 circuit today isn't likely to be repurposed for a 480 circuit later on. I check 208 or 240 at 500 volts, and 480 at 1000 volts.
 
SmithBuilt said:
What voltage are you testing at.

For 208 volts?

For 480 volts?


And do you use the same criteria for both voltages? I see Brian stated his 480 volt cirteria. What would it be for 208 volt system or would it be the same since the wire is rated for 600v?
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the stitch in time book has a table. i like to go by the rated voltage of wire and equipment. some people go by the voltage that the equipment will use. i double the voltage that the equipment operates at. 120 i put the megger on 250. 208volts i put the megger on 500 etc. romex is rated for 600 volts same with THHN so i put the megger on 1000volts. in new cable you should see infinity but as others stated an acceptable measurment is above 50megaohms
 
mdshunk said:
I test at whatever setting is closest to twice the regular operating voltage, without regard to any higher rating the wire might have. Reason being, wire used on a 208 circuit today isn't likely to be repurposed for a 480 circuit later on. I check 208 or 240 at 500 volts, and 480 at 1000 volts.
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you beat me!
 
electricalperson said:
in new cable you should see infinity but as others stated an acceptable measurment is above 50megaohms
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With old BX, you're darned lucky most times if you get over 50 megohms. The fabric insulated "code conductors" pulled in pipe in the 30's and 40's will almost always be 10 or less; often less than 1.
 
mdshunk said:
With old BX, you're darned lucky most times if you get over 50 megohms. The fabric insulated "code conductors" pulled in pipe in the 30's and 40's will almost always be 10 or less; often less than 1.
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i recently ripped out some old BX. the insulation was perfect and meggered infinity. it was rubber with cloth over it and it looked almost new
 
electricalperson said:
i recently ripped out some old BX. the insulation was perfect and meggered infinity. it was rubber with cloth over it and it looked almost new
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There are exceptions to every rule of thumb. ;)

I have a feeling so much of it megs bad is because it's cooked in ceiling boxes.
 
mdshunk said:
There are exceptions to every rule of thumb. ;)

I have a feeling so much of it megs bad is because it's cooked in ceiling boxes.
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im not sure how old it was but the connections in the boxes were soldered with friction tape over it. the cable looked pretty new inside and it had a tape tracer wire inside the sheath
 
zog 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.
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My mistake , I meant page 45. The formulas you are referring to are for NEW cable, I took the OP as asking about an installed cable. If you have a new cable I would expect "infinity" if you had 1megohm on a new cable I don't think I would go pulling it in.
 
electricalperson said:
the cable looked pretty new inside and it had a tape tracer wire inside the sheath
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If it had the aluminium strip, it couldn't have been that old. When'd they start doing that? I forget. 1960-something?
 
mdshunk said:
If it had the aluminium strip, it couldn't have been that old. When'd they start doing that? I forget. 1960-something?
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the reason why i replaced it was because some hack was using the sheath to carry neutral current and all the other wiring was messed up:grin:
 
electricalperson said:
the reason why i replaced it was because some hack was using the sheath to carry neutral current and all the other wiring was messed up:grin:
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You can't fix stupid, but you can sure get paid to follow it around and repair its wiring.
 
acrwc10 said:
My mistake , I meant page 45. The formulas you are referring to are for NEW cable, I took the OP as asking about an installed cable. If you have a new cable I would expect "infinity" if you had 1megohm on a new cable I don't think I would go pulling it in.
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I was asking about installed cable. there was a reading where we went from each phase to ground (on the load side) to see what we would get (in the disco box). It was jut under 1 megohm. It was 480v powered equipment. the megger we used was an old style that was a hand crank, I guess im spoiled im use to the new ones where you can select the voltage on them. On this old one you just crank it and no selection of voltage.
 
brian john said:
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.
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That should be in the customers: "Best replies, don't know sh*t chapter":wink:
 
jameselectric said:
I was asking about installed cable. there was a reading where we went from each phase to ground (on the load side) to see what we would get (in the disco box). It was jut under 1 megohm. It was 480v powered equipment. the megger we used was an old style that was a hand crank, I guess im spoiled im use to the new ones where you can select the voltage on them. On this old one you just crank it and no selection of voltage.
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if your meggering installed wiring and its under 1 megaohm check for connected loads. if there is no loads connected you have a problem and dont turn it on
 
electricalperson said:
if your meggering installed wiring and its under 1 megaohm check for connected loads. if there is no loads connected you have a problem and dont turn it on
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When I come across this situation, I do not reconnect that circuit to the circuit breaker.
I make a note of it and in the report, I tell the customer (for me the customer is almost always the EC) to trouble shoot the circuit. There are many times that they will call me back and say they found a lamp in the far side of the attic or some closet that they missed during the prep, sometimes it is a a bell transformer - the door bell stops working after my testing ;)
Other times the circuit is abandoned and they install new circuitry.
 
Pierre C Belarge said:
When I come across this situation, I do not reconnect that circuit to the circuit breaker.
I make a note of it and in the report, I tell the customer (for me the customer is almost always the EC) to trouble shoot the circuit.
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Why are you doing those tests?

Does NY require it?

Never seen a EI with any type of meter .... ever.
 
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