Megohmmeter Voltage

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xformer

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Master Electrician
Hello,
When Testing 600V conductors with a megohmmeter, is it permissible to use the 1000V setting on the meter?
 
Carultch said:
Perhaps it would help the OP, if you elaborate on why this is an acceptable practice.
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Because, ITYS.

Not the right answer, but that is a a typical test for motors. Use twice the test voltage as the motor rating.

Peak voltage on a 600 volt system would be about 846. Hopefully the insulation does not break down at 847.
 
Carultch said:
Perhaps it would help the OP, if you elaborate on why this is an acceptable practice.
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Doubt very much if you applied 1.000 volts DC from a Megger that produces very little current that you would do any harm to at least the most commonly used wires: THWN, THHN, & XHHW. Just look at rating for flimsy 16 guage control wires. They are rated for either 600 volts ( general use ) and 1,000 volts for luminares ( example = Some high output 8'long fluorescent luminare ballast produce over 600 volts and operate in cold, hot &/ or damp locations for years with no problems ). We always tested very thin varnished 480 volt motor windings with 1,000 volts for up to 10 minutes.
 
Carultch said:
Perhaps it would help the OP, if you elaborate on why this is an acceptable practice.
Click to expand...
I have used 1,000 volts to test both conductors & 480 volt motors. On old wire would apply voltage for up to 10 minutes & record first reading, 5 & 10 minute mark. If you look at very thin insulation on #16 guage TFFN ( hope that is the correct type ) ,wire it states that in signs good for 1,00 volts. Just think when you perform the first Megger reading on a possible grounded motor you test from load side of motor starter or drive. If a ground is found them you remove wires at motor pecker head and Megger motor then wires. Old rule that I was taught back in the 1960's on meggering still holds up. To figure out a Megger voltage you must double the test voltage and add a hundred volts. Every 480 volt motor should be tested using 1,000 volt DC by a megger.
 
UL 83, the product standard for thermoplastic conductors, requires that the conductor pass a 2kV AC dielectric test while having been in water for at least one hour before the test. The conductor remains in the water for the test. If you do the test using DC, the voltage to be used is 6kV.

Based on that, there should be no issue testing at 1kV using a megger.
 
don_resqcapt19 said:
UL 83, the product standard for thermoplastic conductors, requires that the conductor pass a 2kV AC dielectric test while having been in water for at least one hour before the test. The conductor remains in the water for the test. If you do the test using DC, the voltage to be used is 6kV.

Based on that, there should be no issue testing at 1kV using a megger.
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Thanks. Life long sparky but never heard of this product standard. The only small issues that I came across in my 50 years was the nylon jacket on #2 THHN/THWN was peeling off on a 2,500 spool but was okay to use. While pulling 20 to 40 #14 &16 gauge control wires a few times found insulation appeared to be stretched so cut that small maybe 4 to 6" piece out.
 
garbo said:
Thanks. Life long sparky but never heard of this product standard. The only small issues that I came across in my 50 years was the nylon jacket on #2 THHN/THWN was peeling off on a 2,500 spool but was okay to use. While pulling 20 to 40 #14 &16 gauge control wires a few times found insulation appeared to be stretched so cut that small maybe 4 to 6" piece out.
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It is my understanding that the nylon jacket only provides the gas and oil resistance, and reduces the pulling friction, but is not really part of the electrical insulation for the conductor.
 
Ahh...Insulation Resistance (IR) testing! I love this topic, particularly when it comes to wires only. I understand that it's performed all the time to ensure the cables and wires are good after install, but does it really tell you anything? That's the real question. If a wire (not a cable) has some type of shielding on it, an IR test will give you a definitive result. The reason is you are applying voltage to a conductive material (like the metal conductor) and measuring the voltage on another conductive material (like the shielding) through the insulation. What if the wire has a jacket over the insulation? Well the shielding MUST be beneath the jacket so you are ONLY measuring the insulation and not the jacket as well. But what if it's a typical building wire like THWN or XHHW? Well there is no shielding on those typically thus you don't have that second conductive plane to measure against. So people often measure to a conduit or tray or other wires. Well to measure properly you must have the second plane in constant contact with the insulation for the entire length. I'm willing to bet that wires run in conduit don't always lay perfectly flat and will cross over one another inside that conduit......which makes the IR test useless. Or if you measure between 2 wires then you are actually measuring more than 1 insulation and again they probably aren't touching each other through the circuit either. The NETA standard even tells us this and is the gold standard for insulation resistance testing. It will refer you to table 100.1 for values of passing results of certain voltage ratings. Look carefully at the notes in that table, as it will tell you that unshielded cables will not give pass or fail results and a result should be used as a data-point for a trending analysis. Which essentially means the test won't give a good or bad result and you would need to test several times (with the EXACT same test set up, humidity and temperature). Theoretically possible to do that, but not realistic at all. So shielded cables CAN be tested accurately, but unshielded cables cannot and you are simply wasting time, spending money on a result that means nothing. You will get better information simply checking for shorts with an Ohmmeter.
 
Boingo1972 said:
You will get better information simply checking for shorts with an Ohmmeter.
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Interesting but I disagree, recently there was a gas pump at a gas station that was tripping a breaker occasionally, first time one of the guys went out all the wires ohmed out just fine, he replaced the breaker checked connections, everything worked.
Breaker randomly tripped again, I suggested he bring a megger, one of the wires tested wayyy different than the others so he broke the seals and pulled out the wire, you could see a funny burn spot thru the black wire.
similar situation again about a month later with a sign circuit at a shopping center.
So in both cases the megger helped where a regular fluke meter showed everythings fine.
I have also used a megger to find and solve countless AFCI problems in NM cable where other contractors threw up their hands.
 
tortuga said:
Interesting but I disagree, recently there was a gas pump at a gas station that was tripping a breaker occasionally, first time one of the guys went out all the wires ohmed out just fine, he replaced the breaker checked connections, everything worked.
Breaker randomly tripped again, I suggested he bring a megger, one of the wires tested wayyy different than the others so he broke the seals and pulled out the wire, you could see a funny burn spot thru the black wire.
similar situation again about a month later with a sign circuit at a shopping center.
So in both cases the megger helped where a regular fluke meter showed everythings fine.
I have also used a megger to find and solve countless AFCI problems in NM cable where other contractors threw up their hands.
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I've run into this before myself with my guys, I think part of the problem is many of them carry a meter for daily use that has too low of an ohms scale to read a really high resistance short or ground
 
Ken_S said:
I've run into this before myself with my guys, I think part of the problem is many of them carry a meter for daily use that has too low of an ohms scale to read a really high resistance short or ground
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Hi Ken
It's not so much the ohms scale as it is the test voltage applied to test the integrity of the wire or cable insulation. A megger will apply anywhere from 250v to 1000V to the conductor, to stress the insulation properties of the wiring. A Fluke DMM will only put out the battery voltage of the battery, usually 9VDC which will not identify a bad insulation problem. I.E the proper meter for the proper job.
 
don_resqcapt19 said:
It is my understanding that the nylon jacket only provides the gas and oil resistance, and reduces the pulling friction, but is not really part of the electrical insulation for the conductor.
Click to expand...
This was brand new wire so I asked my inspector what to do ( replace ? ) . He said that it was okay to use. Have seen the nylon jacket peeling off on services that have been out in the weather several years. I started out before this type of insulation was around. Was not fun pulling large THW thru hundreds of feet of conduit by hand. My chief electrician dad was the first person to order THWN wire at one of our supply houses back in the 1960's. A wire salesman dropped off some samples and he ordered several thousand feet but the supply house said there was no such wire. Finally they called back and said they can get some and they should have been notified before the people using it. Took a few samples to my VO Tech shop teacher and he did not like it at first due to insulation being so much thinner then TW or THW . Changed his mind in time.
 
garbo said:
This was brand new wire so I asked my inspector what to do ( replace ? ) . He said that it was okay to use. Have seen the nylon jacket peeling off on services that have been out in the weather several years. I started out before this type of insulation was around. Was not fun pulling large THW thru hundreds of feet of conduit by hand. My chief electrician dad was the first person to order THWN wire at one of our supply houses back in the 1960's. A wire salesman dropped off some samples and he ordered several thousand feet but the supply house said there was no such wire. Finally they called back and said they can get some and they should have been notified before the people using it. Took a few samples to my VO Tech shop teacher and he did not like it at first due to insulation being so much thinner then TW or THW . Changed his mind in time.
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I remember trying to get thhn/thwn back when. Special order.
Green? "WTF for? You nuts? No one uses that!"
 
ATSman said:
Hi Ken
It's not so much the ohms scale as it is the test voltage applied to test the integrity of the wire or cable insulation. A megger will apply anywhere from 250v to 1000V to the conductor, to stress the insulation properties of the wiring. A Fluke DMM will only put out the battery voltage of the battery, usually 9VDC which will not identify a bad insulation problem. I.E the proper meter for the proper job.
Click to expand...
I disagree, I've found many issues with just a DMM that another meter showed as open line because of the resistance scale of that meter.
 
It all depends on the type (degree of resistance) you have. Sure, an ohmmeter is fine for dead shorts. A megger not required. But using a ohmmeter on a high resistance short is useless. That's why a megger was invented. I suggest you google "A stitch in time" by megger, you may learn something.
 
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