Extech megohmeter, deciphering values.

Status
Not open for further replies.

StreamlineGT

Senior Member
Long story short..

Condo has aluminum wiring, insurance company wants arc-fault breakers. Installed arc-fault breakers in the first unit, as a guinea pig. Intermittent tripping is occurring, widely scattered, but usually once a week. I purchased the Extech megger upon direction of a previous thread. I tested the three circuits in question. Everything was unplugged, and light switches turned off. All testing was done hot to neutral. No neutral to ground tests were performed at this time.

Tested a circuit that has not been a tripping problem, and it was clear.

Tested the one that trips the most, and it read 141 on the 2000Mohm scale.

Tested the one that trips once a month, and it read 885 on the 2000Mohm scale.

Tested the one that trips often, but not as often as the most, and it was clear.


This leads me to believe that the clear tripping circuit is either a connected appliance, or a straight over current. Likely not an over current, but it could be. There are no lighting loads connected to this circuit, so having switches off was of no consequence.

My question is this, what do the other values mean? Reading the instruction manual got me nowhere. I am not the sharpest tool in the shed, so if someone has some useful information it would be greatly appreciated.

Thanks,

Brendon
 
Last edited:

StreamlineGT

Senior Member
Should I follow up with neutral to ground and hot to ground tests, given the values I already obtained? I guess I just need to understand what I am looking for.
 

Cow

Senior Member
Location
Eastern Oregon
Occupation
Electrician
I always test H-N, N-G, H-G.

What voltage are you testing at? I use a Fluke 1507 and I'd probably test at 250v and 500v, just be sure nothing is plugged in, GFCI's disconnected, etc first. I like testing at 50v first to make sure the circuit is clear and I didn't miss anything.
 

Cow

Senior Member
Location
Eastern Oregon
Occupation
Electrician
That 141 does seem a little low for just a cable test(no motors, etc), but I'd test between all the other wires before coming to any conclusions.

1000v seems a little hot for residential testing too me. Maybe it's alright, seems like I've always heard the rule of thumb is to test at double the system voltage.
 

hurk27

Senior Member
First most AFCI's with GFP in them won't be tripping because of a hot to neutral resistance, the resistance your reading could most likely be a hidden load somewhere like a door bell transformer, heat tape, or something like this, the GFP in a AFCI works just like the one in a GFCI, and resistance between the hot to ground or neutral to ground will trip it, this is where you should be testing, also they don't like multi-wired circuits, so watch for these also, look for outside circuits as being the most cause of high resistance faults, many outside receptacles will get water in them, bad underground runs taped off branch circuits will show the whole branch circuit bad, make sure to isolate these when you find them connected to the inside branch circuits, look in attics for signs of varmint damage, mice love to chew on romex and are a big cause of low megger readings.

To make things easier, map out the wiring, I hope you have some kind of wire tracer equipment, a megger is only one part of a tool bag, without a good wire tracer you are totally blind as to how the circuit your testing is ran or what is on that circuit that could be causing the readings.

Look for hidden neutral taps where one circuit hot might be using a neutral from another circuit, to do this is to use an amp probe turn on one circuit at a time, put a load on that circuit, and clamp the amp probe around both the hot and neutral for that circuit, if you get a reading then you have current flowing back another path, by checking the other neutral for current you will find one that has this lost current on it, then all you have to do is find the common junction box between both circuits, also look for boot legged grounds, and or grounded neutrals, you find grounded neutral the same way above with the amp probe but you will find the lost current on a ground not the neutral. for boot legged neutrals (where someone has connected the neutral return to a EGC to get power or show a grounded circuit where there was none) this will take using an IDEAL sure test to find it at a receptacle, or looking in the light box's or junction box's for this connection.

Simple circuit trouble shooting is to do one circuit at a time, then divide and concur till you find the bad connection.

Also if you come across a circuit with no problems, then look for a motor load, vacuum cleaners, bath fans, magnetic ballast, any inductive loads can trip some AFCI's when the load is turned off, also most surge suppressor strips have a hot to ground and a neutral to ground MOV's that when they get close to the "end of life" will start having some resistance leakage, check these for leakage between the hot and ground and neutral to ground as these also can cause tripping of AFCI's and GFCI's
 
Last edited:

StreamlineGT

Senior Member
All good advice, Hurk. Thank you.

To be clear about the readings, they should show "OL" on The megger if it is a good circuit, correct? When testing from N to G, or H to G, I can use the ground bar for one lead, as long as the neutral or hot is isolated, correct? Common sense is telling me yes, just making sure.
 

hurk27

Senior Member
All good advice, Hurk. Thank you.

To be clear about the readings, they should show "OL" on The megger if it is a good circuit, correct? When testing from N to G, or H to G, I can use the ground bar for one lead, as long as the neutral or hot is isolated, correct? Common sense is telling me yes, just making sure.

You can use the ground bar, but if you do get a low reading then I would isolate even the affected circuit EGC from the ground bar, same as the neutral, the problem is finding this low reading path after isolating, which will take isolating other neutral or grounds till you find the one causing it, then looking at where the two circuits could have been crossed, but you can also do this with an amp probe as pointed out above.
 

StreamlineGT

Senior Member
All of the circuits in question have motor loads or power strips on them.

The one that trips the most has a dehumidifier, and the power strip for the entertainment system. The second is the fridge and the microwave, as well as a fan and air purifier. The last is the washing machine.

The next step is to change wiring to copper, and use a standard breaker, for the washing machine. That is easy enough to do, so I may as just do it. The others, not so easy.
 

tx2step

Senior Member
141 megohms is low?

141 megohms is low?

Are you guys saying that a reading of 141 megohms between 2 insulated #12's is too low? That sounds like a pretty good reading to me. I haven't read the old standard megger reference article in quite a while, but I thought for #12 THHN that any reading over 1 megohms was considered ok?
 

StreamlineGT

Senior Member
I certainly didn't think it was so bad, but that's why I was asking. I guess the real question is, what is the threshold of the GE AFCI? I am going back to test H-G and N-G on the questionable circuits, but I feel like I am chasing my tail. You guys are going to say that it is just a coincidence that the only breakers that trip are the only ones with motor loads, but I am going to think that that is the case. I will post more results soon.

Thanks for the replies,
Brendon
 

tx2step

Senior Member
Sorry it took awhile for me to get back - I've been working out of town. I would think that any megger reading of 100+ Megohms is a great reading. You said the wiring was aluminum -- i assume you mean even the 15A & 20A convenience circuits. If so, that old aluminum romex was notorious for developing bad splices & bad connections. Lots of places caught fire from the arching. I'll bet you have a high resistance connection or splice that is arching and the AFCI is reacting to that. If your H-G & N-G megger readings are ok, look for poor connections & splices. I've also seen aluminum romex where the individual conductors didn't like being flexed very much & caused the conductor to break inside the insulation, causing arching between the two touching ends. The old aluminum romex was bad news. Hope this helps you find the problem. Let us know what you found.
 

jeremysterling

Senior Member
Location
Austin, TX
To be clear about the readings, they should show "OL" on The megger if it is a good circuit, correct?

"OL" simply means that the value being read is outside of the range of the meter, on your Extech it means that the resistance being read is >2000Mohms. I was taught that "OL" does not mean infinity (∞), as there exists a voltage value that will breakdown most insulations (like lightning). So when recording these test results, write the value as >2000Mohms and do not write "OL" or ∞.

Also, "OL" may not indicate a "good circuit". It may indicate an open circuit.

When testing from N to G, or H to G, I can use the ground bar for one lead, as long as the neutral or hot is isolated, correct? Common sense is telling me yes, just making sure.

Yes, generally I'll attach one lead to a deadfront support or enclosure lip or ground bar when I'm meggering to ground.

tx2step is offering excellent analysis of your OP problem. It seems a high-resistance splice or fractured conductor is tripping the AFCI, not insulation breakdown. 141Mohms would seem to be an adequate resistance value for the insulation of your conductors. Good luck.
 
Last edited:
Status
Not open for further replies.
Top