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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => AC Power and Grounding => Topic started by: Tom Bourke on October 15, 2014, 02:08:30 am

Title: Megohmmeter
Post by: Tom Bourke on October 15, 2014, 02:08:30 am
So what would be an acceptable low cost Meg-ohmmeter?  At this point it is more of a curiosity for me.  I have a nice Fluke clamp on for the stuff that counts.  I have never used a megger and would like to learn.
Title: Re: Megohmmeter
Post by: Scott Holtzman on October 15, 2014, 03:47:10 am
So what would be an acceptable low cost Meg-ohmmeter?  At this point it is more of a curiosity for me.  I have a nice Fluke clamp on for the stuff that counts.  I have never used a megger and would like to learn.

Out of curiosity what is "stuff that doesn't count"?

Anyway, a nice crank amprobe meger on eBay can be had for $100 or so.  The generate a nice bit of voltage with the crank gen and I like the 0 latency of the analog meter.

Without spending $1000 your are not going to get more accuracy.

These are designed to test insulation breakdown.

I may have been confusing the issue in several posts.  When I say "Megger" I am not speaking of a megohmeter but a Megger Instruments ground tester.  These are used to test the quality of ground fields and read accurately below 1 Ohm




Title: Re: Megohmmeter
Post by: Mike Sokol on October 15, 2014, 09:45:01 am
Anyway, a nice crank amprobe meger on eBay can be had for $100 or so.  The generate a nice bit of voltage with the crank gen and I like the 0 latency of the analog meter.

Without spending $1000 your are not going to get more accuracy.

I've only used the crank ones a long time ago. But I see that Amprobe make a battery operated digital one you can buy for $250 on Amazon. http://www.amazon.com/Amprobe-AMB-25-Digital-Insulation-Resistance/dp/B0046HY8NS (http://www.amazon.com/Amprobe-AMB-25-Digital-Insulation-Resistance/dp/B0046HY8NS)

I'll ask my RVIA guys what they use for testing new Recreational Vehicles since that's part of their build spec.
Title: Re: Megohmmeter
Post by: Steve M Smith on October 15, 2014, 10:01:17 am
I used to use a hand cranked one in the late 1980s for testing the insulation between power transistors and heatsinks on high power switch mode power supplies.


Steve.
Title: Re: Megohmmeter
Post by: Stephen Swaffer on October 15, 2014, 10:13:23 am
I use a Fluke battery operated-iirc about $350 and it has served me well-motor tests and such. Mine will test at both 500 VDC and 1000 VDC.

Keep in mind you do not want to "meg" solid state loads-inverters, etc-make sure they are disconnected before you test ( for that matter an inverter genny is not a "load", but you don't want that connected either.)

IMO, for most sound/lighting setups a simple ohm meter would be a sufficient test to make sure there are no dead shorts to ground.  I usually only pull mine out for motor tests or if I am dealing with a "high energy"circuit-not a defined term but just where I am comfortable.  A short on a 20 amp 120 V circuit usually doesn't create too much excitement-a 400 amp 480 volt 3 phase in another story-and those will get megged every time!   But it never hurts to be on the safe side.
Title: Re: Megohmmeter
Post by: Tom Bourke on October 15, 2014, 10:40:43 am
Out of curiosity what is "stuff that doesn't count"?
Poor choice of words, sorry I was tired.  I grab my $250 Fluke when doing work with show power and high current/voltage work or when it is handy.  My second meter that goes with me on all jobs and for more general electronics work is my $40 Tenma.  It has a lower cat rating and no clamp but has features handy for low voltage electronics work.  When the Fluke is not handy I will use it on the test points of a distro or trouble shooting stuff down stream of the distro.  All my other meters have been relegated to 12V and under or the garbage.

I have no use for a ground rod tester however an insulation tester sounds very interesting for some of what I do.  However I don't want to spend hundreds on one till I know for sure it will be used a lot.  A lower cost "trainer" that can later be relegated to the gig bag would be cool.
Title: Re: Megohmmeter
Post by: Mike Sokol on October 15, 2014, 11:21:40 am
A lower cost "trainer" that can later be relegated to the gig bag would be cool.

Supro makes one for $89 but I don't know if it's junk or not... http://www.zoro.com/i/G0712695/?utm_source=google_shopping&utm_medium=cpc&utm_campaign=Google_Shopping_Feed&gclid=CNqI59r6rsECFW4F7AodWm4ApQ (http://www.zoro.com/i/G0712695/?utm_source=google_shopping&utm_medium=cpc&utm_campaign=Google_Shopping_Feed&gclid=CNqI59r6rsECFW4F7AodWm4ApQ)
Title: Re: Megohmmeter
Post by: Lyle Williams on October 15, 2014, 04:47:38 pm
The Victor VC-60B+ found commonly on ebay works fine.
Title: Re: Megohmmeter
Post by: Tom Bourke on October 16, 2014, 03:57:26 am
The Victor VC-60B+ found commonly on ebay works fine.
I found the Victor and also found a Vichy with the same model number but has a couple of extra features.  Most notably normal resistance in addition to the megger function and VAC measurement.  http://www.amazon.com/NEEWER%C2%AE-Digital-Insulation-Resistance-MegOhmmeter/dp/B00DVFNBPC/ref=pd_sim_sbs_hi_1?ie=UTF8&refRID=02X6J1B6R7RRGND0H3G7#product-description-iframe  Tempting.
Title: Re: Megohmmeter
Post by: Tom Bourke on November 05, 2014, 01:12:14 pm
I bought the Vici VC60B+ I linked to above.  The meg-ohm settings seam ok for accuracy.  I had a pile of 1 to 10 meg resisters here and the meter matches the markings in the 250 and 500V range.  1000V rage was a little higher but that could because I was at the extreme low end of the range for that setting.  The AC voltage readings are high (130/266V when the Fluke says 127/255V) on my house voltage.

Over all I am happy.  It is good enough for my hobby stuff. At a 10th the cost I can live with lower accuracy.
Title: Re: Megohmmeter
Post by: Stephen Swaffer on November 05, 2014, 01:45:15 pm
99% of the time I use a megger for "real" work I really don't care about accuracy that much.  If it is a new wire run I want an open reading.  Motors are different-they can show some conductance, but I really am just looking for a ballpark idea-same thing with an underground feeder.  IMO accuracy would really only be a concern for production testing when you need to meet a spec.
Title: Re: Megohmmeter
Post by: Mac Kerr on November 05, 2014, 02:19:33 pm
The AC voltage readings are high (130/266V when the Fluke says 127/255V) on my house voltage.

A man with one meter is always sure, a man with two meters is never sure.

Mac
Title: Re: Megohmmeter
Post by: Jay Barracato on November 05, 2014, 02:48:53 pm
A man with one meter is always sure, a man with two meters is never sure.

Mac

Depends on his calibration protocols.

Sent from my DROID RAZR HD using Tapatalk

Title: Re: Megohmmeter
Post by: Tom Bourke on November 05, 2014, 03:07:55 pm
A man with one meter is always sure, a man with two meters is never sure.

Mac
220 221, what ever it takes.
Title: Re: Megohmmeter
Post by: Lyle Williams on November 06, 2014, 03:10:55 am
While in theory for some tests 1.001Mohm might be a pass and .999Mohm might be a fail, the reality is you are generally looking for hundreds of Mohms and anything vaguely close to  the expected threshold needs investigation.

Insulation also varies significantly with temperature and moisture/humidity.  If you aren't adjusting readings for temperature and humidity, there is little point caring about calibration certificates etc.  It is worth having some high value resistors in the bag with the megger so you can verify the device is operating correctly.