HVAC Bad Megger readings on Trane were due to crud

Compressor had bad Megger Supco MM500 readings until wire entrance area vacuumed and washed and dried. Katrina salt water marsh grass was still under this cover. Via the Supco MM500 original reading BAD less than 20 meg. After vacuuming 100 megs. After washing and drying 300 to 400 megs.

[@Ajax2341]

Would it be better to connect the mega ohm to the compressor terminals compressor housing?

[@marriagepartnersministry5942]

If you attach the alligator clip to an anodized, painted, or poor ground surface you are adding resistance thereby giving a false reading on the meger in the good range when actually it was not good.

[@bayouflats5054]

Thanks. I used those old hand crack types in the Navy on aircraft systems

[@brian3942]

Thanks

[@kinggenius6660]

on the supco M500 I am thinking the red wire should go to the terminal and the black wire should go to the ground? you had the red wire go to ground and the black wire go to terminal?

[@3beltwesty]

It does not matter. The PDF page 4 item 2 says use any wire one megger.
Regards orders.sidharvey.com/IMAGES/specs/m500.pdf

[@kinggenius6660]

3beltwesty thank you! I learn something everyday

[@3beltwesty]

In calibration labs and high end tests there can be a "how it is wired" issue with say a megger or voltmeter. The issue is leakage around the lead area to the tester. Thus one might have a "guard wire" ring say a 3rd wire that pulls? off the leakage so as not to bias the measurement, from 1908 . books.google.com/books?id=LQVRAAAAYAAJ&pg=PA96&lpg=PA96&dq=guard+current+voltmeter+standard+cell&source=bl&ots=pZAFREAued&sig=YNaRnyogha3SPgMf274qLu-D928&hl=en&sa=X&ved=0ahUKEwiSnYHj3M3UAhUFxoMKHfhiAtgQ6AEIRzAH#v=onepage&q=guard%20current%20voltmeter%20standard%20cell&f=false

[@kinggenius6660]

3beltwesty very interesting thank you very much

[@3beltwesty]

www.biddlemegger.com/biddle/Stitch-new.pdf
ruclips.net/video/w07kODisdRY/видео.html
Here an 8000 buck Megger has the guard lead, the blue input wire. The PDF on page 51 (50 on the image) goes into the guard terminal. Regards That PDF is loaded with stuff about testing.

[@rafaellopez5817]

Nice tool👍👍

[@ImNotStalkingYou]

I appreciate you following up the "bad" reading with a more precise measurement. The Supco meggers are awesome for quick checks or one hand free to wiggle wires but not the end all be all diagnostic tool