ECN Forum Forums Technical Discussion Electrical Theory and Applications Strange ....Please read

scenario: A barn is fed from the house main panel with 60 amp dp breaker. There is a Sub in the barn. homeowner said breaker in barn was tripping for air compressor. They had their son check it, he tried to wire the 240v motor from from one leg. After smelling horse s$#t for an hour I finally found he wired it from one leg in panel.
Compressor was wired correctly, everything ran fine, no trips. I watched it for a while on amp probe. I was there most of the day installing fans etc etc. compresor worked fine.
I get called back a week later, compressor was not running. I checked voltage at main, 240 across, 120 to ground. I checked at sub panel , I had 240 to ground and 0 on other leg, 175 across??
I pulled all breakers from panel and checked:everything was fine:120 to ground, 240 across. When I replace the breakers one by one untill i get to the lighting circuit (by the way its a new panel, new breakers)I get 175 on one leg and like 30 on the other. Everything in the barn reads like this. it is still reading normal at the main when it does this. what is going on?? I have isolated all circuits and metered them for shorts, everything looks fine.
I am going back in the morning to change out the 40 year old ceramic light fixtures.
any pointers would be greatly appreciated.
I will check in the morning before I go to the site,

Sound like an open neutral, or a neutral with a corroded connection (or loose) that would cause a large impedance.

How good of condition is the sub-panel in?

Signs of corrosion? Anti-oxidant used?

the panel is new sqareD qp line. at which connection are you talking about? I added a ground rod at barn since the span for the service to the barn was about 150' and I thought the ground size was too small and yes possibly aged since it is 30 years old and is a stranded bare #8. I am in a rural area of south TX where heat and humidity do a number on everything, even me :O)

[This message has been edited by Sean WB (edited 09-16-2002).]

2nd 66's response there......

Sean,
i think 66's had a lot of experience with an article 547, agricultural buildings, which details the finer points here.....

turns out they (UT CO)kernied the neutral at service drop for the barn.there are three weather heads on the pole and it looks like a birds nest up there.I continued to have same problems today, so I am going to run a new underground service tomorrow for the barn and discontinue the overhead service.
Its either an insulation breakdown in the ungrounded conductors feeding the barn (which are 30+ years old) or I have lost a neutral at the bird nest.
Either way, this was definitely a learning experience.
I need to invest in a megger, whats a good name to get ??

Lots of Extech meters here...
https://www.electrical-contractor.net/The_Store/EX/GRT_Main.htm

Can't go wrong with Fluke either...

which megohm meter should i look for? I just do mostly residential, light commercial . these exteck say they are for anaolog and communication cables.I would just like to get hold of a plain ol simpson crank type megger, you know? no fancy digital malarky

Woops! My bad!

Telcom indeed...

Can't seem to find any crank type for less than $600!

Buy a unit with both 1 AND .5kV ranges. It would be money well spent on a tester with built-in 600VAC measurement, to be damn sure the circuit’s deenergized before trying to meg. [Escaping smoke is expensive.]

Biddle/AVO's "A Stitch in Time" booklet is excellent reading. Also stuff at: http://www.avointl.com/us/publications/Index.asp




[This message has been edited by Bjarney (edited 09-17-2002).]

I ran a new underground service this morning to the barn. Evrything lined out great (Thank GOD)

Man , there was 50 volts to ground on the sub panel in that barn!!
I had to use a suicide receptacle from that S-P to run my hole saw for the new service, It was very nerve racking. Thanks for all the pointers, I think I like it here:O)

[This message has been edited by Sean WB (edited 09-17-2002).]