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Amp meter on 2300 volts?

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gene6029

Member
Location
Wilson NC
Recently the company I work for had an outside contractor install CTs on our electrical equipment. Today I was told one of the motors legs was 20 amps lower than the other 2 legs. One leg was roughly 20 amps the other 2 around 30 amps each.( this may be a faulty CT, because the motor runs fine) My question is, can I use a hand held volt meter rated for 600volts and snap the amp clamp portion on the meter around a leg at a time safely? The legs are in an over head raceway in open air. The meter is a DMM. I would not be exposed to any open cabinets to come in contact with the 2300. Thanks....Gene
 
Besoeker

Besoeker

Senior Member
Location
UK
gene6029 said:
Recently the company I work for had an outside contractor install CTs on our electrical equipment. Today I was told one of the motors legs was 20 amps lower than the other 2 legs. One leg was roughly 20 amps the other 2 around 30 amps each.( this may be a faulty CT, because the motor runs fine) My question is, can I use a hand held volt meter rated for 600volts and snap the amp clamp portion on the meter around a leg at a time safely? The legs are in an over head raceway in open air. The meter is a DMM. I would not be exposed to any open cabinets to come in contact with the 2300. Thanks....Gene
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I wouldn't.
 
Besoeker

Besoeker

Senior Member
Location
UK
zbang said:
If you don't trust the instrumentation, schedule a shutdown and have that tested/calibrated.

(And if the MV conductors are shielded, you won't get a good reading anyway.)
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What sort of shielding would there be?
 
Jraef

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
I'm glad you heeded the advice. But just for your enlightenment:
gene6029 said:
Recently the company I work for had an outside contractor install CTs on our electrical equipment. Today I was told one of the motors legs was 20 amps lower than the other 2 legs. One leg was roughly 20 amps the other 2 around 30 amps each.
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That's not 20A LOWER, that's 10A lower. But 10A out of 30 is a VERY SIGNIFICANT imbalance and should absolutely be investigated, so you were right to question it.

( this may be a faulty CT, because the motor runs fine)
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NEVER assume that meters are wrong... "Runs fine" is highly subjective. If you really have that much of an imbalance, the only thing you might notice is extreme heat showing up on an IR scanner, or total meltdown after being left running like that for too long if you didn't look for that. Side note though, one of the CTs may have the wrong ratio and the installer failed to notice. I've also seen it PRINTED WRONG on the CTs! You would think that can't happen, but I've been in a CT manufacturing facility... believe me, it's not uncommon for it to happen, but most of them get caught in the final QC (workers in most CT factories are typically illiterate 3rd world manual labor people).

My question is, can I use a hand held volt meter rated for 600volts and snap the amp clamp portion on the meter around a leg at a time safely? The legs are in an over head raceway in open air. The meter is a DMM. I would not be exposed to any open cabinets to come in contact with the 2300. Thanks....Gene
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Those two statements are actually contradictory. Those conductors, despite being in open raceway, are STILL potentially lethal if there is any insulation leakage, it's very different from LV conductors. I've seen 2300V conductors in a dark room look like they are covered in living blue spider webs simply because (as it turned out) someone had strapped down clamps on the conductors too hard and compressed the foam core, decreasing the dielectric properties and causing a "partial discharge". Had anyone touched those conductors, they could have been killed. We couldn't see it in the light, but we could hear a "ticking" sound that turned out to be that stray voltage going jumping a 1" gap to get to ground. The sound was shrugged off by the plant manager as "normal stuff", but I knew better and had them turn off the lights. The plant manager was embarrassed to say the least. So the best practice when having to live / work around MV conductors is to ALWAYS assume they are potentially lethal.

To educate yourself more on this, watch this video. The "spider web" situation I described shows at 1:09.
https://www.youtube.com/watch?v=GRmOyUGAODA
 
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Besoeker

Besoeker

Senior Member
Location
UK
GoldDigger said:
To affect the current measured by either a clamp-on ammeter or a current transformer, the "shielding" would have to actually be a current carrying concentric neutral.
Click to expand...

Yes, that was the basis of my question. I worked a bit with MV up to 11kV but I have never seen that sort of shielding. Mostly, we use multi-core SWA but the point of measurement would be on individual conductors which are not usually shielded.
 
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