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Trust your instruments? #33998 01/31/04 12:24 AM
Joined: Jan 2004
Posts: 2
Airtime Offline OP
Junior Member
I'm an electrical contractor in Central California and I'd like to relay a strange situation I came across on a trouble call.
A large Hi Tech company called me in to one of their data centers to check out the reason for burning a 120-volt 2KVA UPS system after it was plugged into some recently installed receptacles. After verifying that all the effected loads were disconnected, I energized the receptacles on the new circuits, there were only (5) 120v branch circuits on a 225 amp, 3-phase 208/120Y panel, but several 208v single-phase loads. None of the circuits were multi-wire branch type.
I read the Line to Neutral voltage and it was 115.6, 117.8 and 117.6 on A, B and C phases respectively. Line to Ground was 118.1 on all phases +/- 0.1, and 207 volts between all phases. I suspected a Neutral problem but had no idea what I was about to find. I checked the voltage at the panel mains and found the exact same readings. All the neutrals were tight, including the main. So what happened? I traced the main neutral back to source, a PDU in the room fed by an 80KVA UPS. Within the PDU I found a 100-amp feed to the panel in question, a good ground but the neutral was never hooked up. It was safed off during install, tucked away and forgotten. Without the neutral in the panel, 208v was able to find it's way through the loads when they were plugged in and burn power supplies.
The real question is, where did my Fluke 87 get it's zero reference when I was checking the line to neutral voltage. I might mention that I megged the open neutral and it was >2,000 megohm at 250 volts, no grounds. To prove it I stuck a good old wiggy on each phase to neutral and it showed zero.
I got together with the on-site electrician and we tested several digital meters on the circuit and found each one had similar readings. I connected the main neutral in the PDU and all readings returned to normal and loads were connected successfully.
Is the moral of the story not to trust your Digital Meter? Where do you think it got it's zero reference? I don't have the answers and neither did an electrical engineer I spoke with. Any high impedance digital meter we put on the circuit showed voltages from Line to Neutral within spec, but low impedance equipment showed low or no voltage, phase to neutral.
This is a new one for me.


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Re: Trust your instruments? #33999 01/31/04 03:11 AM
Joined: Nov 2002
Posts: 790
wa2ise Offline
There was probably enough stray capacitence from the unconnected neutral to ground and the
three phases. The three phases would cancel out to near zero volts. If the impedance of the DVMs is 10 megohms, and the stray capacitence is around 0.002 microfarads the DVM would measure about 99% of the true voltage. The voltage across a cap in series with a resistor (the DVM looks like a high value resistance) would be 90 degrees phase shifted compared with the voltage across the resistor. The voltage across the capacitence is around 15 volts. The 90 degree phase shift is why 15v + 117v not= 118v.

High impedance digital meters are great for electronics work but for what you're doing you probably want lower impedance. You could parallel a 100 kilohm 5 watt resistor with the DVM to get the impedance down. Oh, you'll still see some voltage, but the numbers will be so way off (like 50V) that you'll know something is broken.

Re: Trust your instruments? #34000 01/31/04 03:49 AM
Joined: Jul 2002
Posts: 8,337
Trumpy Offline
Welcome to ECN mate!. [Linked Image]
The answer to your question lies in the actual impedance of the meter that you are using.
Now, it's a situation of horses for courses.
Using a high Z (impedance) meter is one thing, using a meter that will draw enough current to give a decent reading is another.
I've been called out to I don't know how many faults caused by Electricians that don't how to use the right type of Test Gear.
The problem is Ghost Voltages caused by the overall Reactance and Inductance of the circuit will upset a high Z meter where as a low impedance one will discount Ghost Voltages.

Re: Trust your instruments? #34001 01/31/04 03:52 AM
Joined: Jul 2002
Posts: 8,337
Trumpy Offline
Also Airtime,
Never trust your test gear until you have verified it against a known source of supply, both before and after you have used it!. [Linked Image]

Re: Trust your instruments? #34002 01/31/04 09:46 AM
Joined: Oct 2000
Posts: 5,389
sparky Offline
i like this part;
a good old wiggy
couldn't agree more Airtime.

having had my share of digital 'ghost voltages' as well i suppose asking here for a brief synopsis of digital/analog differences might help



Re: Trust your instruments? #34003 01/31/04 10:18 AM
Joined: Jan 2004
Posts: 2
Airtime Offline OP
Junior Member
Thanks for all your input. I had a feeling capacitance in the wire may have been the culprit but I couldn't prove it. There's only about a 40' run of #2 between panels, needless to say, I was scratching my head trying to figure out how I could develope enough capacitance in that short of run to effect my readings.
I suspected the neutral as soon as my L-N voltage was lower than my L-Gnd readings by a volt or so. I just got a kick out of having all this expensive, calibrated instrumentation in my rig and to prove my point I just needed to pull out the ol wiggy.
This is the first time I've posted anything on this site, great response time and good info, thanks.

Re: Trust your instruments? #34004 01/31/04 11:13 AM
Joined: May 2001
Posts: 376
frank Offline
Ya I remember being told long ago about digital/analog differences and that analog meters had that advantage but I thought I was imagining it because no one had actually said it yet.......Waiting

[This message has been edited by frank (edited 01-31-2004).]

Re: Trust your instruments? #34005 01/31/04 06:01 PM
Joined: Nov 2003
Posts: 269
electricman2 Offline
I have got a Simpson 260 analog VOM that I use anytime digital readings are suspect. The high input impedance of digital instruments while it is more accurate makes for some strange readings sometimes.

Re: Trust your instruments? #34006 02/01/04 12:16 AM
Joined: Apr 2002
Posts: 2,527
Bjarney Offline
Part of trusting your instruments is knowing their limitations. Sometimes 10-megohm meters need to be properly accessorized.

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