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#36048 03/30/04 12:50 AM
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[There is a good spoof on Nixie displays at http://www.gpstime.com/nixie.html]

On high-impedance voltmeters and "ghost" readings, NEMA engineering bulletin 88 dated Oct 1998 suggesting a parallel resistor. The concept is valid but their recommended RESISTOR CAN BE DEADLY.
http://www.nema.org/engineering/papers/PhanBull88.html

Their Feb 2003 revision of engineering bulletin 88 quietly axed the resistor suggestion.
http://www.nema.org/docuploads/97F7D189-813B-4EFB-95A878ADAD82F954//bul88.pdf

#36049 03/30/04 04:51 AM
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Quote
Now how do we get a web page to produce the warmth that tubes gave or the special dusty smell of old tube gear or 60 cycle hum or ...
[Linked Image]

What exactly is the safety concern over a shunt resistor?

The only two things I can see are the following:

1. There's a remote possibility of the resistor shorting-out, resulting in a near zero-ohm condition between the probes. That objection could be overcome by using fused test leads, as are commonly fitted to industrial meters these days anyway.

2. There will be a much lower resistance between the leads, and thus a greater risk of shock if someone is touching one probe while the other is being connected to a hot terminal. But that's just down to proper procedure: I always make sure the second lead is safe while connecting the first anyway.

#36050 03/30/04 06:05 PM
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I have no argument with the concept of a shunt resistor, but someone at NEMA had their head up their hind end when recommending 500-1000 ohms.

From http://www.nema.org/engineering/papers/PhanBull88.html
  • NEMA recommends placing a resistor of approximately 500 to 1000 ohms across high-impedance multimeters.
OK, so 500 ohms across 120V results in a power dissipation of ~29 watts.

And, 1000 ohms across 240V results in power dissipation of ~58 watts.

In either case, it’s very likely that a ½-watt resistor from a Radio Shack blister pack will deflagrate in the process.

Increase the resistor by a factor of 100. Even 50,000 ohms across 240V is a power dissipation of ~1.2 watts, which will cook a ½-watt resistor if left connected for any significant amount of time.

This makes a mockery of kilovolt-impulse withstands in IEC-1010 overvoltage categories, for use with multimeters on all but the smallest of power circuits.




[This message has been edited by Bjarney (edited 03-30-2004).]

#36051 03/31/04 04:57 AM
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Point taken if someone tried to use a 1/2-watt resistor.

The principle is still good though, and it wouldn't be hard to construct a little feed-thru box with terminals each end to insert between meter and probe leads. Inside could be a suitable series-parallel resistor bank of wirewound resistors.

#36052 03/31/04 09:28 AM
Joined: Jan 2003
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I carry a Tripplet 310 in the plant for general troubleshooting of control circuits, after all its free since the co provides and if it falls from a crane I don't see the loss, however if I have to go to the CNCs or any high energy circuit I go get my personal Fluke 87III.

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