Hi Ken and John:
Let me see if I understand this correctly.
The test goes like this. We're testing a bonding conductor, so I
assume that, at least, the resistance at low current (with an
ohmmeter) is near zero.
The objective is to determine that the bonding conductor can
carry sufficent fault current such that the overcurrent
device can open the circuit. If the conductors are too small,
the resistance may be too high, and the fault current too low,
such that the overcurrent device will not operate. Therefore,
the resistance of bonding conductor must be determined at the
worst-case overcurrent device operating current (taken as 200%)
and operating time (taken as 2 minutes).
So, I hook up the test equipment to both ends of the conductor.
Then, I gradually turn up the test current (and voltage) towards
30 amps. Assume the resistance is 0.1 ohm. As I increase the
current to 1 amp, I get 0.1 volt. Then, 2 amps and 0.2 volts.
And so forth until I get 30 amps and 3 volts. I never get to
12 volts. The EUT passes.
Now, let's assume a connection is contaminated and the
resistance is 1 ohm. I gradually turn up the test current (and
voltage). At 1 amp, I get 1 volt. At 2 amps, I get 2 volts.
If the test equipment is limited to 12 volts per the standard,
then I get 12 amps and 12 volts. I can't get to 30 amps, and
the EUT fails the test. Even if I could get to 30 amps and 30
volts, the EUT fails the test.
If the circuit is open, then I can't get any current at any
voltage. The EUT fails the test.
If the circuit is open, then, I suppose, I could apply a high
enough voltage to cause a breakdown at the point where the
circuit is open. The voltage would need to be more than 323
volts as air does not break down below that voltage (Paschen's
Law).
Low-voltage arcing is due to physical opening of a contact,
where the contact area reduces as the contact approaches the
break of connection. When the contact area reduces, the
current density increases to the point where the metal melts
and then sprays away due to the EMF in the molten conductor.
Then, the combination of the molten metal and the current
density superheats the surrounding air to the point where the
air breaks down due to the heat and a visible arc (in air) occurs.
This is not a function of the voltage across the contact, but
the current through the contact.
Of course, if the physical motion re-makes the contact before
the molten metal has solidified, then the contact may very
well be micro-welded. In which case, the contact may very
well pass the 30-amp test, depending on the heat-sinking of
the assembly.
I've tested loose connections at 25 amps. I moved the loose
connection for both break and make. It the parts which are
loose are relatively massive, they provide a heat-sink at
the point of connection, and the loose connection will
easily pass the test.
This is my understanding of the physical phenomenon involved
in the test. I still don't see the effect of voltage on
the test. HELP!
Best regards,
Rich
On 3/29/2013 1:55 PM, Ken Javor wrote:
My ancestors informed me similarly. Corrosion, grease, any thin
non-conductive layer between the part to be grounded and ground is invisible
to a high potential, but detectable by a lower potential that cannot "punch
through it."
Ken Javor
Phone: (256) 650-5261
From: John Woodgate <j...@jmwa.demon.co.uk>
Date: Fri, 29 Mar 2013 20:20:30 +0000
To: <EMC-PSTC@LISTSERV.IEEE.ORG>
Subject: Re: protective bonding test at 12V
In message <5155f037.6080...@ieee.org>, dated Fri, 29 Mar 2013, Richard
Nute <ri...@ieee.org> writes:
The voltage of the test generator is of no consequence.
Well, that isn't what my ancestors told me, which is what I recited.
--
OOO - Own Opinions Only. See www.jmwa.demon.co.uk
Which Thunderbird will David Miliband pilot? Or will he drive Lady Penelope?
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
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