I'll take a stab at this and then Mr. Woodgate can individually. Due to the time difference, he made the first response on this, and his response was precisely what mine would have been, but he got up earlier than I did. Due to the time difference again, I'm up later than he is.
First, I don't know the particulars of your standard and test method. All I was saying is that if there was corrosion, or paint, or any sort of non-conductive material in series with the ground path to be measured, a low potential could see that as an open, but a high potential would arc across it. So my point was that if you tested the bond at a high potential, you would not catch the fqct that there was some sort of "gunk" gumming up the works, to use a technical term. This is not a linear function, as per your example below. To use your numbers, the low potentials would see an open, or nearly so, but above some threshold applied potential, the bond path would suddenly drop from very high to your 0.1 Ohm target value. Ken Javor Phone: (256) 650-5261 > From: Richard Nute <ri...@ieee.org> > Reply-To: <ri...@ieee.org> > Date: Fri, 29 Mar 2013 16:43:14 -0700 > To: Ken Javor <ken.ja...@emccompliance.com>, John Woodgate > <j...@jmwa.demon.co.uk> > Cc: <EMC-PSTC@LISTSERV.IEEE.ORG> > Subject: Re: protective bonding test at 12V > > > > 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 >>> >>> > > - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <emc-p...@ieee.org> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <emcp...@radiusnorth.net> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher: <j.bac...@ieee.org> David Heald: <dhe...@gmail.com>
