Building a Kelvin Bridge for Breaker/ATS Contact Measurements

[akimbojimbo]

Hello all,

I'm wondering if someone could provide insight or experience on constructing and using a Kelvin Bridge? I have the need to measure the contact resistance of many large low voltage breakers (100-3000A, 120-480v) and am a bit put off by the cost of commerical milliohmeters. As such, I want to build myself a device for 4 point measurement of resistance, which I understand to be a Kelvin Bridge. I get the basic layout and concept of it, however all the pics I've seen show it being used on a 4 wire resistor. I can't wrap my head around how I can apply that to the two terminals of the single pole of a breaker?

I'm familiar with doing a caveman style 4 point test: supply a known/controllable current to the item under test, then measure voltage drop across the test item while it is energized. I'm hoping there's something that I can make a bit more unitized to keep with me in the field.

Any suggestions? Thanks!

[Kleinstein]

The main point is using 4 contacts / wires. With the breaker and other normally 2 wire parts this means ataching 2 wires to each side. With could be with something like clamps or just holding some of the tips.  With 4 wires it is no longer impartant that the individual contracs are very stable. With a DC measurement for the low restance range one would however have to keep an eye on thermal EMF and holding the tips by hand is thus not such a good idea.

The simple way is to get a resonable stable constant current source (e.g. lab supply for 1 A - though some of the low cost ones are not very stable) and than use a DMM to measure the current and votlage. One would still prefer a DMM with reasonable good resolution (e.g. 0.01 mV or ideally better).
Instead of a specialized miliohm meter a reasonable good DMM with 4 wire ohms (usually a bench meter) may be sufficient.