brian john

We are often called by electrical contractors to investigate why fuses are blowing or CBs are tripping. Many times the cause is a high resistance connections resulting in sufficient heat to effect the thermal element in the fuse or CB.

A simple method to isolate the high resistance connection and thus the source of the heat is the Fall of Potential Test Method, commonly referenced to as the FOP test. To perform this test, one simply needs a multimeter with a millivolt scale, and an amp clamp.

There needs to be a load on the device to be tested, preferably a balanced load or close to balanced load. In the case of a fused safety switch (FSS). One would measure current across all three phases, then measure from line to load of one pole/phase of the conductor strands (if exposed) for each pole of the FSS. If one phase has a higher that average millivolt measurement (actually the voltage drop across the device under test). Your next measurement would be from line conductor to line of the fuse, if all readings are close to equal move to the next components of the FSS, in this manner you an isolate the high resistance connection.

With an arranged outage repairs can be implemented and a repair FOP measurement taken to verify repairs.

Our thermographers perform this test as part of their IR Scan to isolate to high resistance issue. As sometimes it is not possible to determine from a picture if the issue is a CB connection to the bus or the CB. Additionally it is not feasible to use a DLRO (Digital Low Resistance Ohm Meter)/ Micro ohm-meter to take measurements on small CBs and FSS due to contact point spacing of the test instruments, so our technicians take pre-repair and post-repair measurement s to verify repairs.


An example we IR’d a 200 amp CB this weekend with 155 amps per phase (average), millivolt readings were 38mv, 91mv and 42 mv. The readings were taken from the bus stabs of the CB, negating any possible issue with the CB to bus connection or conductor termination connector to CB connection. B phase had an issue, when we replace the CB we will do further testing and open the CB to see if visual thermal damage has started.

This test can be performed on single pole CB, or any 3-pole devices, we have used this on 4000 amp bolted pressure switches.

As with any testing of exposed energized parts, all safety cautions must be observed, wearing of PPE, isolating the area to be worked in. One issue we have had over the years is customers taking FLASH photography as we are taking measurements. We no longer permit customers to take photos, without prior notice. This minimizes heart attacks.

MDShunk

Thanks a bunch for that narrative, Brian. That will be very helpful to me.

A couple questions:

1. Is the preliminary amp check of each phase to verify a balanced load, or is it to possibly ferret out the problem leg without having to do the millivolt test?
2. Is there any reason to do the millivolt check on a regular volts scale first, in the event that one fuse might be blown (in the case of fuses)?
3. Are the little knobs that companies like Ideal and Bussman make that clamp fuse holder sockets tighter onto fuse blades and ferrules to be considered a permanent repair, ever?

brian john

A couple questions:

1. Is the preliminary amp check of each phase to verify a balanced load, or is it to possibly ferret out the problem leg without having to do the millivolt test?
The amp check is to verify load, even without a balanced load one can do the millivolt drop check, but you need a load.

2.Is there any reason to do the millivolt check on a regular volts scale first, in the event that one fuse might be blown (in the case of fuses)?

With a high impedance meter this should not be an issue the meter should read OL or some variance of that.

3. Are the little knobs that companies like Ideal and Bussman make that clamp fuse holder sockets tighter onto fuse blades and ferrules to be considered a permanent repair, ever?

We use these on a temporary basis, not sure about a full time repair, but then selling a FSS is always a plus.

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Mountain Electrician

I have used the FOP method to check the intergity of contacts in a motor starter when there have been "nuisance tripping" of the overloads. I have also used a fall of potential tester for ground rods where you drive an electrode 80 ft away from the rod you are testing, and then another at different intervals between the first test rod and the rod being tested. This was always the way I tested grounds, but now I'm told a clamp on ground rod tester is as accurate. Does anyone have any experience with these?

brian john

I use several types of ground testers, clamp-on, 3-point test method (FOP) and a combination tester clamp-on/3-point. I still prefer the 3-point test method. The clamp-on method has to be used at the neutral to ground bond jumper and is not useful if you have down stream grounds, which is fairly common unfortunately.

Pierre Belarge

Brian
What type of 'clamp-on' ground tester do you use?
Would you have any information on the use of these other than the instruction manual from the manufacturer?
I am very interested in the use of the clamp-on ground tester. I have the one from AEMC.
Thanks

brian john

Pierre;

I am suppose to be in the office all day Monday, I will research the info then.

OregonYeti

I am not an electrician but in the HVAC business. I often measure current to ground when I have a question about why a system might not be working. I use a Fluke 179 true RMS meter, with its optional clamp for amp readings. I find it very helpful. It's really sensitive and I think Fluke meters are great in general.

I signed up to this site to see what I might learn from you people. A lot of my work involves electrical troubleshooting.