mattsilkwood

I have heard you talk about a fall of potential test many times. I understand the principle behind it but was wondering if there is a general rule of thumb to go by.

Something like a certain percentage based on voltage or current.

MDShunk

Brian surely has some better stuff, but here's my 2 cents.

You're better off comparing the poles of a multi-pole breaker to each other, and individual poles of single pole breakers to other single pole breakers. You want all your readings of similar breakers, or all the poles of a multi-pole breaker to be within 20% of each other. There really isn't any good published data from manufacturers on this, but many test specs will say that your measurements can be as far as 50% from each other. I don't like that, so I examine more thoroughly anything different by more than 20%.

In general, though, if you get a reading over 100mv, you might have a problem. Over 200mv, you definitely have a problem.

When I worked at AMP, I remember that Faston connectors (1/4" female connectors) were tested for a maximum drop of 3mv per amp.

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I'm an electrician, not a magician.

John

Just some basic ohm’s law. If the FOP was 100mv and the breaker amperage was say 1000 amps, the wattage across the breaker would be 100 watts. You just need to keep in perspective what you are measuring.

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Doubt All Before Believing Anything.......

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. You are measuring voltage drop across the item under test.

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.

Zog

Your readings should be with 50% of each other (Phases)

mattsilkwood

Thanks guys.