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Old 06-08-2017, 11:41 AM   #41
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You need to get a data logger on it so you know what's going on when it "randomly" trips.

From what you have posted so far it looks like the feeders, disconnect, and load are the only common demonstrators left. Well the service itself i suppose but if no other machinery is having issues... Def let us know when you figure it out or have more info. GL

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Old 06-08-2017, 01:15 PM   #42
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Are there other 3 phase loads out of that panel? Any issues with them?
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Old 06-08-2017, 01:30 PM   #43
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Quote:
Originally Posted by tersus View Post
At the breaker, 120v to ground off each leg, 208v between black and blue, 208v between red and blue, 0v between black and red. I take the wires off the breaker and try it..same thing. I immediately check voltages on the buss--good. Turn the breaker on and off couple times.. now we're good again.
Did you try reading each leg from the bus to the load side of the breaker with the wires disconnected? Maybe you have multiple issues happening here. Like something on the load causing the breakers to fail, but also some crappy breakers.
My thoughts on the 0V reading between 2 legs would be that these two legs are at the same potential/ phase somehow.
Possibly an issue with incoming line or the bus...
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Old 06-08-2017, 02:05 PM   #44
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Quote:
Originally Posted by tersus View Post
I guess you detected my sarcasm by asking you 'seriously?' I would've typed a longer sentence such as, "seriously?, if you'd read my post you would have known that I checked the voltage before the breaker", but I figured that might be too many words and you'd might get lost in it..
OK, seriously.. I'll admit I'm being a complete ass right now. You're getting the brunt of it which wasn't fair, it just seemed the two guys before you didn't read it all either. I know it's a long and tedious one.
I'll answer this for you. If there is a control inside connected to the phase you are missing the phase to phase voltage, you may be getting the 120 volt to neut, on all three due to feedback.

Now the missing phase needs a FOP test, if you don't know how to do this here goes.

With the load on and running.
Read A phase of your main feed to A phase at the DW, should be real close to 0 Volts.
Repeat for each phase, if you read a voltage move from main feed to bus, If voltage is read move to bolt on screw, so on back to the DW. Between last place you read voltage and no voltage will be your problem spot.
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Old 06-08-2017, 03:14 PM   #45
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Originally Posted by just the cowboy View Post
I'll answer this for you. If there is a control inside connected to the phase you are missing the phase to phase voltage, you may be getting the 120 volt to neut, on all three due to feedback.

Now the missing phase needs a FOP test, if you don't know how to do this here goes.

With the load on and running.
Read A phase of your main feed to A phase at the DW, should be real close to 0 Volts.
Repeat for each phase, if you read a voltage move from main feed to bus, If voltage is read move to bolt on screw, so on back to the DW. Between last place you read voltage and no voltage will be your problem spot.
Ding ding ding! I think we have a possible Winner!

My bet is that you (tersus) are using a DMM. One potential drawback they have is in presenting basically no load to a circuit. So SOMEWHERE in your system there is a phase loss or maybe even a high resistance phase to phase short taking place and when you read the voltage to ground as good, your sensitive meter is reading THROUGH something like a control transformer that has so little resistance that your meter with virtually no burden doesn't show the voltage drop. But once you load the bus up with the DW, that current flow makes the entire system unbalanced and eventually something reacts to it.

I'm an old Wiggy guy for stuff like this. I think if you used an inductive tester like a Wiggy or Knopp on that system when the DMM shows 120V to ground on all three, but no 208 between two, you will find that the Wiggy will not show you 120V to ground on the bad phase.

But the tests that Brian John suggested a while ago are a better and more thorough way to go.
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Old 06-08-2017, 08:08 PM   #46
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Quote:
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thanks for the advice. can you tell me what degree of voltage drop would be a cause for concern? we're talking mV right?
The VD will be in Millivolts and generally between 25-100 mv, BUT the big thing to look for with a 3-phase breaker is a balanced VD if the current is similar across all 3-phases the VD should be similar.
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Old 06-08-2017, 08:10 PM   #47
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Quote:
Originally Posted by just the cowboy View Post
I'll answer this for you. If there is a control inside connected to the phase you are missing the phase to phase voltage, you may be getting the 120 volt to neut, on all three due to feedback.

Now the missing phase needs a FOP test, if you don't know how to do this here goes.

With the load on and running.
Read A phase of your main feed to A phase at the DW, should be real close to 0 Volts.
Repeat for each phase, if you read a voltage move from main feed to bus, If voltage is read move to bolt on screw, so on back to the DW. Between last place you read voltage and no voltage will be your problem spot.

I wrote a sticky for this a while back I thought?
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Old 06-08-2017, 08:13 PM   #48
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Found it.

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 IRd 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.
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