Tripping Breaker

A

arnettda

Senior Member
I repiped and pulled new wire to a 3 phase disconnect and installed a new disconnect. I have a rotation meter so my phasing is the same. The disconnect feeds a transformer and then the machine. I used the existing breaker that fed the disconnect. At the end of the day they shut the disconnect off that feeds the machine. Now the Majority of the mornings when they turn on the disconnect the breaker trips. I am starting with replacing the breaker but am a little lost after that, is it related to the work I did which I inspected the best I could without pulling all the wiring out. Or is it just timing and something is going wrong with the machine or transformer. After they reset the breaker the machine has run perfectly all day every day. Thank you.
 
Maybe with the repiping and new wire there is a lower impedance feeding the transformer and therefore a higher inrush current when the transformer is turned on? Is the new wiring path noticeably shorter than it was before, or with heavier gauge wire?
 
arnettda said:
I repiped and pulled new wire to a 3 phase disconnect and installed a new disconnect. I have a rotation meter so my phasing is the same. The disconnect feeds a transformer and then the machine. I used the existing breaker that fed the disconnect. At the end of the day they shut the disconnect off that feeds the machine. Now the Majority of the mornings when they turn on the disconnect the breaker trips. I am starting with replacing the breaker but am a little lost after that, is it related to the work I did which I inspected the best I could without pulling all the wiring out. Or is it just timing and something is going wrong with the machine or transformer. After they reset the breaker the machine has run perfectly all day every day. Thank you.
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Use data logger, observe inrush current on start up, adjust breaker magnetic settings suit inrush current
as inrush current chan6 at what point supply voltage wave the transformer switched on
 
It sounds like you may be having a problem with residual magnetism or magnetic hysteresis which causes a higher than expected inrush current.
Not only should you try to use the largest breaker allowed by the NEC (250% of FLA) you should look for one with the highest possible Instantaneous pick up point, such as a 400A frame breaker with a 200A long time trip point.

Anecdotally, it sometimes helps to have a load on the transformer when you start it up.
 
If understood correct, breaker >disconnect>transformer>machine.

You are on point with the breaker, especially if you kicked it off during service and it has some age on it. I've had many that get trippy after that.

Probably good to throw your meter on before breaker replacement and check inrush and FLA. That's generally what gives me more confidence in a bad breaker.

Last one a dealt with was a simple water pump circuit. I did nothing to that circuit other than flipped the breaker off because I was working in the box. Then got a call for random trips on the breaker. I could not reproduce the issue but confirmed a 20A circuit with only 8A on it, inrush was spot on what the motor should pull, and the known fact that the pump previously ran on it for many years. All I did was flip the breaker off. I first removed the breaker to inspect all contacts, bars, etc. All good so I swapped it. Never heard from customer again.
 
I agree with the above thinking, namely that you might have created a "stiffer" supply to the transformer - and accidentally started showcasing an inrush problem on power up that might have been close the edge before, and now is just over the edge.

2 quick questions...

1) Was the transformer changed at all, or just the wiring to it?

2) Is this transformer by any chance being backfed? Like a standard step-down 480 Delta to 208 Wye being used in reverse to create 480 from 208?
 
MD Automation said:
I agree with the above thinking, namely that you might have created a "stiffer" supply to the transformer - and accidentally started showcasing an inrush problem on power up that might have been close the edge before, and now is just over the edge.

2 quick questions...

1) Was the transformer changed at all, or just the wiring to it?

2) Is this transformer by any chance being backfed? Like a standard step-down 480 Delta to 208 Wye being used in reverse to create 480 from 208?
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The transformer was not changed at all. I even reused the flex that fed down to it. The transformer is being backfed to form 480 out of 208 volts. I did increase the wire size from what is was mainly to account for derating with more wires in the same pipe. The run is probably shorter as well compared to the original circuit that went all over the place.
I will start with all of this when I go back tomorrow,
 
OK. So I would think that your shorter + bigger wire has created a lower impedance supply that is capable of supply more current than before, particularly for inrush at switch on.

In addition, reverse feeding a transformer creates additional problems with inrush current. Normally, the primary windings are placed on the outside of the inner (secondary) coils. So the typical design is steel core on the inside, secondary windings in the middle, then primary windings on the outside.

One obvious reason for the primary coils on the outside is that is where you want the voltage adjustment taps. But winding the primary on the outside means they are also farther away from the inner steel core and that distance helps to slow the buildup of the magnetic field when you first energize the transformer.

Slower is good from an inrush current pov. This is the reason why, when you backfeed a normal step down transformer, the inrush current can be (more) problematic. It’s because you are energizing the inner windings, and being closer to the steel core, they can really magnetize the core quickly - and draw more initial current while doing it. And, if the residual magnetization left over in the core from the last shutdown is in the same orientation / direction as the first cycle of energizing current wants to push it, the core can get driven quite quickly into saturation for a cycle or 2. It's a random thing, but it's worse when you energize the inner coils as the primary windings.
 
arnettda said:
The transformer was not changed at all. I even reused the flex that fed down to it. The transformer is being backfed to form 480 out of 208 volts. I did increase the wire size from what is was mainly to account for derating with more wires in the same pipe. The run is probably shorter as well compared to the original circuit that went all over the place.
I will start with all of this when I go back tomorrow,
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There is roughly between 9 and 21 amps of inrush on the different phases when just turning on the transformer and power to the Machine. It seems like it changes as well. As 2 of the phases read pretty much around 8 amps and then jumped to 13 amps after I tried it 5 or 6 times and then can back to it.
 
arnettda said:
There is roughly between 9 and 21 amps of inrush on the different phases when just turning on the transformer and power to the Machine. It seems like it changes as well. As 2 of the phases read pretty much around 8 amps and then jumped to 13 amps after I tried it 5 or 6 times and then can back to it.
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Typical transformer inrush is 6-10 times rated current. Your transformers appear to be very small, or you do not have the correct means to measure the inrush.
IDK how that relates to a transformer that is reverse fed in a non-compliant way.
 
arnettda said:
There is roughly between 9 and 21 amps of inrush on the different phases when just turning on the transformer and power to the Machine. It seems like it changes as well. As 2 of the phases read pretty much around 8 amps and then jumped to 13 amps after I tried it 5 or 6 times and then can back to it.
Click to expand...
This sounds like a residual magnetism issue, where the inrush depends on what the transformer core was doing when it was turned off.

Reverse fed transformers can have inrush currents much higher (>10X) than a standard unit as explained by MD in post #11.

A clamp on meter will typically not respond fast enough to see the highest magnetizing current, but a peak capture might be able to.
 
jim dungar said:
This sounds like a residual magnetism issue, where the inrush depends on what the transformer core was doing when it was turned off.

Reverse fed transformers can have inrush currents much higher (>10X) than a standard unit as explained by MD in post #11.

A clamp on meter will typically not respond fast enough to see the highest magnetizing current, but a peak capture might be able to.
Click to expand...
I just saw a sentence in the Square D catalog that states transformers may require 250% primary overcurrent.
 
A bit of apples to oranges. I have small single phase transformer I used for bench top when I needed 480 at my shop. 240v in 480v out.
I used a Fluke 43B power quality to record the Inrush of it.
The left img shows the current of 96 amps at .05 of a second after power on. It was overscale @ start. Current .21 seconds later was 6 amp.
My Fluke 337 showed an inrush of 20.8 amps.

IDK what the size of my transformer is. Lable was long gone before I scavenged it.


eta: Reverse fed with 15 amp breaker at 240 volt
 
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