Phase Loss

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mull982

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I am having a problem I was hoping someone could help with. Will a three phase molded case circuit breaker trip on the loss of a phase? I have a 480V three phase circuit breaker that keeps tripping and I cannot figure out why. I have an electronic relay monitoring the 480V circuit that indicates there is a phase loss when the breaker is tripping. (Please note that the relay cannot trip the breaker, the breaker only trips as a result of what is on the 480V line.) The 480V breaker is feeding a starter which is going out and powering a transformer connected to a DC rectifier. The 480V breaker is a 30A rated breaker. It will run fine for an hour or so and then trip througout the day.

Since I dont know what is causing the breaker to trip my questions are these. Will a phase loss cause a 3 phase breaker to trip. What happens to the voltage and current on the other two phases if the feed was to loose one of the phases. Normally the current on all three phases is about 4.5A. If the feed lost one of the phases would that cause the current on the other two phases to increase thus tripping the breaker? Will the voltage on the other two phases still be 480V line-line. What happens during a phase loss?

Can anyone help?

Mull982
 

480sparky

Senior Member
Location
Iowegia
mull982 said:
Will a three phase molded case circuit breaker trip on the loss of a phase? Mull982

To answer this part: No.

As to 'what happens' when you lose a phase, that depends on what it's hooked up to. It's possible there's excess amperage draw on either or both of the other phases that could trip the breaker, but without knowing exactly what you've got going on, it's hard to say.

Here's some links to similar threads:

http://forums.mikeholt.com/showthread.php?t=88021&highlight=%22phase+loss%22
http://forums.mikeholt.com/showthread.php?t=90608&highlight=phase+loss
 
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mull982

Senior Member
The three phase feed is hooked up to a 480V AC to 115V AC transformer. What will happen in this case?
 

480sparky

Senior Member
Location
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All three phases go to a step-down control transformer? Usually it's just two phases on the primary side, 480/120. If the lost phase is one that doesn't feed the transformer, then it won't be affected. If the lost phase is one that feeds the transformer, then the tranny will not function.

Is there a three-phase ouput on the 120v side of the transformer?
 

Jraef

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If it is feeding a 3 phase DC rectifier, then when you lose a phase on the AC side the current drawn by the rectifier will increase by the square root of 3, in other words it will pull 173% current on the two remaining phases, assuming of course that the load on the DC side of the rectifier remains the same. If the rectifier current is now above the rating of the breaker,That SHOULD cause the breaker to trip!
 

mull982

Senior Member
The three phase circuit is feeding a three phase Delta-Wye AC transformer with the rectifier connected to the secondary wye side of the transformer. As I mentioned in my previous post this is a three phase transformer stepping down 480V AC to 115V AC. (This is not a control power transformer) During normal operation the current draw on the three phases is about 4.5A.

I'm beginning to understand why loosing one phase will cause the current on the other two phases to increase, but I am still trying to see it mathematically. With the loss of a phase, will the voltage on the other two phases stay the same. For instance if there was a phase loss on phase C on my circuit would the line to line voltage between phases A and B still be 480V? Would the line to ground voltages on phases A and B still be 277V?
 

jim dungar

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mull982 said:
The three phase circuit is feeding a three phase Delta-Wye AC transformer with the rectifier connected to the secondary wye side of the transformer. As I mentioned in my previous post this is a three phase transformer stepping down 480V AC to 115V AC. (This is not a control power transformer) During normal operation the current draw on the three phases is about 4.5A.

The correct terminology for your delta-wye transformer is 480-208Y/120V. Voltage systems are defined by their line-line voltage.

I'm beginning to understand why loosing one phase will cause the current on the other two phases to increase, but I am still trying to see it mathematically. With the loss of a phase, will the voltage on the other two phases stay the same. For instance if there was a phase loss on phase C on my circuit would the line to line voltage between phases A and B still be 480V? Would the line to ground voltages on phases A and B still be 277V?

In your example if Phase C opens then there would be no change to the L-L nor L-N voltages on A & B.
 

Jraef

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The voltage does not change as Jim Dungar said.

What is happening is that because of the 3 phase bridge rectifier, the power consumed (pulled) by the DC load remains the same. So ALL of that power through the rectifier must now come from a single phase source, i.e. 2 of the previous 3 phases. The mathematical power relationship is just the basic 1 phase vs 3 phase power because you have gone FROM 3 phase TO single phase. To get VA in a 1 phase circuit, it's Amps x Volts. VA in a 3 phase circuit is Amps x Volts x 1.732. If you have the same Voltage and the same VA in both circuits, the 1 phase Amps is the 3 phase Amps x 1.732.

Take your system for example: assuming the 4.5A is on the primary side (although it really doesn't matter).

3 phase VA was 480V x 4.5A x 1.732 = 3741VA
The power draw is the same, but make it 1 phase and find the 1 phase A (A1);
So now 3741 = 480 x A1;
Divide both sides by 480 since that stays the same, so A1 = 3741/480 = 7.794 Amps

And 7.794 / 4.5 = ???

Answer: 1.732!

So back to your original issue;
If you had a 5A circuit breaker on that 480V side and you lose a phase, the rectifier will draw 7.794A and trip the breaker.

I should note that this is not unique to systems having a rectifier, it happens in any 3 phase load where the power circuit is common to all 3 phases, such as is the case with 3 phase motors. But in motors, you lose average torque because 2/3 of the windings are not being fully energized (assuming delta connected). When you lose torque and the load stays the same, the motor slows down, the slip increases and the motor draws A LOT MORE than 173% current and hopefully trips the OL relay before the breaker would trip. But that too depends on loading of the motor; an unloaded motor may run all day on 1 phase power. In a system with a rectifier, there is no change in the load since the rectifier is like a new source, somewhat isolated from the incoming AC power, at least from a mathematical sense.
 
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mull982

Senior Member
Jraef thanks for the response, I am now seeing how the math works. The thing that is puzzling me however is that the breaker is a 30A breaker. I checked some nameplate ratings ant the FLC AC current draw on the 480V side is 6.7A. So with a phase loss you would expect to see the following:

6.7A * 1.73= 11.59A

11.59A is still well under 30A so I dont understand why the breaker would trip. Any ideas?

Some other namplate information that i got was that the magnet drew 5000 DC at 43.3A. Remember that this load is coming through a rectifier which is coming off of the secondary side of the 480/208-120V transformer. So doing the math on the low side of the transformer I found the following.

5000W / 115V = 43.4A.

This makes sense with the nameplate amperate for the DC output coming off of the low-side of the transformer.

Taking that same 5000W and calculating the current draw on the 480V side of the transformer I get the following:

5000W / 480V / 1.73 = 6.02A

This is close to the nameplate FLA. Transformer is probably a bit oversized. So with these numbers I cannot see how the 30A circuit breaker would trip even with a phase loss causing the current to draw 1.73 times higher. Any ideas?
 

coulter

Senior Member
mull -

Where do you have the monitoring relay connected? Line side of the CB or Load side of the CB?

carl
 

mull982

Senior Member
The monitoring relay is on the Load side of the 480V circuit breaker. The relay is an Allen Bradley E3 relay which is which is a part of a DeviceNet Smart MCC bucket. The relay is essentially a solid state overload with circuit diagnostics capibility (thus the phase loss alarm). The relay is on the load side of the circuit breaker just before the contactor.
 

coulter

Senior Member
I'm familiar with AB E3, E3+.

I'm guessing this is part of an MCC. If that is correct, then the rest of the post will make sense. Otherwise, (bleep) I'm all wet - which happens regularly.

Where I'm headed is this is not phase loss. If it were, you would be see a lot of other things hppening in the MCC. I suspect the E3 is seeing the CB opening as phase loss.

Or if it is truely phase loss, you have a bad bucket-to-bus connection. Pull the bucket and you will see it on the stabs, or stab-to-conductor connections.

If not that, then the CB is likely heating up and tripping. Check the connections. If they look good - replace the CB. If any look bad, fix them and replace the CB.

A couple of questions on the bucket:

Is this a mag-only cb? As I recall AB marks them MCP (maybe?). I don't know how familiar you are. If it's a mag-only, it will have three little dials at the bottom

Is the E3 set up to provide overload protection? Yes, I know the E3 will not trip the the CB, but if it is a mag-only cb the E3 will be wired for overload trip of the contactor.

The reason I mention this is what you are describing is a normal configuration for a combination motor starter. If the MCC (or replacement bucket) were specified for this task, it most likely would have been a thermal cb. But contactors feeding xfmrs are unusual so nothing says the specifier didn't screw this one up. I suspect a bucket with a contactor and E3 is being used to get PLC control of the rectifier feeding the magnet. If it is a mag-only cb, give some thought to replacing it witha thermal.

You have an interesting one here. let us know.

carl
 

mull982

Senior Member
Coulter: Yes the 30A circuit breaker is a magnetic circuit breaker type HMCPS. It does have the adjustment knobs on the breaker which I believe our electricians have turned all the way up after the last trip. It seems to have been doing fine since they did that yesterday. Do the knobs on these breakers just add a delay time on the breaker or do the actually lower the setpoint for the breaker tripping? I'm just curious why such a big breaker was engineered for this small current draw.

You are correct about the relay as well. It is given an output from a PLC to pull in or drop out the motor contactor. No relay however to trip the 480V circuit breaker.
 

coulter

Senior Member
A 30A mag-only with the dials turned all the way up is likely 400A to 500A. There is no delay. You don't have any over current protection. The E3 may be providing overload protection - depends on how it is programmed.

You have a couple of possibilities;

1. The CB is not lying and the equipment has a huge current spike that is tripping the
cb. And you have just set it up to where the CB is ignoring the current spike. Not good.

2. The CB is broke. Also not good.

My inclination is to go with the lesson from Three Mile Island, "Believe your instrumentation." Hold to that unless proven defective. (figure the cb is not broken until proven otherwise)

Recomendations:
1. Schedule a panic. You have non-motor loads hooked up to a mag-only CB and its tripping.

2. Get the mag-only out of there. Call the equipment MFG and get a recomendation for OCP. Apply the transformer ratio, get the right CB and get it installed. I'd guess a thermal-mag, 15A. Or, pick out a cb size that adequately protects the transformer (art. 450) and then make sure the secondary protection is right for the equipment.

If the equipment hasn't burned up yet cancel the panic. If it has burned up cancel anyway.

Could be the equipment draws current in pulsed high current spikes and the right cb fixes the issue. But , I don't know that. You said the cb held for an hour then trips - your are resetting and it trips through the day. If it not a bad cb, the equipment is heating up and malfunctioning.

When you call the mfg about the ocp, ask to speak to one of the techs. Ask about current spikes, and heating effects. Maybe you will get lucky. If not, then:

3. Get some monitoring equipment and see what the equipment is drawing. Get something good enough to see and record high current spikes.

Just as aside: If you can do this without getting screechy-voiced, red-faced, or threatening-looking, ask whoever okayed this, what their plan was in connecting a non-motor load to a mag-only CB. I would be curious.

good luck, keep us informed - it is interesting.

carl
 

mull982

Senior Member
Coulter: I called the manufacturer and they were really not much help. Basically as far as OCP they recommended a dual element time delay fuse or a time delay breaker. However they did not say anything about the magnetic breaker that we were using. They also said that there was nothing under normal operating conditions that would cause current spikes or surges or anything of that nature that could trip the breaker.

I located a spec on the breaker, and it is a 30A HMCP Cutler-Hammer Breaker. It has instantaneous adjustment settings on it from A-H ranging from 90A to 300A. Like I said in a previous post we dialed it all the way up and now have it at a 90A instantaneous setting. I believe it came from the factory defaulted to its lowest setting which would be 90A but I'm not sure. Even with it set to its lowest setting of 90A I cant see would would be tripping the breaker? This brings up another question: With a 30A continuous breaker having settings of 90A - 300A, what makes it a 30A breaker then. If it is a 30A breaker but set for 90 or 200A then what is the point of having a 30A breaker? Why not just by a 90 or 200A breaker?

I checked with the engineers who speced the breaker size and asked why they chose such a large breaker (I would think a 15A breaker would be enough) but I have not heard back. Do you think that because this is an instantaneous breaker only they had to size that big in order to get over the transformer inrush of 6-8 FLA. (Typically a thermal or time delay breaker would be speced for this as you mentioned to get over inrush) Even so, it is not tripping on inrush, it is tripping during normal operation.

The manufacturer also mentioned that with a phase loss the current will rise and possibly trip the breaker but based on my calculation in a previous post I only calculate the current to rise to aprox 11.4A and dont see this tripping the breaker? Do you agree?

Thanks for all the help.
 

jim dungar

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PE (Retired) - Power Systems
Per NEC 430.51 conductors feeding a motor may use a short circuit protective device (i.e. you instantaneous trip HMCP) and an overload device (i.e. your AB E3 relay). This is the standard construction of a combination motor starter like you have in your MCC bucket.

The HMCP and overload relay combination do not provide protection for the conductors feeding your transformer per NEC 240.9.

Your HMCP has a nominal continuous current rating of 30A (this is similar to the rating of a piece of wire). It trips instantly when the current exceeds the setting of 90-300A. It is adjustable so that it can accomodate the inrush current of a motor. Its setting should be based on NEC 430.52(C)(3).
 

mull982

Senior Member
jim dungar said:
Your HMCP has a nominal continuous current rating of 30A (this is similar to the rating of a piece of wire). It trips instantly when the current exceeds the setting of 90-300A. It is adjustable so that it can accomodate the inrush current of a motor. Its setting should be based on NEC 430.52(C)(3).

Will the breaker trip if the continuous current reaches a level of 35 or 40A? What will cause the breaker to trip in this case since it is an instantaneous breaker?
 

coulter

Senior Member
Mull -
Glad to help. However, you really need to read all of my posts - they sort of go together.

Assuming you checked the connections as recommended, forget the phase loss - it isn't that. You are getting side tracked from the problem.

mull982 said:
...I called the manufacturer and they were really not much help. Basically as far as OCP they recommended a dual element time delay fuse or a time delay breaker. ... They also said that there was nothing under normal operating conditions that would cause current spikes or surges or anything of that nature that could trip the breaker. ...
The mfg did fine by you. What they said is code for, "Use a normal inverse time CB." They didn't say anything about using an instantaneous cause nobody would do that.

Read Jim's post about instantaneous/ mag-only CBs. They don't trip until the current exceeds the setting. There is no delay. There is no continuous rating. That 30A cb will hold in continuous at 299A. That's is why they are only used in a listed combination starter. The overloads provide overload protection. Why do the mfgs use 30A as a rating?. Don't know. I guess the user is susposed to know that the instantaneous rating is 3x to 10x of the cb rating.

The mfg also said there is nothing that would cause current spikes. Okay, (repeating what I said earlier) either the CB is bad or the equipment is malfunctioning.

mull982 said:
...I checked with the engineers who speced the breaker size and asked why they chose such a large breaker (I would think a 15A breaker would be enough) but I have not heard back. Do you think that because this is an instantaneous breaker only they had to size that big in order to get over the transformer inrush of 6-8 FLA. ...
You asked the question wrong. Yes 30A is too big, instantaneous is just wrong. Ask, "Why did you use a mag-only CB for a transformer feeder?"

If you get a blank stare, the next thing to say is, "We are using a listed combination motor starter. It comes with a mag-only/instantaneous CB. Is that what we want to use for the transformer feeder?

If you still get a blank stare, find another engineer. If you get red-faced denials, get another engineer. If you get, "Oh $h--, what was I thinking, thanks - good catch." You got a good one that will work with you.

The recomendations I gave you previously are still good ones.

carl
 

coulter

Senior Member
mull -
Instantaneous/Mag-only CBs:

Get some curves from ABs website. Look at inverse time/thermal-magnetic and instantaneous trip. Stick with the small frame CBs.

The one you have in your bucket is likely a 100A frame. I suspect that all on the mag-only CBs in 100A frame size use the same contacts and trip circuit. The only difference will be spring tension on the magnetic trip coil (maybe the number of turns on the sense coil). These CBs are pretty dumb. The trip is a coil of wire wrapped around the pole bus, connected to solenoid, the armature is connected to the CB trip lever. Armature is held back by a spring. You adjust the spring tension with the dials. There is no long-time trip element.

carl
 

mull982

Senior Member
Coulter:

Thanks for all the help. We have checked all of the connections to the circuit breaker, and the breaker itself and everything seems to be fine. I have also gone back and asked the engineer why he used a instantaneous breaker when feeding a transformer. You recommended using a thermal inverse time delay breaker so I'm assuming that it is accepatble by code to have this type of breaker in a combination starter. I am going to wait to hear the engineers answer and depending on it attempt to replace the breaker.

I find it interesting the information that you provided for instantaneous breakers. It doesn't make sense to me like you said why the manufacturer would have a 30A continuous rating, if this really didn't even mean anything. Same with every other breaker for that matter. Why not just say or make a breaker that is 90A, or 300A etc.... As I mentioned we now have the breaker set at 300A and it has not tripped at all in the past two days. I guess I can assume that whatever is tripping the breaker is less than 300A. Like you also mentioned we may have found a way around the cause of the trip but this is not the correct solution, and I would like to fix it. So am I right by saying that whatever was causing the breaker to trip was above 90A, (since this is where we had our origional setting when it was tripping) and below 300A (since it is no longer tripping assuming the current still exists). I looked at some curves online and saw that these curves were completely vertical for each setting, indicating a instant trip with no time delay (as opposed to thermal or inverse which have curves) I find it totally misleading how the manufactures rate these breaker, so I guess I will have to be careful in the future.


Just out of curosity, since the breaker in a combination motor circuit is typically instantaneous what protectecs your feeders (T-Leads) going to the motor? Is this the job of the thermal overload? In this case the feeders to the transformer are #10.

Thanks again for the help.
 
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