xfmr primary/secondary device coordination

[lielec11]

In the attached image I have a TCC showing a simple 208V-480V step up transformer arrangement. There is a fuse on the primary side (brown) and a fuse (green) as well as an MCB (blue) on the secondary side. My question is, do you need to worry about the MCB on the secondary side overlapping with the primary side fuse (blue over brown curve)? Or will this not be an issue in the field due to the transformer isolating the fault currents?

 

[iceworm]

Picture is not real clear. However, if I am understanding correctly, the overlap is right at the corner at the end of the CB LT curve and again ion the instantaneous range.
Not much you can do about the instantaneous.
For the overlap at the corner, at the begining of the ST, say 220A - 250A, the time can be anywhere from 30 seconds to instantaneous. I would not see this as an issue. In this range, the secondary fuse should go first.

Or will this not be an issue in the field due to the transformer isolating the fault currents?

No, any fault current on the seondary reflects back to the primary

And I'm thinking you already knew all this. What am I missing?

 

[lielec11]

Picture is not real clear. However, if I am understanding correctly, the overlap is right at the corner at the end of the CB LT curve and again ion the instantaneous range.
Not much you can do about the instantaneous.
For the overlap at the corner, at the begining of the ST, say 220A - 250A, the time can be anywhere from 30 seconds to instantaneous. I would not see this as an issue. In this range, the secondary fuse should go first.


No, any fault current on the seondary reflects back to the primary

And I'm thinking you already knew all this. What am I missing?

I can never get the images to come out exactly how I want :rant:. But yes, you are correct regarding the overlap in the long-time region, I'm ignoring the instantaneous as this is NYC and the code stipulates selective coordination starts at 0.1s.

Oak so in this case you're right it isn't much of an issue because we also have the fuse on the secondary side which should act first. However, in general, we are still required to coordinate secondary devices to primary no matter how far away from the transformer, correct? This is how I've been attacking these issues but recently I had a stubborn engineer of record come back and make me revise my recommendation because he thought otherwise. Now I'm trying to find ways to prove him otherwise so we don't get in trouble down the line.

 

[iceworm]

... However, in general, we are still required to coordinate secondary devices to primary no matter how far away from the transformer, correct? ....

Disclaimer: I have no knowledge of NYC codes
Hummm .....
So where is the requirement comming from:
240.12 orderly shutdown?
708.54 COPS?
517.31 Healthcare?
700.32 Emergency
701.27 Legally required standby
Someting else?

 

[lielec11]

Disclaimer: I have no knowledge of NYC codes
Hummm .....
So where is the requirement comming from:
240.12 orderly shutdown?
708.54 COPS?
517.31 Healthcare?
700.32 Emergency
701.27 Legally required standby
Someting else?

I'm using the word "requirement" loosely here. What I meant was that it is best practice to be concerned about devices downstream on the secondary of a transformer overlapping with upstream devices on the primary side, correct?

 

[iceworm]

... this is NYC and the code stipulates selective coordination starts at 0.1s....

Using the engineering definition, "Yer screwed"
208/480V step ups are usually a dedicated motor/equipment, or a long run and an attempt to ease the VD. Assuming the currents on the TCC are reflected to the 480V side - this is a 30KVA transformer. Generally that would be in the "Who cares" range. Just protect it such that it doesn't start a fire. As for coordinating between the transformer primary OCP and secondary OCP. What difference does it make as to which one goes first? Either way the transformer and all the loads are out.

However, if your code says "coordinated to .1 sec" and is written to include transformer primary and secondary, you don't have lot of choices:
Ditch the secondary CB just use the secondary fuse.

 

[dkidd]

In the attached image I have a TCC showing a simple 208V-480V step up transformer arrangement. There is a fuse on the primary side (brown) and a fuse (green) as well as an MCB (blue) on the secondary side. My question is, do you need to worry about the MCB on the secondary side overlapping with the primary side fuse (blue over brown curve)? Or will this not be an issue in the field due to the transformer isolating the fault currents?

It isn't a problem.

This is somewhat addressed by

700.28

Exception: Selective coordination shall not be required between
two overcurrent devices located in series if no loads are
connected in parallel with the downstream device.

 

[iceworm]

I'm using the word "requirement" loosely here. What I meant was that it is best practice to be concerned about devices downstream on the secondary of a transformer overlapping with upstream devices on the primary side, correct?

I didn't see your #5 post before I sent my #6 post.

As dkidd noted, it doesn't matter much which goes first.

As you noted, no code issue.
And that makes it a design issue.

If the primary CB is hard to get to, not in my jurisdiction (utility), I would tend to be more selective (pun intended)

In this case, they look equally accesible, so we are back to "who cares".

recently I had a stubborn engineer of record come back and make me revise my recommendation because he thought otherwise. Now I'm trying to find ways to prove him otherwise so we don't get in trouble down the line.

You have the answer: The "engineer of record" has spoken. You have done your duty and expressed your opinion on the nature of the design issue.

This is when I say, "Right on. We got it. It's changed"

 

[lielec11]

It isn't a problem.

This is somewhat addressed by

700.28

Exception: Selective coordination shall not be required between
two overcurrent devices located in series if no loads are
connected in parallel with the downstream device.

You are correct, in this instance it isn't much of an issue since they're all in series. I was asking in general since I have any similar issue on another project where a branch breaker in a downstream panel overlaps with a distribution breaker upstream on the primary side of the transformer. Would this be a concern? Assuming yes it would..

 

[lielec11]

I've attached a totally different case here. Downstream CB overlaps with upstream CB on primary side (purple overlaps with green)... would this be an issue?

https://ibb.co/PhJkvCk

 

[iceworm]

Y... a branch breaker in a downstream panel overlaps with a distribution breaker upstream on the primary side of the transformer. Would this be a concern? Assuming yes it would..

I would agree. Yes, it is a concern. This is classic for a load being large in porportion to the panel - say a 100A branch in a 200A main

 

[lielec11]

Thanks for sticking with me on this... I just wanted to make sure I'm not crazy.

 

[dkidd]

Thanks for sticking with me on this... I just wanted to make sure I'm not crazy.

I'm and engineer, not a doctor.

 

[topgone]

I can never get the images to come out exactly how I want :rant:. But yes, you are correct regarding the overlap in the long-time region, I'm ignoring the instantaneous as this is NYC and the code stipulates selective coordination starts at 0.1s.

Oak so in this case you're right it isn't much of an issue because we also have the fuse on the secondary side which should act first. However, in general, we are still required to coordinate secondary devices to primary no matter how far away from the transformer, correct? This is how I've been attacking these issues but recently I had a stubborn engineer of record come back and make me revise my recommendation because he thought otherwise. Now I'm trying to find ways to prove him otherwise so we don't get in trouble down the line.

When you said you have to coordinate the primary protection with the secondary protection, think again. If your primary and secondary protection breakers are serving the same load, it's useless trying to "coordinate" both as the same effect will happen--> either of the protection device tripping will result to no power to the faulty load.

What you should worry instead will be losing power to other loads sharing the same source when other loads fault out when the healthy circuits are supposed to get power from the main source. You need to "coordinate" how your main breaker trips, to allow other loads to operate while the faulty equipment gets isolated.

 

[lielec11]

When you said you have to coordinate the primary protection with the secondary protection, think again. If your primary and secondary protection breakers are serving the same load, it's useless trying to "coordinate" both as the same effect will happen--> either of the protection device tripping will result to no power to the faulty load.

What you should worry instead will be losing power to other loads sharing the same source when other loads fault out when the healthy circuits are supposed to get power from the main source. You need to "coordinate" how your main breaker trips, to allow other loads to operate while the faulty equipment gets isolated.

I wasn't specific enough in some of my descriptions. What I meant when i said coordinate both sides I didn't mean the two OCPDs in series with the transformer, I meant other breakers downstream on the secondary side. Say, a branch circuit breaker in the first panel downstream of the transformer.

 

[steve66]

I've attached a totally different case here. Downstream CB overlaps with upstream CB on primary side (purple overlaps with green)... would this be an issue?

https://ibb.co/PhJkvCk


View attachment 23255

It also looks like one of the breakers on the line side of the transformer may trip on transformer inrush current.

 

[lielec11]

It also looks like one of the breakers on the line side of the transformer may trip on transformer inrush current.

Yes that is a whole other issue we have with these breakers. I'm not a fan of these GE SP/SK/SL/SF etc. series breakers, they never seem to have enough give in the instantaneous region, even on their maximum settings.

 

[topgone]

Yes that is a whole other issue we have with these breakers. I'm not a fan of these GE SP/SK/SL/SF etc. series breakers, they never seem to have enough give in the instantaneous region, even on their maximum settings.

I printed your TCC plot on A3 paper in color and found the settings are well coordinated.
Just look at the intercepts of the various TCCs of the breakers at 0.1s. They are graded by fault value, not time separation. It's hard when the upstream curve's lower band plot overlaps with the higher band plot of the downstream protection, Just try to compare the lower band trips of both adjacent protections and make sure the downstream breaker trips first. Also, the damage curves of lines/equipment should never be located below and to the left of the protecting breaker trip curve plot.

In your case, here are the trip values in order of coordination @ 0.1 seconds:

1. PD-RP-EM-2A-2 trips @ 45A
2. PD-RP-EM-2A-1 trips @ 90A
3. PD-T-RP-EM-2A-1 trips AT 100A
4. PD-DP-EM-2A MCB trips @ 320A

All your lines/equipment damage curve plots fall below their respective protective device trip curves also.

 

[lielec11]

I printed your TCC plot on A3 paper in color and found the settings are well coordinated.
Just look at the intercepts of the various TCCs of the breakers at 0.1s. They are graded by fault value, not time separation. It's hard when the upstream curve's lower band plot overlaps with the higher band plot of the downstream protection, Just try to compare the lower band trips of both adjacent protections and make sure the downstream breaker trips first. Also, the damage curves of lines/equipment should never be located below and to the left of the protecting breaker trip curve plot.

In your case, here are the trip values in order of coordination @ 0.1 seconds:

1. PD-RP-EM-2A-2 trips @ 45A
2. PD-RP-EM-2A-1 trips @ 90A
3. PD-T-RP-EM-2A-1 trips AT 100A
4. PD-DP-EM-2A MCB trips @ 320A

All your lines/equipment damage curve plots fall below their respective protective device trip curves also.

I'm not so sure I agree with your assessment. Either that or I'm not understand your point of view. As far as I know, the lower band represents the unlatching time and the upper band represents the fault clearing time. For fuses the lower banded is the melt start time, and the upper is the fault clearing time. With that said, in my curve there is a portion where bands for PD-RP-EM-2A-2 (purple) overlap with the lower band of PD-T-RP-EM-2A-1 (green). Look roughly at the 65A-80A region. I understand that to mean there are certain fault currents that will cause the green to being unlatching (lower band), before the purple curve clears the fault (upper band), which would in turn eventually lead the upstream breaker clearing/opening as well once it starts to unlatch.

Thoughts?

 

[topgone]

I'm not so sure I agree with your assessment. Either that or I'm not understand your point of view. As far as I know, the lower band represents the unlatching time and the upper band represents the fault clearing time. For fuses the lower banded is the melt start time, and the upper is the fault clearing time. With that said, in my curve there is a portion where bands for PD-RP-EM-2A-2 (purple) overlap with the lower band of PD-T-RP-EM-2A-1 (green). Look roughly at the 65A-80A region. I understand that to mean there are certain fault currents that will cause the green to being unlatching (lower band), before the purple curve clears the fault (upper band), which would in turn eventually lead the upstream breaker clearing/opening as well once it starts to unlatch.

Thoughts?

Please review your electrical setup. There are no fuses involved. A CB will either trip or not, no melting time, only mechanical lag. Just remember that CB's actuation time will depend on how big a difference the fault is to the CB's trip setting (inversely proportional) plus a little mechanical lag.

What you have are 4(four) circuit breaker and the solid line curves are the corresponding damage curves of your lines and the transformer. CB TCCs will give you the earliest trip time and the longest trip time possible after the CB senses the fault.

When coordinating, you either separate the trip curves by time-grading (vertical separation) or amplitude separation (horizontal separation). That is why, it could have been better if you can tweak the fault value settings and add time delays to the CB curve to lift the tripping characteristics. If not, just keep the faith that your horizontal separation works as designed, IMO.