Equipment Sequence for Coordination

[timm333]

A question please. if I have a utility supply, then breaker with OC-relay which will act as primary protection of transformer, then transformer, then breaker with OC-relay which will act as the secondary protection of transformer. then MCC, then overload-relay plus breaker for motor, and then motor. In this combination it becomes hard to coordinate all three relays.

can I omit the breaker+relay of transformer secondary so that the primary of the transformer protects the cable and MCC on the secondary side of the transformer. The reason I was thinking to do it is because if i can omit one relay, then it will be easier to coordinate the remaining two relays (which will be motor relay and transformer primary relay.)

 

[JoeStillman]

The rules for transformer protectio let you eliminate the secondary protection under some circumstances. However, there is no exception to the rule that requires protection for whatever cables you have on the secondary.

You shouldn't worry too much about the transformer primary relay coordinating with the secondary protection (assuming the primary relay protects ONLY the one transformer.) When either one trips, you're just as dead. You must have a really big motor on that transformer though, if you can't get cordination with the overload relay.

 

[timm333]

The problem that I am facing is that the TCC of motor-relaybecomes horizontal straight line at 250A and then it stays horizontal till100000A. This horizontal part touches TCC curves of other upstream relays. Is theresome way to remove this horizontal part?
Also in addition to the TCC of motor relay, is it necessaryto draw the TCC curves of the motor (starting curve, stator damage, rotor damage,hot start curve, cold start curve, safe stall) ?

 

[templdl]

The problem that I am facing is that the TCC of motor-relaybecomes horizontal straight line at 250A and then it stays horizontal till100000A. This horizontal part touches TCC curves of other upstream relays. Is theresome way to remove this horizontal part?
Also in addition to the TCC of motor relay, is it necessaryto draw the TCC curves of the motor (starting curve, stator damage, rotor damage,hot start curve, cold start curve, safe stall) ?

By being horizontal what it appears that you have described is the instantaneous short circuit part of the curve. Is there a "inverse time current curve" up to 250 and if so where does it start?
From what I can determine from you post It is difficult to coordinate the instantaneous because all protection up stream see it at the same time and it then becomes a face to trip. This is where a time delay is useful. In the area of instantaneous you would have to have an instantaneous pick up and delay setting. When you get into power breakers they have a higher short circuit current rating previously called withstand rating. Whe umn there is a fault you have the ability to delay tripping of the up stream breakers just enough to allow the down stream breaker to clear.

 

[timm333]

For motorrelay, up to 250A is a thermal inverse time curve which starts at 30A, and itbecomes straight horizontal line a 250A/8-sec. if I add the instantaneouselement to this thermal element, then the horizontal part shifts down from 8sec to 1 sec. But still it touches the instantaneous part of transformersecondary relay; this instantaneous part of transformer secondary relay is avertical straight line at 2750A.
The onlyway to coordinate these two curves is if I keep both instantaneous and thermalparts of the motor relay, and keep only the thermal part (instantaneous removed)of the transformer secondary relay. But is it allowed to remove the instantaneouspart of the transformer secondary relay?

 

[GoldDigger]

For motorrelay, up to 250A is a thermal inverse time curve which starts at 30A, and itbecomes straight horizontal line a 250A/8-sec. if I add the instantaneouselement to this thermal element, then the horizontal part shifts down from 8sec to 1 sec. But still it touches the instantaneous part of transformersecondary relay; this instantaneous part of transformer secondary relay is avertical straight line at 2750A.
The onlyway to coordinate these two curves is if I keep both instantaneous and thermalparts of the motor relay, and keep only the thermal part (instantaneous removed)of the transformer secondary relay. But is it allowed to remove the instantaneouspart of the transformer secondary relay?

Since it is the instantaneous part of the secondary (or primary) protection at thethe transformer that is protecting the conductors from short circuit, you would have to keep that protection active (also to keep Arc Flash incident energy down.)

What is it about this situation that makes coordination more important to you than safety?

 

[templdl]

For motorrelay, up to 250A is a thermal inverse time curve which starts at 30A, and itbecomes straight horizontal line a 250A/8-sec. if I add the instantaneouselement to this thermal element, then the horizontal part shifts down from 8sec to 1 sec. But still it touches the instantaneous part of transformersecondary relay; this instantaneous part of transformer secondary - is avertical straight line at 2750A.
The onlyway to coordinate these two curves is if I keep both instantaneous and thermalparts of the motor relay, and keep only the thermal part (instantaneous removed)of the transformer secondary relay. But is it allowed to remove the instantaneouspart of the transformer secondary relay?

Now that make more sense.
But you with think that setting you motor relay to a shorter time hopefully that setting would be quicker than that for the transformer
I would seriously consider a pick up and delay for coordination for your transformer. If you omit the instantaneous part you are actually defeating your short circuit protection.
I believe that I am understanding your application.

 

[timm333]

I was doing the arc flash study and ascoordination study is preceded by arc flash so I was trying to find out a wayto avoid intersection of curves. But yes, the safety takes priority.

I tried adding and then removing the instantaneous part of the transformersecondary relay but arc flash incident energy does not change. Maybe I am doingsomething wrong. Also changing the time delay of the transformer secondarycircuit breaker does not change incident energy.

 

[GoldDigger]

I was doing the arc flash study and ascoordination study is preceded by arc flash so I was trying to find out a wayto avoid intersection of curves. But yes, the safety takes priority.

I tried adding and then removing the instantaneous part of the transformersecondary relay but arc flash incident energy does not change. Maybe I am doingsomething wrong. Also changing the time delay of the transformer secondarycircuit breaker does not change incident energy.

Your Arc Flash software may be by default only considering the first upstream OCPD, assuming that you have coordination.
But the IE calculation I am concerned about is at the termination of the secondary to motor feeder where the second OCPD should not even be in the circuit.
Possibly the IE is bring limited by the combination of the primary OCPD and the transformer impedance?

 

[jim dungar]

For motorrelay, up to 250A is a thermal inverse time curve which starts at 30A, and itbecomes straight horizontal line a 250A/8-sec. if I add the instantaneouselement to this thermal element, then the horizontal part shifts down from 8sec to 1 sec. But still it touches the instantaneous part of transformersecondary relay; this instantaneous part of transformer secondary relay is avertical straight line at 2750A.
The onlyway to coordinate these two curves is if I keep both instantaneous and thermalparts of the motor relay, and keep only the thermal part (instantaneous removed)of the transformer secondary relay. But is it allowed to remove the instantaneouspart of the transformer secondary relay?

You need to 'cut off' the protective device at the point where fault current is maxed out, which is usually the available short circuit current at the load.
Motor running thermal protection will never respond as fast as short protective devices. The NEC requires you to provide Branch Short Circuit and Ground Fault protection for your motor, this is the function you would normally be coordinating with upstream devices.

 

[timm333]

The short circuit current is about 11 kA so the relay should be cut off at 11kA. But I have tried every option and it does not cut off at 11 kA, instead the horizontal straight line of TCC of motor relay keeps going and touches the time axis on the right side.

It makes sense to perform IE calcultaions at transformer secondary junction box, but I think arc flash is mandated only for enclosed panels like MCC.

 

[jim dungar]

The short circuit current is about 11 kA so the relay should be cut off at 11kA. But I have tried every option and it does not cut off at 11 kA, instead the horizontal straight line of TCC of motor relay keeps going and touches the time axis on the right side.

It makes sense to perform IE calcultaions at transformer secondary junction box, but I think arc flash is mandated only for enclosed panels like MCC.

You really have 11kA flowing through your motor running protection, wow!

You calculate IE whever you might be exposed to an AF.

 

[timm333]

No not through motor running protection, but through power cable, lol. sorry ...

I am still wondering why the curve of motor relay cannot be stopped at 11kA, because the horizontal part of the curve keeps going to the right and touches the time axis on the right at 100kA.

Also should the motor starting curve, hot/cold start, rotor/stator damage curve be plotted on the TCC chart, or are these motor curves not important?

 

[GoldDigger]

No not through motor running protection, but through power cable, lol. sorry ...

I am still wondering why the curve of motor relay cannot be stopped at 11kA, because the horizontal part of the curve keeps going to the right and touches the time axis on the right at 100kA.

Also should the motor starting curve, hot/cold start, rotor/stator damage curve be plotted on the TCC chart, or are these motor curves not important?

I have not seen the curve in question, but I would guess that what you are seeing is that the instantaneous trip is spring driven and cannot not open any faster as the fault current rises above the point where it trips at the beginning of the first cycle.
A thermal trip will interrupt sooner the higher the fault current. A magnetic instant trip cannot.
What is the value on the time axis?

It is hard to stop a relay from tripping just because the input that is supposed to actuate it has gone too high.

 

[timm333]

I have attached two curves. First curve is motor relay without instantaneous element and the time of the horizontal part of this curve is 8sec. The second curve is the same motor relay with instantaneous element added, in this case the horizontal part shifts down to 0.018 sec. I am wondering how to cut off these horizontal lines at maximum short circuit (11kA); because these horizontal lines should not keep extending to the right and should not touch the time axis to the right.

 

[JoeStillman]

The portion of the overload relay curve that falls to the right of your available fault current is meaningless. The manufacturer who produces the curve has no way of knowing what your available fault current will be.

SKM will cut it off at the available fault current, if you have completed the SC study.

 

[templdl]

You need to 'cut off' the protective device at the point where fault current is maxed out, which is usually the available short circuit current at the load.
Motor running thermal protection will never respond as fast as short protective devices. The NEC requires you to provide Branch Short Circuit and Ground Fault protection for your motor, this is the function you would normally be coordinating with upstream devices.

This is why it would be nice if one lines were included with the OP showing the actual wiring scheme. I sort of doubt if he just has thermal for his motor but his verbal description implys that that what he has. Good old art. 430-52.

 

[timm333]

Here is the one line attached.