Selective Coordination

[necnotevenclose]

I have (2) questions this time.

1) I'm new to this selective coordination thing could anyone refer me to a good selective coordination program that is compatible with dumass 1.0?

2) I was looking at a previously engineered project and I notice that a panel was feeding another panel via circuit breaker, but then there was also an external fuseable disconnect. Thinking that this was excessive to have multiple disconnects I asked the enginner and he said the fuse disconnect was added because of the selective coordination? Is there any other way to accomplish selective coordination without using additional equipment?

 

[don_resqcapt19]

With breakers it is sometimes impossible to have selective coordination for short circuits and ground faults with breaker ratings of less than ~400 amps. This only happens where the available fault current is high and where the fault on the load side of the branch breaker flows enough current to get into the magnetic (instantaneous)[FONT=&quot] trip range of both the branch breaker and the feeder breaker(s). In this case both breakers will trip. Remember, if it was always easy to selctively coordinate breakers, Bussmann would not have submitted the proposals that resulted in 700.27 and 701.18 in the 2005 NEC.
Don
[/FONT]

 

[kingpb]

If it was a breaker feeding a sub-panel, the extra fuses may have been unncessary. I say maybe, because if the main panel feeding the sub-panel had some critical loads on it, you would want the sub-panel protection to go first. Without the fuses, depending on the main CB and sub-panel feeder CB ratings, it could have been a race as to which one tripped first. The fuses would become the primary protection in this case, the breaker becomes the back-up. As back-up, selective coordination usually goes out the window, you just want it to trip.

That's just one scenario, without more details it is only speculation.

In many cases where you have a branch breaker feeding another breaker with only cable in between, then selective coordination is a mute point. Who cares if the upstream breaker trips first. You can add 10 fused disconnects in between the breakers, and all you get is a race as to which one goes first. In that case, the fuses are added cost for nothing and potential for later problems, because unfortunately, fuses are quite often replaced with the wrong one's when they blow, simply because that's what they had at the time to get back on line. Then they are never replaced with the right one(s).

I'm sure no one here has ever done that, right!

 

[LarryFine]

[FONT=&quot]In this case both breakers will trip.[/FONT]

I've always been under the impression that it's very difficult to get cascaded breakers to trip simultaneously, even under bolted-fault conditions.

 

[don_resqcapt19]

Larry,

I've always been under the impression that it's very difficult to get cascaded breakers to trip simultaneously, even under bolted-fault conditions.

It is common. One breaker detects and starts to open the fault but while this breaker is trying to clear the fault, the other breakers in the circuit as still seeing the fault current and they may also "unlatch". Once the trip has "unlatched" the breaker is going to open. One breaker may very well have opened first, but while the first one was opening, the second has started to open.
Don

 

[ryan_618]

Larry, think about series rated systems. Quite common for both breakers to trip.

 

[jimioy]

coordination study

coordination study

I know this is a little off track but connected...I'm under the impression that when I get started with the coordination study, I will be able to use the info to get my facilities compliant and more importantly - safe! Am I right?

 

[W6SJK]

You can add 10 fused disconnects in between the breakers, and all you get is a race as to which one goes first. In that case, the fuses are added cost for nothing and potential for later problems, because unfortunately, fuses are quite often replaced with the wrong one's when they blow, simply because that's what they had at the time to get back on line. Then they are never replaced with the right one(s).

And most of those 30 fuses (10x3phase) would be weakened and no longer accurate.

 

[don_resqcapt19]

sparkiee001,

And most of those 30 fuses (10x3phase) would be weakened and no longer accurate.

How does current flow through the fuse "weaken" them?
Don

 

[kingpb]

I know this is a little off track but connected...I'm under the impression that when I get started with the coordination study, I will be able to use the info to get my facilities compliant and more importantly - safe! Am I right?

If you are referring to NFPA 70E, then yes the coordination study is required to be done before arc flash analysis can be performed.

Don't forget, the best safety program is where equipment is always worked de-energized!

 

[steve66]

I know this is a little off track but connected...I'm under the impression that when I get started with the coordination study, I will be able to use the info to get my facilities compliant and more importantly - safe! Am I right?

I'm not sure what you are trying to comply with. Selective Corrdination is only required is certain cases. For example, elevators, or emergency systems where it would be unsafe to have the power shut off. Another example might be an industrial process that could become dangerous to shut the power off on.

However, your typical run of the mill electrical system is not required to be coordinated.

 

[steve66]

If you are referring to NFPA 70E, then yes the coordination study is required to be done before arc flash analysis can be performed.

When you are doing an arc flash analysis, of course it helps to know which breaker is going to shut off the arc. But beyond that, I don't see any reason a coordination study is required?

If an upstream breaker unlatches at the same time the branch breaker is opening, I don't see that as affecting the arc flash.

Steve

 

[necnotevenclose]

Thanks to everyone for the replies.

 

[kingpb]

When you are doing an arc flash analysis, of course it helps to know which breaker is going to shut off the arc. But beyond that, I don't see any reason a coordination study is required?

If an upstream breaker unlatches at the same time the branch breaker is opening, I don't see that as affecting the arc flash. Steve

Arc flash analysis is based on current and time of the devices. You need to know the minimum and maximum fault currents. Therefore, to do it correctly, you have to model the system, perform short circuit study and find minimum and maximum fault current, set your protective devices (need time aspect), then run arc flash.

After arc flash, you may need to readjust the device settings if you are trying to limit arc flash to a specific level. Sort of an iterative process.

 

[W6SJK]

How does current flow through the fuse "weaken" them?
Don

Well, in the (GE?) seminar that I took on selective coordination many moons ago they mentioned that fuses may be taken to near their melting point by overloads but not blow. After that they are no longer accurate. Isn't that why you replace all three fuses in a disconnect even if only one blows?

 

[don_resqcapt19]

Steve,

If an upstream breaker unlatches at the same time the branch breaker is opening, I don't see that as affecting the arc flash.

That is dynamic interaction between the breakers and I know it plays a big part in series ratings. I expect that it also plays a part in the arc flash issue.
Don