Using general purpose relays downstream to current limiting fuses

[petersonra]

It’s the difference of a coil failing and taking out some PB and/or overload contacts along with it or no damage.

Again I would point out that control circuits rarely have all that much short-circuit current available. So it doesn't really matter whether you use a current limiting fuse or not because even a dead short probably won't get into the current limiting range of the fuse.

 

[ptonsparky]

Again I would point out that control circuits rarely have all that much short-circuit current available. So it doesn't really matter whether you use a current limiting fuse or not because even a dead short probably won't get into the current limiting range of the fuse.

Damage vs No Damage is my observation. Scientific? No.
Granted, I do not have four parallel 500s going directly to a 5 amp class CC fuse then on to a shorted coil via standard PB contacts.

 

[petersonra]

Damage vs No Damage is my observation. Scientific? No.
Granted, I do not have four parallel 500s going directly to a 5 amp class CC fuse then on to a shorted coil via standard PB contacts.

There would be no difference in damage because a dead short or an overload would cause the same amount of current flow without regard to what kind of fuse you have. They're just is not enough available short-circuit current in most control circuits for a current loading fuse to operate in the current limiting range.

Push button contacts are only designed for pilot duty anyway so it's not like they are particularly robust to begin with.

 

[Jraef]

Control circuits are not part of the SCCR requirement, only power.

 

[ptonsparky]

Control circuits are not part of the SCCR requirement, only power.

So the OP problem should not be a concern?

 

[petersonra]

So the OP problem should not be a concern?

I am not exactly sure what the OP's concern actually is.

 

[lielec11]

I think the let-through charts apply to everything, including buses and the contactor you're referencing.

John, I am in this camp as well. Why can't the let-thru tables be used to indicate the lower fault current when these fuses are installed? For instance upstream of a switchboard?

In New York City we see this all the time. Con Edison provides 200kAIC at various locations within their network, but they do not make a switchboard higher than 200kAIC. If you use software to perform a short circuit analysis, including the impedance (X/R), you'll likely get values over 200k. So what are you supposed to do in that situation? Do you tell your client there is nothing they can do? In reality, when there are CL fuses installed, the fault current will not be the 200kAIC they claim.

 

[petersonra]

John, I am in this camp as well. Why can't the let-thru tables be used to indicate the lower fault current when these fuses are installed? For instance upstream of a switchboard?

In New York City we see this all the time. Con Edison provides 200kAIC at various locations within their network, but they do not make a switchboard higher than 200kAIC. If you use software to perform a short circuit analysis, including the impedance (X/R), you'll likely get values over 200k. So what are you supposed to do in that situation? Do you tell your client there is nothing they can do? In reality, when there are CL fuses installed, the fault current will not be the 200kAIC they claim.

This is one of those cases you could ask for special permission to have a PE sign off on some kind of CL solution because there may not be a practical alternative.

However, it is not something just anyone can do, and IMO requires special permission.

OTOH, it is also possible that the switchboard might already be series rated for use with CL fuses. In which case no special permission would be required.

Incidentally, are there any fuses UL listed above 200 kAIC?

 

[lielec11]

This is one of those cases you could ask for special permission to have a PE sign off on some kind of CL solution because there may not be a practical alternative.

However, it is not something just anyone can do, and IMO requires special permission.

OTOH, it is also possible that the switchboard might already be series rated for use with CL fuses. In which case no special permission would be required.

Incidentally, are there any fuses UL listed above 200 kAIC?

Aren't Bussmann KRP-Cs rated for 300k?

As a PE I don't think I'd be willing to sign off on it by itself. I would likely rather list both the maximum theoretical value based on the 200k, and have a separate chart when using the CL value based on the tables. However, this doesn't seem to be normal so maybe I'm crazy?

 

[Jraef]

Hello, it may be a dumb question but can I use general purpose relays downstream to a current limiting fuse? The reason I need the fuse is that I have 50kA available fault on 480V line which is used to control a 480V coil. Short circuit withstand rating for the general purpose relay contactor used to energize the coil is 5kA and fuse RMS let through at 50kA seems to be less than 2000A.

Fuse curve (30A)
Relay

Am I missing something? I expected this to be more difficult.
Thanks

So, remember waaaay back when I said you could use the MANUFACTURER'S LISTING in series combination if there is one?
Turns out that contactor IS LISTED AT 50kA SCCR if protected by Class J fuses.

Page 161 of this document.

https://library.industrialsolutions.abb.com/publibrary/checkout/DEC1005-EN?TNR=Catalogs%20and%20Buyers%20Guides%7CDEC1005-EN%7CPDF&filename=Control%20ad%20Power%20Electronics%20Catalogue-English-Ed0719-DEC1005.pdf

Note: the confusing issue was calling this an "ABB" contactor. It was a GE contactor, then ABB bought GE and discontinued it. But that makes no difference in the UL listing of it.

MOST legitimate power component manufacturers have listed their devices for higher SCCR in Series Combination with fuses and in the case of those who also sell circuit breakers, in series with their own breakers too. You just have to try harder to find the information.

 

[petersonra]

Aren't Bussmann KRP-Cs rated for 300k?

As a PE I don't think I'd be willing to sign off on it by itself. I would likely rather list both the maximum theoretical value based on the 200k, and have a separate chart when using the CL value based on the tables. However, this doesn't seem to be normal so maybe I'm crazy?

I think they are actually self-certified above 200,000 AIC. But I might be in error. I looked into this once a long time ago and that's what I found but things might have changed since then or I might have remembered wrong.