Relation between fuse SCCR & system actual voltage

[Mismail]

Hi all,

I have a simple question in mind. If I have a panel that has a main fused disconnect switch from Eaton-Bussmann rated for max voltage = 600VAC & max. fuse ampere=30 A.

The same switch can hold UL class CC fuse (fuse SCCR=200KA, 600 Vac) or IEC class gG fuse (fuse SCCR=120 KA, 400 Vac).

I have the following questions:
--------------------------------
(1) I understand that, in general, a fuse "rated voltage" like 240, 400, 480 & 600 mean "maximum possible" , i.e I can use 600 vac fuse in 240 vac system (waste of money i know, i just consider technically).. Is that correct ?

(2) In my panel example what is the fused switch "SCCR voltage" that I usually find in data sheets mean ? is that "maximum possible" or is it "should be applied at"?

(3) Does the fuse SCCR vary with applied voltage ? so if the applied voltage is 400 then class CC may have lower SCCR value than 200 KA (something like energy capacity and current increases if voltage decreases , like a circuit breaker case of interrupt current capacity varies with voltage)? I tried the data sheets of both the fuse & the switch and found no details about such a thing.

In short terms, Am i going to get 200KA SCCR protection for my panel if I used class CC fuse in a system that is run under 400 VAC ??

thank you all in advance

 

[Jraef]

Hi all,

I have a simple question in mind. If I have a panel that has a main fused disconnect switch from Eaton-Bussmann rated for max voltage = 600VAC & max. fuse ampere=30 A.

The same switch can hold UL class CC fuse (fuse SCCR=200KA, 600 Vac) or IEC class gG fuse (fuse SCCR=120 KA, 400 Vac).

I have the following questions:
--------------------------------
(1) I understand that, in general, a fuse "rated voltage" like 240, 400, 480 & 600 mean "maximum possible" , i.e I can use 600 vac fuse in 240 vac system (waste of money i know, i just consider technically).. Is that correct ?

Correct.

(2) In my panel example what is the fused switch "SCCR voltage" that I usually find in data sheets mean ? is that "maximum possible" or is it "should be applied at"?

The term "SCCR voltage" is just defining the maximum voltage at which the SCCR was tested at. At a given amount of prospective short circuit current, the amount of energy in a fault increases as voltage goes up. So if a device has an SCCR value listed at 400V, it has that SCCR at any voltage level below that, but may not have it above that voltage.

(3) Does the fuse SCCR vary with applied voltage ? so if the applied voltage is 400 then class CC may have lower SCCR value than 200 KA (something like energy capacity and current increases if voltage decreases , like a circuit breaker case of interrupt current capacity varies with voltage)? I tried the data sheets of both the fuse & the switch and found no details about such a thing.

Not exactly sure what you are saying there, but see above.

In short terms, Am i going to get 200KA SCCR protection for my panel if I used class CC fuse in a system that is run under 400 VAC ??

Different subject. You don't "get" a 200kA SCCR rating on a panel just by using fuses capable of interrupting 200kA (200kAIC), they are different concepts. For an SCCR rating on your panel, ALL components in the power stream must be evaluated in series with each other. The use of Current Limiting fuses can aid in attaining a higher level of SCCR when components down stream are only listed for lower levels, because you can consider the "let through" current of the fuses at the Prospective (Available) Fault Current. But just putting the fuses in does not get you there, the devices must be TESTED in series. Usually this is done by the manufacturers of the components. So for example an Overload Relay (a typically problematic component) may have a stand-alone SCCR of only 5kA, but the manufacturer may have a Series Listing of that OL relay, attached to their Contactor, protected by a Class J fuse, of 65kA, based on series tested combinations saying that the let-through current of the Class J CL fuse is low enough that the 5kA rated OL relay, when attached to the contactor, will not become shrapnel.

 

[Mismail]

Correct.


The term "SCCR voltage" is just defining the maximum voltage at which the SCCR was tested at. At a given amount of prospective short circuit current, the amount of energy in a fault increases as voltage goes up. So if a device has an SCCR value listed at 400V, it has that SCCR at any voltage level below that, but may not have it above that voltage.


Not exactly sure what you are saying there, but see above.


Different subject. You don't "get" a 200kA SCCR rating on a panel just by using fuses capable of interrupting 200kA (200kAIC), they are different concepts. For an SCCR rating on your panel, ALL components in the power stream must be evaluated in series with each other. The use of Current Limiting fuses can aid in attaining a higher level of SCCR when components down stream are only listed for lower levels, because you can consider the "let through" current of the fuses at the Prospective (Available) Fault Current. But just putting the fuses in does not get you there, the devices must be TESTED in series. Usually this is done by the manufacturers of the components. So for example an Overload Relay (a typically problematic component) may have a stand-alone SCCR of only 5kA, but the manufacturer may have a Series Listing of that OL relay, attached to their Contactor, protected by a Class J fuse, of 65kA, based on series tested combinations saying that the let-through current of the Class J CL fuse is low enough that the 5kA rated OL relay, when attached to the contactor, will not become shrapnel.

Well, first of all thank you for you fast reply, Jraef. You always offer help & answers in more than one thread I submit.

(2) Thank you for your helpful & direct answer about the meaning of the "SCCR voltage".

(3) yes i know there is a lot of things behind SCCR estimation. UL-508 has "sweep 1" & "sweep 2" method. Bussmann has a very very helpful paper with example about how to calculate SCCR of panel. It is exactly as you said. In my question I just had some misunderstanding towards SCCR voltage so I thought that the fuse interrupt capacity might vary if used at lower voltage than rated one, hence I was focusing on the effect of voltage on fuse interrupt capacity.

Thank you again for your answer.