current-limiting fuse vs. short circuit calcs

[lielec11]

I remember once reading something in an IEEE reference book that stated they do not recommend using current-limited fault fault current values in a short circuit study. Can someone refresh my memory on where I might have read this?

I have a project where the RMS symmetrical fault will cause certain equipment to be underbraced, however, it is protected by a current limited fuse and therefor it will not see this RMS value, rather an effective values based on the current limiting fuse chart.

 

[ron]

Due to dynamic impedance you can't count on the current limiting nature of such devices in many situations. That's when series rated tested combinations came about.
See this linked article

StackPath

 

[lielec11]

So then why is all this data published regarding current limiting fuses and how they can help limit let-thru current if they can't be trusted? There is math that backs it up, and with dynamic impedance aren't the fault values multiplied using a factor based on the actual vs. text X/R ratio?

I guess what I really would like to know is where this is specifically referenced in an IEEE standard (or elsewhere), because it's not the first time I've come across this.

 

[charlie b]

I see that Ron posted while I was typing. But I will carry on anyway.

It may not be as simple as you could wish. Much depends on whether the equipment has any overcurrent protection devices of its own. If for example you are talking about a controller that has its own fuses, then the reaction time of those fuses can interfere with the reaction time of the upstream current-limiting fuses, so that they won't be able to stop current from exceeding their current-limiting value. That is the "dynamic impedance" that Ron mentioned.

 

[bwat]

To add to what others already said (I apparently don't type very fast):

I know you're asking for IEEE, so this could be hogwash, but for UL508 the current limiting nature of upstream devices in the panel can be used for passive devices like a PDB if I'm not mistaken, but if the device has a OCPD itself, then the current limiting up stream part is irrelevant unless it's a series rated tested combination. I'm not sure, but it seems reasonable that bracing might fall into the category of passive and can rely on an upstream CL fuse.

 

[lielec11]

In my case I'm referencing bracing for a 5kV switch on the primary side of a step-down substation. It's braced for 40kA but there's 50k available.

 

[ron]

So then why is all this data published regarding current limiting fuses and how they can help limit let-thru current if they can't be trusted? There is math that backs it up, and with dynamic impedance aren't the fault values multiplied using a factor based on the actual vs. text X/R ratio?

I guess what I really would like to know is where this is specifically referenced in an IEEE standard (or elsewhere), because it's not the first time I've come across this.

Fuse manufacturers want you to buy fuses, and there is a small limited opportunity to use the curves for UL508, NEC 240.86(A) and for other manufactures trying to establish series combination ratings.

IEEE Standard 1015-1997 (Blue Book), Applying Low-Voltage Circuit Breakers Used in Industrial and Commercial Power Systems. Paragraph 4.4.5.2(d) states:

In cases where increases in available short-circuit current necessitate a system upgrade, a second approach, shown in Figure 4-4, may be used for retrofitting existing older systems where a recognized series rating is not available. A line-side current-limiting circuit breaker or fuse, which limits peak current and let-through energy, may be added only if the existing load-side breakers do not exhibit dynamic impedance within the first half cycle. The distribution of short-circuit energy is shifted away from the slower, load-side circuit breaker to the higher speed current-limiting device. The downstream circuit breaker is then subject to no more short-circuit energy than its rating. The manufacturer of the existing circuit breakers must be contacted to verify that they do not exhibit dynamic impedance.

 

[ron]

In my case I'm referencing bracing for a 5kV switch on the primary side of a step-down substation. It's braced for 40kA but there's 50k available.

So you want to put something upstream of the primary 5kV switch? It might be cheaper just to replace the 5kV switch with a fused version that has a higher SCCR. Or is it fused already and the fuses only have a 40kA rating?

 

[lielec11]

So you want to put something upstream of the primary 5kV switch? It might be cheaper just to replace the 5kV switch with a fused version that has a higher SCCR.

The primary switch is part of a substation, it isn't a free standing piece of equipment. There already is a 5kV CL fuse upstream of this device that's why I'm trying to figure out my options before I tell my client I think they need to increase the AIC rating of the primary side of the substation. I'm trying to exhaust all options and I want to make sure if I tell them something I have backup as to why it doesn't work.

 

[ron]

The primary switch is part of a substation, it isn't a free standing piece of equipment. There already is a 5kV CL fuse upstream of this device that's why I'm trying to figure out my options before I tell my client I think they need to increase the AIC rating of the primary side of the substation. I'm trying to exhaust all options and I want to make sure if I tell them something I have backup as to why it doesn't work.

Sometimes it is worth checking the short circuit calculations to be sure there really is 50kA available where they say it is (not considering the up-over-down BS), before you decide that the equipment is over-dutied.

 

[synchro]

I remember once reading something in an IEEE reference book that stated they do not recommend using current-limited fault fault current values in a short circuit study. Can someone refresh my memory on where I might have read this?

Possibly from this and its link to a page of the IEEE 242-1986 Buff book:

SKM Software Help Desk - Current Limiting Protective Devices and its effects on Fault Calculations - Knowledge base

SKM Software Help Desk Knowledge base - Current Limiting Protective Devices and its effects on Fault Calculations - [u][b]Question:[/b][/u][b]... :help desk software by Jitbit

support.skm.com

... I have a project where the RMS symmetrical fault will cause certain equipment to be underbraced, however, it is protected by a current limited fuse and therefor it will not see this RMS value, rather an effective values based on the current limiting fuse chart.

...In my case I'm referencing bracing for a 5kV switch on the primary side of a step-down substation. It's braced for 40kA but there's 50k available.

From page 155 (pg. 167 in pdf file) of IEEE Std 242-2001:

"5.8.1 Bus-bracing requirements
Reduced bus-bracing requirements may be attained when current-limiting fuses are used.
Figure 5-19 shows an 800 A motor-control center being protected by 800 A Class L fuses.
The maximum available fault current to the motor-control center is 40 000 A rms symmetrical.
If a noncurrent-limiting device were used ahead of the motor-control center, the bracing
requirement would be a minimum of 40 000 A rms symmetrical, with a peak value of
92 000 A (2.3×40 000 A). For this example with current-limiting fuses, the maximum peak
current has been reduced from 92 kA to 38 kA with a corresponding reduction in effective
rms available current from 40 kA to 16.5 kA. As a result, bus bracing of 16.5 kA or greater is
possible rather than requiring the full 40 kA bracing. Most bus is now listed for maximum
short-circuit currents with specific types and sizes of current-limiting fuses. The bus manufacturer
should be consulted for these specific combination ratings."

A copy of IEEE Std 242-2001 is at:
[Moderator's note: removed link to copyrighted material.]

 

[lielec11]

Possibly from this and its link to a page of the IEEE 242-1986 Buff book:

SKM Software Help Desk - Current Limiting Protective Devices and its effects on Fault Calculations - Knowledge base

SKM Software Help Desk Knowledge base - Current Limiting Protective Devices and its effects on Fault Calculations - [u][b]Question:[/b][/u][b]... :help desk software by Jitbit

support.skm.com

From page 155 (pg. 167 in pdf file) of IEEE Std 242-2001:

"5.8.1 Bus-bracing requirements
Reduced bus-bracing requirements may be attained when current-limiting fuses are used.
Figure 5-19 shows an 800 A motor-control center being protected by 800 A Class L fuses.
The maximum available fault current to the motor-control center is 40 000 A rms symmetrical.
If a noncurrent-limiting device were used ahead of the motor-control center, the bracing
requirement would be a minimum of 40 000 A rms symmetrical, with a peak value of
92 000 A (2.3×40 000 A). For this example with current-limiting fuses, the maximum peak
current has been reduced from 92 kA to 38 kA with a corresponding reduction in effective
rms available current from 40 kA to 16.5 kA. As a result, bus bracing of 16.5 kA or greater is
possible rather than requiring the full 40 kA bracing. Most bus is now listed for maximum
short-circuit currents with specific types and sizes of current-limiting fuses. The bus manufacturer
should be consulted for these specific combination ratings."

A copy of IEEE Std 242-2001 is at:
[Moderator's note: removed link to copyrighted material.]

That's it (from 1986)! I just went through my copy of the 2001 and was surprised to find a similar statement does not exist. I did come across the paragraph you quoted from 5.8.1 but that doesn't seem very definitive.

 

[topgone]

Sometimes it is worth checking the short circuit calculations to be sure there really is 50kA available where they say it is (not considering the up-over-down BS), before you decide that the equipment is over-dutied.

To add, a few feet of conductors could drop down the 50kA down to 40 kA! Been there, done that, got the t-shirt thing!

 

[lielec11]

To add, a few feet of conductors could drop down the 50kA down to 40 kA! Been there, done that, got the t-shirt thing!

I hear you but I can't add feeder that isn't there for the sake of fault current. I have to work with what's in the field and/or what the contractor is installing.

 

[Grouch1980]

I see that Ron posted while I was typing. But I will carry on anyway.

It may not be as simple as you could wish. Much depends on whether the equipment has any overcurrent protection devices of its own. If for example you are talking about a controller that has its own fuses, then the reaction time of those fuses can interfere with the reaction time of the upstream current-limiting fuses, so that they won't be able to stop current from exceeding their current-limiting value. That is the "dynamic impedance" that Ron mentioned.

Hi Charlie... I was under the impression, via another thread, that fuses didn't have dynamic impedance. So the fuses in the controller wouldn't interfere with the upstream current limiting fuses. Unless you had circuit breakers in the controller, then those breakers would introduce dynamic impedance. So fuses also have dynamic impedance then? and can affect the upstream CL fuses?