Do I make an assumption or call the power company?

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Tainted

Tainted

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New York
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Engineer (PE)
There is a switchboard that has all the fuses in it without an interrupting rating and they are feeding loads. The fuses are inside switchboard that has 200kA SCCR switches.

There are spare 400A fuses without interrupting ratings inside a 200kA SCCR, 400A switch that I want to use to add a panelboard. This 400A is basically a service switch.

From my understanding, if the fuse kAIC is not labelled, then it must be assumed that the fuse is rated at 10kAIC. But it seems weird, why would there be 10kAIC fuses in a 200k SCCR switchboard?

If I was to utilize the 400AS/400AF to feed a new panelboard, do I need a 10kA SCCR panel? Or do I call utility to make sure the available fault current is not more than 10kA? what would you guys do?

If the available fault current is more than 10kA, does this mean I would have to replace every fuse on that switchboard?

Here's the fuse with no kAIC rating:
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Russs57 said:
Look at the fuse and see if it has a CSA standard/number on it. That may give you a clue.
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I was browsing through pictures and noticed a CSA number on a 600A fuse but not the 400A because it wasn't too clear. If I have the number, what do I do with it? it's my first time hearing about CSA.

Lets say the number is 11573 for a 600A fuse
 
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You are going to spend an inordinate amount of time trying to find interrupt ratings on fuses from a defunct company.
These are the functional equivalent of Bussman NON Class K5 fuses, which would be rated for 10kA at that size range. So no way they are rated for 200kA no matter what. I would just inform the end user that the ratings of the fuses cannot be verified, making them unusable and replace them with new Class RK1 fuses.
 
Jraef said:
You are going to spend an inordinate amount of time trying to find interrupt ratings on fuses from a defunct company.
These are the functional equivalent of Bussman NON Class K5 fuses, which would be rated for 10kA at that size range. So no way they are rated for 200kA no matter what. I would just inform the end user that the ratings of the fuses cannot be verified, making them unusable and replace them with new Class RK1 fuses.
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How did you know they are equivalent to Bussman NON Class K5 fuses? and why would these fuses be in a 200kA SCCR switchboard? Wouldn't you want to put 200kAIC instead of 10kAIC?
 
Those fuses are so old, they date from a period before our current SCCR practices and procedures.

This is a problem of replacement fuse, where people buy the cheapest ones they can find, in the correct amp rating, rather than considering all if the fuse characteristics. Many people specify, and some SCCR require, rejection style fuse clips.
 
Tainted said:
and why would these fuses be in a 200kA SCCR switchboard? Wouldn't you want to put 200kAIC instead of 10kAIC?
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Rarely does the AIC of an OCPD match the SCCR of a panel or switch board. OCPD would generally be sized to the available fault current. For example Siemens P series panelboards generally have a SCCR of 200k, but are often filled with 10k breakers.
 
I would say that anything you are adding to would need to be up to snuf. Any feeders you are not touching, I probably wouldn't worry about. You could inform the customer that their equipment may be overdutied and see if they want to pay to have it investigated and repaired if necessary.
 
electrofelon said:
Rarely does the AIC of an OCPD match the SCCR of a panel or switch board. OCPD would generally be sized to the available fault current. For example Siemens P series panelboards generally have a SCCR of 200k, but are often filled with 10k breakers.
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So let's say the available fault current is 150k but you just want to install a panelboard with main circuit breaker that is 22kAIC. Current limiting fuses can't really cover for that due to dynamic impedance from arc. So what do I do? would the fuse and circuit breaker have to be rated for series combination in order to use a circuit breaker kAIC rating lower than the available fault current?
 
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If the fuse blade has a rejection feature, it's current-limiting. Otherwise I agree with jraef that this is an old "NEC" type fuse - non-current limiting.

If this disconnect switch is labeled for 200 kA, it should not be possible to install a non-current limiting fuse due to the rejection feature requirement. Unless the fuse holder has been tampered with.

Maybe include a photo of the switch nameplate.
 
David Castor said:
If the fuse blade has a rejection feature, it's current-limiting. Otherwise I agree with jraef that this is an old "NEC" type fuse - non-current limiting.

If this disconnect switch is labeled for 200 kA, it should not be possible to install a non-current limiting fuse due to the rejection feature requirement. Unless the fuse holder has been tampered with.

Maybe include a photo of the switch nameplate.
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Its an old switchboard, I don’t think it rejects anything lol
 
To answer the question directly, call the utility. If I were a betting man, I'd guess the fault current is >10kA, but call.
 
Tainted said:
So let's say the available fault current is 150k but you just want to install a panelboard with main circuit breaker that is 22kAIC. Current limiting fuses can't really cover for that due to dynamic impedance from arc. So what do I do? would the fuse and circuit breaker have to be rated for series combination in order to use a circuit breaker kAIC rating lower than the available fault current?
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Yes exactly. Series rated or fully rated, those are you two choices.
 
Tainted said:
How did you know they are equivalent to Bussman NON Class K5 fuses? and why would these fuses be in a 200kA SCCR switchboard? Wouldn't you want to put 200kAIC instead of 10kAIC?
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I used an old cross-reference chart I found that still listed the Cefco fuses and what they crossed over to in Bussman. Most fuse mfrs make these lists, but Cefco is now so old they have dropped off of them. I just happened to have found an old one from when Cefco fuses were still a viable option. You can still buy Cefco fuses from junk dealers, but they are all "NOS" (New Old Stock) at best, used at worst.

Is the switchboard the same age as the fuses? If so, the switchboard might have had a 200kA withstand rating, which was the precursor to what we now call the "SCCR" (Short Circuit Current Rating). All that meant at that time was that the bus bracing was built to be able to handle the mechanical stresses of a 200kA fault taking place. The INTERRUPT rating would be based on what fuse was used. In this case, with those fuses, it could only be used where there was 10kA or less. LATER ON in the world timeline, the switchboard and fuse holder mfrs started making the rejection features so that you could NOT replace an HRC fuse with a cheap junk fuse like this. But a long time ago, you could (which is why they had to come up with a fix).

If it is a newer switchboard and marked as 200kA SCCR, then someone would have had to have rigged up a way to defeat the rejection clips to use these cheap fuses.
 
Jraef said:
I used an old cross-reference chart I found that still listed the Cefco fuses and what they crossed over to in Bussman. Most fuse mfrs make these lists, but Cefco is now so old they have dropped off of them. I just happened to have found an old one from when Cefco fuses were still a viable option. You can still buy Cefco fuses from junk dealers, but they are all "NOS" (New Old Stock) at best, used at worst.

Is the switchboard the same age as the fuses? If so, the switchboard might have had a 200kA withstand rating, which was the precursor to what we now call the "SCCR" (Short Circuit Current Rating). All that meant at that time was that the bus bracing was built to be able to handle the mechanical stresses of a 200kA fault taking place. The INTERRUPT rating would be based on what fuse was used. In this case, with those fuses, it could only be used where there was 10kA or less. LATER ON in the world timeline, the switchboard and fuse holder mfrs started making the rejection features so that you could NOT replace an HRC fuse with a cheap junk fuse like this. But a long time ago, you could (which is why they had to come up with a fix).

If it is a newer switchboard and marked as 200kA SCCR, then someone would have had to have rigged up a way to defeat the rejection clips to use these cheap fuses.
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Hmmm, I’m having second thoughts about this 200kA rating. Yea it’s an old switchboard, probably same age as the fuse. The disconnect switches are vacu break switches. They look like this:
1642830662075.jpeg
 
gadfly56 said:
To answer the question directly, call the utility. If I were a betting man, I'd guess the fault current is >10kA, but call.
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Agreed, first thing you need to do is figure out what the max available fault is from the utility. Then youll be able to tell if the fuses were overrated for what needs to be there or not. Ive worked on a building where most all main gear was called out for 65KAIC, when it came time to add new breaker to MDP first thought was to match other ratings. Utility confirmed max available fault was only ~17000A.
 
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