Disconnect switch short circuit rating

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Grouch1980

Grouch1980

Senior Member
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New York, NY
Hi all,
So I have a disconnect switch rated 10,000 AIC with Class H fuses and 100,000 AIC with Class R, J, and T fuses.

My 2 questions are:
1. How does a switch short circuit rating change with the type of fuses installed inside it? doesn't the switch have copper bus / lugs inside that have a constant short circuit rating? how does the fuse type change the short circuit rating of the switch? I also heard this applies to meters as well.
2. This is probably just semantics, but wouldn't the switch has a SCCR rating, and the fuses have the AIC rating?

Thanks!
 
You are correct.

A non fusible switch has an SCCR, The fuses are the items interrupting the fault current and they have an AIC.
be able to withstand the fault current until the fuses have operated, However the components of the switch need to work in conjunction with the fuses so the fusible switch is said to have an AIC.

Note: the switch only has this rating if rejection style fuse clips have been installed.
 
Grouch1980 said:
Hi all,
So I have a disconnect switch rated 10,000 AIC with Class H fuses and 100,000 AIC with Class R, J, and T fuses.

My 2 questions are:
1. How does a switch short circuit rating change with the type of fuses installed inside it? doesn't the switch have copper bus / lugs inside that have a constant short circuit rating? how does the fuse type change the short circuit rating of the switch? I also heard this applies to meters as well.
2. This is probably just semantics, but wouldn't the switch has a SCCR rating, and the fuses have the AIC rating?

Thanks!
Click to expand...


1. It has to do with the time-current curve defined in the fuse class, as well as the AIC rating of the fuses themselves. The fuse needs to melt its element before the disconnect is damaged by the fault, when the combination is subject to a fault as high as the fault current in question. Unfused disconnects commonly have a default 10 kA SCCR rating, but upstream fuses may allow it to be used where available fault current is higher. Upstream breakers don't let you take credit for the same thing, unless it is a series-rated combination of devices.

2. Yes. The switch has an SCCR rating, and the fuse/breaker has an AIC rating. In any case, it is the maximum allowable available fault current for the device.
 
so the short circuit rating of a fused disconnect switch is similar to a circuit breaker panel... it's about the rating of the complete assembly. For example, if the panel is rated 65k SCCR, but the main breaker is only rated for 10k AIC, the rating of the complete assembly (panel and breaker) is only 10k. Is this analogy correct? If so, then the switch above in my example (without the fuses) is rated for at least 100,000 SCCR. Once you put the fuses inside, the complete assembly is now lowered to 10k with the Class H fuses, since the fuses have a max AIC of only 10k
 
The switch will have been TESTED AND LISTED with the specific fuses in series. The 10kA SCCR is basically the UNTESTED rating of the switch, because ostensibly you don't know what the SCPD is in that circuit. All disconnect switches are given a 10kA rating unless tested for higher.

For most non-fused switches, the SCCR is higher with the fuses ANYWHERE upstream of the switch, it doesn't actually have to be IN the switch itself, but you would have to be able to document it for an AHJ if asked. So the switch mfr will (should) have a "series listing" document available. This is where you have to be careful with cheap stuff from China purchased on-line, because when it's too cheap to be real, there is usually a reason.

Also in some cases, it can be series listed with breakers too but to get a higher SCCR (albeit not as high as with fuses), the switch mfr must TEST it with the breakers. So when it comes to series listings breakers, the switches are generally only going to be listed with breakers from the same mfr. It's about $50k per line item for the testing, so if Sq. D is going to list their switches with breakers, they are not going to pay another $50k to test them with Siemens breakers, then another $50 to test with Eaton etc. etc.
 
Carultch said:
1. It has to do with the time-current curve defined in the fuse class, as well as the AIC rating of the fuses themselves. The fuse needs to melt its element before the disconnect is damaged by the fault, when the combination is subject to a fault as high as the fault current in question. Unfused disconnects commonly have a default 10 kA SCCR rating, but upstream fuses may allow it to be used where available fault current is higher. Upstream breakers don't let you take credit for the same thing, unless it is a series-rated combination of devices.

2. Yes. The switch has an SCCR rating, and the fuse/breaker has an AIC rating. In any case, it is the maximum allowable available fault current for the device.
Click to expand...
This is where i don't follow... "The fuse needs to melt its element before the disconnect is damaged by the fault, when the combination is subject to a fault as high as the fault current in question"... so to me that says the disconnect has a constant SCCR rating. how does this rating go higher with a different type of fuse?
 
is there some kind of graph that can be uploaded to show this? Doing a google search on these topics is near impossible... its very hard to find explanations.
 
i'm getting it now... and Jraef mentioned it... it's a series rated combination when you have a fused disconnect switch. so it's information only given by the manufacturer. if you have no fuses inside the switch, then you have no series rating... now you're stuck with a 10ka switch... unless something upstream of it properly protects it from higher fault currents.
 
Which leads to another question... the series rated combination of the disconnect switch and internal fuses... do the fuses have to be current limiting or can they make it series rated without current limiting fuses?
 
Grouch1980 said:
Which leads to another question... the series rated combination of the disconnect switch and internal fuses... do the fuses have to be current limiting or can they make it series rated without current limiting fuses?
Click to expand...
It has to be tested with SPECIFIC fuses, and the tests are very expensive, so the manufacturers basically determine mathematically if a given fuse is going to work before they even try, and the ones that give them the mathematical certainty are going to be the current limiting type. A non-current limiting fuse MIGHT work, but for $50,000 per shot, why would they bother if they have one that they KNOW will work? The cost of the fuse is irrelevant to the switch mfr.
 
Grouch1980 said:
Which leads to another question... the series rated combination of the disconnect switch and internal fuses... do the fuses have to be current limiting or can they make it series rated without current limiting fuses?
Click to expand...
Most UL ratings actually require that current limiting fuse clips be installed in the switch to get the higher rating, s this prevents non-current limiting fuses from being installed.
 
and actually.... let's say for argument sake, you have a fused disconnect feeding a main lug only circuit breaker panel... since the fuses inside the disconnect switch are more than likely to be current limiting, now you're forced to also make sure those fuses in the switch are series rated with the downstream breaker panel, because of the dynamic impedance of the breakers. is that correct?
 
Grouch1980 said:
This is where i don't follow... "The fuse needs to melt its element before the disconnect is damaged by the fault, when the combination is subject to a fault as high as the fault current in question"... so to me that says the disconnect has a constant SCCR rating. how does this rating go higher with a different type of fuse?
Click to expand...

It is called a series rating. It means that the combination of products is tested to function together, and take credit for the KAIC rating of the fuses to improve the SCCR rating of the disconnect when in series with them. I would assume they produce a short current on the load side of the product, and inspect everything for possible damage to verify that only fuse elements were destroyed, and nothing else was damaged.

It is more of a matter of a lack of "co-testing", rather than any prediction of product failure, that gives the unfused disconnects a 10 kA rating when not used in combination with fuses. They've tested the disconnect in combination with certain classes of fuses, but not with all the various types of breakers. Subject the disconnect to a 11kA fault while protected by a 22 kaic breaker upstream, and it is a realistic chance that the breaker will trip before the disconnect really is catastrophically damaged. But there is no product testing to back it up, which is why it doesn't get a rating to substantiate such an installation.
 
Grouch1980 said:
and actually.... let's say for argument sake, you have a fused disconnect feeding a main lug only circuit breaker panel... since the fuses inside the disconnect switch are more than likely to be current limiting, now you're forced to also make sure those fuses in the switch are series rated with the downstream breaker panel, because of the dynamic impedance of the breakers. is that correct?
Click to expand...

The fuses can allow you to take credit for series ratings to possibly use 10 KAIC breakers despite the available fault current otherwise being 20 kA at the panel. You need to consider series rating of the brand-specific breaker, and the class of fuses, to know whether there is a series rating or not.

As for vice versa, a breaker doesn't reduce the KAIC rating of a fuse, in the event that the combination is not series rated. The 200 kA rating of a fuse will remain, no matter what breaker is downstream. When you use a not-series-rated combination, all that means is that the breakers have to survive the available fault current in their own right.

As an example, consider 10 kA QOB breakers from Square D, which are series-rated with class J fuses. Suppose the available fault current is 20 kA at the position of the panelboard, and 25kA at the position of the upstream fused disconnect. Available fault current diminishes with circuit length due to conductor impedance, so this is expected. You can use QOB breakers in series with the class J fuses, and take credit for the series rating. The fuses blow before the breakers get damaged, when there is a high current fault. Alternatively, you can use class RK5 fuses that are also 200kA rated, but not series-rated with the QOB breakers. If you did that, you'd have to upgrade the QOB breakers to QOB-VH breakers,so they are 22KAIC rated in their own right, to comply with the 20kA available fault current.
 
Carultch said:
As an example, consider 10 kA QOB breakers from Square D, which are series-rated with class J fuses. Suppose the available fault current is 20 kA at the position of the panelboard, and 25kA at the position of the upstream fused disconnect. Available fault current diminishes with circuit length due to conductor impedance, so this is expected. You can use QOB breakers in series with the class J fuses, and take credit for the series rating. The fuses blow before the breakers get damaged, when there is a high current fault. Alternatively, you can use class RK5 fuses that are also 200kA rated, but not series-rated with the QOB breakers. If you did that, you'd have to upgrade the QOB breakers to QOB-VH breakers,so they are 22KAIC rated in their own right, to comply with the 20kA available fault current.
Click to expand...
Thanks for the example!.... so if we use class RK5 fuses rated for 200kA inside the disc. switch, and the QOB-VH breakers rated for 22KAIC at the downstream panel, we're essentially using a fully rated system, since each component is fully rated for the available short circuit current on their line sides. BUT... is a fully rated system allowed in this case? If the fuses inside the disconnect switch most likely have to be current limited (in order to achieve the series rated combo of the switch and fuses)... wouldn't that force us to use a series rated combination between the disc. switch and downstream panel? so we would have no choice but to use the class J fuses and the 10kA QOB breakers, since they are series rated. Per other discussions i've been in, the downstream breakers have dynamic impedance, and having current limiting fuses on their line side will force a series rating listing between them.
 
Grouch1980 said:
Thanks for the example!.... so if we use class RK5 fuses rated for 200kA inside the disc. switch, and the QOB-VH breakers rated for 22KAIC at the downstream panel, we're essentially using a fully rated system, since each component is fully rated for the available short circuit current on their line sides. BUT... is a fully rated system allowed in this case? If the fuses inside the disconnect switch most likely have to be current limited (in order to achieve the series rated combo of the switch and fuses)... wouldn't that force us to use a series rated combination between the disc. switch and downstream panel? so we would have no choice but to use the class J fuses and the 10kA QOB breakers, since they are series rated. Per other discussions i've been in, the downstream breakers have dynamic impedance, and having current limiting fuses on their line side will force a series rating listing between them.
Click to expand...

A fully rated system is allowed as a possible solution to this example. I don't see why it wouldn't be.

You either have a choice of using a fully rated system, or a series rated system. Series-rated means you are depending on upstream devices to protect downstream devices, due to the time-current curve causing the upstream device to trip first when fault current exceeds the KAIC or SCCR rating of the downstream device. Fully rated means each device is capable in its own right of withstanding the available fault current at its position in the circuit, independently of what other OCPDs are installed elsewhere in the circuit.
 
Carultch said:
A fully rated system is allowed as a possible solution to this example. I don't see why it wouldn't be.
Click to expand...
From other threads i've been on, if you use current limiting fuses to protect a downstream breaker panel, that combo has to be series rated. dynamic impedance from the breakers will cause the CL fuses to behave differently from what you see on their let-thru charts, so you have no control or idea of how the system as a whole will behave... so that forces a series rating listing needed between the fuses and the downstream breaker panel, since it's something that the manufacturer can study. At least that's what i've read.
 
Grouch1980 said:
From other threads i've been on, if you use current limiting fuses to protect a downstream breaker panel, that combo has to be series rated. dynamic impedance from the breakers will cause the CL fuses to behave differently from what you see on their let-thru charts, so you have no control or idea of how the system as a whole will behave... so that forces a series rating listing needed between the fuses and the downstream breaker panel, since it's something that the manufacturer can study. At least that's what i've read.
Click to expand...

You’re a little off here if I understand what you’re saying. Series ratings basically let a downstream device take credit for an upstream device. Having a current limiting fuse upstream from a breaker does not mean you need to have it series rated with a downstream breaker, but it does if you want to take credit for the upstream CL fuse when looking at the downstream breaker’s AIC. The breaker’s “fully rated” AIC that it has by itself still applies even if upstream there’s a CL fuse.

Likewise, the AIC of the upstream CL fuse is not impacted by the dynamic impudence of a downstream fuse. However, and this is where you may have been thinking, the CL fuse may not trip as fast because of the dynamic impedance of the downstream breaker. That’s not a problem unless you were relying on the CL attribute of the fuse.
 
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