Is there any good reason at all to provide fused disconnects?

[ptonsparky]

Yeah that is interesting. There is no NEC rule that NF switches be protected by an OCPD at their rating, but it seems some manufacturers are saying that.

Sq D says you can't use a NF switch without an OCPD ahead of it, such as if using as a meter disconnect switch on a service.

I wish these manufacturers would stick to making things and stop trying to get into the code business.

Mostly it's CYA. They've most likely been sued enough that the lawyers on staff just as well be writing exceptions to everything.

 

[kwired]

Good answer.
There are however a select few NF disconnects that have been tested in series with SPECIFC breakers, but they are always from companies that make BOTH items themselves (as opposed to brand-labeling), and that is becoming more rare. So for example Siemens knife switch type HD rated non-fused disconnects are series listed with specific Siemens circuit breakers at slightly higher than 10kA (14-25kA depending on size). The same is true for Eaton (table 28.1-46 of this link), but NOT for Square D. But even then, these are typically ONLY the knife switch type disconnects, never the inexpensive rotary type (IEC style) disconnects that people like to use now because of price. Those are never rated for more than 10kA SCCR if there is just a circuit breaker ahead of them, they are ONLY going to be listed at more than 10kA if there are specific fuses ahead of them.

Now, IF you can show that the available fault current is 10kA or less at the terminals of the disconnect switch, then it's fine. But in most industrial and large commercial installations, that's a difficult number to get to.

Pretty certain Square D is, at least the heavy duty switches anyway, general duty maybe not. I mostly use the heavy duty, partly because most I use are for 480 volts and they don't make general duty for over 250 volts.

 

[electrofelon]

Mostly it's CYA. They've most likely been sued enough that the lawyers on staff just as well be writing exceptions to everything.

 

[Carultch]

Good answer.
There are however a select few NF disconnects that have been tested in series with SPECIFC breakers, but they are always from companies that make BOTH items themselves (as opposed to brand-labeling), and that is becoming more rare. So for example Siemens knife switch type HD rated non-fused disconnects are series listed with specific Siemens circuit breakers at slightly higher than 10kA (14-25kA depending on size). The same is true for Eaton (table 28.1-46 of this link), but NOT for Square D. But even then, these are typically ONLY the knife switch type disconnects, never the inexpensive rotary type (IEC style) disconnects that people like to use now because of price. Those are never rated for more than 10kA SCCR if there is just a circuit breaker ahead of them, they are ONLY going to be listed at more than 10kA if there are specific fuses ahead of them.

Now, IF you can show that the available fault current is 10kA or less at the terminals of the disconnect switch, then it's fine. But in most industrial and large commercial installations, that's a difficult number to get to.

Good response. I'm glad to see you know about some examples, where the breaker can series-rate with an unfused disconnect.

As for your second paragraph, one place you might see this, is at a disconnect you place immediately adjacent to the load device, after the branch circuit has traveled a significant distance from the service equipment. Like a maintenance disconnect that you co-locate with an HVAC unit. You might have 30kA of fault current at the main distribution panelboard, but with the impedance of the branch circuit length, it may diminish to 8kA, and the unfused disconnect is perfectly acceptable despite no upstream fusing.

 

[Jraef]

Pretty certain Square D is, at least the heavy duty switches anyway, general duty maybe not. I mostly use the heavy duty, partly because most I use are for 480 volts and they don't make general duty for over 250 volts.

I thought so to, but Schneider has an FAQ site that addressed this specifically and it said they do not have series listings on their NF disconnect switches. I was surprised by that to be honest.

Edit:
I just looked at it again to post a link, and I see that it says "general duty", so you are right.

What is the short circuit current rating (SCCR) of non-fused general duty safety switches when used downstream from circuit breakers? | Schneider Electric USA

www.se.com

I looked for specifics on the HD switches on the FAQ site and it says that IF there is a series rating on a specific switch with a specific breaker, it is included in the drawings for that switch. I didn't take the time to look them up though.

What is the Short Circuit Current Rating (SCCR) of Heavy Duty Safety Switches? | Schneider Electric USA

www.se.com

 

[jim dungar]

I thought so to, but Schneider has an FAQ site that addressed this specifically and it said they do not have series listings on their NF disconnect switches. I was surprised by that to be honest.

Then you asked the wrong question of them. Their NF disconnects have been rated in combination with some of their breakers for more than 40 years. Maybe the issue is the specific phrase 'series rating'.

Their on-line Digest says "The UL Listed short circuit current rating for the Square D non-fusible switches is based upon the switch being used in conjuction with fuses or Square D circuit breakers or Mag-Gard motor circuit protectors"

 

[electrofelon]

I thought so to, but Schneider has an FAQ site that addressed this specifically and it said they do not have series listings on their NF disconnect switches. I was surprised by that to be honest.

Edit:
I just looked at it again to post a link, and I see that it says "general duty", so you are right.

What is the short circuit current rating (SCCR) of non-fused general duty safety switches when used downstream from circuit breakers? | Schneider Electric USA

www.se.com

I looked for specifics on the HD switches on the FAQ site and it says that IF there is a series rating on a specific switch with a specific breaker, it is included in the drawings for that switch. I didn't take the time to look them up though.

What is the Short Circuit Current Rating (SCCR) of Heavy Duty Safety Switches? | Schneider Electric USA

www.se.com

I think that first link is the one I have complained about before. It makes no sense. It says:

Resolution:
First, a Non-Fused Safety Switch by itself does NOT provide any short circuit current rating because it does not contain any overcurrent protection.

First, I don't like the word "provide". Then something has a SCCR, I don't see what an OCPD has to do with anything (except of course maybe raising the SCCR).

 

[Jraef]

...
First, I don't like the word "provide". Then something has a SCCR, I don't see what an OCPD has to do with anything (except of course maybe raising the SCCR).

I agree. They are conflating SCCR with Interrupting Capacity. SCCR is what we used to call "withstand rating", in which case the OCPD is involved, but not the only aspect.

Reminds me of the joke about the definition of "involvement" vs "commitment".
In your bacon and eggs breakfast, the chicken was involved, the pig was committed.

 

[Flicker Index]

I'm wondering if I have a mechanical equipment such as an outdoor condenser or maybe a chiller, is there really any purpose or anything gained in providing a local fused disconnect?

I mean the breaker at the panelboard is protecting the unit and the cable so what's the point? Why not make all disconnect switches unfused and get rid of fuses forever like they are shopping malls?

Thoughts?

Thanks

See https://www.eaton.com/content/dam/e...-center/bus-ele-tech-lib-high-speed-fuses.pdf
Circuit breakers have replaced fuses in most places but there are still characteristics of fuses that can not be matched with breakers. Fuses can be specified (usually by the equipment manufacturer) that can reduce or prevent damage to solid state motor drives.

Another reason to use a fused disconnect is where the available short circuit current is greater than 10kA since non-fused disconnects only have a 10kA rating. With a fused disconnect, depending on the fuses, the rating can be as high as 200kA.

Remember that equipment installed where fault current exceeds the equipment's AIC or SCCR is a violation of the NEC and OSHA.

If you're putting fuses for the reason of exceeding AIC, why would you put it at the load? It wouldn't provide any protection for something that may happen along the run between the equipment and the panel.

 

[wbdvt]

If you're putting fuses for the reason of exceeding AIC, why would you put it at the load? It wouldn't provide any protection for something that may happen along the run between the equipment and the panel.

Not entirely sure what your meaning is. The fuses in fused disconnect increase the AIC rating of that disconnect wherever that disconnect is used. Protection is provided by the upstream OCPD but that does not affect the AIC rating of the disconnect switch.

 

[Flicker Index]

What I mean is that if the wiring in between the disconnect and circuit breaker is damaged somehow, such as a screw driven into it, then there's no guarantee that OCPD can clear the fault safely without blowing up if the available fault current exceeds the breaker's rating, but this will like almost never be the case with 120 or 240 due to the added impedance imposed by the wires themselves.

 

[kwired]

What I mean is that if the wiring in between the disconnect and circuit breaker is damaged somehow, such as a screw driven into it, then there's no guarantee that OCPD can clear the fault safely without blowing up if the available fault current exceeds the breaker's rating, but this will like almost never be the case with 120 or 240 due to the added impedance imposed by the wires themselves.

The upstream device needs to have rating to handle whatever fault current is available at that device. Yes impedance because of circuit length will reduce what fault current is at that point where the screw is driven in vs what might be available if fault occurred at/very near the OCPD.

A current limiting fuse doesn't change available fault current at line side of fuse, nor does it change the interrupt rating of switch or withstand rating of other components. It can limit how much current might flow if there is a fault on the load side.

Components that are series tested and listed are deemed safe to use together within guidelines of the testing/listing.

If you have breaker supplying a non fused switch and switch is only listed for 10kA, a lot of times you will not have over 10kA available at that switch location simply because of circuit impedance. If really short distance maybe there is more to be concerned about. But a lot of time when you do have high level available you have higher current distribution equipment and then sub feeders then branch circuits, and often by the time you get to branch level or even for some the lower amperage feeders the available current is under 10kA.

 

[paulengr]

Fuses disconnects are the answer to arc flash silliness. Place a fuses disconnect ahead of a breaker. The disconnect is “death by arc flash” and the breaker will be category 1 or 2.