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

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designer82

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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
 
Is there anything, well 440.14 for one, requires a local disconnect. But it is not required to be fused.
And most panelboard breakers are HCAR rated, but there may be some units that require fuses.
It will be interesting to see what our HVAC technicians say
 
tom baker said:
Is there anything, well 440.14 for one, requires a local disconnect. But it is not required to be fused.
And most panelboard breakers are HCAR rated, but there may be some units that require fuses.
It will be interesting to see what our HVAC technicians say
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Correct. 40+ years ago, many (most?) HVAC units specified fuse protection. Since the panel it was fed from likely had breakers, it was logical to use a
fused disconnect as the local disconnecting means. Many of those installations are still in existence.
 
designer82 said:
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
Click to expand...
I agree with you. I haven't provided a fused disco in forevs.
 
HVACR Techs say that fuses are a single phase condition waiting to happen, and are to be avoided when possible on the main unit service.
Fuses inside the HVAC unit control panel can be a different affair, and many former designs utilize individual circuit breakers particularly on the compressor loads. Carrier did this a major lot and it paid off in spades. Even on a failure a simple as a 6 lead compressor motor starting with 2 contactors and one contactor coil fails. The compressor is fairly protected, and you can change one contactor and be back on line.
 
Can depend in some cases. I can purchase 30 and 60 amp 600 volt fused disconnects plus fuses for less than I can get 480 volt breaker and enclosure, and is used a lot for irrigation loads and other agricultural loads. I often do get just an unfused disconnect if there is proper upstream protection and all we need is a disconnect.
 
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.
 
hillbilly1 said:
Fuse manufactures claim fuse are better protection for motor loads. Don’t know if it’s true or not, but I did stay at a Holiday Inn Express a couple of years ago. LOL!
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Fuses kind of are better than circuit breakers from the perspective there is no moving parts to fail. In worst case even if defective and won't open in the correct trip curve, they likely still will open somewhere near that trip curve.
 
hillbilly1 said:
Fuse manufactures claim fuse are better protection for motor loads. Don’t know if it’s true or not, but I did stay at a Holiday Inn Express a couple of years ago. LOL!
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We used to have a Bussman field rep that was very knowledgeable, (they all are) and that is what he claimed as well. I saw two many motors burnt up due to single phasing and I also stayed at a HIE a few years ago.

Customers would replace fuses with whatever they had so what started out as (y), turned quickly to :poop:.
 
Equipment marked with a simple "Maximum Fuse Size" requires fuses to protect the equipment. Breakers aren't a valid substitution unless there are additional notes on the equipment that allows the substitution.

Equipment marked with "Maximum Overcurrent Protection" can use either fuses or breakers.

The short circuit current rating is another valid point. HVAC equipment is normally only rated for 5KA, unless a higher short circuit current rating is specified. So its not strictly per code to install where the fault current is higher than the units rating. Adding fuses doesn't automatically fix this, but I usually feel better knowing the fuses are there. After all, if you ask for equipment with a higher rating, the manufacturer usually just adds a fused disconnect to the unit. The difference between them adding the fuses, and you adding the fuses is that they have paid to have UL or someone test the combination.

Then there are always the equipment substitutions the contractors make. The HVAC engineer may specify brand A that allows a 60 amp overcurrent protection, but then the contractor will install brand B that requires a 50 amp MOCP. Its usually easier if the fuses can just be changed.
 
A mechanical contractor I frequently sub for recently installed mini-split A/C units on the balconies of several high-rise condo units. These new units use two pole 15 amp breakers. These replaced older split units. The old air handler was in a ceiling space and was powered by a two-pole 30 amp breaker. A new head unit connected to the mini-split was put in the ceiling. I extended the air handler circuit to the balcony. The mini-sized panel and breakers (all half-size) were very old and replacement breakers were not readily available. I left the old 30 in the panel and used fused disconnects to overcome the issue. By using fused disconnects, I didn't have to search for replacement breakers nor wait for them to be shipped. I most likely saved the client money. I also avoided opening up the old panel box because who knows what could happen there.
 
I don't provide fused disconnects, but an hvac once told me he likes to see fuses at the unit that are lower rating than the fuses/breaker in the panel.

His reasing was simple - when it blows, it happens outside. He has access to it right there.

I get it. How much easier it is to change a fuse at the unit, rather than going several stories down a ladder and maybe chasing down a maintenance man to unlock a door to the electrical room.

Or showing up to a house in the morning, the homeowner leaves and locks the door, the breaker trips inside, and now it's a busted day.

Fuses can save a lot of walking and frustration, I suppose. But I still don't
 
We used to use fusible a lot on commercial jobs as the units often required protection other than what was shown in the plans. Print would call for 40 amp circuit and unit would end up with a max OCP of 35 amp or some odd size and breakers were not readily available whereas fuses were.
 
wbdvt said:
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.
Click to expand...

It's more of a matter of the fact that the unfused disconnect is only tested on its own at 10 kA, and isn't "co-tested" with the breaker manufacturer to take credit for the KAIC rating of the breaker as its SCCR rating. The manufacturers have tested for series rating with fuses of particular fuse classes, but not with breakers. The 10 kA default rating of unfused disconnects is more of a matter of lack of proof otherwise, than a prediction of failure. There may be no physical consequence of overlooking this, but a rating is still a rating, and the NEC and OSHA require complying accordingly.

The unfused disconnects have the same blade assembly, same handle, and same housing as their fused counterparts of the same brand & otherwise-same configuration. The essential differences is the lack of fuse holders, lack of factory-installed neutral, lack of neutral bonding screw, and lack of a "suitable for use as service equipment" rating. It is likely that the manufacturer allows you to use it in series with a set of upstream fuses elsewhere in the circuit, that are of a class that would be compatible with the fused counterpart. So if you had a set of fuses in a switchboard where the branch circuit originates, which then lead to the disconnect in question, it could be acceptable for it to be unfused depending on the fuse class, despite an available fault current in excess of 10 kA.
 
Carultch said:
It's more of a matter of the fact that the unfused disconnect is only tested on its own at 10 kA, and isn't "co-tested" with the breaker manufacturer to take credit for the KAIC rating of the breaker as its SCCR rating. The manufacturers have tested for series rating with fuses of particular fuse classes, but not with breakers. The 10 kA default rating of unfused disconnects is more of a matter of lack of proof otherwise, than a prediction of failure. There may be no physical consequence of overlooking this, but a rating is still a rating, and the NEC and OSHA require complying accordingly.

The unfused disconnects have the same blade assembly, same handle, and same housing as their fused counterparts of the same brand & otherwise-same configuration. The essential differences is the lack of fuse holders, lack of factory-installed neutral, lack of neutral bonding screw, and lack of a "suitable for use as service equipment" rating. It is likely that the manufacturer allows you to use it in series with a set of upstream fuses elsewhere in the circuit, that are of a class that would be compatible with the fused counterpart. So if you had a set of fuses in a switchboard where the branch circuit originates, which then lead to the disconnect in question, it could be acceptable for it to be unfused depending on the fuse class, despite an available fault current in excess of 10 kA.
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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.
 
yes, there can be series ratings for non fused disconnects but that requires specific tested combinations which can be unlikely unless it is a system that was designed and built as designed. In many cases, a disconnect is added as a convenient shut off or LOTO point with no thought to fault current. In a Cutler Hammer catalog I have, it notes that the C-H non fused switches can have higher ratings based on upstream fusing but there are no ratings greater than 10kA if the upstream OCPD is a circuit breaker.

An interesting note in the C-H catalog for non fusible disconnect switches is:
For applications that require a UL listed short circuit rating of 10,000 rms symmetrical amperes or less, a Cutler-Hammer non-fusible switch must be properly protected by any overcurrent protective device rated no greater than the ampere rating of the switch.
So that seems to indicate that one cannot have a 100A breaker or fuse that feeds into a 60A non-fusible C-H switch.
 
wbdvt said:
yes, there can be series ratings for non fused disconnects but that requires specific tested combinations which can be unlikely unless it is a system that was designed and built as designed. In many cases, a disconnect is added as a convenient shut off or LOTO point with no thought to fault current. In a Cutler Hammer catalog I have, it notes that the C-H non fused switches can have higher

An interesting note in the C-H catalog for non fusible disconnect switches is:
For applications that require a UL listed short circuit rating of 10,000 rms symmetrical amperes or less, a Cutler-Hammer non-fusible switch must be properly protected by any overcurrent protective device rated no greater than the ampere rating of the switch.
So that seems to indicate that one cannot have a 100A breaker or fuse that feeds into a 60A non-fusible C-H switch.
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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.
 
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