fused disc switch vs. non-fused disc switch

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john37

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I'm a little confued when it comes to fused vs. non-fused disc. switches. When you just have a disc. switch for one a/c unit you would normally use a non-fused disc. switch. Correct? Would there be any reason why you would use a fused disc. switch if it was for just one a/c unit? If I uderstand correctly you would use a fused disc. switch when there are 2 or more a/c (or even other types of motors). Is that correct? I would appreciate it if someone could clarify this issue for me. Thanks.
 
When faced with a design situation that requires a disconnect switch, I will use the fuses when I need branch circuit overcurrent protection, for the device(s) downstream of the switch. A non-fused disconnect can be used when the OCPD is already present in the design, and it properly covers the device(s) downstream of the switch.
 
I almost always call out fused disconnects, even for single units. This allows the overcurrent protection to be easily changed if necessary -- like when a 10 SEER unit gets changed out to a 13 SEER unit.

Fuses come in 1A, 3A, 6A, and 10A, which are sometimes more appropriate protection than the 15A minimum breaker size.

Fuses tend to clear much faster than breakers, so properly applied fuses can be much more effective at limiting damage than a breaker. I know they are partisan, but if you ever have a chance to see the demos in the lab at Bussmann you should. They do a demo with a bolted short, 2 pieces of #4/0 Cu, 90' each if I recall correctly. They start with the conductors on the floor, parallel and a few inches apart. With a breaker the conductors make a circle in the air. With fuses the conductors hardly move. I'm sure they show their products in the best light, but it's still a very impressive demonstration.

Martin
 
hmspe said:
Fuses tend to clear much faster than breakers
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This is not a true statement for most circuits with fault currents below 4,000A. Even though the service entrances and feeders may have high fault levels most branch circuits do not.

It seems most fuse manufacturer demonstrations are done at fault levels above 30,000A.
 
JohnJ0906 said:
Unless the nameplate says"Max fuse or HACR breaker size xx", you must use fuses.
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The use of the word "or" between fuses and HACR breaker eliminates the requirement to use one over the other. An HACR protected circuit would not require fuses to be used.
 
hmspe said:
. I know they are partisan, but if you ever have a chance to see the demos in the lab at Bussmann you should. They do a demo with a bolted short, 2 pieces of #4/0 Cu, 90' each if I recall correctly. They start with the conductors on the floor, parallel and a few inches apart. With a breaker the conductors make a circle in the air. With fuses the conductors hardly move. I'm sure they show their products in the best light, but it's still a very impressive demonstration.

Martin
Click to expand...
While in apprenticship school, a Bussmann rep came in, and showed a film of these tests. Quite intresting and informative.
 
It seems as though there are different opinions on this issue. However, code wise you could use either one if I'm not mistaken. Unless the nameplate specifically calls for a fused or HACR. Am I correct? Thanks for your thoughts...
 
Some A/C units (especially older unit) say "Maximum Fuse Size" with no mention of breakers or HACR breakers. If this is the case a fuse MUST be installed in the circuit. If the distribution panel contains only circuit breakers, then a fused disconnect is the logical place to provide fuse protection. If the unit indicates that it may be protected by HACR Rated Breakers, and the distribution panel contains HACR rated breakers, then an unfused disconnect may be used.
 
jim dungar said:
This is not a true statement for most circuits with fault currents below 4,000A. Even though the service entrances and feeders may have high fault levels most branch circuits do not.

It seems most fuse manufacturer demonstrations are done at fault levels above 30,000A.
Click to expand...

Jim,

Thanks for the input. As I think back I think all the examples I've looked at curves for were 100A or more. I'll have to see if I can find curves for 60A and below fuses and breakers and look into this further.

As to 30,000A demos, I'd do the same thing if I was trying to sell fuses. OTOH, the demos I've seen were in the 125A to 200A range, which would correspond to feeders. 30,000A available fault current makes sense in that case.

It would be interesting to see tests on, say, a 40A breaker or fuse with 30' of #10CU (to match a typical 5 ton, 13 SEER A/C unit) and with 30' of #8CU, with 2,500A available fault current. Probably not as dramatic as the standard demo.

Martin
 
FYI, in a comparison I did last night at a fault of 5.7kA (it was on the secondary of a 480V to 480V isolation transformer) an 800A standard circuit breaker opened in 0.039 sec and a standard fuse in 0.044sec. Actually I was suprised both even opened this fast considering the fault is in the about 7 times FLA, which is usually the time delay portion of the curves.
 
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