Fusable Disconnect
- Thread starter mimo
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barbeer
Senior Member
- Location
- Pinellas County, FL
If you have overcurrent protection at the distribution point(you usually do) you do not need fuses again although some spec them.
Simplistic answer: Use a fusible disconnect when the feeder is protected at a higher overcurrent level than what you want to protect the load at, or (design choice) if you think it may potentially be so in the future, e.g., an air handler is replaced with a smaller/larger air handler.
That are many instances when both are used, usually commercial & industrial. Many times it is due to voltage drop.
For instance, a motor application may only need, say, #10 on a 35amp breaker. But the EE may spec #4 due to the long run, protected by a 60a breaker, and then use a fused disco w/ smaller fuses to satisfy the max. OCD rating of the motor.
For instance, a motor application may only need, say, #10 on a 35amp breaker. But the EE may spec #4 due to the long run, protected by a 60a breaker, and then use a fused disco w/ smaller fuses to satisfy the max. OCD rating of the motor.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Fused disconnects are often used to terminate tap conductors.
other uses
other uses
Fused disconnects are also used at times to reduce fault current and some equipment (coolers for example) are "listed" with "max fuse (only)protection" (as opposed to max over current) thus indicating they are not listed for breaker protection
other uses
Fused disconnects are also used at times to reduce fault current and some equipment (coolers for example) are "listed" with "max fuse (only)protection" (as opposed to max over current) thus indicating they are not listed for breaker protection
mayanees
Senior Member
- Location
- Westminster, MD
- Occupation
- Electrical Engineer and Master Electrician
Fused disconnect requirement
Fused disconnect requirement
I've made this post before, but rather than refer to that post, it's easier for me to just reiterate.
An unfused disconnect is only good for 10,000 amps withstand. So, if the available fault current exceeds 10kA, then the manufacturer of the disconnect must be the same manufacturer of the upstream OCPD, since that would be a listed combination - or use a fused disconnect with appropraitely applied fuses.
Personally, an unfused disconnect is always my first choice, since I think it's an unnecessary addition of a failure point. But you have to look at the fault current situation as described above.
JM
Fused disconnect requirement
I've made this post before, but rather than refer to that post, it's easier for me to just reiterate.
An unfused disconnect is only good for 10,000 amps withstand. So, if the available fault current exceeds 10kA, then the manufacturer of the disconnect must be the same manufacturer of the upstream OCPD, since that would be a listed combination - or use a fused disconnect with appropraitely applied fuses.
Personally, an unfused disconnect is always my first choice, since I think it's an unnecessary addition of a failure point. But you have to look at the fault current situation as described above.
JM
Minuteman
Senior Member
- Location
- The Sooner Nation
A fused disconnect will also generate more A/C service calls.
Say a A/C unit is slightly over charged with refrigerant and has dirty coils. Result, higher head pressure, higher amps. Over-current device opens the circuit.
If it sets a while and cools off, bubba comes by and resets the breaker - it will run a while. If instead, the fuses blow in the disco, he may just call somebody. (Or then again, he might just put 1/2 a roll of tin foil around the blown fuse... so forget what I said)
Say a A/C unit is slightly over charged with refrigerant and has dirty coils. Result, higher head pressure, higher amps. Over-current device opens the circuit.
If it sets a while and cools off, bubba comes by and resets the breaker - it will run a while. If instead, the fuses blow in the disco, he may just call somebody. (Or then again, he might just put 1/2 a roll of tin foil around the blown fuse... so forget what I said)
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
mayanees said:
Actually, unless the switch contains "fuse rejection clips" it is still only UL Listed at 10kA regardless of the fuses installed.
Series ratings do not depend upon equipment being from the same manufacturer. While most manufacturers only test their models of circuit breakers, series testing with upstream fuses always involve a universal "umbrella" fuse that is not manufacturer specific.
mayanees
Senior Member
- Location
- Westminster, MD
- Occupation
- Electrical Engineer and Master Electrician
... okay....mayanees said:
I ammend my post to say upstream OverCurrent Protective Breaker, as opposed to OCP Device.
My experience is that most Motor Control Centers contain breakers, so if you have a Square D MCC, with breakers, use a Square D Safety Switch.
JM
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