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Fusible vs non-fusible disconnects 1
- Thread starter klubie
- Start date
More protection with fusible.
Horrific loss of space in the enclosure with fusible.
Much larger hazard with fusible if no covers available, as the fuse stuff provides an acre of energized metal to touch.
Fusible means energy dissipation in the enclosure, though often not enough to matter.
If the box temperature varies greatly the fuse protection may vary somewhat.
Keith Cress
Flamin Systems, Inc.-
Horrific loss of space in the enclosure with fusible.
Much larger hazard with fusible if no covers available, as the fuse stuff provides an acre of energized metal to touch.
Fusible means energy dissipation in the enclosure, though often not enough to matter.
If the box temperature varies greatly the fuse protection may vary somewhat.
Keith Cress
Flamin Systems, Inc.-
Non-fused disconnect switches often have very limited short circuit withstand capability. For systems with high short circuit current, it is often necessary to use fused switches, just for the fuse to protect the switch.
And the poor wretch who closes into a fault downstream will really thank you for putting fuses in there. (Well, he probably won't actually, but he should).
And the poor wretch who closes into a fault downstream will really thank you for putting fuses in there. (Well, he probably won't actually, but he should).
He asked for the advantages of using a NON-fusible vs fusible, not the other way around.
There is only one "advantage" so to speak. If you ALREADY have a valid Short Circuit protective Device (SCPD) such as fuses or a circuit breaker somewhere upstream in your circuit, DEDICATED to that single circuit, then you do not need an additional SCPD downstream. In fact it becomes a bad idea to have too many SCPDs in the circuit because you create possible confusion as to which one opened, which increases the risk of a safety mishap.
So an example would be, you have a motor controller circuit. Power for it comes from a distribution board via a properly sized circuit breaker. All wiring is correct for that size circuit breaker out to a local control panel. You are required to have a lockable external disconnect at that control panel, but at that point, fuses would be redundant to the upstream circuit breaker. You don't absolutely need them.
That said, some people may put them in anyway because often times, fuses provide other beneficial capabilities, i.e. current limitation and/or faster clearing for semiconductor devices.
There is only one "advantage" so to speak. If you ALREADY have a valid Short Circuit protective Device (SCPD) such as fuses or a circuit breaker somewhere upstream in your circuit, DEDICATED to that single circuit, then you do not need an additional SCPD downstream. In fact it becomes a bad idea to have too many SCPDs in the circuit because you create possible confusion as to which one opened, which increases the risk of a safety mishap.
So an example would be, you have a motor controller circuit. Power for it comes from a distribution board via a properly sized circuit breaker. All wiring is correct for that size circuit breaker out to a local control panel. You are required to have a lockable external disconnect at that control panel, but at that point, fuses would be redundant to the upstream circuit breaker. You don't absolutely need them.
That said, some people may put them in anyway because often times, fuses provide other beneficial capabilities, i.e. current limitation and/or faster clearing for semiconductor devices.
davidbeach
Electrical
jraef, not quite so fast. When I had to worry about such things - up until nearly 3 years ago - I never say a non-fused disconnect having a withstand rating based on an upstream breaker, only upstream fuses. So the only two places for non-fused disconnects are on systems with less than 5kA available fault current or on circuits already protected by appropriately sized fuses. A 3000A service entrance fuse won't protect the non-fused disconnect, it needs to a fuse of a size that would go in the disconnect if it were fused.
Excellent point, you're right, I was being hasty and forgetting about all the new emphasis on series ratings. Although the NF switches my company makes do indeed have a withstand rating when used behind CBs, it is only 10kA, so that caveat must be observed. Adding Class R, J or T fuses brings that same switch to 200kA.
davidbeach
Electrical
jraef, nice to know that there are ratings with breakers, but 10kA is still so low as to be nearly useless. Maybe it would be enough for many 30A and 60A applications, but probably too low for 100A and above.
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On 208 or 240 volt systems when the transformer is under 150 KVA, non-fused should be fine. On nearly every 480 volt system, the interrupting current is over 10kA, so fused should be used unless it's protected by an upstream dedicated fuse sized to protect that switch/circuit.
pooslinger
Electrical
There are a lot of cases where a machine is powered by a MCC (Motor control center) or a stand-alone starter. OSHA requires a lock out point within eyesight of the machine. The machine is already double protected by over current heaters and fuses or circuit breakers. It makes no sense and you are not required to triple protect your machine. Another use for a non fused is when you have a generator with a transfer switch. Transfer switches are not cheap, a nonfused disconnect is installed between the transfer switch and the load protecting the transfer switch from arcs from the surge of a starting motor.
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