when to use a fused disconnect

[sparkyca]

I seem to get mixed answers on this, but when do I use a fused disconnect instead of a non fused disconnect at a unit. My understanding is a fused disconnect can act as secondary protection to the overloads. I have read that on HVAC units you want to use a fused disconnect. Also I have heard that it depends on the size of the motor, not whether it is a HVAC unit or not. So how would I determine what size needs one and which does not. Thanks.

 

[dpc]

If the circuit is protected upstream by a circuit breaker or another fuse, then the general rules of the NEC would not require another fuse at a local disconnect.

For HVAC equipment, many "package" systems are UL tested using a current-limiting fuse upstream of the HVAC system. For these systems, a current-limiting is required at some point in the circuit. So if the upstream device is a circuit breaker, the local disconnect will need to be fused in order to have an installation that meets UL requirements (and hence, NEC requirements).

Also, a non-fused disconnect switch will generally have a lower short circuit withstand rating than a fused switch, so for systems with a high level of available fault current, the fused disconnect may be a practical necessity.

 

[sparkyca]

Thanks. Yes the feeder of this circuit will have a circuit breaker. So for example a 50hp exhaust fan would not need a fused disconnect, just a standard disconnect? Thanks.

 

[dpc]

Probably not, but you need to look at the recommended installation information from the fan manufacturer. In most cases, a simple fan would not require a fuse, but if there were other things power from the same 480 V circuit on the fan assembly (e.g. damper motor) a fuse may be required. For HVAC equipment this is most often an issue for package system such as heat pumps, chillers, air handlers, etc where a single power supply is run to the skid and there are starters, motor controls, breakers, etc provided with the HVAC system.

But for UL-listed equipment, you pretty much have to install per manufacturer's directions, no matter how stupid it seems.

 

[davidbeach]

Watch the ratings of non-fused disconnects. Most have a 5kA withstand rating unless there are current limiting fuses in the circuit; breakers don't get it in this application. So if you have more than 5kA fault current available at the disconnect locations you may well need to have the fuses anyway. Check with the manufacturer's literature.

 

[jraef]

The SCCR rating issue looms large now in all cases. Nobody can second guess the appropriateness of a fuse installation without the risk of violating a UL listing any longer. Combined with a new focus by insurance adjusters to be looking for non-UL compliant equipment and voiding insurance claims if they find them, the issue needs to be taken very seriously.

sparkyca,
Your initial statement regarding the possibility of using fuses as overload protection shows that you are not an EE, so please don't proceed further on your endeavors without consulting one. There are a LOT of extenuating circumstances when determining the proper Short Circuit Protective Device in a system; using a "Tips" website to evaluate them is wholly inadequate and you are asking for trouble.

The important point for you to get from this is that any {b}old[?b] notions you, or anyone else you ask, have about why or why not use fuses, is almost irrelevant now. The NEC has drastically changed in this respect and the new rules are taking some time to be assimilated by the masses. At this point, don't assume anything without consulting a trained professional who is up to speed on the latest code changes.

 

[sparkyca]

jraef
First of all I am a EE and second of all if the overloads fail in the motor there is a chance for a short circuit due to overheating and deterioration of insulation which can result in short circuit.

 

[advidana]

Since you are a EE, check with your buss fuse manufacture representative for a book that they hand out free explaining the use of the fuse and non fused disconnect usage and code compliance. This is pretty basic stuff that you need to know in your profession and they cover it well.

 

[sparkyca]

i was told by the bussman fuse rep that this is can be used as secondary protection in case the overloads fail

 

[dpc]

My understanding is that fuses are acceptable motor overload protection per NEC if sized appropriately. I would not recommend it, but I do think it is allowed per code.

Of course, the Bussman rep will have a strong, heart-felt opinion that a fuse can do just about anything.

 

[jraef]

i was told by the Bussman fuse rep that this is can be used as secondary protection in case the overloads fail

In my opinion, that is an irresponsible statement on the part of that Bussman rep. I have never heard or seen Bussman promote that concept, it would be opening them up to a lot of liability. The restrictions on fuse sizing along with the necessity for allowing motor starting without nuisance clearing precludes the use of using fuses as OL protection in all but the smallest of motor loads. Just because the NEC says you are not restricted from trying something doesn't mean that it will work!

Example: a 50HP 460V motor, 65FLA. The fuse recommended for providing SC protection is a 100A dual element (time delay). That alone is 154% of the motor FLA. A Class 10 I²t overload relay should have dropped out the starter in approx. 45 seconds at 150% current, yet the fuse may never blow! In fact if you look up the melt curve for a dual element fuse at that size, it may not even blow at 190A for 5 minutes, more than enough time to smoke the motor! if you were to drop the fuse size down to 80A so as to be at the 125% point for "motor protection", the inrush current of approx. 390A would blow it in approx. 3 seconds at a dead cold start; not likely to last long at that rate.