Fused Disconnect Question from a Newbie.

[Florida EE]

I've been out of school for a year now, and the more I learn, the more questions I have. I am currently doing consulting work in Florida.

The engineers I work under like to use a fused disconnect as a local means of disconnect, for motor driven devices. We may also use a combination starter/disconnect. I have been told that fuses will provide better protection for the equipment, usually an air handler.

A typical circuit would involve a breaker in the panelboard, wire sized to the breaker, a local fused disconnect, a starter provided by the equipment manufacturer and the equipment.

The starter should provide overload protection to the motor, the breaker should protect the wire, and a nonfused disconnect would provide a local means of disconnect.

Now, does using a fused disconnect present an additional risk in a multi-phase circuit? If a fuse for Phase A opens that phase and Phase B and C are still closed, does it single phase the motor? (I'm not sure if that is the right terminology.) Would that cause more damage? If a breaker was the only overcurrent protection, all 3-phases should open.

Thanks for the help.

 

[bphgravity]

Yes. This is one of the downsides to using fused overcurrent devices. The equipment is subject to "single-phasing". One option is to utilize phase monitors to protect against the loss or reversal of the phase conductors.

 

[kingpb]

Simply use a non-fused switch, if the desire is just to have a local disconnect. Fusing is not required for a properly sized and protected circuit from the panel.

 

[petersonra]

Simply use a non-fused switch, if the desire is just to have a local disconnect. Fusing is not required for a properly sized and protected circuit from the panel.

it also adds in another point of failure that is not always real obvious.

 

[bob]

The engineers I work under like to use a fused disconnect as a local means of disconnect, for motor driven devices. We may also use a combination starter/disconnect. I have been told that fuses will provide better protection for the equipment, usually an air handler.


Now, does using a fused disconnect present an additional risk in a multi-phase circuit? If a fuse for Phase A opens that phase and Phase B and C are still closed, does it single phase the motor? (I'm not sure if that is the right terminology.) Would that cause more damage? If a breaker was the only overcurrent protection, all 3-phases should open.

Thanks for the help.

Yes. The loss of a fuse would cause a single phase condition. However, using
a fuse allows you to fuse closer to the actual full load amps of the motor.
For example, if you have a 10 hp 3 phase 208 motor the FLA is 31 amps.
Bussman recommends a 35 amp fuse for protection. Using a breaker would require a 80 amp if you size it at 250%. During a single phase one leg could
approach 71 amps and never trip the breaker. Generally the motor overloads
would clear the motor but sometimes they do not.

 

[ron]

If this is a large motor, then the calculated fault current at the disconnect may be > 5kA. Most non-fused disconnects only have a 5kA withstand and thus would require a fused switch for local disconnect.
Smaller motors have enough impedance in the small conductors to bring the calculated fault current down low enough to have just a non-fused disco.

If you have a breaker to protect the wires, the short circuit protection is there. The overloads in the stater will offer really tight overload protection, so the fuses at the disco becomes a fault current question.

 

[Jraef]

Yes. The loss of a fuse would cause a single phase condition. However, using a fuse allows you to fuse closer to the actual full load amps of the motor. For example, if you have a 10 hp 3 phase 208 motor the FLA is 31 amps. Bussman recommends a 35 amp fuse for protection. Using a breaker would require a 80 amp if you size it at 250%. During a single phase one leg could approach 71 amps and never trip the breaker. Generally the motor overloads would clear the motor but sometimes they do not.

An 80A breaker on a 31A motor??? Talk about overkill! Just because you CAN use a breaker UP TO 250% of FLA does not make that the right breaker to use. I would start out with looking at a 40A breaker, and if the magnetic trips were not adjustable, go up a size or two only if necessary.

IMHO, the only real valid reasons to use fuses is when
a) the available fault current is higher than what a breaker can interrupt, or
b) the starter is only rated for a withstand that is lower than the breaker let-through.

What a) means is, if you have an available fault current of, say 100,000A, and this starter is all by itself coming off of a pole transformer (Service Entrance), then you have to either pony up for a more expensive breaker rated 100kAIC, or you use fuses, which will be considerably less expensive.

In scenario b), even if your breaker is rated for 100kAIC, if the let-through is 14kA and the starter is only rated for 5kA withstand, you will need current limiting fuses in between the breaker and the starter (or get a much more expensive current limiting breaker).

The beauty of buying combination starters, with breakers OR fuses, is that the manufacturer MUST now provide you with the total interrupting capacity of the unit, not just the breaker or fuse. That makes it much cleaner and simpler.

If you already have a breaker in a panelboard feeding this motor, chances are good that someone in the past did a coordination study to know that this breaker is rated for interrupting the available fault current. So then you only have the withstand rating of the starter issue to deal with. If that works out to be OK too, then fuses would be redundant and add that risk of single phasing that you mentioned. IF however, you had one breaker feeding two motors, i.e. that 80A breaker feeding two 31A motors, then fuses would add closer protection.

In any case, if you use fuses, ALWAYS add in a phase loss relay at a minimum.

 

[ptonsparky]

Many of the newer electronic overloads provide a very good degree of phase loss protection due to the ability to detect and respond to current imbalance when the overloads are set correctly.

 

[Jraef]

True, but not all. You need to check to make sure, i.e. don't Ass-U-Me!

 

[kevin]

"The starter should provide overload protection to the motor, the breaker should protect the wire, and a nonfused disconnect would provide a local means of disconnect."


It would be more accurate to state that the breaker provides either branch circuit (Art. 430, PartIV) or feeder (Art. 430, Part V) "short-circuit and ground-fault protection"; and that the starter's overload relay provides (Art. 430, Part III) "motor and branch-circuit overload protection". Refer to Art. 100 for definitions of "branch circuit", "feeder", and "overload".

The concern about single-phasing can be alleviated by specifying and installing a starter with an "adjustable solid state overload relay with phase loss and unbalance protection". Such starters are readily available and are the same cost as "standard" starters with three heaters or thermal units.