FAULT RATING HP RATED DISCONNECT SWITCH
FAULT RATING HP RATED DISCONNECT SWITCH
(OP)
Non fused HP rated disconnect switches (at motors) are mostly rated 10,000 amps at 480 volts. Assume there is NOT an interlock (micro switch back to the contactor) to make sure that the contactor is dropped out when the switch is open, there is a slim chance the HP rated switch can be closed into a fault if the contactor is picked up.
Assuming the fault current is greater than 10,000 amps at the switch, then this is a bad application?
Based on what I see of designs, this is seldom reviewed, considered, or looked at.
Am I being too careful and detailed or am I missing somethig?
Opinions Please
Thanks
JIM
Assuming the fault current is greater than 10,000 amps at the switch, then this is a bad application?
Based on what I see of designs, this is seldom reviewed, considered, or looked at.
Am I being too careful and detailed or am I missing somethig?
Opinions Please
Thanks
JIM






RE: FAULT RATING HP RATED DISCONNECT SWITCH
I have gone around and around with the manufacturer's of these units about the low AIC rating issue and it is an overlooked (or purposely ignored by the manufacturer) issue.
I usually only use these units for small motors. I have calculated the length of run of #10 wire that it takes to reduce the fault current from 65,000 to 10,000. I then put a note on the drawings that these disconnects are not to be used where the motor lead length is less than "X" feet. The length is not very long.
The heavy duty knife switches have better ratings especially when they have been tested with the same manufacturer's circuit breakers (I am thinking of Square D and Cutler-Hammer).
RE: FAULT RATING HP RATED DISCONNECT SWITCH
RE: FAULT RATING HP RATED DISCONNECT SWITCH
Still, it's a bigger deal than most manufacturers want you to understand, especially when it comes to IEC style rotary disconnects that are taking over in popularity because they are cheap and small. Most of them are rated at 5kA, and those that advertise having 10kA will have a footnote stating that is only with Class J fuses (in most cases). But in the UL98 style safety switches and / or flange mount disconnects, the same switch is usually rated 200kA with fuses, so you know it can withstand more than those little ones. They are only tested to 10kA because they know as a non-fused switch, they are always going to be a downstream device.
RE: FAULT RATING HP RATED DISCONNECT SWITCH
Other times marginally qualified maintenance personnel may change a motor out with the disconnect open and perhaps it could be a bad motor or miswired. When the unit doesn't start with the disconnect open the disconnect is closed with the starter energized.
They work and they are cheap but I still don't use them where I calculate the fault current at the disconnect terminals to be above 10,000A.
I believe that I remember looking at the trip curves of a Cutler-Hammer 15A HMCP versus a 15A Class J fuse at 10,000 fault and I felt that the let through would be less on the HMCP than the fuse. If someone feels differently please comment.
I asked one of the manufacturer's of these devices why they aren't tested with circuit breakers instead of fuses and they said that the UL testing procedures (I believe it is UL508) require testing with fuses not circuit breakers. It is time to get that one updated.
RE: FAULT RATING HP RATED DISCONNECT SWITCH
The NQBs are switching function only, with no ST, LT, INST tripping. NQB = Non-automatic Quenched Break.
The breakers in this application do not need any of the tripping devices. It is only used as a disconnect.
The problem with the NQBs, is that they do not have an arc interrupting capacity rating. That is common for NQBs, since they are usually used for unloaded connecting and disconnecting of loads. In our case it is used as an emergency stop, so it needs to open under load. Since it could could be opening (locally or remotely) or closing (manually locally) on a fault, it should have an AIC above that available at that location.
Will the NQBs operate under that fault without failing? Probably, but they are not tested under those conditions and do not have that rating.
Note that not everyone agrees with my opinion on this and they believe no AIC rating is required since there are no protective features in NQBs. All I can say is that those people will never have to operate that device.
RE: FAULT RATING HP RATED DISCONNECT SWITCH
In your case, why not just put in a breaker with the guts in it and have a tripping function? I don't think it will change the physical size. There must be better places to save a few dollars.
RE: FAULT RATING HP RATED DISCONNECT SWITCH
Yes, correct 100%. What I wrote above was my initial dilema.
Installing an AQB is what I eventually had to do. The problem was a shock qualification (physical shock) as this is a military application (NAVY). I had a qualified NQB, but I didn't want to use it because of the lack of an AIC, and there were no AQBs of the right rating, with a UV and a shock qualification.
The Navy uses the NQB, and just doesn't care about the rating. But I am tasked with building a Navy ship to commercial standards. Instances like this is where the meeting of the two worlds gets ugly.
In the end I got an AQB shock qualified ($$$$).I just had trouble making a case for the need.
RE: FAULT RATING HP RATED DISCONNECT SWITCH
Yes, those military/nuclear certifications can be a PITA. I think you made the right choice, although you probably caught some flak about the cost. With increased concern about arc-flash hazard, they will be happier in the long run.
To be a good engineer, you have to worry a lot. This will leave one less thing to worry about.