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lansford (Electrical) (OP)
14 Jan 08 15:16
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
  
gepman (Electrical)
14 Jan 08 15:30
No you are not missing anything.  You are talking about the rotary type disconnects which typically use a European (IEC) style manual switch.  If you look closely these units are only rated 10,000 AIC when protected by fuses which are not used much in the US for motor control.  I overlook the fuse vs. circuit breaker rating issue.

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).  
davidbeach (Electrical)
14 Jan 08 15:56
Even a standard NEMA non-fused disconnect has a similar rating, many are 5kA.  Upstream fuses of the proper rating will increase the rating to the fuse interrupting rating.  Many/most do not include breaker ratings.  Non-fused disconnects are a problem that is widely overlooked.
jraef (Electrical)
14 Jan 08 17:19
While I agree this is a sadly overlooked issue, an important issue to remember is the intended use. These are designed to be used at or near the motor as part of a motor branch circuit, but they are NOT the SCPD in the circuit. That function is required to be elsewhere upstream of these switches. So your concept of closing a motor disconnect into a fault has limited consequences, especially in that the fault would have had to happen while it was disconnected, otherwise the SCPD upstream would have taken it out. In addition whatever that SCPD is (fuse or circuit breaker), it will have a let-through current, then you have the entire circuit to contend with down to the disconnect switch. So as long as your total available fault current at the terminals of the disconnect is below the switch's SCCR rating, it is fine.

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.
gepman (Electrical)
14 Jan 08 18:14
I think that there is a reasonable chance that someone will close them into a fault at some time.  They are very popular in the food processing industry where the cleanup crew will lock them out in order to clean food processing equipment with high pressure (up to 2,000 psig) hoses.  Then the line is started back up under PLC control (and if the system does not have the auxiliary contact on the disconnect wired back to the PLC), the system will not start, and someone goes around closing the disconnects with the starter energized.

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.
rovineye (Electrical)
15 Jan 08 16:27
I have been struggling with this one for a while now. My application is a hoist motor, that uses an NQB circuit breaker as a disconnect and an E STOP using a UV coil.

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.
dpc (Electrical)
15 Jan 08 16:33
Sounds like a bad idea to me. I like FUSED disconnect switches for motor disconnects.  Actually, I don't like local disconnects at all, but when necessary, I always try to specify fused disconnects unless some genius has to "value engineer" the fuses out.

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.  

rovineye (Electrical)
15 Jan 08 17:28
dpc,
 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.
dpc (Electrical)
15 Jan 08 17:34
rovineye,

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.

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