Jumper in 3 phase 30amp heavy duty switch ( disconnect )

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Jm2008

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This is a 3 phase dis connect but I am using it for mechanical equipment but it's single phase . From the factory out of the box is a jumper of #10 , on B phase line side to A phase load side.
I would like to know WHY they do that ? To me it would still function with no jumper if I I utilized a phase on line and load .
This is non fusable also
thank you .
 
Jm2008 said:
This is a 3 phase dis connect but I am using it for mechanical equipment but it's single phase . From the factory out of the box is a jumper of #10 , on B phase line side to A phase load side.
I would like to know WHY they do that ? To me it would still function with no jumper if I I utilized a phase on line and load .
This is non fusable also
thank you .
Click to expand...

There is another fairly current thread with the same thing (maybe a higher amperage switch though). It was concluded that the switch gets a higher DC interruption rating if you series additional contacts because you essentially are widening the gap for arc when breaking the circuit.
 
kwired said:
There is another fairly current thread with the same thing (maybe a higher amperage switch though). It was concluded that the switch gets a higher DC interruption rating if you series additional contacts because you essentially are widening the gap for arc when breaking the circuit.
Click to expand...

why would it need a "higher DC interruption rating"? are these kind of switches even rated for interrupting a load?
 
petersonra said:
why would it need a "higher DC interruption rating"? are these kind of switches even rated for interrupting a load?
Click to expand...

DC is more difficult to interrupt, especially as voltage increases. So in order to be able to attain any kind of load interrupting rating at 600VDC, which is the goal for solar systems, they must use the contacts in series. If not, the switch "throw" would need to be comparatively huge.

Many switches are rated for 250VDC without having to do that, but a lot of commercial solar systems run at 500-600VDC.
 
petersonra said:
why would it need a "higher DC interruption rating"? are these kind of switches even rated for interrupting a load?
Click to expand...
If it were not rated for interrupting a load what value does it have?

That doesn't mean frequent interrupting of heavy loads cannot shorten their useful life though.
 
petersonra said:
I did not know disconnect switches even had a make or break rating.
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I guess I honestly don't know exactly what they are rated for, but if they are not supposed to make or break a load, without some labeling who besides maybe an electrician is going to know that? We put them there for users in many cases, especially the non fused switches. But they do have an AIC rating don't they and not just a SCCR?
 
kwired said:
I guess I honestly don't know exactly what they are rated for, but if they are not supposed to make or break a load, without some labeling who besides maybe an electrician is going to know that? We put them there for users in many cases, especially the non fused switches. But they do have an AIC rating don't they and not just a SCCR?
Click to expand...

switches do not have an AIC rating, just a sccr. only things that can clear a short circuit have an aic rating.
 
kwired said:
Then same would apply to a contactor which is definitely intended to make/break while loaded.
Click to expand...

Yes, but the AIC rating is definitely a fault interrupting rating, not a normal load break rating.
A contactor which is not part of OCPD will have a load rating (carrying) and usually that will include the ability to break any load it can carry, with the exception of inductive loads like motors, where there will typically be a separate HP rating.
 
Jraef said:
DC is more difficult to interrupt, especially as voltage increases. So in order to be able to attain any kind of load interrupting rating at 600VDC, which is the goal for solar systems, they must use the contacts in series. If not, the switch "throw" would need to be comparatively huge.

Many switches are rated for 250VDC without having to do that, but a lot of commercial solar systems run at 500-600VDC.
Click to expand...


We installed two 600HP 660V DC motors. Our regulations require a local “load break” isolator (disconnect).

It was cheaper and quicker to fit off the shelf 1200A 11kV 3Ph switches than have bespoke units made and type tested (UL listed). DC is evil stuff to break the current.
 
petersonra said:
switches do not have an AIC rating, just a sccr. only things that can clear a short circuit have an aic rating.
Click to expand...
That's right. A non-fused disconnect will have a very low SCCR, usually no more than 10kA. A Fused disconnect will have a higher SCCR because the fuses will clear the fault, so generally the SCCR of a fused disconnect is the same as the IC rating of the fuses in it; i.e. 100kA or 200kA if used with that kind of fuse.

"Disconnect Switches" can be UL listed under UL98 or UL508. The difference is, the cheaper UL508 listed switches cannot be used as branch circuit devices all by themselves, they must have another device ahead of them, such as a UL98 listed switch or a UL489 listed breaker. So the only real use for UL508 listed switches is as a local LO/TO at or near the motor when there is another device, like a combination starter, ahead of it somewhere.

But as far as the switch itself goes, UL98 requires that a disconnect have a HP rating and be capable of interrupting multiple times its rated load (I think it is 200-300%, but I don't have my copy here) repeatedly in rapid succession. UL508 does not have the same test requirements, but it does require that a disconnect used in a motor circuit have a HP rating.
 
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