Load breaking capabilities of 208 V and 480 V switches 5

[bacon4life]

Do all 208 V and 480 V disconnects typically have load breaking capabilities? If not, at what amperage rating do isolation-only switches become available?

At the moment I am reviewing a drawing that shows a 400 A 208 V switch that will be used for lockout/tagout purposes. If large amperage switches might be isolation-only, I would need to revise the specification to include load-breaking ability.

 

[JezNZ]

I cannot speak for the NEC world (assumed by the voltages) but regarding terminology:
In the IEC world a 'switch' is rated for making and breaking, a 'disconnector' is rated for isolating (no-load) and a 'switch-disconnector' is rated for both.

IEC standards used 'utilization category' AC-20 (AC disconnector) or AC-21/22/23 (AC switch-disconnector, depending on load type).

In theory, disconnectors can be of any amperage, but it is rare to find them in commercial/industrial range of products by the major manufacturers. Often some products are marketed like a disconnector only, but will usually be found to have an AC-22 rating.

 

[che12345]

Dear Mr. bacon4life (Electrical)(OP)7 Jun 22 00:49
"... #1. Do all 208 V and 480 V disconnects typically have load breaking capabilities? If not, at what amperage rating do isolation-only switches become available? .....#2. I am reviewing a drawing that shows a 400 A 208 V switch that will be used for lockout/tagout purposes. If large amperage switches might be isolation-only, I would need to revise the specification to include load-breaking ability.... "
#1. In the IEC world, see learned Mr. JezNZ advice. You mentioned 208V/480V which is a US voltage, where NEC is applicable. It is the Law.
Per IEC, a 400A disconnector rated AC20 is capable to carry 400A continuously, but NOT switching with any current. It is only for to close/cpen without current!.
#2. It is fine to replace it with a load-breaking ability device, at a higher cost.
FYI: a) A disconnector may be allowed? per NEC with proper labelling "Warning ...."
b) In IEC a disconnector may be allowed? where there is (an early make/break auxiliary switch) which [switches off/opens the up-stream OCPD] etc., before the main poles close/open, with zero current.
Che Kuan Yau (Singapore)

 

[unclebob]

Spring-loaded mechanisms have usually load breaking capabilities. Pringle and Bolt-Switch are 2 examples.

 

[che12345]

@ Dear Mr. unclebob (Electrical)7 Jun 22 19:03
"...Spring-loaded mechanisms have usually load breaking capabilities. Pringle and Bolt-Switch are 2 examples."
I wish to draw the attention/caution and the limitations that " ... spring-loaded mechanism have usually load breaking capability". It is true that spring-loaded mechanism is helpful in the speed when closing/opening, which is independent of the manual force/speed. It is very important for a switching mechanism to be able to handle the arc produced during closing/opening on load/current. This is evident even a small say 20A contactor, the switching poles are covered with arc-chute. For ACB say >1kA, the poles are covered with very heavy arc-chute, to quench the arc.
Che Kuan Yau (Singapore)

 

[dpc]

NEMA-rated disconnect switches generally are 100% rated for make and break. So a 100 A switch can make or break 100 A. In addition, most are also horsepower-rated for use as motor disconnect switches.

 

[bacon4life]

This is in the USA, so I am looking for info about typical NEMA rated devices.

 

[LionelHutz]

I believe there are cheap commonly available cheap isolation/service type disconnects which are not spring action and are not load break. Basically, they have blades that the handle directly moves into and out of a set of contacts. These are typically marketed more as commercial use, but there is nothing stopping them from being applied at any motor location as the service lockout and they are available as low as 30A.

If the device is spring loaded then it will be load break rated because it'd be stupid for a manufacturer to spend the money putting the spring loaded mechanism into a switch that is not load break rated. The only reason for the spring loaded mechanism is to snap the switch contacts open quickly which is part of the arc control design.

 

[bacon4life]

I now see that NEC 430.109 (e) allows for isolation-only disconnects for stationary motors over 100 hp as long as the disconnect is marked as "Do not operate under load." From what I could tell, this allowance is specifically for large motors, so isolation-only disconnectors would not be allowed for other kinds of applications.

Thanks all.