Non-Fuse Switch as Motor Disconnect

[rbalex]

Right. But ALSO in Article 100:

Switch, Isolating. A switch intended for isolating an electric
circuit from the source of power. It has no interrupting
rating, and it is intended to be operated only after the circuit
has been opened by some other means.

So it is a specific defined device.

I'm not trying to be argumentative here, I'm just making the point that this requirement for a label saying "Do not open under load" is not intended to be applied to anything that can be interpreted as a "disconnect means", it is referring to a SPECIFIC type of device.

I believe my initial post indicated that application of a "Do not open under load" lable is very limited.

 

[Sahib]

there is no interrupting rating for non-ocpd equipment.

Wrong. 110.9 Second para.

The HP rating is what provides the necessary correlation for switching under load. SCCR rating has to do with ability to withstand fault current well in excess of nominal and atypical (e.g. LRC) by many magnitudes... at least for the duration of an ocpd-handled event.

SCCR rating alone also enables a motor to be opened under locked rotor condition in addition to ride through short circuit condition. I think if it has a SCCR rating and motor full load rating, though costlier, the switch would be capable of opening the motor under load also.

 

[kwired]

Also, the SCCR rating has nothing to do with whether the switch is operated under load for either load make or load break. It has to do with whether or not the component will fail catastrophically at short circuit currents exceeding the rated value.

You usually are not opening the switch when such a fault is in progress, you are running away from it, it is when you close the switch into a fault that you are definitely right up close and why people always say "I would't want to be closing the switch into that fault". Truth is even if the switch were rated 100kA, you still will need clean pants afterwards if you closed it into a 50kA fault in most instances.

 

[Jraef]

Actually in my experience in attaining an SCCR for a product, the device is not opened or required to attempt to interrupt the fault current in the test, or even to function afterward. It is quite simply just required to not spew molten metal or shrapnel around. Keep in mind, "SCCR" simply replaced the older term "withstand rating", it has nothing to do with interrupting anything.

 

[Carultch]

You usually are not opening the switch when such a fault is in progress, you are running away from it, it is when you close the switch into a fault that you are definitely right up close and why people always say "I would't want to be closing the switch into that fault". Truth is even if the switch were rated 100kA, you still will need clean pants afterwards if you closed it into a 50kA fault in most instances.

I would think you would first have to clear the fault, before reconnecting the circuit. It is my understanding that the OCPD is supposed to turn off the circuit automatically in the event of a fault, and then the people who go to repare it are supposed to isolate and eliminate the fault.

 

[kwired]

I would think you would first have to clear the fault, before reconnecting the circuit. It is my understanding that the OCPD is supposed to turn off the circuit automatically in the event of a fault, and then the people who go to repare it are supposed to isolate and eliminate the fault.

I guess I was referring to the situation of closing a switch into a fault, but you didn't know beforehand there was a fault.

Usually it is a non electrician that does this the first time it happens anyway.

 

[Besoeker]

I would be more concerned about a non-fused switch closing under a load, rather than opening under one. But that's just me.

In the dim and distant past we used switches that were rated as fault make load break.

 

[Jraef]

I guess I was referring to the situation of closing a switch into a fault, but you didn't know beforehand there was a fault.

Usually it is a non electrician that does this the first time it happens anyway.

I saw a guy wet his pants after closing into a fault once. Big blue flash-bang and the door of the MCC bucket bowed out. It was all over in a second and nobody was injured, at least not physically. He left the job site and never came back.

 

[kwired]

I saw a guy wet his pants after closing into a fault once. Big blue flash-bang and the door of the MCC bucket bowed out. It was all over in a second and nobody was injured, at least not physically. He left the job site and never came back.

That is why even before PPE became a big deal they taught us to stand off to the side and not look at the equipment when closing a switch. I still do that most of the time even when I know the available fault current is low enough that shouldn't be a problem.

I don't think of it as much with "loadcenter" breakers but often will think of that when closing a breaker in larger capacity panels or switchboards.