Disconnect Switch Arc Flash label

[Alley Cat Bad Dog]

Hello everyone,

I was looking into this matter, but I couldn't find the right answer. I'm looking to add a small 3ph motor (less than 1HP) to an existing 480V panel with a disconnect switch, I assume I don't need a different arc flash study for this small motor, but the client is asking for an arc flash label for the disconnect switch which again I couldn't find in the code (NEC 70, NFPA 70E) that confirms or denies that i need one. Any tips here?

 

[don_resqcapt19]

Some places put the arc flash label on all disconnects. It doesn't matter if a code or standard requires it...it matters that the client requires it.

 

[Alley Cat Bad Dog]

Some places put the arc flash label on all disconnects. It doesn't matter if a code or standard requires it...it matters that the client requires it.

And is it just a warning label without data? (incident energy, ...)

 

[don_resqcapt19]

And is it just a warning label without data? (incident energy, ...)

No, the industrial plants that I have worked at require a full 70E compliant label.

 

[David Castor]

Motor disconnects are a grey area for arc-flash labeling. NFPA 70E requires labeling of equipment likely to be serviced or examined while energized. I could argue both sides of this for motor disconnects, but most facilities want to have A-F labels on local motor disconnects.

The small size of the motor doesn't necessarily mean that there will be less arc-flash hazard - the hazard is determined by the available fault current.

 

[Jraef]

The small size of the motor doesn't necessarily mean that there will be less arc-flash hazard - the hazard is determined by the available fault current.

And you might not like what you find out...
When you look at the AFC at the source, and even after the impedance of the wire to the disconnect is factored in, it might be higher than you think. What can happen is that you discover that, say, the AFC is 40kA at the MCC bucket, but still 18kA at the disconnect. Then you discover that non-fused disconnects are only rated 10kA! So you end up having to change to a fused disconnect even though you have a breaker on the motor circuit in the starter already! That's happened to me more than a few times.

 

[JoeStillman]

And you might not like what you find out...
When you look at the AFC at the source, and even after the impedance of the wire to the disconnect is factored in, it might be higher than you think. What can happen is that you discover that, say, the AFC is 40kA at the MCC bucket, but still 18kA at the disconnect. Then you discover that non-fused disconnects are only rated 10kA! So you end up having to change to a fused disconnect even though you have a breaker on the motor circuit in the starter already! That's happened to me more than a few times.

I've been wondering about this question lately. Can we say that a non-fused disconnect is not expected to interrupt current at fault level? 110.9 makes a distinction between devices that are and ones that aren't. For ones that aren't, it just says "equal to the current that must be interrupted", IMHO that means the LRA of the HP that the switch is rated for.

 

[David Castor]

Can we say that a non-fused disconnect is not expected to interrupt current at fault level?

The 10,000 A rating is a withstand rating, not an interrupting rating. If it's HP rated, it can be opened during motor starting or running, but they were never intended to be opened to interrupt a fault. It's much better to use fused disconnects.

 

[Jraef]

The 10,000 A rating is a withstand rating, not an interrupting rating. If it's HP rated, it can be opened during motor starting or running, but they were never intended to be opened to interrupt a fault. It's much better to use fused disconnects.

And the term “withstand rating”, which was not fully defined, has now been supplanted by “Short Circuit Current Rating” , which is shortened to SCCR and is fully defined, including test standards.

 

[JoeStillman]

SCCR ≠ AIC

Here in my Digest 178, the non-fused heavy-duty switches can go as high as 100 kA, depending on the upstream breaker, but it has to be a Sq D breaker. Fuses are indeed better. If you have upstream fuse protection, the catalog says you can be rated as high as 200 kA.

Siemens Speedfax^tm shows some AIC improvement over 10 kA (they say AIC and not SCCR) but only with certain Siemens frames and they don't go as high as Sq D can - 18 kA max for everything up to 400A. With fuses, they're the same as Sq D.

So if you need more than 10 kA, make sure you're the same brand as the upstream breaker or else go fusible.

 

[Jpflex]

Hello everyone,

I was looking into this matter, but I couldn't find the right answer. I'm looking to add a small 3ph motor (less than 1HP) to an existing 480V panel with a disconnect switch, I assume I don't need a different arc flash study for this small motor, but the client is asking for an arc flash label for the disconnect switch which again I couldn't find in the code (NEC 70, NFPA 70E) that confirms or denies that i need one. Any tips here?

Wouldn’t you do a separate label for arc flash warning and maximum available fault current for this motor with its own disconnect?

Take transformer kva / secondary voltage x 1.732 / transformer impedance

Then figure wire run length impedance from transformer to this motor switch? This part of calculation I'm not familiar with?