A job has NF switches at each motor for a maintenance safety disconnect. Due to the 10K AIC rating on Non-Fuse switches, engineer proposes to tag all the NF disconnects "Do Not Open Under Load". Is that an acceptable practice ?
Non-Fuse Switch as Motor Disconnect
- Thread starter augie47
- Start date
- Status
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
I don't think they should be used to open loads but I don't think that changes the SCCR any.
It does not take a lot of wire in many cases to get the SCC down to 10kA. It may be that out by the motor the SCC is already low enough that it does not matter. Someone would need to run the numbers.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Apples and oranges. First, I think you meant "SCCR" not AIC rating, because a device that does not automatically interrupt a fault does not have an interrupt rating. Fuses and CBs have interrupt ratings, switches do not. The SCCR has to do with it holding up under fault current stresses. So the only valid sign you could put on there would be "Do not open under fault conditions", which is ludicrous.
On the other hand, if the switch has a HP rating, it will be rated to open that full HP under load. In MV it's referred to as a "load break" rating and while we don't generally use that term in LV, it's the same. That's why often you will see that the HP rating of a switch is not the same as the amp rating, i.e. a 30A switch will not be rated to open a 25HP motor under load, even if the FLA is less than 30A. So as long as you use a switch with a HP rating commensurate with the motor, and you must by code anyway, his signage suggestion is pointless in that context as well.
Bottom line, if you can show that the available fault current is 10kA or less by the time you get to that disconnect, it's fine. If it's still more than 10kA, you will need to check if the switch has a series rating. Some do, with up stream fuses. I don't know if any that are series rated with up stream CBs though. If not, just use a fused switch.
K8MHZ
Senior Member
- Occupation
- Electrician
I would be more concerned about a non-fused switch closing under a load, rather than opening under one. But that's just me.
Carultch
Senior Member
- Location
- Massachusetts
The 10kA SCCR rating of standard non-fused safety switches does not make any sense.
The exact same swicth components are used for the otherwise identical fusible switches, and they obtain the SCCR rating of the fuses, possibly 200kA. The unfused switches are often "series rated" for the full SCCR rating of the fuses when used with fuses elsewhere in the circuit. I think the 10kA is more of an ignorance rating, than any indication that a serious problem will happen at 11kA.
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.
The LRA of each motor may be checked. If it is less than 10K, the switch may be used for opening the motor during such condition. Otherwise such a tag is necessary.
SCCR does not correlate with LRC. A short circuit can draw many magnitudes of current over the motor LRC.
Here the interrupt rating of the switch is meant for any disconnecting of motor during locked rotor condition such as motor failing to start.
As mentioned earlier, there is no interrupting rating for non-ocpd equipment. 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.
Actually, if this happens to be a General Duty Square D disconnect, and if you check the notes in the Sq D catalog, the 10KA rating assumes there are class H fuses somewhere in the circuit (even for the non-fusible disconnect.)
So in effect, the Short Circuit withstand ratings for the non-fusible disconnects are exactly the same as for the fusible disconnects.
I don't see anything in the NEC that requires a disconnect to have a UL Listed withstand rating greater than the available fault current. Is this one of those cases where the AHJ can approve the equipment (meaning the disconnect) even if it isn't listed?
After all, most pieces of HVAC equipment are only listed to withstand 5KA of fault current. So should we really worry about the disconnect?
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Well, yes and no.
As to the "code not requiring a withstand rating" for a local disconnect, that's technically correct. Articles 409, 440, 670 refer to having an SCCR for "control panels", defined as "2 or more devices in an assembly". So a site mounted disconnect, because it is only one device, technically doesn't need it. But that's not how I have been seeing AHJs interpreting it lately, basically if there is power flowing through it, it needs an SCCR. In one local case that got someone in trouble on this specific issue last year, the AHJ cited Article 110.3, the so-called "suitable for the intended use" clause, which is hard to argue against (I was not the one arguing by the way).
Non-fused switches MIGHT be "Series Listed", but for that to be the case, the switch must STATE that is is, and with exactly which specific OCPD. It's usually fuses, but some have taken it a little further. Here is what Eaton says:
Eaton goes on to say that they have listed THEIR non-fused switches with THEIR molded case circuit breakers as well. I believe Siemens and Square D have done so also. But to be clear, that will NOT apply to a Sq. D breaker ahead of a Siemens switch or any other non-tested combo.
As to HVAC equipment being listed for 5kA, that is not a reason to assume it's OK, that's actually a problem! It's happening all the time now and more and more AHJs are learning the ins and outs of what a low SCCR label means. It's a big headache for ECs, who get stuck having to deal with equipment suppliers who ignore the issue and just slap on the "courtesy" 5kA listing for untested devices. It's only going to keep getting worse too as more AHJs get up to speed.
Interesting.
I did find the attached document in Sq.D's publications but as far as I know its not on the switch itself (in regard to UL 98)
Looks like Augie beat me to it, but I was going to say on the Sq D side, their general duty disconnects state they haven't been tested with any molded case breakers, so the 10KA only applies if fuses are somewhere in the circuit. However, the heavy duty disconnects say they are rated 10KA when used with any manufacturers breaker.
I know what you mean about the HVAC manufacturers. I had a 500 amp RTU that took about 2000 amps of starting current, but it was only rated 5K. How am I supposed to provide 2000 amps for starting, but not exceed 5000 amps for a short? The owner wound up paying extra for a higher rating, and the manufacturer just stuck a fused disconnect on the unit.
At the same time, I still wonder if the codes are overkill? Has anyone ever heard of a RTU, or a CU, or even a disconnect catching fire, or having a catastrophic failure due to a short circuit, when the unit was protected at its MOCP rating??
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
The basic rule for "Every Disconnecting Means" is Section 430.108 with further references to Sections 430.109 & 110. The specific (and limited) permission to label "Do not operate under load" is Section 430.109(E). (This is a very big deal in classified locations
)
)Excuse my lack of knowledge, but would that allow a 10K switch on a circuit greater than 10k ?The basic rule for "Every Disconnecting Means" is Section 430.108 with further references to Sections 430.109 & 110. The specific (and limited) permission to label "Do not operate under load" is Section 430.109(E). (This is a very big deal in classified locations
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
Are the motors in question rated over 40 hp DC or 100 hp AC or does the switch meet all the other requirements of Section 430.109(A)?
Last edited:
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Please note, that is referring to an ISOLATING SWITCH, which is different from a DISCONNECT Switch. I've never seen one in LV, but that's used in MV all the time, for example where you have a vacuum contactor in conjunction with an isolating (aka "non-load brake" rated) switch....The specific (and limited) permission to label "Do not operate under load" is Section 430.109(E). (This is a very big deal in classified locations
- Location
- Mission Viejo, CA
- Occupation
- Professional Electrical Engineer
It is within the context of Article 430, Part IX and Section 430.109; i.e., it is still a disconnecting means.
Edit Add: It also meets the definition of disconnecting means in Article 100.
Last edited:
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
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.
- Status