UL508 Motor Disconnect SCCR After VFD

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JovialBulge

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I have a scenario in which I have a VFD which is currently being protected by some 60A Class J fuses on the line side. It has been requested of me to add a motor disconnect between the VFD load side and the motor said VFD is controlling. The motor is 25.0HP, 29.50A @ 480 VAC. The motor disconnect that has been specced to use is rated at 30A however it has some conflicting information on the manufacturers website and I'm hoping someone here may be able to clear up this scenario as a whole for me. On the webpage of the device it states that the "SCCR is 65kA RMS Symmetrical when protected with equally sized Class J Fuse", which to me means that if the disconnect is rated at 30A, it would need to be protected by a 30A Class J fuse and all would be well. On the datasheet for the device it reads that its "Suitable for use as a Motor Disconnect per NEC 430-109 (a)(6)Current Rating @ 65,000 RMS Symmetrical Amps up to 600 VAC Max when protected with 40A Class J Fuses, permitting usage at full amperage rating of the switch."

As stated earlier the VFD currently has 60A Class J fuses on its line side, and what I believe we need to do is swap those out for 40A Class J fuses (which will be fine based on the VFD's maximum input current being 33.7A) to adhere to the datasheets specs on the motor disconnect. Is this the correct train of thought? I assume that if we were to keep the 60A Class J fuses protecting the VFD, add the motor disconnect on the load side of the VFD, then the SCCR for that branch and subsequently the entire enclosure would be reduced to only 5kA. Is another option here to keep the 60A Class J fuses on the line side of the VFD but then place another set of 40A Class J fuses on the load side of the VFD prior to the motor disconnect? Which seems not correct to me but was suggested by a colleague and I would like to make sure that I am not the only one whom thinks that's "not ok". Yet another option suggested was to just get a higher amperage rated disconnect whos specs stated that it needed to be protected by 60A Class J fuses, which from this manufacturer would mean a 40A disconnect.


In addition to the above, another VFD within the enclosure is protected by 25A Class CC fuses and is required to have a motor disconnect as well. The issue with this branch is that the 20/25A disconnect from the same manufacturer also requires it to be protected by 40A Class J fuses in order to achieve its 65kA SCCR rating. This problem with this branch is that the maximum allowable fuse size to meet the UL 61800-5-1 requirements of the VFD manufacturer have the fuse size capped at 30A, which is less than the aforementioned 40A requirement of the motor disconnect, so what do I do with this branch to achieve the 65kA SCCR?
 
There is virtually no prospective short circuit current on the output of a VFD (unless there is a bypass starter). The VFD output short circuit current is only what the VFD can supply, similar to a Separately Derived Source. So even though NF disconnects are only going to have a 10kA SCCR, that’s more than enough.
 
Is there a potential issue with having a disconnect after the VFD where opening the disconnect under load can damage the drive?
 
Jraef said:
There is virtually no prospective short circuit current on the output of a VFD (unless there is a bypass starter). The VFD output short circuit current is only what the VFD can supply, similar to a Separately Derived Source. So even though NF disconnects are only going to have a 10kA SCCR, that’s more than enough.
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Yes.
And technically you can have fault contribution from the motor (about 4x FLA) which needs to be added to that from the VFD (about 1.1x full load output), but it is not usually worth the effort to caclulate.
 
Jraef said:
There is virtually no prospective short circuit current on the output of a VFD (unless there is a bypass starter). The VFD output short circuit current is only what the VFD can supply, similar to a Separately Derived Source. So even though NF disconnects are only going to have a 10kA SCCR, that’s more than enough.
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There will be no bypass starter in the branch so that’s not a worry. I did read on here before posting where you referenced something like this in another thread however I couldn’t find anything in 508A where I could reference it. When speaking with my local UL rep the other day they claimed that the disconnect would “need to be installed as per the manufacturers data sheet” in order to still carry the SCCR rating. If I’m following correctly you’re saying that the VFD practically eliminates the SCCR potential on the output side of the drive so the motor disconnect being installed there should just carry the same SCCR that the branch would have without the disconnect being there at all.
 
JovialBulge said:
There will be no bypass starter in the branch so that’s not a worry. I did read on here before posting where you referenced something like this in another thread however I couldn’t find anything in 508A where I could reference it. When speaking with my local UL rep the other day they claimed that the disconnect would “need to be installed as per the manufacturers data sheet” in order to still carry the SCCR rating. If I’m following correctly you’re saying that the VFD practically eliminates the SCCR potential on the output side of the drive so the motor disconnect being installed there should just carry the same SCCR that the branch would have without the disconnect being there at all.
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The VFD becomes a new source of short circuit current. What exists on its input side is much higher than what is on its output.

Any disconnect on the output of the non-bypass VFD only needs to be rated 5kA maximum, unless your VFD output is 1000A.
 
jim dungar said:
The VFD becomes a new source of short circuit current. What exists on its input side is much higher than what is on its output.

Any disconnect on the output of the non-bypass VFD only needs to be rated 5kA maximum, unless your VFD output is 1000A.
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While I very much appreciate this information and trust in what you’re saying, is this spelled out in black and white somewhere where I can show my UL inspector whom claimed that it needed to be protected as I originally outline in my first post?
 
JovialBulge said:
While I very much appreciate this information and trust in what you’re saying, is this spelled out in black and white somewhere where I can show my UL inspector whom claimed that it needed to be protected as I originally outline in my first post?
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What is the fault current output of your drive? What procedure does your inspector want you to use?
I know of no VFD, installed per UL, that has fuses on its output circuit.

What is the fault rating of your switch when the equal size Class J fuse is not upstream?
 
If installed as per the Allen Bradley specs the SCCR of a PF525 is 100kA when protected on the line side by the correct amperage fuses, which is what we currently have.

I do not know what procedure the UL inspector would like to use, only that they stated when I asked my initial questions here that “well the disconnect needs to be installed as per the manufacturers specs in order for it to carry the 65kA SCCR”

The motor disconnect data sheet states that the disconnect can have a 65kA SCCR if the correctly sized Class J fuses are used to protect it, this is dependent on the amperage rating of the disconnect for the size of fuse required. However I assume, as you and others have pointed out in this thread, that those disconnect requirements and specs are listed if the disconnect it just used in line with a standard motor starter and not a VFD, however that is just an assumption by me at this point.

The fault rating of the switch does not seem to have a table in the data sheet that shows a “de-rating” if the incorrect fuse sizes are used. I will have to reach out to the manufacturer Tuesday and see if that information is available else I’d assume the devices SCCR would drop to 5kA.
 
herding_cats said:
Absolutely. Disconnecting the load side when energized will kill the bus in the drive. NEVER should you put any disconnect on the LOAD side of a drive.
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So this is a request of the end user in order to adhere to some NEC code where they need a disconnect within line of site of the motor I believe. I am not versed in the NEC so I’m just trying to regurgitate what I recall from the initial request. We will interlock the disconnect to the PLC using an AUX contact so that the drive would never START with the disconnect opened, but we will not be interlocking it so that it cannot be turned off while the drive is active.
 
A switch between the drive and the motor sounds like a bad idea to me. Can the drive be located to be in site and less than 50' and then use a drive line side disconnect?
 
If I remember correctly we've used disconnects with aux contacts that open the control circuit before the power conductors are disconnected. Also the 50' within sight rule may not apply if you can meet 430.102(B)(2) and Exception 1:

430.102(B) Motor.
A disconnecting means shall be provided for a motor in accordance with 430.102(B)(1) or (B)(2).
430.102(B)(1) Separate Motor Disconnect.
A disconnecting means for the motor shall be located in sight from the motor location and the driven machinery location.
430.102(B)(2) Controller Disconnect.
The controller disconnecting means required in accordance with 430.102(A) shall be permitted to serve as the disconnecting means for the motor if it is in sight from the motor location and the driven machinery location.
Exception to (1) and (2): The disconnecting means for the motor shall not be required under either condition (1) or condition (2), which follow, provided that the controller disconnecting means required in 430.102(A) is lockable in accordance with 110.25.
(1) Where such a location of the disconnecting means for the motor is impracticable or introduces additional or increased hazards to persons or property
Informational Note: Some examples of increased or additional hazards include, but are not limited to, motors rated in excess of 100 hp, multimotor equipment, submersible motors, motors associated with adjustable speed drives, and motors located in hazardous (classified) locations.
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infinity said:
If I remember correctly we've used disconnects with aux contacts that open the control circuit before the power conductors are disconnected. Also the 50' within sight rule may not apply if you can meet 430.102(B)(2) and Exception 1:
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So the disconnect does have early break aux contacts NO/NC, which I would use back in the PLC to disable the logic which would tell the drive to run. However, since I’ve never use these or any others, I’d have to assume that a human turning the disconnect will be faster than the AUX NC contact opening, the PLC commanding the drive to stop etc. As much as I would like to steer this decision at the end user level it’s more or less something I’m being asked to accomplish and not something I’m being asked “hey should we do this”, I will read up on the article you posted above though and see if we can use avoid this all together.
 
JovialBulge said:
So the disconnect does have early break aux contacts NO/NC, which I would use back in the PLC to disable the logic which would tell the drive to run. However, since I’ve never use these or any others, I’d have to assume that a human turning the disconnect will be faster than the AUX NC contact opening, the PLC commanding the drive to stop etc. As much as I would like to steer this decision at the end user level it’s more or less something I’m being asked to accomplish and not something I’m being asked “hey should we do this”, I will read up on the article you posted above though and see if we can use avoid this all together.
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Search for disconnect switches with Aux contacts or disconnect switches for VFD's. If I remember correctly the travel of the handle opens the aux contact before it disengages the power conductors.
 
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