UL508 Motor Disconnect SCCR After VFD

J

JovialBulge

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New England
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Programmer
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.
 
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.
 
infinity said:
Is there a potential issue with having a disconnect after the VFD where opening the disconnect under load can damage the drive?
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YES you will kill the bus in the drive.
JovialBulge said:
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.
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if the line side is loto then how possibly could the load side be active? Having a dc on the load side is a really bad idea.
 
herding_cats said:
YES you will kill the bus in the drive.

if the line side is loto then how possibly could the load side be active? Having a dc on the load side is a really bad idea.
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I’m not saying that this makes any sense and I would not include it within a typical panel design of mine. However, I was asked to accomplish this based on a customer request in which they referenced the NEC requirements for having a disconnect in sight of the motor since the LOTO disconnect on the line side of the VFD will be located on another floor of the plant and therefore very much out of site of the motor.

Additionally, not an argument from me here just an example, I have seen many a design from a decade or two ago where people would put 2 contractors in series on the load side of a PF4 VFD and tie their coils into the overall safety relay output for the machine. These machines have been in service since they were installed in 2005 and the VFDs have never been replaced. Is this good for the bus of the drive? absolutely not; will it kill the drive the first time (if there is one) that the disconnect is opened while the VFD is active, I don’t think so.
 
I BELIEVE this is the case for the disconnect that was specced but I will look into it further once companies open back up on Tuesday this week.
infinity said:
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.
Click to expand...
 
JovialBulge said:
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.
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Starting the drive while disconnected is not the problem. So feeding the disconnect aux. contact to the PLC is not going to help.

OPENING the disconnect while the drive is RUNNING is the problem. That’s because as power contacts open while current is flowing, an arc forms across the contact and that arc, together with the increasing air gap, forms a sort of capacitive tank circuit that rapidly increases the voltage across the gap until the dielectric of the air gap between the contacts is sufficient to interrupt it. This is all happening in fractions of a second, but that rapid dV/dt (delta or change in voltage over delta time) can exceed the voltage withstand rating of the transistors and cause them to fail, which is fatal to small drives or at least very expensive for larger ones that are repairable.

Most drives will have an “Enable” input or can have one of their inputs programmed that way. That will directly kill power to the base of the transistors (also called a “base block” function in some brands), which overrides any decel or braking programming, so that the drive INSTANTLY turns off the output transistors. By wiring the disconnect aux contacts to that drive input, the aux contact will open slightly before the main contacts do, so the drive output will be turned off by the time the main disconnect contacts open, avoiding the risk of this damage.

SOME DRIVES have built-in protection circuits against this, so people say “it’s ok to ignore this issue”. That is generally true of EMERGENCY situations like machine safety systems that require contactor isolation of the output of the drive, which will happen rarely. But those consist of components designed to absorb and dissipate the excess voltage and just like surge suppressors, if you use them too much, they themselves fail, then the drive fails. Once you have a manual disconnect available, my experience is that sooner or later, operators get lazy and start using it every day rather than figure out how to to an orderly shutdown. So it’s always best to AVOID this by interlocking the disconnect aux to the drive.
 
Jraef said:
Starting the drive while disconnected is not the problem. So feeding the disconnect aux. contact to the PLC is not going to help.

OPENING the disconnect while the drive is RUNNING is the problem. That’s because as power contacts open while current is flowing, an arc forms across the contact and that arc, together with the increasing air gap, forms a sort of capacitive tank circuit that rapidly increases the voltage across the gap until the dielectric of the air gap between the contacts is sufficient to interrupt it. This is all happening in fractions of a second, but that rapid dV/dt (delta or change in voltage over delta time) can exceed the voltage withstand rating of the transistors and cause them to fail, which is fatal to small drives or at least very expensive for larger ones that are repairable.

Most drives will have an “Enable” input or can have one of their inputs programmed that way. That will directly kill power to the base of the transistors (also called a “base block” function in some brands), which overrides any decel or braking programming, so that the drive INSTANTLY turns off the output transistors. By wiring the disconnect aux contacts to that drive input, the aux contact will open slightly before the main contacts do, so the drive output will be turned off by the time the main disconnect contacts open, avoiding the risk of this damage.

SOME DRIVES have built-in protection circuits against this, so people say “it’s ok to ignore this issue”. That is generally true of EMERGENCY situations like machine safety systems that require contactor isolation of the output of the drive, which will happen rarely. But those consist of components designed to absorb and dissipate the excess voltage and just like surge suppressors, if you use them too much, they themselves fail, then the drive fails. Once you have a manual disconnect available, my experience is that sooner or later, operators get lazy and start using it every day rather than figure out how to to an orderly shutdown. So it’s always best to AVOID this by interlocking the disconnect aux to the drive.
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I appreciate your in depth response here. The more I thought about this it did occur to me that running the disconnect AUX contact into terminal 1 on the PF525 would be a better option versus my initially stated idea of running it into the PLC and then disabling the logic within the PLC which commanded the PF525 to run. I could then utilize the N.O. AUX contact on the disconnect back at the PLC if I would like to enable the VFD logic so that I do not command the drive to run when the disconnect is opened, or I could monitor the .Ready bit on the PF525 in Studio 5000.

With all of this said, I'm still a bit hazy on how I can put forth an argument to my UL inspector where I can state that I do not need to install the recommended ampere rated Class J fuses ahead of the motor disconnect if it is used after a VFD.
 
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