Motor short-circuit protection when using VFD

[overkill94]

I work as a third-party inspector on electrical equipment and I was taught to size the input fuses for a VFD per its manual but recently a city inspector claimed that the short-circuit protection should be sized to the motor instead. I believe the example was a 1 HP motor rated at 1.6 Amps at 480 Volts with which they installed 10 Amp fuses per the manual for its VFD. I've noticed that the manuals for the VFDs usually say that the NEC and other codes supercede the recommended input fuse sizing so does this make their recommendations useless when it comes to code compliance?

 

[petersonra]

I work as a third-party inspector on electrical equipment and I was taught to size the input fuses for a VFD per its manual but recently a city inspector claimed that the short-circuit protection should be sized to the motor instead. I believe the example was a 1 HP motor rated at 1.6 Amps at 480 Volts with which they installed 10 Amp fuses per the manual for its VFD. I've noticed that the manuals for the VFDs usually say that the NEC and other codes supercede the recommended input fuse sizing so does this make their recommendations useless when it comes to code compliance?

X. Adjustable-Speed Drive Systems
430.120 General. The installation provisions of Part I
through Part IX are applicable unless modified or supplemented
by Part X.

I do not see anywhere in Part X where the requirements for BC OCPD are modified. i would want to look at the drive manual before I commented on why it might say such a thing.

1.8* 1.75 = 3.15. next standard rating for fuses is 6A.

I don't know where the 1.6 FLA came from but for this calculation you use the number in the chart in the code and not the motor nameplate.

It is possible that the drive manufacturer wants you to use CC fuses which can goto 400% if needed to start the motor. Maybe that is their claim.

1.8*4=7.2. next std size 10A.

I tend to use CB for small motors as they are no more expensive usually than fuses and 15A is the smallest I am ever required to use.

 

[Jraef]

430.52,
(C) Rating or Setting.

(5) Power Electronic Devices. Suitable fuses shall be permitted
in lieu of devices listed in Table 430.52 for power
electronic devices in a solid-state motor controller system,
provided that the marking for replacement fuses is provided
adjacent to the fuses.

This allows you to use the fuses listed by the VFD mfr for protecting the VFD, as per their UL listing and SCCR rating, without having to worry about the effect that might have on sizing based on the motor HP. This was done because UL requirements for listing of VFDs changed in around 2005 to make it a requirement that the VFD protect against short circuits on the MOTOR side, because there is too indirect of a connection to the Line side OCPD. So bottom line, the VFD is now covering the SCPD protection on the load side, you only have to deal with the size fuse they require to protect the VFD itself.

By the way, this does NOT apply to breakers (although I haven't seen the 2014 yet to check if they modified this). That's why you see in MCCs and other factory built systems where even though there is a breaker feeding the drive, there are fuses too. that gets ignored in the field all the time, but technically, the fuses need to be there, or the breaker needs to be sized to the motor, which might not work for the VFD SCCR requirements. It's a bit of a dance sometimes. That gets even more complex if there is a bypass system for the VFD, I'll warn you, but it's not impossible to keep everyone happy, as long as they truly understand all of the rules and follow them.

Bottom line, if you had fuses and they were sized per the VFD requirements, your city inspector was wrong, he didn't read the entire section.

 

[GoldDigger]

"Too indirect of a connection", as for example that a single line fault in the motor side would effectively be driven by all three supply lines in parallel? Not to mention the energy storage in the DC bus capacitor bank

 

[petersonra]

This allows you to use the fuses listed by the VFD mfr for protecting the VFD, as per their UL listing and SCCR rating, without having to worry about the effect that might have on sizing based on the motor HP. This was done because UL requirements for listing of VFDs changed in around 2005 to make it a requirement that the VFD protect against short circuits on the MOTOR side, because there is too indirect of a connection to the Line side OCPD. So bottom line, the VFD is now covering the SCPD protection on the load side, you only have to deal with the size fuse they require to protect the VFD itself.

By the way, this does NOT apply to breakers (although I haven't seen the 2014 yet to check if they modified this). That's why you see in MCCs and other factory built systems where even though there is a breaker feeding the drive, there are fuses too. that gets ignored in the field all the time, but technically, the fuses need to be there, or the breaker needs to be sized to the motor, which might not work for the VFD SCCR requirements. It's a bit of a dance sometimes. That gets even more complex if there is a bypass system for the VFD, I'll warn you, but it's not impossible to keep everyone happy, as long as they truly understand all of the rules and follow them.

Bottom line, if you had fuses and they were sized per the VFD requirements, your city inspector was wrong, he didn't read the entire section.

I have seen this provision before and never really thought much about it. I have noticed that UL508a has a blurb that is a lot clearer along these lines.

 

[don_resqcapt19]

...
By the way, this does NOT apply to breakers (although I haven't seen the 2014 yet to check if they modified this). ...

Now you have to use "semiconductor" fuses, not just "suitable" fuses.

(5) Power Electronic Devices. Semiconductor fuses intended for the protection of electronic devices shall be permitted in lieu of d evices listed in Table 430.52 for power electronic devices, associated electromechanical devices (such as bypass contactors and isolation contactors), and conductors in a solid-state motor controller system, provided that the marking for replacement fuses is provided adjacent to the fuses.

 

[Jraef]

Now you have to use "semiconductor" fuses, not just "suitable" fuses.

Thanks for that.

But wow, that's all that changed? I know several drive mfrs were pushing for the inclusion of tested combinations of circuit breakers, but I guess it didn't fly. In fact, changing from "suitable" to the more specific "semiconductor" fuse designation is even a little more restrictive, and the fuses are more expensive.

Were I more of a conspiracy theorist, I might think the fuse industry lobbiests might have had a hand in this, but now that Bussman is Eaton and Eaton sells VFDs and CBs so would benefit from the CB inclusion as well, I'm sure that they would have not contributed any pressure to change the fuse requirements. I doubt that Mersen and Liitlefuse can still hold such sway on their own, so I'll have to concede that there must be a legitimate reason for that... Having witnessed short circuit testing of VFDs I can't think of what it might be, but I was not privy to all testing, only the final acceptance tests. The last stage of the test was to make the fuses clear a fault to demonstrate that the drive does not become a bomb. But in order to make the fuses on the line side clear from a short on the load side, we had to jump out the transistors and turn on the power. In the real world, I have never seen a transistor short (become a conductor), it either opens or explodes, which is an opening of sorts as well.

 

[overkill94]

Sorry to bring up a dead subject, but I just noticed the blurb in UL 508A that someone else mentioned (31.3.2). It simply claims that you should follow the manufacturer's instructions and if it's not specified then you should go by the normal calculations.

With this installation being in a control panel rather than a standalone VFD, would 508A take precedence?

 

[Aleman]

By the way, this does NOT apply to breakers (although I haven't seen the 2014 yet to check if they modified this). That's why you see in MCCs and other factory built systems where even though there
is a breaker feeding the drive, there are fuses too. that gets ignored in the field all the time, but technically, the fuses need to be there, or the breaker needs to be sized to the motor, which might not
work for the VFD SCCR requirements.

We have been using breakers on some and fuses on others. We paid a listing agency (TUV) to come out and look over what we were building and our only problems were labeling; SCCR and fuse values etc.
All drives were on breakers and he didn't mention that. I see plenty of panels with motor protectors both feeding and protecting drives. The ones with the adjustable overloads and a lockable rotary switch.
I see these especially in European built panels.