Frequency Drives

[danbel57]

Why is ground fault protection required for motors driven by VFD's ? Article 430.130

 

[infinity]

Here is the substantiation for the code change in the 2014 NEC:

Substantiation: Currently, sections 430.52 and 430.53 describe the allowable
overcurrent protective devices for motor circuits and their ratings and settings.
While the existing text is adequate for electromechanical motor starting
equipment, adjustable speed drive controllers may have special requirements
that pertain to overcurrent protective devices.
New sections 430.130 and 430.131 are proposed to deal with the special
requirements of the adjustable speed drive controller overcurrent protection
functions. The proposed text contains both mandatory and informative
information on where to find the required overcurrent protection for a drive
controller as well as the rationale for adjustable speed drive controller
apparatus overcurrent protection requirements.
New section 430.130 (A)(3) is proposed to clarify that self-protected
combination controllers are only permitted when the listing of the power
conversion equipment specifically allows for protection by those devices. This
would prevent the application of 430.52 (c) (6) unless the drive controller has
been investigated specifically for use with self-protected combination
controllers.

 

[Ingenieur]

one reason I can think of is a gnd fault on the vfd output may be current limited and not trip an input cb

 

[Ingenieur]

Here is the substantiation for the code change in the 2014 NEC:

do you have the rationale?
The proposed text contains both mandatory and informative information on where to find the required overcurrent protection for a drive controller as well as the rationale for adjustable speed drive controller apparatus overcurrent protection requirements.

 

[Jraef]

Are you misinterpreting the term "ground-fault protection" perchance? In this context, "branch-circuit short-circuit and ground-fault protective device" is just how the NEC refers to a thermal-magnetic or electronic circuit breaker or Motor Protection Circuit Breaker or Fuses as the SCPD. That should not be confused with "Ground-Fault Protection of Equipment" or "Ground-Fault Circuit Interrupter". All three terms have specific meanings in the NEC in spite of colloquial use of the term "Ground Fault" to mean only the last two (or one). It's important to understand that. Go to the Definitions section of the NEC to see the differences.

A "branch-circuit short-circuit and ground-fault protective device" just means it will open on short circuit current going from line-to-line, or line-to-ground. This term is used consistently throughout Parts IV and V of Article 430 as well, the only change is that they extended it to Part X, where it hadn't been specifically addressed before, leading to people sometimes not using a "branch-circuit short-circuit and ground-fault protective device" because the Code didn't specifically state that it was necessary.

 

[danbel57]

one reason I can think of is a gnd fault on the vfd output may be current limited and not trip an input cb

Thank you for your input. I was looking for an explanation other than what I was reading in the code, and you've given me some insight as to how these drives function. So, from what you're telling me, the drives by their design will not present the fault current that may actually be present, which would allow a normal breaker to trip.

 

[Jraef]

Thank you for your input. I was looking for an explanation other than what I was reading in the code, and you've given me some insight as to how these drives function. So, from what you're telling me, the drives by their design will not present the fault current that may actually be present, which would allow a normal breaker to trip.

Anything that happens on the load side of the drive is not directly connected to the line side protection. The drive protects the load, the breaker or fuses are only there to protect the drive. Since 2005, UL listing of VFDs requires that the VFD now provides Short Circuit and Overload protection for the load connected to it.

 

[kwired]

Anything that happens on the load side of the drive is not directly connected to the line side protection. The drive protects the load, the breaker or fuses are only there to protect the drive. Since 2005, UL listing of VFDs requires that the VFD now provides Short Circuit and Overload protection for the load connected to it.

I agree, and will say that the breaker or fuses are mostly there to protect the supply conductors, if the front end of the drive fails, it is usually toast anyway, fast acting fuse or breaker will only reduce how spectacular of a failure one may encounter.

The most minor ground fault on the output will trigger a fault and drive will cease supplying any power to the output terminals. The ground fault protection OP is asking about is short circuit and ground fault protection and not GFPE or GFCI protection.

 

[Jraef]

I agree, and will say that the breaker or fuses are mostly there to protect the supply conductors, if the front end of the drive fails, it is usually toast anyway, fast acting fuse or breaker will only reduce how spectacular of a failure one may encounter.

:thumbsup:
I can't tell you how many times I've tried to get that point across to people when they complain that the expensive high speed semiconductor fuses didn't "save the drive". They will only clear AFTER the drive has already let out its magic smoke, the semiconductor fuses simply limit the fireworks show, then cost 4x as much to replace. The only people who benefit are the fuse suppliers.

I've noticed that most drive mfrs have stopped recommending these expensive fuses and just list standard Current Limiting fuses (if any) in order to get their SCCR ratings to be reasonable. The only place you see the semiconductor fuses being recommended for drives any more are... fuse catalogs!

 

[danbel57]

Thank you everyone for your comments.