dezertdweller
Member
- Location
- Dallas, Texas
What section of NEC states that the manufacturers recommended fuse sizes must be used to protect the VFD? I am certain I have read this before...
VFD Fuse Protection
- Thread starter dezertdweller
- Start date
- Status
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
There is none specifically. There is 110.3(B) which more or less says you have to follow the instructions included with the listing.
Whether that covers the situation you are asking about is questionable. It is doubtful that the entire manual is part of the listing.
dezertdweller
Member
- Location
- Dallas, Texas
I think Article 110.3 is enough to prove that the VFD manufacturer's recommended fuse size must be used to maintain the UL Listing.
Thanks
Thanks
dezertdweller
Member
- Location
- Dallas, Texas
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
The device itself is listed reqardless of whether you put the right fuses in place or not.
Incidentally, has anyone ever seen a VFD manual that says you can use circuit breakers instead of fuses?
Here is what a typical AB manual says.
It goes on to say:
The use of the word guide seems to imply they are optional and use of the word minimum implies they can be larger.
The table this is a note to is for using certain AB motor circuit protectors, so it would appear not to apply generally.
As best I can tell by reading the AB manual for this drive, unless you are using a particular motor circuit protector, any CB or fuse on the input suffices.
Last edited:
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
I know it appears a bit confusing, but remember that not all areas of the country require NRTL listing (i.e. UL). So here's what it is saying:
1) The first paragraph is saying that you can meet NEC requirements with fuses or circuit breakers.
2) If you need a UL listed installation, you have one of those fuse types listed and the minimum size in the chart. So for instance if you buy a PF40 in an MCC and you want CBs, you will open the bucket and see fuses down stream of the CB. This is because the fuses are essentially being used as current limiters in the circuit. The fuse SIZE they put in there is coordinated with the CB size so that the CB trips first under most common fault conditions. But for the Short Circuit Current Rating testing (and UL845 in the case of MCCs), they needed the fuse current limiting capability.
That is pretty much the case for all small VFDs I am aware of like the PF40. Breakers are just too slow for short circuit testing of small VFDs. I think it's because the diodes in the Intelligent Power Modules that everyone uses on small drives now will pop by the time the breaker clears and it fails the test. If you have ever witnessed a UL sort circuit test you would understand. Basically they place the drive in a box, stuff the box full of cotton and connect the short. If ANY of the cotton is even a little too brown, it fails. larger VFDs will usually pass because they use more robust devices, so for instance the PF70 and 700 drives are UL listed with breakers.
But the only breaker-like devices the PF40 has been tested and listed with for SCCR ratings are their own Buletin 140M motor protectors (you didn't say that, I'm reading my manual). I can't explain how they got those listed that way, but it's in their file. (PS, it might be because unlike MCCBs, they can be adjusted to trip at lower levels, which would reduce the total clearing time.)
Here is a link that will allow you to look up the SCCR rating of any A-B device (that has it) and what it must be used with. Most of the major mfrs have this kind of information available now in one form or another.
Last edited:
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Most likely, they only want to pay for testing of their devices.
They could realize a monetary loss if they tested using someone else's circuit breaker. UL testing with fuses, requires a universal umbrella fuse so the manufacturer is not important.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Yes I figured that the why part was because they don't want to support the competition, that's pretty much par for the course. I was referring to how I was surprised that they could get those little 140M Motor Protector switches listed to protect the VFDs, but not the MCCBs. Before I looked I was expecting it to be fuses only.
I later ammended what I think may be the case. If you are trying to list a 1/2HP VFD rated 1.4A and the smallest MCCB is 15A which will have a 150A magnetic setting, there is a little more time before the MCCB begins to open compared to the Motor Protector Switch which, having an adjustable trip, may begin opening at 14A. Still an assumtion though.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Circuit breakers are not always as slow as many believe, even though they do not meet the strict parameters to be UL Listed as Current Limiting.
So a manufacturer might be able to create a specific breaker, wire, VFD combination that passes a UL short circuit test.
15 to 20 years ago, most drives I ran accros required fuses to maintain their UL Listing (if you need it). Now, most that I see dont require fuses to maintain the UL listing. SCCR is another matter. If the mfg. bothers to get a coordinated rating, they usally do it with their own breaker and the like. They all do it that way.
- Status