VFD and GFI 1

[TZellers]

I've had a little bit of experience with VFDs - enough to know about the characteristic leakage current to the ground circuit. If fed a single phase input, is there anyway to get a GFI style protection?

To further state the issue: using VFDs in residential applications with either 115 or 230 volt single phase input driving a 3 phase motor for pump use. Since water is involved, I'm fairly certain that GFI are required per NFPA 70. But VFDs trip standard GFIs, so it seems a catch-22.

 

[rbulsara]

Personnel GFCI's are not required (by NEC) for permanently wired circuits.

If you really want to have such protection, you need to used the equipment protective GFCI which essentially are less sensitive, trips at 30mA or more , compared to 4-6mA sensitivity for personnnel GFCIs required in bathrooms or kitchen.

 

[jraef]

To further rbulsara's comments, Bender Systems makes a GFI that states it can be used in front of VFDs, and they tried making a splash about this very issue a few years back when targeting the Fountain Pump and Spa markets. However what they found out is that if permanently wired, as most that would use a VFD were, GFI's are not required. It is however still an issue if you are going to use the VFD on "portable" equipment that will plug into a wall. Here is a link to their unit.

http://www.bender.org/lifeguardgfci.html

"Venditori de oleum-vipera non vigere excordis populi"

 

[jbartos]

Comment on the original posting "catch 22":
The GFCIs are for a personal protection. VFD is not considered in areas of GFCI protectors or protection. Ground fault relays or monitors, which are different from GFCI, take over. Visit

http://www.startco.ca/se701m.pdf

etc. for ground-fault monitor

 

[TZellers]

The issue with GFI and personal protection is the perceived problem. These pumps would be for decorative/ornamental ponds and often have fish (koi or goldfish) in them. People do get in them to clean, feed, de-weed or whatever. And often, the equipment is within 20 feet of the body of water. That is why I thought there would be a bigger stink as it relates to GFCI and electrical equipment. Not all pumps are isolated from the pumpage (i.e. motor shaft contacts water). Most of ours are isolated.

Thanks for the links and info.

 

[jbartos]

Suggestion: Application of isolation transformer should not be ruled out. It is used for electric lawn mowers and similar electrical appliances outdoor. It should also be adequate for electrical pumps. However, the ground fault monitoring is needed.

 

[ruggedscot]

An isolation transformer would add to the risks I think, would prevent the protection operating effectifely to discriminate the fault out of the network. A GFI will not see through a transformer and thus the load would not be earth fault protected. A GFI after the Drive also adds to the problem of VFD damage when running and being open circuited. see posts on this.

Some VFD's have earth protection built into the electronics anyways so this might give you the protection that you require. Trouble is that some shut the motor down but you still can have potential at the output.

Check the drive manual and contact the drive supplier if possible.

Rugged

 

[aolalde]

Per NFPA 70 “Receptacles that provide power for a SPA or hot tub shall be ground-fault circuit interrupter protected”.

I suggest your VFD be integrated to the pond and pump; the receptacle that provides the input power has the GFCI. The cord shall be unplugged for servicing the pond. I assume the protection is for humans since the fishes are at the water potential.

 

[peebee]

You could put a ground fault relay downstream of the VFD, and use the contact from the relay as a control input to the VFD to stop it.

There's no way you could reliably do GF tripping upstream of the VFD, you'd get too much nuisance tripping, you'd never be able to discriminate VFD leakage current from fish-electrocuting current.

As ruggedscot has pointed out, you could buy a VFD with built-in GFI.

jbartos -- I'm scratching my head on the iso tx suggestion. How would that provide GF protection?

 

[Trogdor]

I had a similar problem in which I wanted GFCI protection (even though it's not required by NEC) for workers using portable equipment with VFDs in a wet area. I purchased the Bender GFCI devices and they work fine. We haven't had any nuicance trips.

Then I noticed the VFD manual specifically says to use an adjustable GFCI and set to 200mA and not less than 0.1 second. This might be a cheaper solution as the Bender LifeGuard GFCI units are pricy.

 

[Skogsgurra]

Trogdor,

A GFI with more than 30 mA tripping current will probably not save the life of the person thet gets zapped. The 200 mA recommendation is murder waiting to happen.

Have a look at this link for more information;

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/gfi.html

 

[ruggedscot]

This is quite a difficult thing to work out, 30mA GFI's do save many lifes, if you think on the natural leakage on a 110 or 240 circuit you will see that there is always a component lost through capacitance, then if you add the imbalance on the circuit you will see that it is usually less than 30mA to trip the cicuit, due to the natural imbalance, then you take time into consideration the trip time is usually within a few cycles so the effects of this are limited, the let through is reduced so you do save a life and the only result is the person gets a bit of a fright ! Now if he was up a ladder when he contacted the hot wire and fell that could be different ! The main thing is that is trips out the supply within a relatively short time, fuses would have fried the person.

300mA GFI are for system protection, they are designed to protect an installation from ground faults, not for human life preservation.

Rugged

 

[Skogsgurra]

Yes, rugged. That is the problem. 300 mA GFIs are for system protection.

The human body seldom/never lets as much as 300 mA through - it would require a very low resistance - so the 300 mA GFI never trips. The lethal current (somewhere between 30 and 300 mA) continues to flow and kills the poor person.

One could say that the tripping condition is not met. So, 300 mA GFIs are accidents waiting to happen. They are dangerous.

 

[ruggedscot]

any GFI greater than 30mA is NEVER marketed for human life preservation. It is always for system protection. Any company that tells you different is going against the grain.

Rugged

 

[Skogsgurra]

Right. But it still happens. There seems to be a very thin line between the two zeroes in 300 mA.

 

[ruggedscot]

If someone fits a 300mA GFI to protect life then they are incompetent and should not be working as an electrician.

If there is any risk then that risk should be removed but this is not a perfect world so we need protection. 10mA GFI's can be fitted at outlet but really the sparkie working should be well versed with all types of GFI's.

Rugged

 

[Skogsgurra]

Agree again. Problem is that when 30 mA nuisance trips, then 300 mA is used instead. That seems to be a very natural thing to do - at least on building sites.

 

[ruggedscot]

No its not natural....

Sorry that fuse keeps blowing lets change it to a 6 inch nail....

that is the same logic. Clearly if it is changed to a 300mA GFI then the guy changing it doesnt know his job, or is being forced to do so by someone else, maybe he hates getting disturbed from his coffee and donuts.

There is no excuse for this one, better education is required. I had one a few years back guys fitting GFI 3 poles in as main breakers thinking that they were the main protction isolator. It was only when I hooked one up with a dead short on the output that they saw that it wasnt an overcurrent device. Blew the ass out of it but it probably helped educate them. Some people really need such things explained to them in a simple and constructive manner so that they dont go making ill informed decisions.

Rugged

 

[Skogsgurra]

Good thread, this. You are so right, Rugged. How do we educate those people? The nail example is good. Same thinking can be seen in many places. Money-driven thinking. Or no thinking at all.

 

[peebee]

As indicated in the hyperphysics site posted by skogsgurra, UL only recognizes 5mA GFI devices for personnel protection. Even 10mA is too high for UL.

However, I am under the impression that European GFI devices operate at a much higher level -- maybe 30mA? -- even for personnel protection. Can anyone confirm the IEC (or whatever is applicable) requirements for GFI in European installations?