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hjens56 (Mechanical) (OP)
5 Jul 07 17:40
I have a question about installing a GFCI on a machine which has a number of components, some of which run on 208V 3-phase and others on 120V single-phase.  Is it possible/safe to install a GFCI at a single 208V 3-phase input to the machine and split the 120V loads off after the input using a single hot wire and the neutral? Would a GFCI made for 208V three-phase work?  Thanks, in advance.
JensenDrive (Electrical)
5 Jul 07 18:03
I am not familiar with 208 GFCI. Does the model you have in mind sum all 3 phases as well as neutral? If yes, then single phase loads should not bother it. As long as Ia+Ib+Ic+In are 0, then all is well. If it istead assumes a delta load, and really only sums Ia+Ib+Ic, and ignores In, and you connect a load that will cause In > 0, you are going to trip.
hjens56 (Mechanical) (OP)
5 Jul 07 18:29
That's the conclusion that I had arrived at, but I wanted to check with others to see what they think.  However, another part of my problem is where to find such a device. I had found a GFCI that claimed to work for 208-240V 3-phase, but it has very limited documentation and is a bit of a black box. As far as I can tell, it seems like it is assuming a delta load (and input for that mattter), as it functions correctly for a three-phase load, but trips when any 120V single phase load is connected.
Anyone know of a GFCI that would work in this situation?
JensenDrive (Electrical)
5 Jul 07 18:43
We need to have more experienced people chime in on this  one. But for my 2 cents worth, I have seen people put zero sequence CTs on three phases + neutral and run the output to rather some inexpensive relay (Crompton or such) and then to a breaker shunt trip, but I do not think they were highly sensitive. Mostly at 208V you get breakers with ground fault functions built in, but again I do not think they are commonly as sensitive as GFCI.
dpc (Electrical)
5 Jul 07 18:43
I don't think you're going to find what you are looking for.  What is the purpose of the ground fault protection - for operator safety or equipment protection?  

If you are just trying to detect ground faults, a molded case breaker with a solid-state trip unit including ground fault would do what you want, but you would need a neutral sensor in addition to phase sensor.

If you are trying to match the function of a standard single-phase GFCI breaker or receptacle, that is another matter.  These operate at a much lower current level to prevent electrocution.  Best bet for this would be to add GFCI receptacles or breakers for your single-phase loads downstream of the three-phase breaker.  
LionelHutz (Electrical)
9 Jul 07 10:40
There are two options that I know of.

#1 - Startco SE-502 relay (startco.ca). This relay basically resistance grounds the neutral of a Y supply transformer and trips a under-voltage coil in a breaker. I have used one of these and tested it works with 6mA of ground current.

#2 - Bender LifeGuard GFCI (bender.org). This is a complete relay/breaker/transformer UL listed unit that provides the ground fault protection.

thebutler007 (Electrical)
1 Sep 07 23:01
check out square d's web site....they offer three phase ground fault relays for great for motor applications...we had to hook up a bunch of mitsubishi ductless split a/c units.....big warning on their literature to provide "ground leakage detection"...sounded like gfci to me....relays have under voltage detection too....
davidbeach (Electrical)
2 Sep 07 0:59
The Eaton (Cutler-Hammer) family of "Earth Leakage" breakers would be a consideration.  These will often be the easiest to implement compared to the external ground fault modules such as Square D.  Also, they probably have a better range of settings.
ScottyUK (Electrical)
2 Sep 07 4:15
30mA three pole or 3-pole + neutral RCDs are fairly common in the UK. They use the same principle as the single pole RCDs - a core balance CT with a sensitive tripping circuit. I don't recall seeing them with a 10mA rating because under UK legislation that is only required in higher risk environments. The standard RCD (Residual Current Device - GFCI to North Americans) for personnel protection is a 30mA setting.

Here's Schneider's offering:

http://www.merlingerin.com/MG/en/products/index_fon1_fam52_M9_ID_RCCB.htm
 

----------------------------------
  
Sometimes I wake up Grumpy.
Other times I just let her sleep!

thebutler007 (Electrical)
2 Sep 07 22:26
We had some 3 phase applications of for the mits a/c outdoor a/c units....ocp required like 60A/3p could not find any commercially available cb's(us) with gfci....went with the relay....

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