Three Phase Ground Fault Protection

[gabany]

I have a question concerning ground fault protection on aerator motors in a lagoon at a water treatment facility. These are 3 phase 480V motors, floating throughout the lagoon, connected via a submersed 4 Wire cord. Of course, no one actually enters the lagoons, but maintenance is often performed by boating out to the motors. GF protection is not required for the equipment. Should (or is it) required for personnel safety? I cannot find any thing to say it is.
Thanks

 

[bphgravity]

Re: Three Phase Ground Fault Protection

I don't feel gfci protection is needed if safe working practices are performed, like deenergizing the equipment before entering the lagoon.

 

[charlie b]

Re: Three Phase Ground Fault Protection

Although it may not be required, it is worth careful consideration. Specifically, it is worth considering the following two "proposed design criteria":
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• <font size="2" face="Verdana, Helvetica, sans-serif">They will be turning off a given motor before working on that motor. But the other motors will continue running.</font>

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• <font size="2" face="Verdana, Helvetica, sans-serif">Somebody is going to fall out of the boat and into the water.</font>

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[gabany]

Re: Three Phase Ground Fault Protection

All the aerators are not shut off to go work on one. Although that does sound like the right thing to do, the process cannot support this (IOW, shutting down all the aeration for a lenghty time would upset the lagoon conditions).

 

[ron]

Re: Three Phase Ground Fault Protection

Many motor starters can be purchased with GF protection. This will offer equipment protection but not personnel protection, as the setting is too high.
Boating out to the motor in a metallic boat will place the worker at the same potential as the water, which may offer a level of safety when the motor under service is turned off and disconnected from the starter (no ground continuity out to the motor).

There was an interesting program on the discovery channel showing a helicopter lifting a lineman to some high voltage conductors to perform maintenance. All was OK, because the helicopter was at the same potential as the conductors.

[ January 31, 2006, 04:08 PM: Message edited by: ron ]

 

[templdl]

Re: Three Phase Ground Fault Protection

You would never get a 4-6ma people protection GF to work as it would be prone to nuisance tripping. You may be able to apply a 30ma equipment protection rated GF device.

 

[electricman2]

Re: Three Phase Ground Fault Protection

Did this very thing at a wastewater treatment plant where I worked. Worked from an aluminum boat to lubricate aerator motors. We only shut off the one we were working on. Electrical service to the plant was 480V 3 wire ungrounded system monitored by a ground fault indicator. As I remember no one ever considered shutting off all 4 motors at once.

 

[gabany]

Re: Three Phase Ground Fault Protection

This is turning into an interesting debate. I called the manufacturer, and they said "ground fault protection is not neccessary for the equipment, check your local codes. if there is a short to ground, the breaker will trip anyway right?" I then read these 2 article from this web site and it makes me think even more.

http://www.mikeholt.com/mojonewsarchive/Safety-HTML/HTML/Freshwater-Marinas-A-Ground-Fault-Analysis~20040826.php

http://www.mikeholt.com/mojonewsarchive/SV-HTML/HTML/Hot-Marina-Lethal-AC-Ground-Faults~20031117.htm

 

[bphgravity]

Re: Three Phase Ground Fault Protection

GFCI protection does not protect workers from being injured or killed, safe working practices do. The GFPE protection the manufacturer is suggesting will not protect persons, only the equipment.

 

[charlie b]

Re: Three Phase Ground Fault Protection

Originally posted by gabany:. . . they said ". . . if there is a short to ground, the breaker will trip anyway right?"

True, but irrelevant.

Please take note that a current of far less than one amp can be fatal. However, adding one more amp of current to the load seen by a breaker is not going to cause the breaker to trip. A GFCI is not there to protect against a "short to ground." It is there to protect against a small amount of current flowing through a person's body.

 

[ramsy]

Re: Three Phase Ground Fault Protection

Originally posted by ron:
There was an interesting program on the discovery channel showing a helicopter lifting a lineman to some high voltage conductors to perform maintenance. All was OK, because the helicopter was at the same potential as the conductors.

I saw that too. As the helecopter approached they used a botton to cautch the electric arc that jumped several feet to meat them. They called it "bonding on".

For bonding on the lagoon, wonder what happens when one hand touches the aluminum boat, and the other a grounded guard rail?

Falling in the sess pool would agravate me more than the 480 volt shock.

 

[realolman]

Re: Three Phase Ground Fault Protection

Originally posted by ramsy:

Originally posted by ron:

Falling in the sess pool would agravate me more than the 480 volt shock.

That's funny. But I doubt it.

 

[gabany]

Re: Three Phase Ground Fault Protection

GFCI protection does protect workers from being injured or killed, however, it sounds like it is not feasible in this application. Looks like we are only fooling ourselves by installing GF equipment protection (fooling ourselves in thinking it provides personell protection).

 

[ramsy]

Re: Three Phase Ground Fault Protection

Check out Mike Holts "Grounding vs Bonding Online Training 24" the accompanying video 250-54-000.ram describes line-to-line Residual Current Monitor/Detector(s) and one product called "Marina Guard" with an adjustable trip from 10mA - 10A. I believe 10mA is survivable.

You can search the general catagory of Mike Holt Newsletters

 

[gabany]

Re: Three Phase Ground Fault Protection

10mA may be survivable, but I believe that may be so low it will cause excessive nuisance tripping. Would you agree?

 

[templdl]

Re: Three Phase Ground Fault Protection

gabany,
Kind of hard to think it would work when one considers .005ma sensitivity and .010a is not that much less. (Kind of hard to think that more is less.)
Remember the nuisance tripping problems that one can have with those 1ph 20a-60a 2P GFCIs for spas and whirlpools due to all sorts of weird phenomena? Just think of trying to do it with 208, 240. and 480v. It would be easy for circuit to have a just a small enough imbalance such that the breaker is going to respond as a GF.
The next is the 30ma ELCI (earth leakage) but still 120/240 2p as close as I can tell.
The lowest 3ph device that appears to be common is 1a but Eaton may still have one that can go lower. I have a sample with selectable setting from .1 to 3.2a and time delay adjustments. It is a window type CT that you would run all the phase conductors through, and neutral if applicable, with the electronics packaged with. It requires control power to operate and has contacts that close to for use in conjunction with a breaker shunt trip.
Dave

 

[ramsy]

Re: Three Phase Ground Fault Protection

Thats probably why its adjustable. Anything could be survivable, but the chance is better when it trips as some point vs never.

 

[rcwilson]

Re: Three Phase Ground Fault Protection

GF protection on 480 V motors can lead to false trips when the current transformer(s)saturate during motor starting current. The tripping currents are so low compared to the starting and running currents that it is difficult to provide good ground fault protection and be able to reliably start the motor. Some motor GF protection modules get around this by inhibiting tripping for the first 4-10 seconds of motor running time.

Probably the safest thing for the pond aerator motors is a good maintenance & inspection program on the ground wire (equipment bonding wire) in the cable. Make sure it is connected well at each end and that the trailing cable has not been damaged. Maybe run a two-point continuity test between the MCC ground and the motor frame every year.

Theoretically, if the motor is properly bonded, there should be no danger.

 

[mprairie]

Re: Three Phase Ground Fault Protection

Check out this site:
http://www.ktec.org/

My company supplied 58 hp 480v submersible pumps to a movie set for use in water tanks with divers.
At the time, they were the only company that could come up with a system to satisfy the inspectors in Hollywood.
This site contains a lot of info on 3 phase GFI's for personnel protection.
Mark

 

[templdl]

Re: Three Phase Ground Fault Protection

mprairie,
Thanks. That product is certainly unique.
They must have done a good job in their design if you think about it. A device that has a 3ph 480v, 100a rating with a 5ma sensitivity that is classified UL 943 Class "A."
Just think of having to deal with up to a 100a load at 480v 3ph an still and a .005a sensitivity.
You would think that at that voltage and current capacitance of inductance would very likely create enough of an imbalance to fool a .005a sensitive device into nuisance tripping.