underwater 480V 1

[pierguppy]

we are a diving company we use a electro-hydraulic cleaning machine.
the motor is a 15HP 480VAC 3 phase.
what i am looking for are some guidlines in setting up a GFCI circiut for this system.
Most importantly the trip level (ma) and the trip delay (ms)
i cant seem to find any references on this type of system.
PS. we use a 600 foot umbilical which tends to complicate

thanx

 

[redtrumpet]

You will not be able to make this circuit as safe as a GFCI circuit, due to the voltage, current, and the fact it is a three-phase system. GFCI circuits trip at 4-6 mA and therefore ensure safety of people. A ground-fault protection scheme for this circuit will likely have a minimum trip level of a few hundred milliamps, depending on how sensitive a ground fault relay you can find and the charging current of the circuit. A 600 foot cable in water will probably have a high charging current. The trip setting must be higher than the charging current. A ground-fault protection scheme in this situation will reduce the hazard to people, but will not ensure their safety in a shock situation.

My recommendation would be a separate ground fault relay with a window-type (core balance, zero sequence) current transformer surrounding all three phase conductors. This relay should feed a circuit breaker with a UV release; this will make the circuit fail-safe as opposed to using a shunt trip.

The other option is to use an electronic circuit breaker with internal current sensors. However, these breakers use residual sensing to determine ground fault current (not quite as accurate as the window CT) and tend to have minimum settings in the ampere, not milliampere, range.

These schemes will work only if the pump is fed from a wye secondary system, not a delta system. It may also be advantageous to resistance-ground the neutral to limit ground fault current to 5 A or less and improve the sensitivity of the ground-fault detection.

Either solution will be quite expensive, certainly several hundred dollars, probably more than $1000. Perhaps there are some manufacturers of pump protection panels that incorporate ground fault detection that could help you out; I seem to recall seeing these panels on the market a few years ago for submersible and swimming pool pumps.

 

[busbar]

That’s an interesting challenge. A 600-foot cable with acceptable leakage characteristics would seem to be very speciallized. There is an odd little “class-A” (5-milliamp) CFCI that has an integral ‘donut’ CT for use with an external contactor, but I don’t think it is intended for a marine environtmemt, or for voltage above 250.

http://www.leviton.com/pdfs/6895gfci.pdf

There a some zero-sequence ground-fault relays, like the ABB GRD/GKT/GKC-series, but their minimum pickup seems to be 2 amperes.

A ground-check (continuity monitor) feature like that used in mining electrical distribution should be investigated also.

Is a deck-mounted air compressor with a pneumatic motor at the subsurface worksite out of the question?

 

[redtrumpet]

More on this from some literature I have from Startco Engineering Ltd. (

http://www.startco.ca

), circa 1996:

"Some work is being done to develop a ground-fault circuit with GFCI sensitivity for 3-phase circuits. To date, such units are not commercially available. In the coal mining industry, 750 mA neutral-grounding resistors are used and the operating value of the ground-fault relays is 80 mA."

I suggest you contact Startco, either through the web link above or by phoning 306-373-5505 and asking for Blair Baldwin. Blair is more knowledgeable about ground-fault protection than any other engineer I know.