I'm installing 18KW 480V 3-phase heaters in a liquid chemical storage tank. The manufacturer's instructions say to GFP the circuit. These will 30 amp circuits.
Well the prospect of doing this has thrown one of my suppliers for a loop, and the other one is quoting a Square D method that utilizes a shunt trip breaker and a GFI module that triggers the breaker. I trust Square D to be top of the line, but not always the most economical option.
Does anyone have a manufacturer or method they could recommend to accomplish this?
Does the manufacturer actually want ground fault protection for people (very low current, 4ma range),or are they looking for grounding and bonding of the tank or heater mounting assembly in such a way as to insure that the breaker will trip in case of a fault (equipment protection)? Or are they asking for a trip based on an imbalance on the phases indicating a blown element? Feed us some more information. It seems to me that if this manufacturer is requiring it, they should have some idea of how to do it.
Re: 480 volt 3-phase GFI?#47642 01/21/0510:58 AM01/21/0510:58 AM
Marc, thanks for the link, but I can't find anything 480V.
GamecockEE, the manufacturer rep. tells me it is for equipment protection. I asked him what they wanted to see the trip value at and he told me he had it written down somewhere, he thought it was 500ma, but it is written down somewhere. This kind of told me the level of expertise I was dealing with, so I decided I would pursue finding GFI protection at 30ma and know I was covered.
I am still waiting to hear from Square D to see if their method will adjust down to anything below 20 amps as the book spec.'s state. I am wondering if it is even the right thing.
Then JBD gives me the link that saves the day. I knew I brought this to the right place. These are available immediately and are exactly what I was picturing in the back my mind as I made the original inquiry. Thanks Guys!
Re: 480 volt 3-phase GFI?#47644 01/21/0506:58 PM01/21/0506:58 PM
A capacitor is simply two electrodes separated by an insulator. The capacitance depends upon the surface area of the electrodes and the spacing between them. When you apply an AC voltage to a capacitor, some current will flow, even though (in the ideal case), there are _zero_ charge carriers (electrons) moving between the electrodes. This is because at least a few electrons have to move into or out of the electrodes in order to match the voltage of the electrodes to that of the source. Apply a continuously changing source, and you end up with a continuous current flow.
The current flow will be proportional to the capacitance, the applied voltage, and the frequency. The higher the voltage the higher the capacitive current to ground.
Thinking about it, I don't know what would happen in a perfectly balanced system; clearly the system will still have capacitive coupling to ground, but if the charging current is perfectly balanced, I don't think the ground fault system will detect any imbalance.
I've been buried in ground fault detection on systems with variable speed drives, where this is significant common mode capacitive current at the switching frequency. Ground fault pickup needs to be set above any expected and normal ground current flow.
I would still suggest hounding the heater supplier until they provide you a firm number for their recommended trip level, and then get ground fault protection set at that trip level.
Re: 480 volt 3-phase GFI?#47647 01/23/0510:32 AM01/23/0510:32 AM
I have installed these in an industrial heat transfer liquid ("Syltherm") system for a chemical reaction plant. Even though one of the heating element sheaths split open and exposed the bare element, the resistance of the Syltherm was so high that the CB (100 amp) didn't trip.