GFCI breaker, single pole switching and 2pole cable

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Brownsound

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Toronto Ontario Canada
Hi everyone,
i have several runs of 208v heat trace cable (single phase 2 pole, think of it as 220v or 240v, whatever makes sense to you based off where you're reading this from), which will be controlled by a contactor. The cct is being fed from a 2pole GFCI breaker set to trip at 30mA. They have added additional runs of heat trace cable and now are out of poles on the contactor. The engineer has suggested we only switch one of the legs of the heat trace cable on the contactor to allow us twice the contactor capability (so our 12P contactor could switch as many as 24 runs of heat trace cabling). I think switching one leg of the circuit might cause the GFCI to trip on inrush (if the 'cold' leg of the cable draws more than the 30mA difference the GFCI allows). Has anyone run into this ? I've never worked with heat trace, 30mA GFCIs OR 'single legged switching' so I might be overreacting - if anyone has had this experience please advise :)
 
Brownsound said:
Hi everyone,
i have several runs of 208v heat trace cable (single phase 2 pole, think of it as 220v or 240v, whatever makes sense to you based off where you're reading this from), which will be controlled by a contactor. The cct is being fed from a 2pole GFCI breaker set to trip at 30mA. They have added additional runs of heat trace cable and now are out of poles on the contactor. The engineer has suggested we only switch one of the legs of the heat trace cable on the contactor to allow us twice the contactor capability (so our 12P contactor could switch as many as 24 runs of heat trace cabling). I think switching one leg of the circuit might cause the GFCI to trip on inrush (if the 'cold' leg of the cable draws more than the 30mA difference the GFCI allows). Has anyone run into this ? I've never worked with heat trace, 30mA GFCIs OR 'single legged switching' so I might be overreacting - if anyone has had this experience please advise :)
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The Cold leg will not draw any more than the Hot one because there should not be any current flow until both legs are energized. No different than any other load where you switch one of the conductors for control purposes.
 
I think there may even be less chance of tripping if one lead is hot all the time, especially on a 30 mA trip device. A little moisture finds a way to an energized component but the fault current remains below 30mA, it will likley dry that moisture up. Switch both lines and that fault may reach trip level when you do apply voltage. You also are have different capacitive effects if one line is "always on" The "capacitor" will have an "inrush" current when energized, but one that is always energized will have more of a steady state current, though it will see some change when closing the contactor to the other line.
 
kwired said:
You also are have different capacitive effects if one line is "always on" The "capacitor" will have an "inrush" current when energized, but one that is always energized will have more of a steady state current, though it will see some change when closing the contactor to the other line.
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It's that inrush that I'm worried about. If the one leg has the steady state (pretty much throughout its entire 250' length) and the other leg once energized has to make it all around the circuit, and the difference between the two amounts to greater than 30mA (even for that short time) the GFI could trip thinking that someone is absorbing that difference in current. But, since I so far have 2 votes saying I'm overreacting, I probably am !
 
Brownsound said:
It's that inrush that I'm worried about. If the one leg has the steady state (pretty much throughout its entire 250' length) and the other leg once energized has to make it all around the circuit, and the difference between the two amounts to greater than 30mA (even for that short time) the GFI could trip thinking that someone is absorbing that difference in current. But, since I so far have 2 votes saying I'm overreacting, I probably am !
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I'm saying there is a chance you have less surge current if you already have 120 volts across the "capacitor" then if you energize the capacitor from a zero volt reference level. You still have 120 volts max to ground whether you have one pole or both poles closed, but there is probably still going to be a surge of current when the contactor closes, just not sure exactly what level it will reach, I don't usually have to deal with this type of calculation and the theory behind this is not fresh enough in my mind on how to determine exactly what will happen here.
 
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