Manufacturer instructions II

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megloff11x

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And similarly, we have a heater system that needs to use a phase fired SCR. All parts are listed, but the suggested hookup is having the "single phase" 230V heater between two legs of a 208V wye, and the SCR is controlling one leg. This means when you turn it off, the heater has one side still live and the other open circuit. I prefer to remove all power when it's off. Say no to "neutral switching" or in this case, switching the other phase. Adding an intermediate disconnect adds cost - there is a main switch on the system which removes all power. The phase firing gets more precise control than zero cross switching, and is needed in this case. They don't seem to be able to implement it two legs at a time, which also bothers me; why not?

I was wondering if anyone has dealt with this issue. The manufacturer instructions say it's OK and I see nothing in the code on it, but I would prefer to cut all power when it's controller says it is off.

In some cases it's OK not to fuse all legs of a motor, but I don't get a warm and fuzzy there either.

Matt
 
I don't see a problem either. Like Petersonra said, the SCR isn't a safety device. If the main switch removes power from both lines, that is your disconnect. If not, then you may need to add a disconnect (or see if the circuit breaker qualifies).

If the current draw isn't too large, often a double pole toggle switch (i.e. double pole light switch) can often be the disconnect.

Putting an SCR in each leg probably wouldn't work. An SCR turns off when the current through it goes to zero. So you would have to turn them both on at the same time, but they would need different gate voltages (since they are on opposite sides of the load), and one would have the anode connected to the load, and the other to the line, and you get a lot of complication for nothing.

I can't remermber if "anode" is the right term for an SCR, but I'm sure someone will correct me if necessary:)
STeve
 
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