Switched neutral?

[AberleTec]

I have a problem with an industrial heating element control circuit.
This circuit uses a relay on the "hot side" to apply voltage to the heater and then a triac on the "neutral side" to control power through the heater via an electronic temperature controller.
The problem is that the triac failed, causing an overload to trip. So far, so good. When I arrived at this service call, the machine operator was holding onto the heating element wondering why it was stone cold.
So what happens if that triac had failed and left an open circuit?
That machine operator would have definately been the first to know.
Is this still considered a safe circuit?

 

[hillbilly]

You can't build them idiot proof.

Although it doesn't sound like a very good design, I personally don't see anything wrong with it.

It sounds more like there should be a guard around the heating element to keep anyone from touching it....along with a few caution signs.

This sounds like a industrial installation.
OSHA would have a few ideas.
Different rules apply.


What if the heating element simply burned in two? It would be the same scenario as the triac failing in a open condition open.


Just my opinion
steve

 

[AberleTec]

It is an industrial installation.
The heating element does have a guard on it.
I'm not sure why it never had an interlock circuit to remove power from the heating element if the guard was ever removed (It does now). Maybe because it was built in Italy over twenty years ago.
I've heard of this control circuit being used in residential lighting as a dimmer control and as a heater control in ovens and toasters, but never actually had any experience with it first hand.
Just the idea of the control circuit being on the grounded side of the load seemed a little strange to me.

 

[dbuckley]

Talk of power control and loads invokes thoughts of dimming.

Turn the clock back far enough, and all power control (dimming) was in the neutral. Well, it was actually originally in the negative, and as theatres (being the only places dimmers existed at this time) converted to AC the dimmer was in the neutral, with the switch in the hot.

Ugh.

I wouldn't have thought sticking a triac in the neutral for power control was acceptable in Italy, even 20 years ago...