SSR & contactor

[the blur]

I'm building a circuit that was designed by a retired engineer, that has since died.

We are powering a high wattage heating element, by a low voltage controller.
The engineer designed an SSR, and a contactor in series to control the load.

So both the SSR, and contactor need to close to power the load. Is this necessary??

I remember him mentioning that sometimes the SSR fail in a closed position, and thus he uses a contactor as well to elimate this problem.

The heating elements are 20 amps each. I'm considering using only a contactor, or only and SSR.

What is your opinion ?

 

[shrtrnd]

I haven't seen the circuit, but it sounds like your design engineer knew it well.
He specifically said there is the potential of an SSR failure, and his solution was
adding the contactor.
Who knows if there's a newer circuit or component available to do the job now (since
he made the design).
I spent most of my life in industrial electronics. If you have a high power failure and
no protection for your circuit, you're going to have a fire.
That's my opinion.
Unless somebody comes-up with a better solution, I'd stick with the original design.

 

[the blur]

If you can see the .pdf, you'll see what he did. I'm changing it to 3 phase, so I'm adding in another heating element. Also, the temperture contoller obiously controls the heat output. There is also a blower, but I don't see what is controlling the blower relay. I know the blower runs for 2 reasons. a) once the temperature is over a certain setpoint, that blower will run, and also, it runs immediately when the heaters are on.

Because I am adding a third heating element, I'm wondering if you can suggest either SSR's or contactors. I can easily get a 3 pole 20 amp contactor from the air conditioning supply house.