SSR and contactors

[dex0807]

So I have been looking over some of P-J's wiring diagrams as I am planning a 50A 2 element 2 pump HERMS setup and I am confused as to why you need to have an SSR switched with a PID then fed to a magnetic contactor. Couldn't you just use a relay output PID fed directly to the contactor and do away with the SSR all together?

I work with this stuff a little as I am in the plastics industry and work around ovens and melt tanks so I understand most of what is necessary but the redundancy seems unnecessary.

Thoughts?

 

[jeffmeh]

Mechanical contactors are not made to switch as frequently as SSRs.

 

[jeffmeh]

And with both you can switch the mechanical contactor off and be sure that the element is off, even if the SSR fails closed or leaks power.

 

[BadNewsBrewery]

jeffmeh nailed the two main reasons we use SSRs and contactors.
1 - SSRs are designed to switch on and off frequently, contactors aren't. Your PID will switch the element on and off continusously throughout the brew day. You'll kill your contactor doing that.
2 - SSRs have two nasty faults... they can leak voltage through even when you think they're off (IE the PID hasn't turned them on) and they can fail closed even when you think they're off. Both of these could result in you getting a surprising nasty shock.

Here are a few other reasons...
3 - SSRs only control 1 leg of the supply to the element. If you're using 240v, that means one leg will always be hot to the element. The contactor disconnects both legs (typically). This is an added level of safety - when you turn the element off, you can be SURE it's off.

4 - Most of the PIDs we use and are familiar with (like the 2352) are SSR output. You'd have to get one that is relay output, and is rated to provide the amperage required to actuate the relay. You might have a PID that few people have ever used, which means getting help with setup or wiring may be difficult.

-Kevin