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Old 03-08-2010, 07:12 PM   #1
egurney
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Default Switch/Relay/Contactor Question

I'm finishing up version 1.0 of my CP diagram. I had a couple of questions on the best way to wire something up.
Due to Amp restrictions, I am going to have a selector switch to switch between my HLT and BK. Right now I have that switch controlling 2 2-pole 40A 120V contactors (BK, and HLT) to give power to each respective SSR set. From a safety aspect I think that is the best way to go, as opposed to switching the DC from the PID.

In conjunction with that, on the HLT (10G cooler), I am going to put in a float switch to turn off the element if the water level gets to close. The output there is low amp 120V, and I am hooking that up to another contactor again to cut power to the respective SSR set.

Looks kind of like this:
Contactor 1 (HLT/BK Selector) -> Contactor 2 (Float Switch) - > SSRs
Contactor 3 (HLT/BK Selector) -> SSRs


From reading the millions of threads on this subject, it seems that when a SSR fails, it fails in the open position. By using the contactors, there is no way for an element that should be off to get power. That's my understanding and thinking anyway...

Does this seem to be the best way to go, or is there a better route.

Thanks,
Eric

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Old 03-08-2010, 07:37 PM   #2
goatchze
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Sounds reasonable. To make sure I'm understanding, your switch is a DPDT with ON-OFF-ON? Middle means nothing is on, one way means one is on, the other way means the other?

IMO it should work fine; I've got something similar for my RIMS heater where a contactor/relay is only ON if the pump is on. I have it drawn out just like you do.

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Old 03-08-2010, 07:41 PM   #3
egurney
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Quote:
your switch is a DPDT with ON-OFF-ON?
Exactly.

I figured it would work, just wondering if it is overkill.
I figured I could have the switch control the output from the PIDs rather that the power to the SSRs. To me for the small extra investment, this way seems safer.
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