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goybar 11-30-2012 07:45 PM

Contactor Question
 
Hi All,

I'm trying to figure out what the contactor in many(/all) of PJs diagrams is for.

If the SSR is turning on/off in conjunction with the PID, what is the purpose of the contactor?

I'm sure it is obvious, but I can't seem to wrap my mind around it.

-chris

jeffmeh 11-30-2012 07:59 PM

An SSR can leak current, can fail closed (on), and is only controlling one leg of your 240V circuit. The contactor between the SSR and the element is there so that when you set the switch controlling the contactor off, both legs to the element are truly off.

stlbeer 11-30-2012 08:11 PM

Also, the SSR can switch the power off/on at a much higher rate than a contactor can. The two work in conjunction with each other.

goybar 11-30-2012 11:27 PM

OK, so it really is more of a fail safe or safety item. Not really required but foolish not to use one.

Rarian 11-30-2012 11:33 PM

Quote:

Originally Posted by goybar (Post 4637526)
OK, so it really is more of a fail safe or safety item. Not really required but foolish not to use one.

Without a contactor in a 240v system your PID would only be able to output between 50%-100% power; one leg would continually be Hot.

milesvdustin 11-30-2012 11:38 PM

I don't use one. I always make sure the element is under water before I switch it on just in case.

whoaru99 12-01-2012 01:45 AM

Quote:

Originally Posted by Rarian (Post 4637544)
Without a contactor in a 240v system your PID would only be able to output between 50%-100% power; one leg would continually be Hot.

Not sure that matters from a heating standpoint. Even if only one wire is switched the element only has two states...on or off. 0% or 100%.

From a hazard standpoint it matters through. If only one leg is broken you still have 120V to ground or neutral on the element, potentially a shock hazard.

jgadbois 12-01-2012 05:58 PM

Quote:

Originally Posted by Rarian (Post 4637544)
Without a contactor in a 240v system your PID would only be able to output between 50%-100% power; one leg would continually be Hot.

I think you are misunderstanding 240v.

Current flows from one leg thru the element and thru the other leg. If the SSR is off there is no current flow and therefore no power. With the SSR off there is a 120v potential to ground at any point in the circuit which may be what you were thinking.

stlbeer 12-02-2012 12:21 PM

Quote:

Originally Posted by goybar (Post 4637526)
OK, so it really is more of a fail safe or safety item. Not really required but foolish not to use one.

I'd add that when SSR's fail, they usually fail closed - allowing full current to flow through them - regardless of what the control circuit is doing. Since we only need to control 1 leg of a 240v system a contactor is a requirement to ensure the circuit is off.

Your life is more important than $10 your going to save by not putting in a contactor or the $50 you'll save by not buying the next larger sized panel.

gbrewing 12-02-2012 03:25 PM

You can also use the contactor as an emergency off for the entire system. The big red mushroom looking button on control panels is usually wired to the power in contactor.

If things go wrong, you depress the mushroom button and it kills all power instantly.


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