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Old 06-08-2011, 04:00 PM   #1
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Default Disconnect Switches - safe to pull under load?

I have one of these:

http://www.homedepot.ca/product/air-...-switch/943214

This is a non-fused, 60amp disconnect switch, with a plastic pullout attached to the contacts. I've purchased it to cut the load between my SSR (240v, 30amp lines) and my element. Or once I put it all together, that's what I purchased it FOR.

Is this safe to pull while there's load on the lines, like say in a worse case my PID screwed up or my SSR stuck closed and 100% power is flowing through non-stop to my element. Can I yank this disconnect safely?

I wanted a proper 30amp DPST toggle switch but they're impossible to find here. Ditto to contactors, crazy expensive and special order only. Nuts.

I'd be thinking I'd want this in order to fire up my system without the element having a chance of being "on", then when I want to monitor my kettle cooling after the boil, pull it so I can rest assured I'm not firing the element while trying to cool the wort.

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Old 06-08-2011, 04:48 PM   #2
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Safe? Yes. It should handle that in the event you are talking about. HOWEVER....I wouldn't get in a habit of pulling it under load. The contacts are not designed to handle the arcing. Every time you get an arc, it burns the contacts to the point of eroding them and creating a high-impedance connection. As the resistance increases, the amount of voltage dropped across it also increases. This creates a "load" on your source and will cause the contacts to heat. Eventually the connection will either fail, or worst case, heat up enough to melt the surrounding material and cause an ugly failure, at which point it would no longer be safe at all.
Point is: an occasional use as a disconnect under load is acceptable. Frequent/repeated use is not. Don't use it as a switch.

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Old 06-08-2011, 04:55 PM   #3
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Here is one source: toggle switch, no enclosure, 30A, 600VAC, 3-pole
Cheaper - Ferraz Shawmut Load Break Disconnect Switch, DIN-rail mount, UL508, 3 Pole, 600V, 40A, Rotary (don't forget to get a handle & shaft if you go this route...

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Old 06-08-2011, 05:00 PM   #4
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Thanks for the clear response, that's unfortunate but good information.

So if I can't truly break (unless under emergency situations) both the hots between my SSR and the element, is it generally safe to let the PID handle keeping the element off? Can I set it to 0 and be pretty confident that it's not firing anything?

I wanted a hard switch between the SSR and element plug that could physically disconnect both the 240 lines so I could be sure nothing was energizing the element. I'm concerned about initially plugging the control panel into the wall and the PID telling the element to heat immediately and suddenly I'm plugging something in that's immediately pulling 19 amps, arc, fried wall plug, panic ensues...that's my fear. Justified?

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Old 06-08-2011, 05:08 PM   #5
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So if I can't truly break (unless under emergency situations) both the hots between my SSR and the element, is it generally safe to let the PID handle keeping the element off? Can I set it to 0 and be pretty confident that it's not firing anything?

Sort of... Set your PID to 0 so the SSR is off, or at a very low duty cycle, then pull the disconnect. That would be just fine. I would just try to prevent pulling the disconnect when it is handling more than a few amps on a regular basis.

I wanted a hard switch between the SSR and element plug that could physically disconnect both the 240 lines so I could be sure nothing was energizing the element. I'm concerned about initially plugging the control panel into the wall and the PID telling the element to heat immediately and suddenly I'm plugging something in that's immediately pulling 19 amps, arc, fried wall plug, panic ensues...that's my fear. Justified?

I would prefer the disconnect to be pulled when energizing the controller as well. So, just leave the disconnect out until your panel is powered and you have the SSR off, then energize the disconnect. Again, that would be fine as well.

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Old 06-08-2011, 05:09 PM   #6
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I doubt you would fry a plug but I never like to plug/unplug anything that could have a potential large load. I put a master switch (120v toggle) in my control panel that is wired to control a definate purpose contactor which switches the 50A line in. It was a pretty cheap solution.

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Old 06-08-2011, 05:13 PM   #7
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Yeah the contactor sounded like a good way to go but it was far from cheap here and the only electrical shops that carried them had to order them in, they also seemed pretty sketchy on how to hook it up and if it would work. Too far out of my comfort zone, though I guess this is as well!

OK, so yeah that's a good call, Bull. Pull the disconnect prior to plugging it in, I have a switch for my PID so technically nothing should get any power. Plug in master plug to dryer outlet, flip PID switch on, set PID to 0, plug in disconnect, off to the races. Reverse for cooling down.

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