Contactors necessary?

Williams 728
Homebrew Talk - Beer, Wine, Mead, & Cider Brewing Discussion Forum

Help Support Homebrew Talk - Beer, Wine, Mead, & Cider Brewing Discussion Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
W

wuttheheeck

Well-Known Member
Joined
Aug 7, 2013
Messages
62
Reaction score
0
Planning out my first build here. I am going to have a 5500w BK, 5500w HLT and 1500 seperate HEX for HERMS.

My question is, are contactors necessary? I understand that even though a PID will be telling elements to fire, without a switch in the ON pos. they will not fire.

What if I had a low voltage switch that either completed or cut off signal between PID and SSR.

As a fail safe I could just unplug that element.

This would save me $100 as I originally had a contactor for each element.


Thanks for your advice and opinions
 
Alternatively, you could use a 30a rated double pole switch, like this one. http://www.homedepot.com/p/Leviton-...ch-White-R62-03032-2WS/100356941#.UooxbpUo61s

The reason for switching the two hot lines before the element is that an SSR can fail closed (on). With a switch, you know that off means off. Switching the low voltage control lines into the SSR would not solve the problem.

That margin of safety is worth the price of the contactors and switches, IMO.
 
In the USA 220V is provided by 2 hot lines instead of one hot line and one return line (neutral). For safety reasons, both hot lines need to be switched when turning the power off. You can either do this using a contactor or with a double pole switch. SSR's are single pole switches but you can use one on each hot line.
 
alien said:
In the USA 220V is provided by 2 hot lines instead of one hot line and one return line (neutral). For safety reasons, both hot lines need to be switched when turning the power off. You can either do this using a contactor or with a double pole switch. SSR's are single pole switches but you can use one on each hot line.
Click to expand...


Thanks for your reply. Would I need 2 SSRs for each element in that case? for both hot wires?

Regards,
Sean.

EDIT:
I have not seen this in the case of many wiring diagrams, specifically PJ's and theelectricbrewery.com (main reference source)
 
For the "off means off" rationale, I highly favor a double pole, single throw, normally open, mechanical contactor with both hot lines running through it, with one SSR controlling one line, over skipping the contactor and using 2 SSRs. With 2 SSRs, if either fails closed, there will be a non-obvious, hot 120v line at the element. While highly unlikely, if both SSRs were to failed closed the element will be powered at 240v. With the contactor switched off, there is no such potential, and the second SSR is unnecessary.
 
It is true: SSRs can and do fail when they overheat, and could both fail together if they were both attached to an undersized heatsink. For this reason I use contactors myself.

But 2x SSRs is an option. (Oscsys sell systems like the one I linked to.) You just have to be absolutely sure that your SSRs are not going to overheat.
 
I suppose you'd better let Oscsys know before too many people die.

Grounding, breakers, fuses, and GFCI are the *main* safety features that people rely on.
 
kenh said:
I looked at the drawing in the link and I don't understand. You only need one SSR to interrupt current. Using two just doubles the heat load on your heat sinks & because SSRs don't switch all the way off relying on just SSRs to turn off power could get you killed.
Click to expand...

And it adds an additional point of failure. It's bad design, but only one SSR with the other hot line always live is worse.
 
kenh said:
I looked at the drawing in the link and I don't understand. You only need one SSR to interrupt current. Using two just doubles the heat load on your heat sinks & because SSRs don't switch all the way off relying on just SSRs to turn off power could get you killed.
Click to expand...

I've been bite by them little bastards more than once thinking the power has been removed. They call it leakage . . . :eek:

Main disconnect or contactors are your friend.

'da Kid
 
jeffmeh said:
And it adds an additional point of failure. It's bad design, but only one SSR with the other hot line always live is worse.
Click to expand...

Not really. SSRs interrupt current while disconnects and contactors remove voltage. You only need one device in-line to interrupt current. And as far as "one hot line" is concerned, if your system has two SSRs you should try this:

Unplug your element from your box.
Turn your SSRs off.
Measure the voltage across the two hot pins of the socket.
You'll measure full line voltage!!!!
 
kenh said:
Not really. SSRs interrupt current while disconnects and contactors remove voltage. You only need one device in-line to interrupt current. And as far as "one hot line" is concerned, if your system has two SSRs you should try this:

Unplug your element from your box.
Turn your SSRs off.
Measure the voltage across the two hot pins of the socket.
You'll measure full line voltage!!!!
Click to expand...

I'm not sure which point you are disputing:
1) That a second SSR introduces a second point of failure
2) That a design where a switch or contactor breaks both hot lines so you are sure there is neither current nor voltage to the element is superior to a design where you rely on two SSRs to neither fail closed or leak
3) That a design where you do not have a contactor as described above, and you have one hot line always going to the element with the other passing through an SSR that can fail close or leak is the worst design of the three alternatives

I'm not trying to be snarky, but I am trying to understand how what you described qualifies as a "not really." I'm always willing to learn. :)
 
jeffmeh said:
I'm not sure which point you are disputing:
1) That a second SSR introduces a second point of failure
2) That a design where a switch or contactor breaks both hot lines so you are sure there is neither current nor voltage to the element is superior to a design where you rely on two SSRs to neither fail closed or leak
3) That a design where you do not have a contactor as described above, and you have one hot line always going to the element with the other passing through an SSR that can fail close or leak is the worst design of the three alternatives

I'm not trying to be snarky, but I am trying to understand how what you described qualifies as a "not really." I'm always willing to learn. :)
Click to expand...

All three - you just summarized everything.

To be safe you need a way to disconnect both legs of the main power coming in. This would be a contactor or a incoming main power switch. A SSR won't work because it is never all the way off.

Then once it's safe you only need one SSR to control current to the element. Adding a second SSR does not do a thing, it won't turn the current off any better or faster. But a second SSR adds complexity & cost & another failure point & another heat source.
 
kenh said:
All three - you just summarized everything.

To be safe you need a way to disconnect both legs of the main power coming in. This would be a contactor or a incoming main power switch. A SSR won't work because it is never all the way off.

Then once it's safe you only need one SSR to control current to the element. Adding a second SSR does not do a thing, it won't turn the current off any better or faster. But a second SSR adds complexity & cost & another failure point & another heat source.
Click to expand...

You dispute all three? From your text, it appears to me that you are agreeing with me.
 
For safety's sake you need a positive means of disconnection. That means a plug or a (usually) manual switch. This should be opened (or unplugged) whenever you are working on the gear. A circuit breaker is a disconnect. A contactor is not a disconnect nor is an SSR nor pair of SSRs as any of these can fail on and, as has been pointed out, SSR's leak so there is voltage present even when SSRs are gated off.

As for whether one or two SSR's is desirable in a single circuit derived from a biphase system (240 volt circuit with one SSR in each phase) the following may be of interest:
If the probability of failure of 1 SSR is p then then probability that both will fail is p*p but the probability that one or the other will fail is approximately 2*p. For example, if the SSR's probability of failure in the on mode (turns on when you don't ask it to or doesn't turn off when you remove the gate) is 0.01% then the probability that both will fail on is 0.0001% and you are more secure in your sense that the heater won't come on unless the controller asks it to. Now if either one or the other of the two does fail, and the probability of that is 0.02%, your level of protection against getting heat when you don't want it halves because given that one has failed the probability that the other does is still 0.01% but note that the circuit still works. In fact you will be unaware of the failure unless you do something like put a lamp between the load side of each SSR and the opposite phase. These lamps will glow brightly when the SSR is 'on' whether this be from an applied gate signal or no and less brightly or not at all when an SSR is in the off state (whether the gate is applied or not).

The other side of this coin is that the probability of a failure in which you cannot turn the system on also doubles (approximately) because heat requires that both SSRs conduct.

I'd probably use two if there were a downside to having heat stuck on and I wanted to be absent during the parts of that process where this was so (e.g. automated mashing). OTOH if I were worried about getting ready to boil and not being able to I would probably have one. Actually, given how inexpensive the things are I would probably have 2 with the indicator lights and manual switches in parallel with each. Then I'd be covered for any kind of failure and could chose whether I want lower probability of on failure or off failure.
 
The10mmKid said:
I've been bite by them little bastards more than once thinking the power has been removed. They call it leakage . . . :eek:

Main disconnect or contactors are your friend.

'da Kid
Click to expand...

SSR leakage is in the microamp range. You must be especially sensitive.
 
It's understood that we are for the most part hobbyists and DIYers putting these panels together.

But keep in mind you would not be able to purchase a commercially marketed control panel unless it had some engineered safety controls like a simple line-disconnect-contactor.

Features like this are engineered into equipment to keep users alive and keep their property from burning down.

A commercial panel couldn't get agency certification and wouldn't be available for sale if those features weren't there.

It just makes sense to build your gear so its reasonably safe to use.
 
jeffmeh said:
You dispute all three? From your text, it appears to me that you are agreeing with me.
Click to expand...

Sorry, too much homebrew mixed with typing the other day. I agree with all three points.

But I shouldn't be harping safety too much. My electrical controls are attached to a piece of plywood hung on my three tier brew stand in the basement. It was all in the open until my wife made me build a frame around it & hang a cabinet door on the frame with a lock to keep the kids from touching. But it's still all open when I have the door unlocked & open to brew.

But I do have a incoming disconnect for safety......
 
Well, just in case I wasn't clear on my angle, I'm talking about shutting the element off due to wanting to fully drain the vessel. If you rely only on the PID, such as dropping the setpoint below ambient or setting to manual and 0% output as a means to prevent dry fire, an SSR failure can result in a dry fire situation. With liquid in there, you may have missed it. I guess there are a lot of separate reasons why, but I think we all agree that a secondary and confirmable disconnect of power from the elements is more than practical.
 

Similar threads

B
240v : Scary start of brew
Replies
9
Views
778
passedpawn
passedpawn
Sean_SA
Issues calibrating Inkbird ITC106VH
Replies
2
Views
380
Deadalus
Deadalus
L
Building a potentiometer controller
Replies
25
Views
1K
doug293cz
doug293cz
ALAN C
Need help to confirm wiring .
Replies
17
Views
2K
ALAN C
ALAN C
B
EZ Boil wiring for heating element (240v, no neutral)
Replies
14
Views
2K
doug293cz
doug293cz
BrewSwag 300

Latest posts

Back
Top