SSRs and leakage

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sandyeggoxj

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Leakage sounds somewhat gross, unless you are sick and twisted and then it probably sounds yummy. Anyway...

So I have been putting together parts for my 1/2bbl eherms build and I need to order the rest of the stuff for the control panel. From what I understand SSRs have some sort of current leakage. It has something to do with the solid state switching mechanism and it is inherent to the design. That is the reason that people use contactors on the heating elements. The contactors provide a safety factor because they mechanically open or close the circuit. Once closed the SSR can handle all the requirements of the PID or BCS/BT.

Here are my questions:
1. Why not use a high-current rated switch to replace the contactor?
2. Why don't people use contactors on the pump SSRs?
3. Why don't SSRs exist that do not have current leakage?
4. Can one of these switches handle (2) contactors, or can (1) contactor be used for (2) heating elements? Or do I have to have (1) contactor per heating element, always?
5. Are these just Best Practices or are they UL or NEC requirements?
 
"Here are my questions:
1. Why not use a high-current rated switch to replace the contactor?
2. Why don't people use contactors on the pump SSRs?
3. Why don't SSRs exist that do not have current leakage?
4. Can one of these switches handle (2) contactors, or can (1) contactor be used for (2) heating elements? Or do I have to have (1) contactor per heating element, always?
5. Are these just Best Practices or are they UL or NEC requirements?"

1. A switch could be used, but they are expensive and fairly uncommon at the high current rating that is required.

2. In many applications, including pumps, SSR leakage is simply not an issue because it isn't a safety or functional issues with the pump or associated wiring.

3. It is just the nature of the beast. There may be zero-leakage SSRs out there, but I am not aware of any.

4. In theory you could use a contactor with the proper current rating as a master power cut-off for your whole panel. The cost of this approach would be some loss of flexibility ie you couldn't shut off just one load.

5. These circuit often are designed using principals from the NEC or UL, but I am not aware of any that have an actual agency certification.
he different circuits you see on this forum can offer many things: Safety, Flexibility of operation
or just Bling factor.

IMO some of the panel circuits shown here offer more complexity than is required to actually get the job done.
 
I have actually brought this up before on HBT. The answers were pretty much exactly what you would read from this wikipedia page.

SSRs work through the use of doped compounds that are placed together in a particular order. When you apply a very small amount of current to the middle compound it allows the gate to open for whatever circuit you are switching.

The leakage comes from the fact that even though the doped compounds create an extremely high resistance to current when the gate is not open. There is still an extremely small amount of current that can pass. Extremely small as in if you had a pool filled with 100K gallons of water, the leak would pass maybe 1/10th of a gallon a day.

Even the insulation on electrical wire leaks current. The same concept applies though.

I hope I have explained leakage current to you properly. I believe that wikipedia page will answer most all your other questions.
 
And now for the part where I get flamed!

If the current leakage is SO small, then why do some people consider it dangerous to not use a contactor? Is it simply because the SSR might fail closed and not because of current leakage? If a SSR fails and closes the only recourse would be powering down the panel. Has anybody on this forum had an SSR fail closed?

Realistically I will be brewing every 3-weeks and 1/2bbl at a time. That is a ton of beer. My panel will be powered up for about 10-hours a month. I have the occasional event where I will need to brew a lot, but that will not add too much to the average time per month that the panel is powered. Over the years I have seen a lot of things come and go, many of these trends are treated as absolute truths while they are popular. Is that the category that contactors are right now? If they are absolutey required for safe operation then I will use them. But if they are just "bling" or "feel good crap" then I might pass. I don't want to waste the resources and space in my panel on something that just make people feel good about themselves. I am out to have a safe and functional panel that will reliably let me brew the best, most consistent beer that my skills allow me to.
 
When it comes to failure, SSRs and mechanical relays both fail in the same "closed" position... typically.

Mechanical relays fail closed because the contacts that makes the switch actually weld themselves together. You could have a coil failure also, that would just make the switch inoperable.

I don't really believe a brewing design, when done properly, would cause safety concerns if you were to use either type of switch.

The general use of SSRs to control heating elements for electric brewing is because most setups are using a PID temperature controller to do the switching of the element. The temperature controller may switch off and on the element hundreds of times during the hour you are mashing, and then the hour you are boiling.

Mechanical relays typically have a switching life span of 50k to 100k operations. You may get more, you may get less. So if you just assume 1000 operations per brew session, (just to make the math easy) you could expect to get 50 to 100 brews done before failure.

With an SSR you can expect the life span to be in the millions of operations though. So if you have the same number of operations during a brewing session, your life expectancy of the relay would be drastically increased. Drastically as in the relay should last the life of your setup.
 
I will be controlling things with a bcs, but I do get the concept of relay cycles as well as relay actuation speed. The SSRs actuate extremely fast and will cycle many more times than mechanical relays.

I guess I have missed the whole point about contactors being intrinsically safer. PJ got grumpy when I questioned it but didn't explain why they were safer.
 
just think of the contactor as a very inexpensive version OF a very big expensive on-off switch (one that can handle 20-30 or more amps @ 240v with ease)

A switch on it's own is unlikely to be able to handle the 20A+ @250v a 5500W heater element draws BUT
link the coil of the contactor (relay) to a switch and you get to control when power is allowed to go all the way from the PID to the Heater element...
 
Another thing to keep in mind is that most people use a single SSR to control their 240V heating element.

This means that if you don't use a relay/contactor to physically disconnect the element from power, one side of the heating element will always be energized while you're cleaning your kettle (assuming you don't turn off the control panel).

I sponge down my HLT while I'm boiling. I have my hands in there and while it's unlikely, I wouldn't want to bump into/break the element in such a way that I get zapped. While I'm not in there very long I'm not overly comfortable with the idea that the SSR may fail closed too and fire up. So I use a contactor.

Is is required? No. It's purely a question of $$$$. Nobody else can tell you whether the expense is worth the reduced risk or not.

Has anybody on this forum had an SSR fail closed? Yes. I certainly don't follow every post but I've seen a few threads, some with pictures. It's probably why it's an issue that gets raised here.

Kal
 
Floppy-

That makes sense. My big welders do the same thing for power input. And those can pull 40-amps at 100% duty cycle.

Kal-

I am going to use dual SSRs, so both hot legs will be switched on each element. On the pump side of things I was just planning on switching the hot leg via the relay and leaving the neutral in place.

I will have to search more for a failed SSR. I would be interested in seeing that.
 
sandyeggoxj said:
I am going to use dual SSRs, so both hot legs will be switched on each element.
Click to expand...
Once you factor in the cost of the SSR and the heat sink, you'll probably find that a contactor (or relay) is about the same price. I would go with the reduced heat & better safety of a single SSR + 2-pole contactor (or relay) than a dual SSR. Same price probably.

On the pump side of things I was just planning on switching the hot leg via the relay and leaving the neutral in place.
Click to expand...
Pumps are low current (relatively speaking) and usually 120V. No relay required. Just switch the HOT line with a simple switch (contactors in these switches are usually good to 10A and must pumps are 1-2A max).

Kal
 
You need an one SSR and a 2-pole contactor (or relay).

I assumed you were going to use 2 SSRs instead (and not contactor or relay).

This is how I do it with a single SSR and a contactor - note that there are actually 2 kettle here so everything is doubled:

elements.jpg


(Not all wires are shown - just the ones relevant to the SSR/contactor/heating elements).

Kal
 

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