240v Elements -Single SSR or 2? Pros / Cons?

I've looked at a number of simple electrical designs for simple 240v electric systems and most that I've seen only implement a single SSR even for a 240v element and essentially keep one leg "hot" during operation. -Yes most of these have a switch that cuts power to both legs but when you turn down the leg connected to the SSR using either a dial or a PID controller -doesn't this mean that "0%" output on your controller / dial is actually 25% power output during operation? (If you're cutting the voltage to the element from 240v to 120v thats 1/4 the wattage output, right?)


I also saw an example system on the Brewtroller site and their solution used 2 SSRs for a single element - one for each element with the low voltage control signal from the PID being connected to both SSRs simultaneously.

What are the pros and cons of going with 2 SSRs for a single 240v element vs. just one SSR?

-Obviously cost and the likelihood of losing power to an element due to an SSR failure is doubled with 2 SSRs, but what else? (Double the heat output for the 2 SSR solution, too so there's plenty of "CONS" for the 2 SSR solution; what are some of the PROs?)


Adam

All of this is my opinion, please do not take as fact.

From someone whom currently uses 1x SSR to control only one leg and a switch for the other, I am changing this design in my BT build.
One SSR works, but if I forget to adjust my switch I could have a hot leg without be knowing. Running an SSR on each leg just makes me feel safe.

As for heat, I would imagine 1x SSR would be less than 2. However you would likely have a larger enclosure which would help with the heat issue as well.

I think it's fairly pointless to use two SSR's for a single element. It's just over complicating things, and the old 'if an SSR fails, it fails full ON not off' thing would mean if both somehow failed/blew/whatever your element would be full on anyhow.

I use a single SSR and a contactor (because I couldn't find an appropriately rated switch) for everything past the SSR.

It's simply enough, not sure how two SSR's each on their own leg would work, if the timing was somehow off for any reason.

no need to rebuild the wheel. The ssr is soley used as a control mechanism. The contactor is your on & off.

I have a huge opinion on this.. I do not want to go into a rant about it, so I'll shut up.

You all do what you want but you are over thinking the whole thing. When a SSR is not fireing (off) there is enough voltage/current still available to take you out.

It is not a switch.!!!

Edit: (What the hell! I said I would shut up and I still went into a rant.)

Thanks PJ,

I appreciate your input as its one that I trust. I will likely be adding contractors to my brewtroller build.

SSRs control current, not voltage. This means 3 things.

Because SSRs control current, installing two does not add anything to your design. One SSR will control current to your element just as well as two will.

2. With two SSRs & both SSRs off, you still have line voltage going to your element. With your element connected, the voltage you can measure with a meter to ground will be somewhere between 0 and 120 VAC. With your element unplugged or a wire disconnected from the element you will instantly have full line voltage where the wire is disconnected. The bottom line is - there is still enough voltage there to kill you!

3. To be safe you have to include some sort of mechanical primary disconnect in your system. This can be a contactor or a breaker & can be ahead of or behind your SSRs. And with 240V, whatever you use must disconnect both sides of the circuit.

thargrav said:

To be safe you have to include some sort of mechanical primary disconnect in your system. This can be a contactor or a breaker & can be ahead of or behind your SSRs. And with 240V, whatever you use must disconnect both sides of the circuit.

Click to expand...

+1.

The SSR to control the circuit. The mechnical disconnect (either a relay or a contactor) is for safety. This is what I did.

Using two SSRs is pointless. It complicates the build, adds extra parts, and has zero benefit.

Kal

This is easy -

For 220-240v, power should flow this way

From your source terminals to

Dual pole breaker (for HOT1 and HOT2) to protect the wiring then to

Either HOT1 or HOT2 through an SSR to control the current then to

Dual pole contactor (for HOT1 and HOT2) as the mechanical disconnect (use either a 120v or 240v coil for actuation) then to

Your elements.

EDIT: Don't forget to add the GROUND. Also, the neutral if needed for 120v circuits.

P-J and Kal both have great experience and schematics. Audger, Thargrave, grandqueso and others are also well versed with how to wire these up.

Thanks guys. The tried and true designs seemed to all use just a single SSR but I realized quickly that I didn't understand WHY they had only a single SSR.

I still don't understand how a single SSR on only one leg can control the current going through the other leg that is directly powered without going through the SSR at all, though... -Can someone help explain this? -If you've got one wire going from one of your 120v lines directly to the kettle element and the other 120v line going through the SSR how can the SSR turn off the current of the line that's directly connected to the kettle?

My design currently calls for a double pole switch to turn off both legs going to the element -so I don't believe I have a safety issue here, I'm just trying to understand how it works. (I'll post my schematic before I start wiring and I'll take an actual photo of the wiring and post it here too.) -I'm pretty sure I'm going to pay an electrician to give it a quick review before I turn it on the first time, too. -I just don't want to mess around with a 40amp 240v circuit, boiling sugar water, and metal -it gives me the heebie jeebies!

Thanks again!

Adam

Adam: Current only flows when the loop is completed. The SSR is the "gate keeper". With only one side connected and the SSR "open", no current will flow.

This is simply the way electricity works.

As mentioned above, I recommend a 2-pole relay or contactor to completely break both sides when not is use for safety.

Kal

Don't bother with two SSR's (one for each hot). There's leakage current anyway so using two SSR's buys you nothing, it just adds more waste heat, inefficiency and another potential point of failure.

I still don't understand how a single SSR on only one leg can control the current going through the other leg that is directly powered without going through the SSR at all, though... -Can someone help explain this? -If you've got one wire going from one of your 120v lines directly to the kettle element and the other 120v line going through the SSR how can the SSR turn off the current of the line that's directly connected to the kettle?

Click to expand...

The answer is it 'doesn't turn off' the voltage from the 'unswitched' leg. It does however interrupt current flow. So at zero current flow (sans a little leakage), it doesn't matter how much voltage is at the element, P=I*V and with I at zero P is at zero no matter the V.

The reverse would be to measure the voltage across the element with the SSR is off. What will you see? Zero volts, because there is no current flowing through it (except the [hopefully negligible] SSR leakage current). Yes if you measure with respect to neutral or ground you would see voltage but the does not matter because that is not the path current is taking through the heating element anyway - it flows only between the two hots.

Sure, with a 240V element wired across 240V the current flows from one hot side to the other. And interrupting the current flow anywhere in the path breaks the current flow from one hot side to the other.

If you were to wire two 120V elements, the first between one hot leg & ground and the second between the other hot leg & ground, you would need two SSRs because even though the total voltage is the same (120V + 120V = 240V) each leg is now separate.

Yes provided you connected the conductor between the elements to neutral. If not, and it were left floating, one SSR would still work.

porcupine73 said:

Yes provided you connected the conductor between the elements to neutral. If not, and it were left floating, one SSR would still work.

Click to expand...

Re-read my second paragraph - I said that the two 120V elements were connected to ground.

Ok well you shouldn't be connecting them to ground in the middle if that's what you mean, it should be connected to neutral if you're connecting the 'middle' at all, and normally in this setup the neutral is grounded at exactly one point at the service entrance. I don't know I might just be understanding what you mean, I was taking it to be a MWBC basically.

I didn't understand that the power flows from one 120v leg and back through the other one in a 240v circuit...

I only have a very basic understanding of electricity based upon the "plumbing" analogy on a simple 120v circuit where power "flows" from the Hot and back down the neutral line as if the hot was the water in a plumbing system and the neutral were the drain. -I sort of logically extended this to the 240v world and thought that the two "hot" wires were the equivalent to a hot water tap and a cold water tap and that the neutral was still the "drain" -I didn't understand that in reality in a 240v circuit the hot water uses the cold water in as the drain and vice versa...


Adam

OK, using your plumbing comparison:

Consider that instead of the faucet in the middle maybe the water flowed out the hot side & back in the cold side?

If this were the way your plumbing worked then you could stop the flow of water by shutting off the cold or hot side. The hot faucet would stop the flow out from the hot side & the cold faucet would stop the flow back into the cold side - but the result would be that the flow would stop when you turned off either faucet.

And in this case you would no longer need both faucets - you could use either one.

The same logic applies to one or two SSRs.

Thanks Thargrav! I thought this was where this was going but I needed it dumbed down to that level to make sure I actually understood.

Adam

biertourist said:

Thanks Thargrav! I thought this was where this was going but I needed it dumbed down to that level to make sure I actually understood.

Adam

Click to expand...

...which is why the neutral is only being used on the 120V side of the circuit to energize the 120V coil.

That's the sound ME having an A-HA moment as well!

Thanks for asking the question, Adam!!

biertourist said:

Thanks guys. The tried and true designs seemed to all use just a single SSR but I realized quickly that I didn't understand WHY they had only a single SSR.

I still don't understand how a single SSR on only one leg can control the current going through the other leg that is directly powered without going through the SSR at all, though... -Can someone help explain this? -If you've got one wire going from one of your 120v lines directly to the kettle element and the other 120v line going through the SSR how can the SSR turn off the current of the line that's directly connected to the kettle?

My design currently calls for a double pole switch to turn off both legs going to the element -so I don't believe I have a safety issue here, I'm just trying to understand how it works. (I'll post my schematic before I start wiring and I'll take an actual photo of the wiring and post it here too.) -I'm pretty sure I'm going to pay an electrician to give it a quick review before I turn it on the first time, too. -I just don't want to mess around with a 40amp 240v circuit, boiling sugar water, and metal -it gives me the heebie jeebies!

Thanks again!

Adam

Click to expand...

Well a 220v circuit needs both legs to be a complete circiut so if you remove one of those legs theres no power being used by the element at all, Its just like removing either the hot or neutral from a 120v circuit and also like removing either the positive or negative terminal from a car battery in a car... it will go completely dead because the circuit is incomplete. now if you grounded one of those hot leads you would have an active 120v line and can still electricute yourself this way if the circuit were to get grounded... GFCI should help with this but relays or contractors are a safer sure way to prevent this from even happening.

augiedoggy said:

Well a 220v circuit needs both legs to be a complete circiut so if you remove one of those legs theres no power being used by the element at all, Its just like removing either the hot or neutral from a 120v circuit and also like removing either the positive or negative terminal from a car battery in a car... it will go completely dead because the circuit is incomplete. now if you grounded one of those hot leads you would have an active 120v line and can still electricute yourself this way if the circuit were to get grounded... GFCI should help with this but relays or contractors are a safer sure way to prevent this from even happening.

Click to expand...

Thanks, I eventually figured that out.

I had only a very basic understanding of 120v electricity using the "plumbing analogy" and the plumbing analogy breaks horribly as soon as you start thinking about US 240v electricity.

From my 120v days doing massive amounts of electrical work on my house I was told to think of the "hot" line of a 120v circuit as the water coming from a tap and the neutral as the "drain" -I mistakingly applied this analogy to a 240v circuit and assumed that each 120v "tap" then "drained" through the common neutral line in a 4 wire 240v circuit; I didn't realize that 120v "tap" #1 "drains" through the 2nd 120v "tap" and that 120v "tap" #2 "drains" through the 1st 120v "tap". -Obviously that makes no sense in plumbing, but that's how electricity works. I didn't realize that the analogy only really works well for simple 120v circuits and it made it hard to understand US 240v circuits.

-I also lived in Ireland for a while and their 240v circuits behave just like our 120v circuits -2 wires and a ground where the tap and drain analogy still works; it's only our weird 240v 2 phase circuits that break this. (I don't even want to THINK about how 3 phase circuits work -I had to deal with a 3 phase "wye" electrical configuration broken down into 2 phase 208v electricity in an apartment a couple years ago and my head almost exploded.)


Adam

Just to clarify: North American 240v power is still only single phase. The number of phases of a circuit pertains to the number of individual voltage sine waves in a circuit. Your power hookup to the electrical utility is fed directly from a transformer that transforms transmission voltage to 240v, and the 120v neutral is a centre tap in the transformer secondary windings.

To equate the relation between 120 and 240v power to the plumping analogy; if you have a water line the pressure of the water at both ends would be the 240v, but if at the centre of the line you had a "t" with a smaller diameter, the lower pressure you received there would be the 120v.

There is only one reason to use two SSR's and that is so that if one fails ON (and everyone assumes that SSR's always fail on) having the other one turn off properly will interrupt the current flow to the heater thus, potentially, saving your wort or mash or whatever, from getting too hot from SSR failure. Now as a circuit is a circuit it doesn't matter where you put the two SSR's in the circuit. IOW you could put one in the black wire and one in the red wire or both in the red wire or both in the black wire.

If the probability that an SSR is going to fail during a given brew session is p then the probability that your controller will try to turn off your heater and it won't turn off is p. If you have two in series the probability that at least one fails is 2*p(1-p) + p^2 (iow, if the probability of an SSR failing is 10% then the probability that at least one will fail if two are connected in series is 2*.09 + .01 i.e. 19% but it is only when they both fail (probability 1%) that the heater does not get turned off.

eric_pwb said:

Just to clarify: North American 240v power is still only single phase. The number of phases of a circuit pertains to the number of individual voltage sine waves in a circuit. Your power hookup to the electrical utility is fed directly from a transformer that transforms transmission voltage to 240v, and the 120v neutral is a centre tap in the transformer secondary windings.

To equate the relation between 120 and 240v power to the plumping analogy; if you have a water line the pressure of the water at both ends would be the 240v, but if at the centre of the line you had a "t" with a smaller diameter, the lower pressure you received there would be the 120v.

Click to expand...

Here's a mech eng clarification, terminology may be a bit off and feel free to correct any errors....

True statement, NA is "single phase". I'll try to add some clarification since it never made sense to me how hooking 120 to 120 should = 240. Fundamentally hooking up 2 wires with the same voltage will result in no current flow. I remember in HS taking an electrical course and lots of students would wire up a 3way light switch incorrectly where in some cases the light would be off because the hot and neutral wires to the light were both hot.
Current will flow from high voltage to low, just as water will flow from high pressure to low pressure. The single phase terminology is what irks me. If you open up your main breaker panel, you'll find 2 hot wires(H1, H2) that come from the pole(transformer) into your main breaker. Neutral is always grounded (0 volts). Half the house circuit breakers are fed by H1 and the other half are from H2 (each breaker slot alternates H1, H2). These are actually both 120VAC (ref to ground), but are 180 degrees out of phase. Since we're dealing with alternating current, that means when line H1 is at max positive voltage line H2 is at the same voltage but it negative. While not exactly correct, its easy to think about it as the difference between the 2 voltages +120VAC - -120vac =240vac. The water pressure analogy is tougher to do but think about in terms of air pressure where you have pressure on one side(+120V) and a vacuum(-120V) on the other. Back in your breaker panel, every other breaker slot is 180 degrees out of phase, that's why the double breaker is needed to produce 240.

At least the math is easier than 3 phase systems. Clear as mud?

I agree with the other comments where you want a contactor or other mech disconnect upstream of the SSR.

BigBlock said:

True statement, NA is "single phase".

Click to expand...

On the pole (primary side) it is, of course, 3 phase (unless you are in one of those rare localities where it is single phase/earth ground). On the secondary side it is actually biphase.

BigBlock said:

The single phase terminology is what irks me.

Click to expand...

It should, when the neutral is considered, because it isn't single phase in that case. If the neutral is ignored or not connected (as would be the case with a 240 load) then, and only then, is it a single phase system.

BigBlock said:

If you open up your main breaker panel, you'll find 2 hot wires(H1, H2) that come from the pole(transformer) into your main breaker. Neutral is always grounded (0 volts). Half the house circuit breakers are fed by H1 and the other half are from H2 (each breaker slot alternates H1, H2). These are actually both 120VAC (ref to ground), but are 180 degrees out of phase.

Click to expand...

The magnitudes of the voltages are the same (or nearly the same, around 120 V) and the phases are 180 ° apart (or nearly 180° apart). Thus it is, in reality, a two phase system. The voltage between two wires is the phasor difference equal to ∆V = |V1| -|V2|e^j(ø2 - ø1). I won't go into the e^j[] stuff because all the engineers on here already know what it means and the poets would not be likely to follow the amount of explanation that would reasonably fit here. If ø2 - ø1 = 180° exactly then e^j(ø2 - ø1) = -1.0000 and ∆V = |V1| - -|V2| = |V1| + |V2| = 240 V.

If you happen to be in an office building in the US or a country where 3ø is distributed to houses (and there are more of these than I thought, aparently) then is given panel you have alternating single pole breakers that connect you to one or the other of two phases that are, referenced to ground, 120° apart 2 pole breakers that give you a single phase whose voltage magnitude is sqrt(3) = 1.73 times the phase to neutral voltage magnitudes of of the single pole breaker phases. In the biphase system you have exactly the same thing except that the phase angle is 180 ° and the voltage is twice the single phase voltage.

BigBlock said:

Since we're dealing with alternating current, that means when line H1 is at max positive voltage line H2 is at the same voltage but it negative. While not exactly correct, its easy to think about it as the difference between the 2 voltages +120VAC - -120vac =240vac.

Click to expand...

That is indeed exactly correct. But only when the phase difference is exactly 180°. Note that it isn't exactly 180° unless all the loads on the secondary are pure resistive loads. Any motors, for example, will change that perfect phase relationship but not usually, in a household, to an appreciable extent.

BigBlock said:

The water pressure analogy is tougher to do...

Click to expand...

Which is why it is not a very good analogy but I'm sure I could contrive something. The voltage between A and B is the work required to move a unit charge from A to B. If it is negative that means that energy is given up in moving from A to B. Voltage can be measured with respect to any reference point, say C. The voltage between A and C is thus the work required to go from A to C plus that required to go from C to B.

BigBlock said:

At least the math is easier than 3 phase systems. Clear as mud?

Click to expand...

Same math. Trivial to those who are familiar with it. Incomprehensible to those who have never met it before.
[/QUOTE]

Great summary ajdelange.

As for the terminology of single phase, I agree it should be known as biphase. Unfortunately I've never heard an electrician refer to it as such. As a controls engineer, anything that is phase shifted I consider out of phase.

Since the OP indicated that they had limited electrical experience, I was attempting to simplify the answer. I learned the basic residential contractor way of wiring in HS, but that really just teaches what to do, not why it works. Since most people don't even have that much background, I did my best to boil down to the basics (although maybe not very successfully). I tried explaining this to a buddy of mine by drawing the sine waves and whatnot once without much success. So like you mentioned, no need to discuss phasors, unbalanced loads, etc.

It wasn't until I went back to school for automation and was modeling a hybrid stepper motor that the basic e&m concepts really clicked. These days I develop controls for power gen, and having that electrical experience is very helpful for me not to be that ignorant mech eng . On a humorous side note, I had spend a good amount of time learning about SSR's for my rims setup which came in handy when we redesigned a fuel heater using several 100KW SCR's. The basic system components and layout are the same, just bigger. Is there any problem that beer can't help to solve??