3 phase SSR vs single SSR for controlling 240v appliances

Other Hardware General Electronics
1

I'm planning to set up a remote on/off activation for a water heater in a home that only is visited on vacation. Either using a ethernet shield or maybe a timer or something. But I can figure all that out.

The question I have is about controlling the water heater. It is a 240v system. It has two hot lines in from a doublepole circuit breaker. My question is what type of relay to use.

Option 1 -- Single phase relay similar to this one

I already have a bunch of these laying around. I would probably use two of them, for each hot wire and control them each to turn on/off at basically the same time when needed. They are rated for 380V, but I won't need anywhere near that much since I will be using two, one for each line. This is probably my preferred option, since I already have all the parts.

Option 2 --Buy a 3 phase SSR, such as this one

I've never used a 3 phase one before. And other than the convenience of a single control side, I'm not sure this is necessary. BUT. Is it 'safer' or 'better' to use this 3 phase instead? I'm not sure I can see how it would be. But like I said, I'm not familiar with 3 phase relays, and I've never switched something at 240v before, I've only done regular 120v lines.

...

So, is the a good reason to use one option over the other? Or will both be fine? (in which case I'll probably go with option 1)

Thanks!

2

In the home environment it's typically OK to use a single pole device for on/off. My HVAC unit switches the compressor on and off with a single pole contactor so it would seem a water heater is no different. Also look at the elements in your heater. The thermostats only switch ONE wire also.

As far as a disconnect or breaker though, it is customary to break both sides of the line. Of course there is no harm in switching off both legs so either approach you've outlined is OK.

3

I do believe only one of the elements is on at a time, if that is what you mean. At least that is my understanding.

But I would probably always want to 'break both sides of the line'. Because I wouldn't want to leave one side on. I know the water heater would be 'off' at that point. But for safety reasons, I wouldn't want to leave half of the line on, in case of some crazy accident where something shorted out. IF that was to happen, you would still have some live wire still, and I want to prevent that, just to be extra cautious.

So, because of that, I would plan BOTH option 1 and option 2 to switch of both sides of the line. I would just use TWO solid states for option 1. Where as option 2 would just need the one 3 phase ssr (leaving one phase unused I guess).

But, in the end. What it sounds like you are suggesting is that either way is fine and safe. Which is kind of what I assumed, but wanted to get input from others to make sure I wasn't missing anything.

4

The two wire, 240 VAC connection to a hot water heater (in North America) is single phase, with 120 VAC on either side of neutral.
I agree with rmetzner; there is no real need to switch both sides of the line as internally, as only one is switched by the thermostat.

However, if it would make you feel better, use a DPDT relay (rated for at least 20 amps at 240 VAC) to switch the heater electromechanically. You can buy latching relays if you don't want the relay coil to be energized all the time.

5

Allright, that makes sense.

So it sounds like switching just one side is good enough. That the safety concern I had is not a serious concern.

So to be clear (sorry, but I want to be sure). My plan is to use a single phase, solid state relay on just ONE of the hot wires (from one half of the circuit breaker effectively) and then switch that relay with the arduino. And then just leave the other side always 'live' to the water heater. Then the water heater will be 'off' due to the lack of one side coming in and 'on' when I switch on the relay.

And I'll probably be using this type of relay

Does that all sound like a reasonable plan? Any flaws or dangers I'm missing?

Thanks all!!

6

That looks reasonable to me. If it was me, I might add a couple of little neon indicator lamps to indicate when each side of the line is hot.

The SSR has a couple of advantages over a mechanical relay... It will run directly from the Arduino (most higher-power mechanical relays require more coil voltage and current).

And, the screw connections make wiring easier, and it's easier to mount.

And, it will never wear-out. (Mechanical relays generally have a very long life, so that's not a big deal.)

7

IMO, an electromechanical relay is the correct device selection.

  1. it is very low duty cycle. 2) SSR's fail in the shorted condition. 3) SSR's will require a heatsink due to their higher losses.
8

I agree an electromechanical would be fine. But since I already have a SSR available, is it really that big of a problem to use the SSR instead.

  1. Low duty cycle is great, but I will be switching on/off VERY infrequently, so I'm not sure I see a huge problem there.

  2. If it fails closed, no big deal. It just turns on the water heater. Yeah it may waste some money/electricity. But I don't see it being 'dangerous'. Plus, my understanding is SSRs have a MUCH longer life expectancy than electromechanical ones, so in terms of failing, I would probably be better with an SSR anyway..

  3. This is a valid point, I am slightly worried about heat loss. But with a solid heat sink, I think I'll be okay. I definitely will monitor this though while the heater is on and heating water. I'll check it frequently initially to make sure it is dissipating heat sufficiently. I have a 40 amp rated relay, and a water heater will only draw 25-30 max. So with a voltage drop of ~2Volts, I'm looking at dissipating somewhere around 60 Watts. I suspect that will be doable, but like I said, I definitely will be watching heat generation both early on, and then re-evaluating as the relay gets old.

So, with all that, is it really best to forgo the SSR and buy a mechanical one right now? Or do you think an SSR can be a decent starting point, and only change to a new one if the SSR 'can't do the job'.??

Thanks

9

This sounds like a us installation.
In the uk we use 240 v phase and routinely switch one side except for isolating switches.
Contacter would be the normal route with ssr normally only used if power control was needed.

10

jremington:
) is single phase, with 120 VAC on either side of neutral.

I think we are the same except that we bond our neutral to one end rather than the middle, in domestic installations it often earth as well.

11

IMO, an electromechanical relay is the correct device selection.

The electromechanical relay is exactly that, a mechanical part with moving parts and contacts and is absolutely the WRONG way to go for reliability. Not to mention the increase in parts count. Most HVAC grades are only available with a 24VAC coil which makes driving them from an Arduino very messy. I suppose you could drive an SSR from the Arduino just to run the coil but you still need to get 24VAC from somewhere.

To power the Arduino you could use a "desktop" style AC/DC brick, cut the cord, and tie it between one leg and ground. Or just buy a 240V version.

Water heater elements are 5000W and they are not both on at once. So you really only have 21A x 1.5V (spec on the SSR) which is 33 watts. If you mount it on a piece of 3x5x1/4 aluminum and mount that to the water heater (I assume you will use the top by the input j-box) you will not completely waste the heat since it goes back to heating the water heater. Maybe buy a plastic box to go over the components.

12

I'm an electrician.

You need to use a double pole relay, it doesn't have to be three phase, but it can be.

13

just a point of correcton. on a US water heater, the thermostat breaks both sides of the line. it is a double pole relay that is activated by water temperature.

and

my first choice would be the pair of SSR's.

second would be a contractor that was rated for twice the load. hard to find with a low voltage DC coil.

I would also install a water shut off valve with alarm.
http://www.getfloodstop.com/
but TEST the valve before you install. I have about a 5% failure rate with these. have had at least 5 go bad. I use these for water heater installations in retail stores.

I had looked into a wireless dial out device for the alarm with a pre-paid phone card. seemed the cheapest way out, but the client did not want to move forward with that unit.
it was for an oil heater oil tank level. the alarm would send out a signal when at half full to indicate level, then again at 1/4 so as to schedule a delivery and if they both went off on the same day, it would be to indicate either a leak or theft.

14

just a point of correcton. on a US water heater, the thermostat breaks both sides of the line. it is a double pole relay that is activated by water temperature.

Just a point of correction: The thermostat does NOT break both sides of the line. The Upper-Limit protection does, but the lower element switches only ONE line. Just a point of correction: It's not a RELAY either, it's just a bi-metallic switch. The circuit is already protected by the circuit breaker/disconnect so no one will be unsafe but it is not necessary to switch both lines since the manufacturer doesn't either. I also stand by my original post wherein I stated an HVAC compressor only switches one side.

In this instance, switching both sides of the line doubles the heat dissipation.

15

Boardburner2:
This sounds like a us installation.
In the uk we use 240 v phase and routinely switch one side except for isolating switches.
Contacter would be the normal route with ssr normally only used if power control was needed.

Yeah it is a US installation. And I have been looking into a Contacter relay. But they are not as easily switched with arduino low voltages. It could be done though, so we'll see.

rmetzner49:
Water heater elements are 5000W and they are not both on at once. So you really only have 21A x 1.5V (spec on the SSR) which is 33 watts. If you mount it on a piece of 3x5x1/4 aluminum and mount that to the water heater (I assume you will use the top by the input j-box) you will not completely waste the heat since it goes back to heating the water heater. Maybe buy a plastic box to go over the components.

That's an interesting idea to 'sink' the heat from the relay back into the water heater. I gotta admit I'm not sure I'm on board with that. It seems that it would cool far less efficiently, since it would be cooling into a warm tank, in stead of room temp air. I guess if the tank, even at max temperature, is still less than what the SSR would heat up to, it would still 'send' heat in the right direction.

Just to be overly cautious, I'm pretty sure I'll just heat sink the relay and attach it somewhere nearby, but not on the tank. Unless someone feels pretty strongly that using the water heater as a heat sink is a great solution.

Qdeathstar:
I'm an electrician.

You need to use a double pole relay, it doesn't have to be three phase, but it can be.

I guess I don't know what to respond to that. I totally see where the advice comes from. I just don't know that using a double pole relay is necessarily that much better than two single relays. The arduino would switch both on well within a few seconds of eachother. And as others have said, you could potentially just get away with one anyway. But I do appreciate the thought and will probably price some double pole relays just to see if it would be worth it.

dave-in-nj:
just a point of correcton. on a US water heater, the thermostat breaks both sides of the line. it is a double pole relay that is activated by water temperature.

my first choice would be the pair of SSR's.

second would be a contractor that was rated for twice the load. hard to find with a low voltage DC coil.

Yeah I'm kind of slightly leaning towards two SSRs right now. Don't know why, just 'feels' right. But I haven't decided for sure yet.

I don't know about whether my water heater breaks both sides or not. I would think that would depend on brand as well, not just country (US). But maybe there are US regulations that say they all need to be the same. I'll have to see if I can find more detailed information about this old water heater and what it does specifically.

dave-in-nj:
I would also install a water shut off valve with alarm.

I gotta admit, I'm not seeing the connection to this with my current set up. As a piece of 'general advice' it sounds great. But I'm not seeing what it has to do with switching it on/off with a relay. But as a general advice, yeah it seems great. I actually have a cheap version of something that alerts me to spills in their already. Nothing fancy like you linked, but a small little alarm device I just put on the floor.

rmetzner49:
Just a point of correction: The thermostat does NOT break both sides of the line. The Upper-Limit protection does, but the lower element switches only ONE line. Just a point of correction: It's not a RELAY either, it's just a bi-metallic switch. The circuit is already protected by the circuit breaker/disconnect so no one will be unsafe but it is not necessary to switch both lines since the manufacturer doesn't either. I also stand by my original post wherein I stated an HVAC compressor only switches one side.

In this instance, switching both sides of the line doubles the heat dissipation.

I'm just going to have to see if I can figure out for sure what my thermostate/water heater does.

But you do raise a valid point about switching both sides leading to double the heat dissipation. Since I haven't tested this yet, I don't know if that really will be an issue. Don't know how hot they will get. But I do have to fairly decent sized heat sinks for both my relays. So I'm happy to hook both up to a heat sink, if I end up going with both.

.......

In the end, it sounds like any of these ideas will work (two relays, one relay, double pole/contactor switching, 3 phase, etc). I'm kind of being left with the impression that as long as I wire it safely, make sure I dissipate heat, do my due diligence, etc., that I'll be okay with any option.

The debate in the thread kind of leads me to infer that there is no 'one right way only' and that I should just pick one, monitor it a bit, and see how it goes.

I do appreciate everyone's replies though.

16

If you're passionate about switching both sides of the line (which I understand) then the 3-phase SSR will be cleaner. One less part to mount & heatsink. In my above response, I was not implying you should mount the heatsink to the tank, I meant the sheet metal wrapper. It's not very thick, but there is a lot of it and it may cut down on heat sink size.

I like your idea of just making temporary connections first and monitoring temperature. It's always good to know the expected result and then measure. No surprises that way. As one of my Professors used to emphasize, "Lab experiment should only be used to verify that which you believe will happen". Just shooting from the hip (like some people on this forum) is dangerous.

Best of luck on your project!

17

Hi, if your hot water heater a decent one, there should be a circuit or connection diagram on it somewhere.
Even on the inspection cover.
If not google the make and model and see what you are working with.

If you cannot see how the thermostat works then I would leave it alone until I knew what I was working with.

Sorry but advising about this unit without knowing its layout is courting disaster.

Tom...... :slight_smile:

18

Hi all!
I also intend to setup a remote controlled ArduinoUno + Ethernet shield + a 3 phase SSR for my water heater. I have a 6KW water heater and operating current is 380V and this results in ~16Amps but the SSR that I'll buy is a 3 phase 40A SSR+heatsink)

I understand that the way I can command the SSR to close the circuit is by feeding it with 5V-32v current. I want to start the heater remotely and keep it ON for 2-3hours (while I'm driving to the vacation house).

The thing is that I don't know how to do that from the arduino. The way I see it (I have no experience) is this:
I'll have the 3 phase SSR interrupting the three phase cable that goes from the 380V mains to the water heater and at the other end of the SSR I would use a transformer that outputs anything from 6 to 30V).
So with arduino I would need to control the +cable that is connected between the transformer's output and SSR's input(+), by receiving a command through the ethernet shield. But I want to keep that contact ON for about 2-3 hours then stop it be sending another command to the arduino via its ethernet shield. Can you please advise me about how can I do that?

And I also need to have some power source for the arduino itself.

19

For a water tank, using one SSR on one electrical leg is perfectly adequate for control purposes. I would probably use it in SERIES with the existing thermostat. Set the Arduino to a middle temperature, such as 115F, and set the water heater thermostat to a higher temp such as 135F. The Arduino does the day-to-day control, and the thermostat does a "kill switch" at a higher temperature for safer operation.

You need to use a proper heat sink on the SSR. Anything greater than a few amps needs a heat sink. For 25 - 60 amp SSRs, an aluminum heat sink of this size will dissipate the thermal rise. Steel, or painted steel, is a weaker heat sink and will not perform as well. If you do not heat sink the SSR, it will fail at some point.
SSR heat sink

Since a SSR has no wear-and-tear, it is ok to cycle it on/off frequently. Load shed it OFF when other heavy electrical products are in use, load shed it ON when power costs drop such as during the night. Turn it on or off with your cell phone when you are away from the house.

20

Failure to use a heatsink for the SSR will result in the SSR failing in the ON condition.

Also, a properly rated 3-phase contactor is also acceptable. I am a technician who works on high power
(10kW - 1 MW ) equipment made for the utility companies. We use SSRs occassionally for in-house test
fixtures but never for something that we plan to ship. As long as you are aware that ALL switch closures
are accompanied by arcing accross the contacts ( mostly on turn-OFF). The only difference is the degree
of arcing. Snubber networks are sometimes added to suppress thie arcing through an RC network (properly rated). You can witness thid phenomenon of electricity firsthand by cycling a high power load in a completely dark room. Same applies to the master ON/OFF blade switch. All manufacturers of such equipment assign a rating of "n" contact closures to the equipment based on melting point and degradation of the metals used that has been verified by lab tests and certified by UL. The one detail that I don't think has been mentioned is that this phenomenon does not apply to SSRs because they have "zero-crossing" circuitry that only switches the Triac at the zero-crossing point which might be why Metzner prefers the SSR solution. Both methods will work but SSRs do not requie a contact closure "cycles" rating since there is no arcing. You choose.