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I've created a complete hardware platform using a Raspberry Pi as master/controller, an Arduino nano and two RF24l01 transceivers, which should control some household electrical appliances.
The software part and the "low voltage" circuitry is ready and tested but now I'm stumbled on the appropriate relay for higher power appliances, i.e an electric water heater of 50 liters rated for 1500Watt, taking account of a 220V mains voltage.

Currently I have two Arduino compatible modules, one rated for 16A at 250VAC (normally-open) and a second one with two channels each rated for 10A at 250VAC as the attached picture below.

enter image description here

The doubt comes in considering that normally water heater uses bipolar switches (I would leave the existent one in place), so the question is simply, should I use:

  • the first relay (16A, left side of the picture) interrupting only the phase line

or

  • should I take advantage of the two channels using them one for phase and one for neutral lines?


Any consideration will be really appreciated!

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    \$\begingroup\$ Since we don't know what is your mains voltage, we don't know how many amps your "1500 W" load means? If you are talking about 120V, then 1500W is 12.5A and exceeds the capacity of your 10A relays. In most cases, it would be safe to interrupt only the "phase" ("hot") side of the power assuming the other side ("neutral") is ground. But if you are in North America, then 230V is typically "phase-to-phase" and neither side is "neutral". If I were doing this project, I would use a more appropriate double-pole, 20A relay. Note that "short circuiting neutral". is NOT an acceptable method. \$\endgroup\$ – Richard Crowley Jun 25 '16 at 18:12
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    \$\begingroup\$ What is the current rating of the water heater? Must be less than or equal to relay rating. It would also be a good idea if the relay current rating was the same or higher than the current rating of the circuit breaker or fuse supplying power for the heater. If the existing switch is bipolar, I would suggest using a bipolar relay (not two separate relays). DPST or DPDT. \$\endgroup\$ – mkeith Jun 25 '16 at 18:13
  • \$\begingroup\$ Sorry, I'll edit the question with the missing information! Anyway I'm in Italy and main voltage is 220V. \$\endgroup\$ – wiredolphin Jun 25 '16 at 18:24
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My hot water cylinder is controlled by an arrangement similar to what you describe (using a Particle (now Photon) Arduino compatible ARM + WiFi module).
My system is "fail energised" - ie if you depower the electronics and/or the relay the relay is NOT operated and the hot water circuit is powered ON. For my purposes this allows the controlling system to be turned off (or to fail in many cases) and the hotwater system operates as it would usually do. This may not be a suitable arrangement for a general controller where you may wish to actively enable powering the load.

If you ignore requlatory requirements, which probably say you are not allowed to do what you want to do) then:

(1) Double pole switching is a very good idea safety wise regardless of regulations.

(2) Any low cost hobbyist ("hacker") market targeted relay is highly likely to be dangerously unsafe for mains switching applications, regardless of the standards marks printed on them. It is usual for low cost Asian sourced devices to have such markings and all too usual for them to have no formal basis.

I looked at a number of "Arduino"/hacker targeted relay modules and none that I saw met required safety standards for isolation. Some used Songle" brand relays as seen in your photo. That is not to say that Songle do not make relays which are mains-safe - just that mine weren't and it is likely that yours aren't. The most obvious failing is that they placed the common terminals of the DPDT relays on the same side of the relay base as the coil contacts, so that mains to low voltage clearances were small. The PCB makers compounded the problem by running tracks without regard to their relative voltages. Under situations within the range of normal such relays could kill.

Another major issue is the genuineness of the contact ratings. I'm operating a 3 kW element which is a more severe challenge than yours, but as you talk about "higher power appliances", if this is used in 'plug in' applications you can expect it may be used for a maximum legal load (in many places) of 10A at 230 VAC = 2.3 kW. Murphy says that some Turkey will then add at least a 1 kW bar heater or a 1.5 kW kettle or just maybe another 2 bar 2.3 kW heater. While the circuit fuse of circuit breaker will (or should) not allow this load for long, you do not want to use a relay whose ratings are suspect.

As others have said, it may be convenient to use a relay module such as you have shown to control a well rated quality relay (which is what I do). In that case the power supply and related circuitry to operate the relay board is floating relative to the microcontroller and driven by a "proper" isolator. Optocouplers are available which have triac outputs suited to driving small relays or larger triacs with which to drive the main relay.

While it may seem attractive to use a TRIAC as the main switch, you will find that the power dissipation is annoyingly high. eg at even 10 A a 1 volt drop yields 10 Watts dissipation, and TRIACS on state voltages will be in the 1.2 - 1.6V range.

Quality relays from known acceptable manufacturers and suppliers are available for probably under $10.00. OMRON & TE Connectivity come to mind but there are numerous trustable brands. Asian products can be top class and many top brands are manufactured in China. But supply via a supply channel whose bona fides are not absolutely impeccable may lead to disaster - regardless of the (apparent) brand on the relay.


These three TE Connectivity relays are SPST but otherwise highly suitable. They only cost about $US3.50 in 1 or 500 depending which part of Digikeys site you read. They are out of stock with large lead time (and probably an MOQ to reorder) so not a final product you'd use from Digikey but show what's available. Ratings are 250 VAC minimum and 20A to 30A.

____________________________________________

Here's a near ideal relay for your purpose. OMRON G7L-2A-TUB
DPST, 250 VAC, 25A carry and break at resistive load or up to power factor of 0.4. $US11.75/1 in stock Digikey US.
Digikey product page
OMRON data sheet Specification and endurance chart on page 3.
100,000 cycles at 25A.
500,000 cycles at 5A.

G7L series
2 x SP
A NO
T quick connect terminals UB mounts with 2 screw ears and tags vertical

enter image description here

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  • \$\begingroup\$ Impressive answer, I really appreciate the detailed explanation and not last the product suggestion. I'm searching some more info about the OMRON but cannot find any ideal circuit to drive it with a micro controller. A really simple schematic could be very helpful. Thank you in advance if you decide to dedicate some more time to your already helpful answer. \$\endgroup\$ – wiredolphin Jun 26 '16 at 9:57
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    \$\begingroup\$ @Vincent From above "As others have said, it may be convenient to use a relay module such as you have shown to control a well rated quality relay (which is what I do)." In this case the coils requires 12V at 158 mA. You can also get 6V 317 mA versions . The spec sheet says the "must operate" voltage is 75% of nominal = 4.5V here so it would be naughty but within spec to run the 6V version on a GOOD 5V supply with a suitable driver. You could use a transistor driven by the Arduino or, very easily, a basic relay module similar to what you have shown to switch the (say) 12V coil voltage. \$\endgroup\$ – Russell McMahon Jun 26 '16 at 16:15
  • \$\begingroup\$ That's the hint I was waiting for!! \$\endgroup\$ – wiredolphin Jun 26 '16 at 16:22
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Inductive switching can cause extremley high voltages to appear across the contacts. This can cause damage to the contacts, but worse still, it could jump across the pcb and damage your low voltage circuit.

Personally, I wouldn't use either of these to drive a mains load; instead I would use one of these to trigger another relay that is better equipped to deal with mains voltage/current and inductive load switching and is away from your low voltage circuit.

Double pole switching is great for full isolation, but if you are using two different relays to control this, then you would need to be very careful to check the mains was not live before working on it. For instance, if the relay controlling the phase was stuck on and the neutral relay switched off, the circuit would be broken(off), but would still be live.

This is only my opinion. I recommend that only experienced electricians work with mains electricity.

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    \$\begingroup\$ (1) The water heater would be a resistive load - no inductance so "extremely high voltages" would not appear. (2) Since neutral is grounded there is probably no need to switch it. Your points are valid, however. Welcome to EE.SE. \$\endgroup\$ – Transistor Jun 25 '16 at 18:32
  • \$\begingroup\$ Fair point, but I thought some heating elements could be coiled and therefore have some inductance. I'm still learning and playing it safe. Thanks. \$\endgroup\$ – Apple Pirate Jun 25 '16 at 18:45
  • \$\begingroup\$ Op is in Italy on 220V, so the double sided switching is a good idea. \$\endgroup\$ – Passerby Jun 25 '16 at 19:18
  • \$\begingroup\$ +1 For sensible solution. Drive external 250vac double-pole relay with 30 amp contacts for long life. Arc snubbers are optional. \$\endgroup\$ – Sparky256 Jun 25 '16 at 22:13
  • \$\begingroup\$ I really don't think you're going to have a big snubbing problem on a resistive load. \$\endgroup\$ – Harper - Reinstate Monica Jun 26 '16 at 10:29
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Safe design of something to control a 1500 watt (6.5A @ 230V) water heater load would be to use a heavy-duty mains-rated relay. This would mean a DUAL POLE relay rated at at least 15-20A at 250V. You cannot assume that either side of the power is "neutral". It is safest to switch BOTH sides of the power.

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  • \$\begingroup\$ Drive such kind of relay is not possible for an Arduino, could in your opinion be a valid and safe approach drive a dual pole power relay with a compatible Arduino relay? And also, assuming that in my country mains is single-phase is your last statement still valid? I mean, what can happen if I use the 16A relay only on the phase line, checking firstly i's really the phase bus? Sorry, I'm really newbie to high-voltage. \$\endgroup\$ – wiredolphin Jun 25 '16 at 19:13
  • \$\begingroup\$ @Vincent 1) You do not have to drive it directly - you can have a driver to energise the relay. 2) Do you really want to ask yourself how lucky you feel today every time you use a hot water tap or take a shower? \$\endgroup\$ – Andrew Morton Jun 25 '16 at 20:44
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    \$\begingroup\$ A relay of ANY size can be controlled with the proper driving circuit by any logic-level signal such as from an Arduino or other micro-controller. Yes, if you can clearly identify one side of the mains power as neutral, then it is safe to control only the "phase" or "hot" side of the power. But remember to treat mains power with proper caution to keep everyone safe. Including people who may come after you to work on the system. And it is always safer to over-rate things by at least 2x. So use at least a 15A relay for a 6.8A load. And a 20A relay would be even better. \$\endgroup\$ – Richard Crowley Jun 25 '16 at 20:59
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If you're planning to connect something such as ~6.5A, then I'd say you need something at least for 16 or 20A, because the beggining peak current can be more than 100% higher than it is rated.

For the second thing, as someone before me said: you should have double pole relay, since you don't know which is line and which is neutral wire, unless you're planning onto wiring thing nearly forever - in that case, you can use single pole relay, but be sure to measure the wires before you connect and separate the proper wire (line!).

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  • \$\begingroup\$ :-}....... \$\endgroup\$ – Russell McMahon Jun 26 '16 at 8:52
  • \$\begingroup\$ You googled me? \$\endgroup\$ – Jakey Jun 26 '16 at 12:32
  • \$\begingroup\$ Start up surge current for almost resistive heating elements is about the same as steady state. But, as you note, if he was switching eg a 1000 Watt incandescent lamp the start current would be many times the run current. \$\endgroup\$ – Russell McMahon Jun 26 '16 at 16:19
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Your water-heater load should be resistive, so it's the easiest load to switch: you don't have inrush current or inductive kick. Nonetheless, you do want to significantly over-spec the relay.

One big problem with working in 120/240VAC is you must be Code compliant with whatever your code is in your country. This means you must enclose things, do quality work, and use parts listed for the purpose. And also, do work using materials and methods that an electrical inspector would recognize as proper. Switching directly with the Arduino module is out of the question.

Parts intended for control of air conditioners are a good option, and cheap because the stuff is made by the millions. Air conditioners are normally switched by a thermostat using (in America, anyway) 24 VAC, which must be kept totally separate from line-voltage circuits. Transformers are readily available which install onto a junction box in a Code-legal way, $12 in America. 2-pole air conditioner relays listed for switching 30-40A motors are also in the $12 range. Common thermostat cable can be used for the hookups. An electrical inspector who saw this setup would be unlikely to question it.

I would use the Arduino module to switch the thermostat-voltage supply, which in turn would operate the large relay. The large relay can be anywhere - thermostat wire is cheap.

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