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I have a water pump rated 650 W and the attached relay board. enter image description here

Is it ok to modify a power socket to use one relay to control the phase and one to control the neutral wire?

Thank you!

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  • \$\begingroup\$ It's a single phase pump? Doesn't sound like you need to switch neutral. \$\endgroup\$ – uint128_t Mar 9 '16 at 6:25
  • \$\begingroup\$ Said another way, if you switch the hot line off, the pump will be off. \$\endgroup\$ – tokamak Mar 9 '16 at 6:34
  • \$\begingroup\$ Yes, it is a single phase pump but the issue is that I know now which is the hot line, but what if the wife/kid will put the power cord in the other way in the power socket. (then the old hot wire will be neutral and neutral will become phase). \$\endgroup\$ – Sorin Trimbitas Mar 9 '16 at 6:38
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    \$\begingroup\$ You should be using a polarized (3 pin) plug on a high power device like that to insure it cannot be inserted incorrectly . \$\endgroup\$ – Nedd Mar 9 '16 at 7:08
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    \$\begingroup\$ For Romania there should be a round polarized type F plug available. Though I suppose in some areas you have to use what is available. In that case the two relay option should be the better idea. Just be sure to connect the enable signal pins together to insure both relays switch at the same time. But you should still provide an unbroken ground wire to the metal frame of the unit, even if it is separate from the line cord. \$\endgroup\$ – Nedd Mar 9 '16 at 8:58
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[Edit] Another user points out this relay is unsuitable for switching 240VAC motors.

You only need one relay to cut the power to the pump. But you can use two if you want to be sure there's no live voltage at the other side of the relay when you switch it off. In continental Europe, for instance, it's impossible to know which line is neutral and which wire is phase.

In any case, be sure to connect the earth, always.

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  • \$\begingroup\$ (I live in Europe). I was thinking on 2 relays because I know now which is the hot line, but what if the wife/kid will put the power cord in the other way in the power socket. (then the old hot wire will be neutral and neutral will become phase) \$\endgroup\$ – Sorin Trimbitas Mar 9 '16 at 6:39
  • \$\begingroup\$ I thought only the US had unpolarized plugs. I don't see any reason why you shouldn't switch both wires. In the US you are usually not allowed to switch the ground wire. Not sure of the code in your country or if you even have a ground conductor. \$\endgroup\$ – mkeith Mar 9 '16 at 7:17
  • \$\begingroup\$ @mkeith there is no ground wire in this particular connection. \$\endgroup\$ – Sorin Trimbitas Mar 9 '16 at 7:29
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    \$\begingroup\$ @mkeith .. yes, i have one installed in the house and this circuit is also part of the main circuit. \$\endgroup\$ – Sorin Trimbitas Mar 9 '16 at 7:40
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    \$\begingroup\$ @SorinTrimbitas The earth connection to the pump is the most important, it saves lives. \$\endgroup\$ – Marko Buršič Mar 9 '16 at 8:14
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The relay appears to be unsuitable for controlling a 220 v motor. The manufacturer's specifications list inductive load ratings only for 120 volts.

enter image description here

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  • \$\begingroup\$ Ahh, i also forgot to check the specs, i saw the 10A 240V and thought it was ok for such a small pump. Thank you for your sharp thinking! \$\endgroup\$ – Sorin Trimbitas Mar 10 '16 at 12:23
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You just need to open the circuit in order for the motor to turn off. It doesn't matter if you accidentally swap the Phase and Neutral as long as you open the circuit.

The relay rating seems fine, just Go Ahead with it. If you feel like, you can just have 2 Relays.

It will work just fine, both ways.

Note: Motors will usually have a peak current during startup that could go 10 times its actual rating. Make sure the peak lies within the Specs of the relay. Look up the specs of the motor.

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  • \$\begingroup\$ It seems that you have failed to understand or address the concern of the OP, which is that if the "neutral" wire is switched, some parts of the pump will be at elevated potential. Possibly creating a hazard. \$\endgroup\$ – mkeith Mar 9 '16 at 7:15
  • \$\begingroup\$ That's true. Ideally it's a good practice to switch Phase line. You can also do that(switching neutral line)when you know that the body is earthed properly. In this case it's better to use 2 switches. \$\endgroup\$ – seetharaman Mar 9 '16 at 8:04

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