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Looking at this datasheet for a mains switch, the 'preferred option' (5503 on the sheet) is called 'switched neutral'. I cannot see how both the live and the neutral can be switched with only 3 pins, as surely you need four (two for input live/neutral and two for output live/neutral) which leads me to believe that only the neutral is switched in this arrangement.

I also don't see how the circuit diagram makes sense, as if the mains plug is connected to pins 2 and 3 then this should just give a short across them when the switch is closed. If instead the mains plug is connected to pins 1 and 2 then only one of the live/neutral can be switched (depending on which one is connected to pin 2).

As only switching the neutral sounds like a terrible idea, combined with the lack of understanding of the circuit diagram, this leads me to assume I have misunderstood something quite badly. Any help you could give me in this matter would be greatly appreciated.

EDIT: The only way I can see the switch working properly is if we have the following setup:

      Pin 1          Pin 2         Pin 3
       (*)            (*)           (*)
       / \             |             |
      /   \            |             |
     /     \           |             |
    /       \         live          live
   /         \        mains         circuit
  /           \
neutral    neutral
mains      circuit

as when the switch is closed pins 2 and 3 become continuous. This seems to disagree with the answers below, where pin 2 is the common neutral pin. Have I misunderstood the answers as well as the datasheet?

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  • \$\begingroup\$ The switch Neutral Pole has nothing to do with Neutral Power. You can use this series of switches for 20A 24Vdc or 240Vac (10,12,16A depending on load type). Note: It says consult factory on lamp voltage. This means they will tell you which value of resistor to connect pin1 to AC Neutral or DC ground to power the diode protected LED on pin 1, depending on voltage on Pin 2 (external R desired to yield say 10mA in Lamp or perhaps 20mA for brighter illumination) I suspect Pin 1 will be rated for 20mA MAX. I think it was a mistake for them to call it Neutral pin. It is the Power input pole. \$\endgroup\$ – user11355 Aug 10 '12 at 9:49
  • \$\begingroup\$ Sorry, I still don't fully understand. So you're saying that I should have live circuit on pin 3, live mains on pin 2 (or the other way around) and then connect pin 1 to mains neutral using some value of resistor, correct? If so, how does the resistor connected to mains neutral work? If there's an internal connection between the live and pin 1 won't this just create a short to neutral, drawing massive current? \$\endgroup\$ – rbobbington Aug 10 '12 at 10:16
  • \$\begingroup\$ Sorry, I got my pins confused above (looking at it the wrong way around). I meant to say live circuit on pin 1, live mains on pin 2 and the resistor on pin 3. \$\endgroup\$ – rbobbington Aug 10 '12 at 10:24
  • \$\begingroup\$ The lamp indicator symbol between pin 2 and pin 1 means it will be a lamp of unknown voltage.. perhaps an LED of 2V with protection diode, so the external resistor might be 120v/.01A= 12KΩ 2watt resistor. for 10mA. CONSULT FACTORY FOR DETAILS Hopefully they use 2 LEDs back to back to double the flicker rate for AC. and hopefully not a plasma nixie mini tube as symbol shows. \$\endgroup\$ – user11355 Aug 10 '12 at 10:30
  • \$\begingroup\$ I understand that I need to ask the factory for details on the lamp, but I don't understand where I'm supposed to connect things to. \$\endgroup\$ – rbobbington Aug 10 '12 at 11:04
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enter image description here

Left goes to mains, middle to your appliance, right goes to ground (maybe via a resistor??). The difference between 5503 and 5508 is that 5503 disconnects both sides of the lamp when off, while 5508 keeps the ground side connected. So the '(un)switched neutral' refers to the neutral of the lamp only.

I don't see the purpose of the 'disconnect the lamp's neutral': the other side of the lamp is still connected to the neutral via the load, so for safety purposes it is still connected.

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  • \$\begingroup\$ Thanks. It's certainly non-obvious (at least to me) that switched neutral refers to the neutral of the indicator lamp. I don't follow your pin assignments of left -> mains, middle -> appliance and right -> ground, though. \$\endgroup\$ – rbobbington Aug 10 '12 at 9:09
  • \$\begingroup\$ You could use mains-middle and load-left, that would leave the lamp connected at one side when the load is off. I am not sure what difference that would make. (except that failure of the left switch would not be detectable from the lamp) \$\endgroup\$ – Wouter van Ooijen Aug 10 '12 at 10:46
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Essentially what is confusing is that the switched neutral means switched Pole for Indicator. NOT SWITCHING NEUTRAL LINE VOLTAGE. In this context they think the pole in the center is the neutral point that feeds both the load on pin 3 and the LED on pin 1.

That indicator pole contact is for low current LED and MUST use gold plated contacts only for that internal circuit typcially using <10 µ" flash gold plating to prevent corrosion resistance from lack of wetting current. Most people do not realize this until they try to do it for the 1st time and it fails or they can read between the lines. I believe that is what the logo symbol for µ is intended to indicate. That not only is it µ" flash gold but it can handle µA currents for a long time. Regular tungsten pole contacts on switches or relays would oxidize and fail quickly on LED loads or Logic level loads.

So the Unswitched Neutral really means unswitched LED pole for special applications where you want the LED ON ALL THE TIME even if the main switch is off.

The other answer is correct too. I just tried to rationalize it for you.

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  • \$\begingroup\$ Thanks for the answer. Unfortunately I still don't fully understand what's going on. When the switch is closed, pins 2 and 3 are electrically continuous so if pin 2 is the neutral point for both the load and the indicator then this appears to not work. I don't seem to be able to pull off the desired formatting of the next part of my reply here, so I'll make an edit in the original question instead. \$\endgroup\$ – rbobbington Aug 10 '12 at 9:00

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