When to use X2 rated capacitors

[Dave#2]

I am making some mains powered equipment and I am wondering if every single capacitor that is not isolated from the mains must be X2 rated?

For example in this circuit (attached), C8 is connected to the mains, but only in series with a resistor. If the capacitor failed surely it would burn out the resistor before catching fire wouldn't it? And therefore, it is not necessary to be X2 rated? (in diagram A = connected to active)

Thanks,
David

 

[Nigel Goodwin]

I am making some mains powered equipment and I am wondering if every single capacitor that is not isolated from the mains must be X2 rated?

For example in this circuit (attached), C8 is connected to the mains, but only in series with a resistor. If the capacitor failed surely it would burn out the resistor before catching fire wouldn't it? And therefore, it is not necessary to be X2 rated? (in diagram A = connected to active)

I would agree that C8 doesn't need to be X2 rated - but that's my personal opinion, I certainly wouldn't bother with an X2 there.

 

[Dave#2]

Thanks Nigel. Anyone know for sure?

 

[G8RPI]

Thanks Nigel. Anyone know for sure?

Hi Dave,
Only capacitors that could present a hazard need to be X or Y rated. Thechnically C8 does not have to be X rated (Watch out for voltage ratings on your resistors though, some small ones are only rated for 250V peak), however I'd use an X2 capacitor for two reasons.
1/ there can be no argument if somthing does go wrong
2/ An X2 47nF 250V AC capacitor will be easier to buy and CHEAPER than a standard foil type.

The peak voltage for 230V AC is 325V without any spikes or surges. you would need a capacitor of at least 630V DC working in this circuit. Here in the UK this would cost 36 pence as opposed to 27 pence for an X2 (or X1 tested to 2500V (4000V).

Robert.

 

[Dave#2]

Thanks G8RPI. X2 are indeed cheaper and more common. Hadn't even though about the resistor rating. Thanks.