capacitors

[reddish75]

hi, was just wondering could i replace a couple of capacitors that are rated at 400v that go over the mains 240v, with some that are rated at 250v? obviously a massive difference in voltage and just want to be sure.

[AndrewT]

There are special mains rated capacitors for connecting to the mains.
X rated for Live to Neutral and Y rated for Live to Earth or Neutral to earth.

The earth connected capacitors pass some current to earth. This is equivalent to earth leakage current.
The value of Y rated Earth connected capacitance must be kept low to ensure your total earth leakage current is kept within sensible limits.

If you have RCCB in your distribution board then the total earth leakage passing each RCCB must be kept below 25% of the RCCB rating i.e. leakage current <=7.5mA for each 30mA RCCB protected circuit.

[teemuk]

You can't: 250V is the maximum voltage of the capacitors while 240V is only the RMS voltage of the mains. Multiply that by 1.414 to get the peak voltage: 339.4V! Add some additional tolerance for voltage and the 400V rating starts to look quite correct.

Naturally, there will be higher transients in the mains than that which means you really need to focus on this issue more. You need to select the proper type of caps that can withstand those transient peaks safely and even more, you need to select caps that, in the case of failure, will fail safely. You can't just stick any cap with "proper" voltage rating there.

[AndrewT]

BTW,
those X & Y rated caps are available in 250Vac and 275Vac.

For the UK you must use the 275Vac versions since our mains maximum tolerance is 254Vac (360Vpk).

[jaycee]

it's also a requirement that the caps used MUST be X-rated (for across the line) and Y-rated (from line to ground).

In an amp or any device not having a switch mode supply, generally the X capacitor is the only one neccesary, and is best placed over any switch contacts.