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Spurious RCD trip due to capacitance

Stephen Biddle
Hi All


I've been asked a question re RCD trip, as follows:  

The issue is that in some cases where we have really long runs of cables the capacitive current is calculated to be greater than 100mA. Therefore setting the earth leakage protection down to 100mA would cause nuisance tripping.


By my calcuation for 60 Hz and 400 V you'd need about 700 nF


Has anyone got any experience of this?
  • 0
    Back of a f** packet calc says you'll need about 27km of 6mm twin to get that kind of leakage current.


    Even so, as long as the leakage is balanced the RCD will not notice.
  • 0
    Customer says: The capacitance for a 3c185mm2 cable 450m run is 600nf
  • 0
    I've got a suspicion that the 3 phase might balance out the resultant electric field to zero.  Or is that just wishfull thinking?
  • 0
    Interesting question, depends on the configuration. 


    In a 3ph SWA cable what is lost to earth (armour) by capacitive leakage on one phase will be sucked up by the other phases so there will not be a nett loss to earth, similarly between phases, the capacitive effects will be localised.
  • 0
    The protection has to be relooked. If it is so important to have the 100mA sensitivity the RCD should be relocated nearer to the equipment.


    Regards

    Chris Chew
  • 0
    Thanks guys I thought we had a serious problem for a moment.

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