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RCD trip time on TT 3 Phase system 0.2 or 0.07

Don246

Hi All,

I'm going round in circles and confusing my self, looking at Table 41.1 max disconnect times.

For single phase system 0.2 seconds but for 3 phase would it be 0.2 or 0.07 seconds, its so simple yet I'm unsure.

I'd say 0.2 seconds because it's 230V to earth not line to line..

if it is 0.2 seconds what scenarios would the other two columns be used in ( 230V < Uo < 400v ) and ( Uo > 400V )

  • +1
    Don246 said:
    I'd say 0.2 seconds because it's 230V to earth not line to line.

    Correct

  • +1

    if it is 0.2 seconds what scenarios would the other two columns be used in ( 230V < Uo < 400v ) and ( Uo > 400V )

    Where the nominal voltage to Earth is above 230V... Such situations aren't common, but they do exist - there are 400/690V 3-phase systems for example, and perhaps all sorts of odd "homebrew" situations where there's no specific equipment standard, so BS 7671 has to deal with it by default.

        - Andy.

  • 0 in reply to AJJewsbury

    That makes a lot of sense, guess because I haven't personally seen it doesn't mean its not out there.

    Thank you for your reply.

  • 0

    BS 7671 (over) simplifies this, by saying that you need to look for a Zs (that is, fault to Earth) per Table 41.5 in TT systems, where RCDs are used for ADS.

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