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Error in on site guide maximum ZS

Newfutile

The maximum ZS for a type B 6 amp mcb  to BS EN 60898  is listed as 5.87 ohms by my calculations (and NICEIC , iCertify) this is incorrect.

6 x 5 (from 3-5 current trip) = 30 , 230/30= 7.666666 (I believe this is correct with BS7671 brown book , 7.66666 x 0.95 (cMin)=7.283333 x 80% (conductor temperature adjustment) = 5.826666

therefore 5.82 should not be exceeded .

  • Thank you, this has already been corrected.

    I've just checked Page 145 of the latest version ("Eighth Ed Reprinted with minor corrections 2022"), and it says '5.83' which matches your calculation (give or take whether you believe you can round up or not).

    I agree it was incorrect (5.87) in the original 2022 "Eighth Edition" and also P 131 in the 2018 Seventh Edition.

    Your approach is spot-on. Formula from Appendix 3:

    And for a B6, Ia = 5 × 6 = 30

    so Zs(m) ≤ 0.8 × 0.95 × 230 ÷ 30 = 5.82666...


    Round up or down? I can see both sides of this argument, but since it's a "rule of thumb" and given the measurement conditions and resolution and accuracy of most loop test instruments,rounding up in this case probably won't make much difference out there in the real world.

  • in reply to gkenyon

    It is indeed numerically wrong, in the third significant figure, but I hope it really should not be changing anyone's conclusions about if a given circuit is too long for the breaker or needs RCD backup or not.

    An error of about 1% is easily lost in the noise of is the breaker next to another one running a bit warm or not, or the exact supply voltage on the day.

    If you had been using moving needle instruments of a few years ago, it is rather less the sort of error associated with a good pub lunch...

    Mike.

  • Don't worry about it. It's all approximations and rounding up and down.

    For a start, 0.95 should be 0.94 but it isn't.

    The reciprocal of 1.2 is 0.8333r.

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