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Zs on TT


paul b b

Question

paul b b

Hi chaps,

Is there a reg for max Zs on a TT?

many thanks

paul

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8 answers to this question

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Andy™

1667 if 30mA RCD'd. although any Ze over 200 should be considered unstable

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paul b b

what if it is a shower circuit on a 40A mcb?

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Andy™

Zs is 1667 for anything with 30mA RCD (but you should never get anything that high)

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paul b b
Zs is 1667 for anything with 30mA RCD (but you should never get anything that high)

Table 41.5 (page 50 in the big red) says Max EFLI not exceeding 32A.

sorry if i am missing something andy, just paniking about my assessment!

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I think a l-n loop test is a good idea on tt systems at least that way you know circuit breaker will trip if you have l-n fault.

Batty

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Andy™

411.5.3 (ii) still applies (which works out at the same Zs as table 41.5)

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paul b b
411.5.3 (ii) still applies (which works out at the same Zs as table 41.5)

ahhh ok, thanks andy

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binky

I look at this way, whatever you would normally expect for Zs still applies plus the rod resistance. But in theory, as Andy says you can have some very big numbers.

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