TT Systems. Zs reading higher than tabulated

by Steve on March 22, 2009


I am measuring a Zs of 140 on my final circuits.

Table 41.3 gives a maximum of 1.44Ω.

I know that a Ze below 200 on a TT system is acceptable (Table 41.5 note 2).

Is my Zs ok?


Yes it is acceptable. As long as your circuit is protected by an RCD.

In this case (at the front end) they should ideally be protected by RCBOs.

The maximum Zs which ensures the operation of a 30mA RCD (circuits not exceeding 32A) is 1,667. (Table 41.5) .

Regarding your Ze measurement. A Ze greater than 200 is considered unstable and it may be prudent to install further electrodes.

Read more…  Why do TT systems require RCDs to achieve 0.2s disconnection time?

{ 2 comments… read them below or add one }

Steve November 19, 2011 at 09:23

Yes. I would put 1667 ohms in the Maximum pertmitted Zs box as per TABLE 41.5 – Maximum earth fault loop impedance (Zs) for non-delayed RCDs to BS EN 61008-1 and BS EN 61009-1. Also see Regulation 411.5.3.

tony November 16, 2011 at 16:44

When entering the Max Zs of a circuit on a TT System do i put the reading as 1667 ohms on circuits 32A and below?


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