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When and which Residual Current Device to use?
Total earth loop impedance
We know how (very) important the total earth loop impedance
Zs is in the reduction of shock risk.
However, in TT systems where the mass of earth is part of the
fault path, the maximum values of Zs given in the IEE Regulations
may be hard to satisfy. Added to this, climatic conditions will alter
the resistance of the earth in such a way that Ze may be
satisfactory in wet weather, but not in very dry.
Figure 10.2 - – Operating principle of the residual current
device
The regulations recommend, therefore, that the preferred
method of earth fault protection for installations in TT systems be
achieved by RCDs, such that the product of its residual operating
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