Calculations
Protection against Indirect contact
If a fault occurs between the phase 1 and the frame of the equipment, a leakage current will exist though the protective wire to the earth. This current meets wire impedance, ground impedance and fault impedance.
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– Installation ground earth impedance
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– Transformer (supply) ground earth impedance
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- Fault impedance
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– Limit voltage (50 V or 25 V)
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– Fault voltage
In case of fault, the fault current is
and the fault voltage 
The security device should open the circuit if
, in this case the rating value of the RCD/ELCB is 
with 𝐼∆𝑛 as the sensibility of the RCD/ELCB
Protection against direct contact
The protection against direct contact is defined by the IEC 60449-1. The RCD/ELCB has to have a sensibility according to the threshold of sensation, 30 mA.
