For earth loop impedance, we can provide the method and the customer calculates it according to his need. Our colleagues of Schneider UK have made Zs calculations for Compact NS and NSX, see the attached file with 5 seconds and 0,4 seconds disconnection times, for a voltage of 230V to earth. We've got loop impedance values for MCBs as well.

Please find below the method to calculate the Breaker Earth Loop Impedance (Zs).

We calculate the Breaker Earth Loop Impedance (Zs) as follows; Zs = Uo / Ia
Where Uo is the voltage to earth and Ia is the current to trip the breaker in a given breaker disconnection time.

Usually we consider two breaker disconnection times: 5 seconds and 0.4 seconds, depending of the application.
Disconnection time of 5 seconds is used for fixed equipment. i.e. a machine in a factory or a fixed appliance (electric oven) in a house.
Disconnection time of 0.4 seconds for portable equipment. i.e. power drill or electric lawn mower. This disconnection time is usually used for final circuits (MCBs)

A. For a 5 seconds disconnection time:

1. Get to know the voltage to earth (Uo)
2. Determine Ia, see the breaker tripping curve, to find the cuurent needed to trip in 5 seconds (see example on the attached file “Earth Fault Loop Impedance Summary.ppt”, slide 2 and 3)
3. Calculate Zs = Uo / Ia

B. For a 0.4 seconds disconnection time:

1. Breakers with electronic trip units – Micrologic (Compact NSb, Compact NSX)
   Zs = (Voltage Phase to earth) / (In x Io x Ir x Isd x 1.1)
   On some trip units “Io” doesn’t exist, in this case consider Io = 1.
2. Breakers with thermal-magnetic trip units (Compact NS, Compact NB, EasyPact EZC, Compact NSX)
   Zs = (Voltage Phase to earth) / (In x Io x Ir x Isd x 1.2)
   On some trip units “Io” doesn’t exist, in this case consider Io = 1.

Released for: Schneider Electric Saudi Arabia