The Breaker Earth Loop Impedance (Zs) can be calculated 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 five 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 current 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” does not exist, in this case consider Io = 1.