Maximum Zs Calculator
Work out the highest earth fault loop impedance a protective device will allow and still disconnect in time, then check a measured Zs against the temperature-corrected limit.
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How maximum Zs is worked out
For the fault loop to disconnect in the required time, the earth fault loop impedance has to be low enough to drive the current that operates the protective device. For a breaker that means reaching its instantaneous magnetic trip:
where U0 is the nominal line voltage to earth (230 V), Cmin is the 0.95 voltage factor, and Ia is the current that causes disconnection in the required time. For a BS EN 60898 breaker (and the equivalent BS EN 61009 RCBO) that current is a fixed multiple of the rating:
The 0.8 rule of thumb
The tabulated maxima assume the conductors are at their normal operating temperature. When you test, the installation is usually colder, so its resistance is lower than it will be in service. A widely used rule of thumb is to compare the measured value against 0.8 of the tabulated maximum, which builds in the temperature rise. BS 7671 also gives the more precise 0.95 correction approach in its tables.
MCCBs
A moulded-case circuit breaker to BS EN 60947-2 trips instantaneously at its magnetic setting Im, which is usually adjustable rather than fixed like a Type B, C or D. Choose MCCB, enter the rating and the instantaneous trip as a multiple of In from the device data, commonly around 10, and the maximum Zs follows the same equation.
Fuses and other devices
For fuses (BS 88, BS 3036, BS 1362 and similar) the disconnection current is not a simple multiple of the rating, it is read from the device's time/current curve for the 0.4 s or 5 s disconnection time. Read Ia off that curve and enter it under “Custom” to get the maximum Zs. The device standard sets which table applies, see the BS numbers reference.