Maximum permitted Zs B20

Maximum permitted Zs B20

The maximum permitted Zs for a B20 circuit breaker is 2.3 Ω. Below we explain how this value is calculated, why relying on it as an absolute limit can be risky in practice, and which measuring instruments should be used to verify ZS.

Maximum permitted Zs B20
Maximum permitted Zs B20

MCB trip characteristic - Type B

When performing the calculation, you must take into account the tripping characteristics of type B MCBs. According to technical literature, a type B breaker should trip at a current between three and five times the rated current IN (for B20, IN = 20 A).

For a safe calculation, the worst-case scenario is assumed - tripping at five times the rated current: IN: 5 × IN = 5 × 20 A = 100 A.

Earth fault loop impedance formula for B16

The formula for calculating the maximum permitted Zs for a B20 breaker is as follows:

Z S = U 0 I a
Z S = 230 V 100 A
Z S = 2.3 Ω

ZS represents the maximum permitted earth fault loop impedance, U0 is the nominal voltage to earth, and Ia is the current required to trip the MCB within the required disconnection time.

Interpreting the result & the 2/3 factor

In practice, you should not rely solely on the theoretical maximum permitted Zs value. You must also consider possible measurement tolerances - BS EN 61557-3 specifies an allowable error of ±30%.

Real-world circuit conditions should also be taken into account. For example, in a socket circuit that may supply multiple devices (such as a vacuum cleaner or iron), the cable length and cross-sectional area may vary. This means the measurement result should be treated with a margin to ensure reliable and rapid disconnection.

For this reason, a safety factor of 2/3 is commonly used in practice - the maximum permitted Zs for B20 is multiplied by 2/3, and this adjusted value is treated as the practical limit to guarantee proper MCB operation.

Loop impedance testers

Accurate Zs measurement for B20 requires appropriate instruments. The choice of tester depends on the rating of the overcurrent protection and whether RCDs are present in the installation. Below we recommend selected models from our range that provide reliable and repeatable test results.