Maximum permitted Zs B16
The maximum permitted Zs for B16 is 2.875 Ω. Below, we explain how this value is derived, why allowing it in practice can be a risky mistake, and which instruments should be used to measure ZS.
MCB trip characteristic - Type B
First, we need to consider the trip characteristic of type B MCBs (Miniature Circuit Breakers). According to the literature, a type B MCB should trip within a range between three to five times the rated current IN (in our case, IN = 16 A).
For calculations, we should assume the worst-case scenario, i.e. the B16 MCB trips only at five times the rated current IN: 5 × IN = 5 × 16 A = 80 A.
Earth fault loop impedance formula for B16
The formula for calculating the maximum permitted Zs for a B16 MCB is as follows:
ZS is the maximum permitted Zs, U0 is the nominal voltage to earth, and Ia is the current required to safely trip the MCB.
Interpreting the result & the 2/3 factor
In practice, you should not rely solely on the theoretical maximum permitted Zs for B16. This is primarily due to the permissible tolerance of the calibrated measuring instrument, which according to BS 61557-3 is ±30%.
You must also consider the real operating conditions of the circuit. In practice, this means that the measured Zs for a circuit supplying, for example, a socket outlet — where various appliances (vacuum cleaner, iron, etc.) with different cable lengths and cross-sections may be connected — must be assessed with a suitable safety margin to ensure that the B16 MCB can still trip quickly.
For this reason, it is common practice to apply a 2/3 factor. This means multiplying the theoretical maximum permitted Zs for B16 by 2/3 and taking the resulting value as the safe limit for measurements.
Loop impedance testers
To effectively measure the maximum permitted Zs for B16, suitable equipment is required. Depending on the MCB rating and the presence of RCDs in the installation, we recommend the following models from our portfolio.
