A description of loop impedance and typical values that would be expected on smaller UK electrical installations.
External loop impedance, Ze is the impedance external to the electrical installation - that of the electricity supplier's wiring and equipment.
Total loop impedance, Zs is the total impedance, and is the external impedance Ze added to the impedance of the circuit within the installation.
For smaller UK electrical installations, typical values for external loop impedance (Ze) for the three main supply earthing types are:
21 ohms for TT is a typical value provided by electricity network operators based only on their equipment (supply transformer and it's associated earth electrode). In practice, the value may be higher and depends on the type and number of earth electrode(s) at the installation site.
Larger installations (over 100A) will typically have much lower impedance values.
Fault current can be approximated from the impedance and supply voltage. As an example, with a 230V supply and Zs of 0.6 ohms, the fault current is 230 / 0.6 = 383 amps.
This value is important when selecting the circuit breaker or fuse for a circuit - the fault current must be high enough to ensure the circuit breaker disconnects if a short circuit fault occurs.
If the impedance is high, fault current will be low. Too low will result in cables overheating if a fault occurs as the circuit breaker will not disconnect in time (or at all).
For normal MCBs / circuit breakers, the current required to disconnect in the event of a fault is:
For calculations, the higher values are used. As an example, a Type C 32A MCB will require 10 x 32 = 320 amps to ensure disconnection.
These values only refer to the magnetic part of the circuit breaker which is used to disconnect near-instantly in the event of a short circuit fault.
Circuit breakers also contain a thermal trip mechanism to disconnect on moderate overloads, such as 30A through a 20A device. However this is slow to react and cannot be relied on for short circuit faults, as the cables would have melted long before the thermal trip had operated.
Similar principles apply to fuses - on moderate overloads the fuse wire element will heat up and eventually melt through. A short circuit will result in a much higher current which will destroy the fuse wire very quickly and disconnect before the circuit wiring is damaged.