Earth Fault-Loop Impedance Calculation

What is "Earth Fault-Loop Impedance"? Why is it important?

Standards define "Earth Fault-Loop Impedance" as the impedance of the earth fault-current loop (active-to-earth loop) starting and ending at the point-of-earth fault.

It is important to know the earth fault-loop impedance of a circuit to ensure that during the occurence of a phase-to-earth fault, normally an insulation fault, the magnitude of the fault current is enough to cause the operation of the protective device within the required time particularly where the touch potential exceeds 50V a.c. or 120V ripple-free d.c.

The earth fault-loop in a MEN system comprises the following parts:

  • The active conductor as far as the point of the fault, including supply mains service line, consumers mains, sub-mains (if any) and the final sub-circuit.
  • The protective earthing conductor (PE), including the main earthing terminal/connection or bar and MEN connection.
  • The neutral-return path, consisting of the neutral conductor (N) between the main neutral terminal or bar and the neutral point at the transformer, including supply mains, service line and consumers mains.
  • The path through the neutral point of the transformer and the transformer winding

The earth fault-loop impedance in its simplest form is the impedance of the active conductor olus the impedance of the earth conductor. In this case, the fault is a bolted fault, the impedance of which is zero.

The maximum disconnection time for a 230 V supply voltage shall not exceed the following:

0.4 s for final subcircuits that supply socket-outlets having rated currents not exceed ing 63A; or hand-held Class I equipment; or portable equipment intended for manual movement during use.

5 s for other circuits where it can be shown that people are not exposed to touch voltages that exceed safe values.

Please note that the maximum disconnection times will vary for other operating voltages or installation conditions, such as wet locations. etc. The fault current must be of sufficient magnitude to cause automate disconnection within the required times.

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