Filipino Engineer

As mentioned in Part 1, the value of the fault loop impedance shall be able to generate enough fault current to activate any protective device.

What does this mean? The earth fault current must be high enough to trigger the automatic disconnection of a protective device within an specified time. If the value of the earth fault current is not within the tripping current of the protective device, then there will be no means of protective device will provide protection against touch voltage. At the location of the fault, there is a possibility that the full voltage is present in the equipment enclosure posing hazard to people with the increased touch voltage higher than 50 Vac.

If in cases where there is no way to increase the fault current, say the distance of the fault from the supply is too far - cable impedance is limiting the earth fault current, the only solution will be using a residual current (earth leakage) protection. It is not always advisable to have this solution though. There are applications that when a residual current protection is used, annoying erratic device operation is possible particularly when the protection device setting is not correct.

In the event that a reduction of the earth fault-loop impedance required to ensure correct disconnection times of the protective device is not possible due to distance factor, additional bonding to earthing system may be used.

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:

1. The active conductor as far as the point of the fault, including supply mains service line, consumers mains, submains (if any) and the final subcircuit.
2. The protective earthing conductor (PE), including the main earthing terminal/connection or bar and MEN connection.
3. 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.
4. The path through the neutral point of the transformer and the transformer winding