Earth Leackage

The Line Leakage test
The Line Leakage test is the most misunderstood of the Safety tests. It was devised in order to give some measure of the quality of the insulation resistance at mains potential where it is inappropriate or impossible to perform either Flash or IR tests.

In any domestic or industrial supply a substantial current normally flows from the Phase (or Live) conductor through the various loads - motors, heating elements etc. - returning to the Neutral conductor. This current then flows, via the Neutral conductor, to the local substation where it is bonded to True Earth. This is actually the ‘Ground’ or Planet Earth. To provide protection and screening from the Live and Neutral conductors, a third conductor, Earth, is included. In a perfect world, where conductors had no resistance and insulators had infinite resistance, the current drawn by an appliance would remain in the L to N loop and no current would flow in the Earth conductor. However, in the real - imperfect - world, the current in the Live conductor can ‘leak’, via insulation, to any other conductor. The Neutral can also become elevated to 40V above True Earth so current can also ‘leak’ from this conductor. The net result is that these ‘leakage’ currents flow back to True Earth via the Earth conductor.

If the Earth conductor in a Class 1 appliance were to become disconnected then this leakage current, or part of it, would flow via the lowest impedance path to true earth. Unfortunately, this could even include the person holding or touching the appliance if they are in direct contact with the Ground.

In the case of Class 2 appliances, where no Earth wire is present, the leakage current would normally flow to True Earth via the Neutral conductor. Therefore, if the Neutral conductor becomes disconnected, the leakage current will again take the lowest impedance path to True Earth - once again possibly via personnel in contact with the appliance and Ground.

MEASUREMENT OF LEAKAGE IN AN APPLIANCE

wo distinct methods of measurement are employed.
Insertion of a sensitive, current measuring device directly between the Earth wire of the appliance and the supply Earth. A reading of the current flow in the Earth path is taken with the appliance powered-up. The power connections are then reversed and a second reading taken - this is to take account of possible polarity reversal in manufacture and/or installation. In some instances the impedance of the measuring circuit is designed to replicate the impedance of the human body.

For Class 2 appliances, where there is no earth wire, the measurement is taken between the outer surface of the appliance, wrapped in conductive foil, and True Earth.

NOTE - this first method can be hazardous and must only be undertaken by skilled personnel and under strictly controlled conditions. The second method uses current transformers and ‘Hall Effect’ devices, which do no require disconnection of the earth conductor. Class 1 appliances can have the earth leakage current directly monitored using this method.