Good Earthing and Shield in Transformer

Shield Earthing in Transformer 

A transformer generally has two separate windings - the primary and the secondary - although some may have multiple secondary windings. The primary winding is driven by the incoming power, while the secondary winding delivers power to the load device(s). In some transformers there is a metal, typically copper, between the actual primary and the secondary. This shield is earthed to the supply ground connection.

If a fault develops in the insulation of the primary, possibly due to overheating, and this gets through to the secondary, then the output will have a direct electrical connection to the power input and this could create a dangerous situation.

This type of transformer might be used in such places as an electric shaver socket in a bathroom. The idea of these sockets is that neither side of the output has any connection to any form of voltage reference such as the input from the local mains supply. If someone comes into contact with one of the output conductors, possibly through having wet hands or accidentally dropping the shaver into water, then with no path to complete the circuit the person does not experience an electric shock. The shield provides a barrier to stop the primary from reaching the secondary winding.

These transformers also have extra insulation surrounding the secondary winding so that the whole winding is floating and has no reference voltage superimposed on the AC output.

Another application is in power systems fed by overhead power cables. If lightning strikes the cable, a grounded shield in the transformed can help protect against the lightning discharge jumping from the primary to the secondary and causing damage further down the supply chain. If the discharge arcs to the shield instead of the secondary then there is reduced risk of it reaching devices connected to the secondary side of the transformer.

Good Earthing for a Transformer

As general rule and safety operatives, never use a earth colored wire to other tasks than grounding a transformer. Neutral phase is not a non-voltage phase.

An insulation measuring decide (dc current supply) does give us an idea of correct grounding of transformer. Normally, values above 300 ohms are correct, being above 1 giga ohm a very good earthing status of transformer. Test voltage to apply to transformer should be lower, of course, than winding that is being tested.

There are many testing configurations (High voltage winding to voltage supply and vs any point of earth, Low voltage winding to voltage supply and vs any point of earth, high voltage winding to voltage supply and vs low voltage winding…)

Years of explotation of transformer reduce it’s operative live, as normally happens, and insulation of transformer becomes also reduced. Periodical checking should give clues about the status of system (transformer). Also insulation liquid ageing (mineral oil) drives us to insulation deviations and possible failures.

Very low values (100, 80, 30…ohms) refer to very bad status and risk of total failure and damage.