The electrical separation of circuits

The electrical separation of circuits is suitable for relatively short cable lengths and high levels of insulation resistance. It is preferably used for an individual appliance

(see Figure F69)

The principle of the electrical separation of circuits (generally single-phase circuits) for safety purposes is based on the following rationale.

The two conductors from the unearthed single-phase secondary winding of a separation transformer are insulated from earth.

If a direct contact is made with one conductor, a very small current only will flow into the person making contact, through the earth and back to the other conductor, via the inherent capacitance of that conductor with respect to earth. Since the conductor capacitance to earth is very small, the current is generally below the level of perception. As the length of circuit cable increases, the direct contact current will progressively increase to a point where a dangerous electric shock will be experienced.

Fig. F69 – Safety supply from a class II separation transformer

Even if a short length of cable precludes any danger from capacitive current, a low value of insulation resistance with respect to earth can result in danger, since the current path is then via the person making contact, through the earth and back to the other conductor through the low conductor-to-earth insulation resistance.

For these reasons, relatively short lengths of well insulated cables are essential in separation systems.

Transformers are specially designed for this duty, with a high degree of insulation between primary and secondary windings, or with equivalent protection, such as an earthed metal screen between the windings.

Construction of the transformer is to class II insulation standards.

As indicated before, successful exploitation of the principle requires that:

• No conductor or exposed conductive part of the secondary circuit must be connected to earth,
• The length of secondary cabling must be limited to avoid large capacitance values,
• A high insulation-resistance value must be maintained for the cabling and appliances.

These conditions generally limit the application of this safety measure to an individual appliance.

In the case where several appliances are supplied from a separation transformer, it is necessary to observe the following requirements:

• The exposed conductive parts of all appliances must be connected together by an insulated protective conductor, but not connected to earth,
• The socket outlets must be provided with an earth-pin connection. The earth-pin connection is used in this case only to ensure the interconnection (bonding) of all exposed conductive parts.

In the case of a second fault, overcurrent protection must provide automatic disconnection in the same conditions as those required for an IT system of power system earthing.