3.3.4   Semiconductor isolators

Semiconductors are very widely used in electrical installations, from simple domestic light dimmers (usually using triacs) to complex speed controllers for three phase motors (using thyristors). Whilst the semiconductors themselves are functional switches, operating very rapidly to control the circuit voltage, they must NOT be used as isolators.

This is because when not conducting (in the OFF position) they still allow a very small leakage current to flow, and have not totally isolated the circuit they control. {Figure 3.5} shows semiconductors in a typical speed control circuit.

Fig 3.5 - Speed control for a dc motor fed from a three-phase supply