Table 1 - Shows the leading initiators of electrical faults.

As an example, in the toaster circuit above, the black or hot wire is shorted to the metal casing of the toaster. When the circuit closes, all or part of the current is channeled through the toaster frame and then the green ground wire. When sufficient current flows (typically 6 x 15 A = 90 A), the circuit breaker will open. A protection relay could be installed to detect currents as low as 5 mA, which would open the circuit breaker at a significantly lower level, hence, much quicker than the traditional circuit breaker.

Although the example above shows a solidly grounded single-phase circuit, the philosophy is the same on three-phase circuits discussed later. Relays and monitors are specifically designed to look for the leading initiators shown in Table 1 by detecting low-level changes in current, voltage, resistance or temperature.

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