Importance

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Abstract

Circuitry is described for isolating a section of a loop circuit (L) from a bi-directional supply unit (20) in the event of a short circuit in the section. One or more circuit elements (21), such as fire detectors, are connected in respective portions of the loop circuit and between respective pairs of bi-directional isolators (1,2,3,4). Each isolator includes switching means (TR1,TR3) providing low and high impedance paths for respectively supplying current to said circuit elements and isolating said circuit elements from the supply unit (20). Each isolator also includes sensing means (TR2, D1, D7) for sensing a short circuit in any one of the circuit elements (21) whereby the sensing means causes a change in state of the switching means from the low to the high impedance path thereby isolating the circuit elements in the respective section of the loop from the supply unit.

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First Claim