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Hybrid ring/radial final circuits NICEIC is sometimes asked whether a ring final circuit has to be supplied directly from a protective device in the distribution board or consumer unit, or whether it is permitted to supply the two ends of the ring via a dedicated radial circuit cable, as shown in Fig. 1, thereby forming a hybrid ring/radial circuit.
his question can arise where, for example, the contractor wishes to make use of a redundant radial circuit cable previously installed for another purpose (such as a cooker circuit) to supply a ring final circuit, or where a Periodic Inspection of an existing installation reveals such an arrangement. The same question may also arise where the contractor wishes to control a ring final circuit by means of a switch for functional purposes, or possibly by means of a contactor, controlled by an emergency stop push button (such as in a teaching situation), the supply being brought to the switch or contactor via a dedicated radial circuit cable. The answer is that it is permissible to form a hybrid ring/radial circuit, even though BS 7671 defines a ring final circuit as ‘A final circuit arranged in the form of a ring and connected to a single point of supply’. For such a hybrid
Fig 1 Typical ring final circuit supplied via a radial circuit cable, forming a hybrid ring/radial circuit
Ring circuit cable (installed rating 20 A min) Ring final circuit
Junction box (or possibly double-pole switch or contactor) rated at 30/32 A min Cable connected to 30/32 A overcurrent protective device in consumer unit
Radial circuit cable (installed rating 30/32 A min)
L OUT
N OUT
Consumer unit
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Summer 2011 NICEIC Connections
50_63_technical.2.vb.indd 52
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14/6/11 15:21:14