SUBCIRCUIT PROTECTION

VERIFYING THE SUB-CIRCUIT PROTECTION RATINGS

This test determines if the sub-circuit protection will operate in under 0.4 seconds if there is Phase – Earth fault of negligible impedance. This test must be done to verify each overcurrent device (fuse/MCB) on the M.E.N and other switchboards.

First, we connect the standard lead (with 3-pin plug) to an outlet at the end of the sub-circuit being tested – this being the worst place to have such a fault, as it is here that the loop resistance is highest, and thus where over-current device will take the longest time to “blow”.

This result should now be verified against the protection ratings Table 8.1 of AS/NZS 3000:2007

In this case, assume the overcurrent device is a Type-C miniature circuit breaker (MCB) rated at 16 Amps. As the reading of 0.23 Ohms is less than the maximum value allowed (as shown in the table), we can be certain that the device will operate in under 0.4 seconds if there is a phase to earth fault anywhere on this circuit.

NOTE: Table 1 lists values for “fuses” - please be aware that the figures refer to HRC fuses only (i.e made to AS2005.21.1 - equivalent to BS 88).

If you are checking the safety of any existing circuits protected by re-wireable fuses, then the table of maximum loop resistances on the below should be used.

However, it’s advisable to replace rewireable fuses wherever practicable.

UP-RATING INSTALLED PROTECTIONS

Should a situation arise where it is considered necessary to increase the rating of an existing over-current protection device, the procedure we have just followed may be used to determine the options available so as to still maintain an installation safety.

CAUTION: The prodcedure outlined only relates to loop resistance and protection ratings. It takes no account of the ratings of the sub-circuit wiring concerned or the fittings connected to it. ensure each of these will also allow the safe up-rating to the intended over-current rating.

Plug the loop tester in at the end of the sub-circuit needing up-rating. Perform a P-E loop test. Consult Table 8.1 of AS/NZS 3000:2007; ensure the installed protection is safe (i.e that the max. loop value allowed is under that from the above test) Use the table to see if the next higher rating (or the one that you wish to upgrade to) will still preserve installation safety