Wiring and Conductors
[ Last Updated 27 March 2006 ]
Q. Are Neutrals Required to be Separately Terminated on a Neutral Bar when Supplied from the RCD, or Can all the Neutral Conductors of Final Subcircuits be Connected to the Neutral Terminal on the RCD?
A. Either solution is correct. AS/NZS 3000 Clause 2.9.3.2(d) requires that the neutral bar is provided with a separate terminal for the incoming and for each outgoing neutral conductor associated with active conductors originating or terminating at the switchboard.
All final subcircuits that are fed from an RCD can have all the neutrals terminated into the outgoing neutral terminal of the RCD; or have separate bars to terminate the neutrals of each RCD final sub-circuit, with a single neutral connected to the outgoing neutral terminal on the RCD. In both cases, the neutral feed from the main neutral bar must be connected to the incoming neutral terminal on the RCD.
Because the main neutral bar is numbered, the way to identify the circuits is to match up the neutrals (and earths) of the subcircuits on their respective bars, i.e. range terminal 2, hot water terminal 3, RCD1 terminal 4, etc. Likewise, where an MCB (4) feeds three final sub-circuits then both the neutrals and earths must be twisted together and screwed into terminal 4 of their respective bars.
Bars (three-way) that can be screwed into the outgoing neutral terminal on the RCD to terminate the multiple neutral conductors are available from electrical wholesalers.
Terminating the multiple neutrals into the RCD appears to contradict clause 2.9.3.2 of AS/NZS 3000:2000 but it is an acceptable alternative.
Q. What are the Options for Two-way Switching if a Green Conductor Can't be Sleeved?
A. Use a three core, R-W-B, TPS cable as the switch straps between two-way switches and intermediate switches (if any), or use two core and a single red core cable.
Q. AS/NZS 3000 Table 3.4 Specifies a Nominal Minimal Conductor Size of 2.5mm² for Socket-outlet Circuits. Is a Smaller Cable Size Permitted?
A. Yes, refer to Note 1 of Table 3.4.
Q. Why is the Sleeving of Green wWre Prohibited?
A. The prohibition in AS/NZS 3000 was based on a number of electrical accidents that occurred in Australia when a cable run was broken in mid-span and re-terminated. The main safety outcome of this provision is to ensure that green coloured conductors of fixed wiring are always earth conductors.
Q. Can a Black Conductor be Sleeved for Use as an Active Conductor?
A. Sleeving of conductors (other than green) for use as active or neutral conductors is permitted by AS/NZS 3000.
Q. What are the Requirements for Neutral Conductors for Motor Control Centres where the Neutral Connection is Required for Control Purposes Only?
A. The neutral conductor may be sized to match the control system protective device rating.
Q. Why Shouldn't TPS Cable be Installed in Contact with Polystyrene?
A. Polystyrene and similar thermal insulation materials such as polyurethane, in contact with TPS cable, causes migration of the plasticiser in the PVC cable sheath, leaving it hard and brittle. The cable should be separated from the polystyrene by a physical barrier such as PVC conduit. The PVC conduit does not contain plasticiser, so is unaffected. Alternatively, cable with a non-migratory PVC sheath can be used. A similar requirement also exists with TPS cable in contact with bitumenised building paper.
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