RCD arrangement check

How this is calculated#

You list the final subcircuits on one switchboard. For each circuit you give what it supplies, its rating and the RCD it sits on. An RCBO is an RCD with one circuit, so give each RCBO its own number. A circuit on no RCD has the number 0.

First the tool asks, circuit by circuit, whether the clauses call for additional protection by an RCD, and at what rated residual current. It reads the same rows as the RCD selection tool. In New Zealand homes that is every circuit that supplies socket-outlets, lighting points or hand-held equipment. In Australian homes it is every final subcircuit. Schools, childcare, outdoor and public places and other buildings follow their own items. A circuit that needs an RCD and sits on none fails.

Then it groups the circuits by RCD and checks the arrangement clause. In every installation with more than one RCD and more than one lighting circuit, the lighting circuits must be on more than one RCD, so one trip does not put out every light. In a residential installation, one RCD may protect three final subcircuits at most. Every circuit on that RCD counts, even one that would not need it. A residential board with more than one circuit that needs an RCD must have at least two RCDs.

The result lists each circuit, each RCD with its count, and the four checks with their clause.

Worked examples#

• A house board with four circuits on RCD 1. The arrangement fails: 4 on the fullest RCD, one more than a residential RCD may protect. Move one circuit to RCD 2.
• A larger house on three RCDs. The arrangement passes every check, with 3 on the fullest RCD and a lighting circuit on each RCD. The car charger counts as one of the three on RCD 3.
• An Australian home of RCBOs. The arrangement passes every check, with 1 on the fullest RCD. Every final subcircuit needs an RCD in an Australian home, the oven and water heater too, and each has its own.
• An office with both lighting circuits on RCD 1. The arrangement fails the lighting spread, with 3 on the fullest RCD. The count limit does not apply outside a residential installation, but the lighting spread does.

Limits#

The tool checks one board. It takes each socket-outlet as 10 A; use the RCD selection tool for a larger socket-outlet, a trip time or a device type. It does not check the exceptions in Cl 2.6.3, such as a dedicated cooking socket-outlet or equipment where a trip is more dangerous. It does not check discrimination between RCDs, or standing leakage on one RCD.

See also: How many circuits can go on one RCD, RCD arrangement for a house board and RCD selection.