Tahua rerenga tūturu RCD

How this is calculated#

Every item on a circuit leaks a little current to earth in normal use, through its insulation and through the filters on its supply. An RCD cannot tell that current from a fault. On one RCD the leakage of everything it supplies adds up, and enough of it trips the RCD with nothing wrong. AS/NZS 3000 asks for circuits to be split so the leakage expected in normal use is unlikely to trip the device. This tool adds up the leakage on one RCD and checks it.

You list what the RCD supplies. For each item you type a measured or maker's leakage, or you take the in-service test limit for its class from AS/NZS 3760: the most a Class I or Class II item may leak and still pass its test. The class limit is a worst case. Most items leak much less, so a measured figure gives a truer total.

The total is checked against two points of the clause's first note. The leakage should not pass one-third of the RCD's rated residual current, and an RCD may operate at any residual current above half of it. Past one-third the tool warns that unwanted trips are likely, past half that the RCD may trip, and at the rating it fails, because the RCD is made to trip there. It then places the items, largest first, on the fewest RCDs that keep each within the one-third budget, without splitting an item.

Worked examples#

• A home office: a heat pump, three computers and a fridge leak 7.3 mA in total. That is within the 10 mA budget of a 30 mA RCD, so 1 RCD keeps it in budget.
• Four Class I power tools at their test limit could leak 20 mA, over half the rating, where the RCD may trip. Spread over 2 RCDs, each stays within its 10 mA budget.
• An office: six computers and six Class II monitors leak 15 mA, over the 10 mA budget. They need 2 RCDs.
• A heat pump and four chargers on a 10 mA RCD leak 6 mA, over half of 10 mA. The budget is 3.33 mA, so 2 RCDs keep each within it.
• An Australian laundry: washing machine, dryer and freezer leak 8.5 mA, within the 10 mA budget on 1 RCD.

Limits#

The tool adds standing leakage, the current that flows all the time. It does not model the short surges as filters charge at switch-on, leakage from a frequency drive at its switching frequency, or a fault. Type F and Type B RCDs, and where each is needed, are for the RCD selection tool.

The class limits are the most an item may leak and still pass. A measurement with a leakage clamp on the circuit gives the real figure.

See also: RCD selection, RCD arrangement and Test and tag.