Switchboard Components

CHRIS HALLIDAY: A life-saving trip


RCDsDo we need RCDs on all circuits, including stoves and hot water units, above and beyond the rules? Chris Halliday examines an important safety matter.

Electricians, as the experts in their trade, have an obligation to clients to provide safe outcomes, now and into the future.

The installation of safety switches is one very easy and important way of achieving this.

Residential installations

Residual current devices (RCDs) have not always been installed on fixed equipment such as stoves, hot water systems or air-conditioning circuits. However, they should be fitted to all circuits in residential installations to ensure safer outcomes.

Further, Clause 3.9.4 and Figure 3.4 of the Wiring Rules looks at clearances from wall linings. Where cables are fixed and concealed within 50mm of the surface of a wall, floor, ceiling or roof (neglecting for the moment the area within 150mm of internal wall-to-wall and wall-to-ceiling corners – see Figure 3.3), there are four ways of achieving compliance.

You could drill bigger holes to allow the cable to move freely, provide adequate mechanical protection, use earthed metallic armouring, etc, and/or protect the circuit with an RCD (refer Clause

Bigger holes would allow mice to enter wall cavities. These creatures often chew the insulation off cables to get a little extra room to fit through – which is not a good result.

Unless you use one of the options it would be difficult to achieve compliance with 75mm wall studs, or with larger cables such as stove cables and 90mm wall studs.

In the Wiring Rules (Clause, RCDs are recommended for main switches in residential premises, with a trip current of 100mA to 300mA and a selective delay to ensure grading (to prevent fires).

If it’s good enough for the Wiring Rules, then it’s good enough for your clients. (A protected neutral link will be needed as an extra).


Other installations

For non-residential applications, Clause of the Wiring Rules specifies where RCDs should be fitted.

The clause states: Additional protection by RCDs with a maximum rated residual current of 30mA shall be provided for (a) final sub-circuits supplying socket-outlets where the rated current of any individual socket-outlet does not exceed 20A.

This generally means that circuits supplying socket outlets above 20A in other installations do not get RCDs. However, at the end of the aforementioned clause, the Wiring Rules refers you to workplace health and safety (WH&S) legislation, and does the lack of an RCD ensure safety? No!

More on this in the next section.


Harmonised legislation

The harmonised WH&S legislation discusses “hostile environments” and stipulates RCD protection above and beyond Wiring Rules requirements.

If you are just following the Wiring Rules to where RCDs in “other electrical installations” are to be installed, then you may be breaking the law.

The harmonised WH&S legislation requires appropriate RCDs for socket outlets where electrical equipment:

  • in normal use is exposed to conditions likely to damage it or reduce its expected life span, including moisture, heat, vibration, mechanical damage, corrosive chemicals or dust;
  • is moved between locations, and damage to the unit or flexible supply cord is reasonably likely;
  • is frequently moved during normal use;
  • forms part of, or used in connection with, an amusement device.

Check the legal requirements in your state and industry, as not all states have adopted the harmonised legislation. Some industry sectors, such as mining, are likely to have additional requirements.


Medical patient areas

You may be asked to perform electrical installation work in patient areas of hospitals, medical and dental practices, dialysis areas, home care, and self-harm areas.

The Standard has special installation requirements, including RCD protection, above and beyond the Wiring Rules.

You will need a copy of AS/NZS3003 to ensure all additional requirements are fulfilled. These requirements include additional certification by experts, as noted in the Standard, and regular inspections.


Testing and tagging

RCDs can and do fail, so testing is extremely important.

The RCD push button allows twice the rated residual current to flow, so this is a reasonably good test. It is particularly important for homes in which this may be the only RCD test ever carried out.

You could add additional revenue to your business by testing RCDs for residential customers, and by doing so you help to ensure safety. (It’s always good to retain customers – especially by keeping them alive.)

AS/NZS3760 In-service safety inspection and testing of electrical equipment specifies that RCDs also be tested for trip time and must be tagged for places of work, public places, or equipment offered for sale.

This includes any residential areas such as hotels, motels, boarding houses, halls, hostels and accommodation houses. Again, this may be an additional revenue stream that you are not cashing in on.


Other issues

It is important to select the right type of RCD for the load.

Much of our equipment is electronic in nature. It will probably need to include DC sensitivity for an RCD or residual current breaker with overcurrent (RCBO) to effectively operate when required.

An RCD or RCBO categorised as Class A will have sine wave and DC pulse sensitivity. Check the manufacturer’s information to ensure you are using the right version for the situation.

Some makes and models of RCDs and RCBOs have electronic circuitry, so their life may be shortened by:

  • overvoltage;
  • transients such as those from lightning or load switching;
  • harmonics; and
  • heat (particularly shortens the life of capacitors).

With electronic circuitry, it will be more important than ever to test RCDs and RCBOs to ensure the electronics are operative.

With the drive to ensure safety, the installation of RCDs and RCBOs is relatively easy in small switchboards but can be problematic in larger boards.

Some switchboard manufacturers do supply a four-busbar carcass, making things easier, but four-pole RCBOs don’t seem to be the norm.

Some manufacturers supply an additional pole and toroidal block to a standard miniature circuit breaker (MCB) to make a four-pole RCBO, and some of these even take up five poles on the switchboard.

Other makes of RCBO take up four poles and insulate the fourth pole from the three busbars, so only two poles are available for the next circuit breaker. Those two poles can then be used for single-phase circuits only, or become spare slots.

To get around the problem of available circuit breaker space on switchboards and the cost of replacing boards, some electricians are simply installing an MCB on the switchboard and using RCD-protected three-phase socket outlets (see accompanying photo).



It seems that RCDs are now a must for most situations, and you would be silly not to install them.

For the residential sector, this will include RCD protection on stoves, hot water systems and air-conditioners, with an RCD as a main switch. Some might say this will result in nuisance tripping, but there is no such thing. If an RCD is tripping, there’s a problem.

Requirements above and beyond the Wiring Rules for RCDs include those detailed in workplace health and safety legislation and in specific Standards such as those for patient areas.

Socket outlets above 20A amps in “other installations”, such as commercial or industrial, should be RCD protected to ensure safety and compliance with legislation.

When testing and tagging, replace faulty RCDs. Once identified, the circuit must be isolated there and then, as RCDs are essential safety equipment.

Read the Wiring Rules, other Standards and the applicable legislation. Become familiar with any additional requirements. If in doubt, fit RCDs to all circuits, or seek further advice.

About Chris Halliday

Chris Halliday

Chris Halliday has be running his own consultancy business since 2005. He has almost 40 years' experience in the electrical industry having started as an apprentice electrical fitter at the age of 16.

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