In the first part of this two-part series, Chris Halliday looked at the more obscure but nevertheless important types of protection. He now examines the conventional forms.

A safe and reliable electrical installation depends on overcurrent, short-circuit and earth leakage protection as required by the Wiring Rules.

This comes at a cost to the customer, but inadequate protection will have serious implications for those using the installation if something goes wrong – and for the electrician responsible.

Let us look at Wiring Rules requirements for the three types of protection mentioned.

Overcurrent protection

This seeks to automatically disconnect the supply if an overload or short circuit occurs (Clause 2.5.1).

Either situation could cause injury or damage due to excessive temperatures or electromechanical stresses in the installation.

Protection these days is more commonly provided by circuit breakers, and with some use of high rupturing capacity (HRC) fuses.

However, the days of the rewireable fuse are long gone. Electricians should always seek to replace rewireable fuses, using a residual current breaker with overcurrent protection (RCBO). This will help to ensure safety and perhaps save a life.

Electricians can never know how many lives are saved by their work, but those they fail to save can be easily tallied.

Overload protection

This must be sized so that it is no larger than the maximum demand of the circuit or the continuous current rating of the conductor (Clause 2.5.3.1).

It doesn’t necessarily have to be at the start of the circuit (Clauses 2.5.3.3). It can even be omitted in certain circumstances, such as where there is a fixed load not capable of causing an overload, eg: a heating appliance (Clause 2.5.3.4).

That also means the load must be suitably matched with the cable and does not negate the need for short-circuit protection.

Short-circuit protection

A short-circuit current must be interrupted to prevent the conductors becoming too hot, and to limit electromechanical forces.

The designer or electrician must know the prospective short-circuit current at the protection location (Clause 2.5.4.1). This can be determined by calculation. However, the easiest way for an electrician to obtain a value is to measure the level using a loop impedance tester (most loop testers are capable of measuring the prospective short-circuit current).

The circuit breaker or fuse used must have a kA rating greater than the calculated or measured prospective short-circuit current at that location (Clause 2.5.4.5).

The Wiring Rules allow for two situations in which short-circuit protection doesn’t have to be at the start of the circuit or a reduction in size of the current-carrying capacity of a cable (Clause 2.5.4.3).

The first is where the length of the circuit is less than 3m, it is mechanically protected, the risk of short-circuit is reduced to a minimum, and it is installed in a way that minimises the risk of fire or other danger.

The second is where the short-circuit device is upstream of a reduction in cross-sectional area or other change, but the protective device must protect the smaller conductor.

Short-circuit protection can even be omitted in certain circumstance (Clause 2.5.4.4) such as:

• when isolating the supply is more dangerous than the short-circuit;
• where consumer mains are installed in accordance with Clause 3.9.7.1; and
• between generators, transformers, rectifiers or batteries and their associated switchboard

However, strict conditions are detailed.

The installation of current-limiting fuses is one way of controlling the available fault energy at a particular location. Downstream protection may then be rated lower than normally allowed (Clause 2.5.4.5 [a]).

Co-ordination

Co-ordination of protective devices is important to ensure safety and minimise the extent of the outage.

We don’t want to isolate parts of the installation that are not involved in the fault (see Clause 2.5.7.1 on the reliability of supply). To this end, the Wiring Rules provides guidance on co-ordination (Clause 2.5.7.2).

Residual current protection

Residual current device (RCD) protection seeks to prevent earth leakages posing a substantial risk of electric shock.

Such protection came to prominence in NSW in the 1970s with earth leakage circuit breakers (ELCBs) being installed on an installation-wide basis. This was problematic and we have seen RCDs become cheaper, more reliable, generally installed on separate circuits, and with increased focus and usage as specified by the Wiring Rules.

I have written extensively on RCDs previously and will briefly sum up the requirements of the Wiring Rules.

The Wiring Rules have been adapted to prevent nuisance tripping by including Clause 2.6.2.1 and 2.6.2.4, which state:

• leakage currents are recommended to be less than one-third the RCD tripping current;
• the number of socket outlets and the nature of the equipment likely to be connected to the RCD is to be considered;
• there will be no more than three final sub-circuits per RCD; and
• if there are more than one final sub-circuit, a minimum of two RCDs must be installed.

Lighting circuits are to be distributed across circuits where there are more than one RCD and more than one lighting circuit.

A Type S, 100-300mA RCD is recommended as a main switch in domestic installations to help prevent electrical fires caused by current leakage across insulation (Clause 2.6.2.3). If it is good enough for the Wiring Rules to recommend this, then it is good enough for you to recommend as well.

Clause 2.6.3.1 specifies that in residential installations RCDs be fitted to final sub-circuits supplying one or more socket outlets, lighting points and directly connected hand-held equipment.

Clause 2.6.3.2 specifies RCDs in “other installations” for final sub-circuits supplying socket outlets not exceeding 20A, lighting not exceeding 20A and directly connected hand-held equipment.

Workplace health and safety legislation may override the Wiring Rules requirements for “other installations” in which an RCD is not required for socket outlets rated at 20A amps or greater, but they are used in hostile environments.

To ensure the safety of workers, an RCD seems to be a minimum requirement for any electrical equipment or situation.

For patient areas in hospitals, medical and dental practices, and dialysis areas – plus home care and ‘self-harm’ areas – the Wiring Rules refer the reader to AS/NZS3003. This Standard has special installation requirements, including RCD protection, above and beyond the Wiring Rules. You will need a copy to ensure all additional requirements are fulfilled.

Gary Busbridge suggested in the autumn issue of Electrical Connection that we will again see increased requirements for RCDs in the next revision of the Wiring Rules, with the installation of RCDs being required on all final sub-circuits.

Conclusions

Overload, short-circuit and RCD protection that operates automatically is a must to ensure safety and prevent damage. Failure to adequately protect an electrical installation will place people within the installation at risk and the electrician will breach the Wiring Rules requirements and the law as Wiring Rules are called up in State-based Regulations.

RCBOs is the most common form of protection to cover overloads, short-circuit and residual current faults. RCBOs are recommended for all final sub-circuits and electricians should take every opportunity to replace rewireable fuses and circuit breakers with them.

Finally, protection options require a little thought and planning by the designer and/or electrician to ensure safety and to minimise the extent of the loss of supply if a protective device was to operate.