Reducing the risk of electrical fires

Short circuit, overcurrent and earth faults are widely understood and protected against in low voltage residential installations, but some hidden sources of ignition — such as arc faults — are not always easy to identify.

Latest statistics show electrical issues are a leading cause of residential fires in Australia, with around 40% of residential fires in NSW caused by electrical appliances or faults. In Tasmania, they are the third leading cause of fires. Complete protection is, therefore, absolutely essential.

Recognising the risk of arc faults, Europe and America have already put in place safety standards relating to the installation of arc fault detection devices. Australia will soon follow suit, with the AS/NZS 3000 electrical installations standard (the Australian/New Zealand Wiring Rules) due to be released early next year, with recommended guidelines around arc fault detection devices to be included.

For the industry, electrical safety risks range from overloaded power points and damaged outlets, right through to inadequately maintained installations. Arc faults occur at nominal or low current, making them difficult to detect manually. Some common causes include: pulling cables out of a power point repeatedly without care; crushing the cable between objects, eg, a door and door jamb; or cables being damaged by rodents or pets. In some cases, an arc fault can also be caused when wiring becomes loose at the terminal connection.

Broken or damaged wires can also lead to small arc currents, burning and degrading insulation over time. They are mostly identified by high frequency (HF) electrical noise and the breakdown of the fault current close to the zero-crossing of the driving voltage.

Serial arc faults are the most common, originating from a fault within the phase or neutral conductor. They can go undetected for a long time. Parallel arc faults originate from a fault between phase and neutral, with the total current in the circuit increasing depending on load impedance and fault impedance.

While safety switches and circuit breakers for overcurrent protection can be effective in reducing the risk of electrically ignited fires, they cannot identify arc faults — therefore, the addition of arc fault detection devices is required to ensure protection against all faults.

Arc fault detection devices divide the measured current of each final subcircuit into a low-frequency and a high-frequency share. These two signals are used as the basis for electric arc identification — they are analysed by a microcontroller to determine whether they display the characteristic HF signals of an arc fault and, if identified, automatic disconnection of the affected subcircuit will be triggered.

They are not designed as an alternative to residual current devices or circuit breakers for overcurrent protection. Instead, they work in partnership with RCDs and MCBs to provide a comprehensive switchboard safety solution. However, some arc fault detection devices also incorporate RCD and MCB functionality in a single compact unit.

A number of standards-related changes are arriving soon in Australia. One expected change relates to guidelines on the use of arc fault detection devices in final subcircuits for locations storing flammable material, fire propagating structures and premises with sleeping accommodation.

The new guidelines are not expected to be mandatory but they present an opportunity to better understand the risk of arc faults. For most electrical contractors, this means understanding the proposed safety standard guidelines for arc fault detection devices — and getting to know what safety products and new protection technologies are available in Australia.

Image courtesy of Eaton.