Standards Australia issues caution on earthing practices

Standards Australia has issued a notice to alert tradespersons of a safety hazard involving the previous practice of using continuous, metallic water reticulation systems as an earthing medium rather than using the currently required method of earthing using an earth electrode.

Possible injury and damage to life and property can be caused by earthing installations that use the outdated practice of earthing to metallic pipes. This hazard can appear in two ways: through the introduction of plastic water pipes into existing metallic water reticulation systems or by faults in the electricity distribution network conducting through metallic pipes.

In both scenarios, it is the outdated method of earthing of earth electrical installations to continuous metallic water reticulation systems that is the problem.

Water reticulation systems have changed markedly in recent years with the introduction of plastic pipes or insulated meters, either as repairs, replacements or alterations to existing services. In many cases, plastic pipes have been fitted to existing metallic systems. This creates insulating sections that no longer have a continuous, effective and low-resistance earthing network and therefore do not provide adequate earthing.

Secondly, the safety hazard can sometimes appear as a flow-on effect from breakdowns in the electricity system in Australia, the multiple earthed neutral (MEN) system.

When a breakdown occurs in the MEN system, the neutral-earth connection becomes open-circuit in either the distributor’s network or in an individual electrical installation. The earth fault is then returned via the earthing system rather than the neutral conductor. As a result, a potential is applied to the water pipe system. When the pipe system is in contact with the earth along its length, this potential is low and less severe. For insulated sections, usually with shorter pipe length in contact with the earth, the potential can sometimes be large and more significant and is often different to that of the water pipe system on the other side of the installation.

If a human is in contact with the metallic water reticulation system at the time of the fault, there could be significant injury or even death caused by the flow of electrical current.

Similarly, if the water pipe system within the premises is continuous, it presents a very low resistance to earth. If local problems exist within the installation, there will be a distinct possibility that the tap connected to the continuous water pipe earth system will be at a different potential to the earth at the base of the sink. There will be a current flow once the gap between the tap and local earth is bridged by the action of attempting to turn on the tap.

Standards Australia committee EL-001 has taken action to provide for detection of the problem with a simple test to detect an open-circuit neutral, which is outlined in the AS/NZS 3017: 2007 installation testing standard. This test can then be called up as part of a periodic testing regime in the reverification standard AS/NZS 3019, which is mandatory in New Zealand but not in Australia.

Standards Australia is also advising enquirers through the Wiring Rules email service, to install an earth stake if there isn’t one already in place, even though this is not a mandatory legal requirement, to ensure that the protective earthing system connecting exposed conductive parts to the earthed neutral of the distribution system is sound and to additionally fit RCDs to disconnect the affected part of the installation.

Use of earth electrodes for earthing electrical installations has been the method outlined in the Wiring Rules for new installations since the 1976 edition. The Wiring Rules and its companion documents apply to new electrical installations and alterations, additions and repairs to existing electrical installations, but do not apply to the maintenance of electrical installations.