The growing number of smart meters installed across Australia has led to an increase in the number of reversed polarity incidents. But what is the correct way of conducting a reverse polarity test?

In the last edition of Electrical Connection we ran an article highlighting the growing number of reported reverse polarity incidents in Victoria in the past year, particularly surrounding the installation of new metering equipment.

While the danger is very real, and the number of incidents is assuredly growing, it turns out that the information we were presented isn’t relevant to the vast majority (99.9%) of Australian installations.

Institute of Electrical Inspectors life member and former Electrical Connection contributor George Bosomworth says: “The test described does not prove what is claimed when applied in Australia or New Zealand (i.e. where the system of distribution used is our multiple earth neutral, MEN, system).”

The test described is only valid where the MEN connection is upstream of the reversal of polarity, he says.

“In the US there are many situations where this test would be valid – but not so in Australia or New Zealand.

“To put it simply, if the neutral and earth (US ‘ground’) are connected together downstream of the meter (as is the case for our MEN system) and the active (US ‘hot’) is transposed for the neutral upstream of that point, both the neutral and earthing conductors within the installation will be at the same (active/hot) potential.

“Therefore, testing voltages between the pins of an outlet (US receptacle) will indicate identical values whether the polarity is reversed or not and the fault will not be detected (i.e. with supply at correct polarity; A – N = 230V, A – E = 230V, N – E =~0V. Polarity reversed upstream of MEN connection; N – A = 230V, N – A = 230V, A – A =~0V.)

“For a test at an outlet to be valid in Australia/NZ, the reference point for testing must be an independent connection to mother earth (the soil) at a point clear of any material that could be connected electrically to the installation, such as underground water piping, etc. Typically a large screwdriver shaft driven into the soil may be used.”

This leads to the next point that often arises in such a discussion, he says: “Reliance on the main earth.”

“Many would say that the main earth will cause a fuse to blow. This would rarely be the case.

“Typical earth resistances for single electrodes driven 1.2m deep are in the range of 50Ω to 100Ω (or more). If we apply the nominal 230V to such a resistance Ohm’s Law dictates that the current flow will be in the range of 2A to 5A. Even if the earth resistance is lowered to 10Ω the current will only be 23A. Hence, relying on the main earth to operate protection is a fallacy.”

A further issue arising from all of this is the MEN system itself.

“When that system of distribution was adopted, the reticulated water system consisted of metallic piping that, when used as the main earthing system, acted as a backup to the supply neutral conductor.

“With the advent of non-conductive water reticulation and the use of single earth electrodes there is now no back up for the supply neutral and hence the rise in neutral related problems such as shocks.

“It may be that this system of distribution needs to be reviewed.

“The requirement to earth the neutral at each installation arises from the need to keep the supply neutral as close to true ground potential as practicable. (If there was no such connection the voltage between ground and the supply neutral would be about half that of the voltage drop along the length of the distribution circuit – think about it).”

Thus this requirement more correctly sits with the distributor.

“If this were to be the case, an RCD with a tripping current in the order of 300 – 400mA used as a main switch would protect the customer’s installation in the event of a supply neutral failure.”