Beware of switched main neutral conductors

We’ve recently identified several installations where the incoming main neutral conductor had been switched via an alternate supply changeover switch. These included battery energy storage systems and standby generator installations.

This resulted in energised earthing systems which could cause an electric shock.

The incoming main neutral is a protective earth neutral (PEN) conductor. When a fault occurs in the electrical installation and the PEN conductor is open circuit, the installation earthing system will become energised as the current return path is via the earthing system through the ground.

As current increases, the voltage on the earthing system will increase due to the impedance of the ground.

In the event of a short circuit to earth, voltage on the earthing system will rise to full supply voltage and it’s unlikely that a sufficient current will pass to operate upstream protection.

Circuit protective devices and RCDs will not provide protection against the voltage rise on the earthing system.

No switch or circuit breaker should be inserted in the neutral conductor of consumer mains, or in PEN conductor. Refer AS/NZS 3000 clause 2.3.2.1.2, clause 7.3.8.1.1(a) and figure 5.1.

AS/NZS 3000 provides examples for the connection of alternate supplies in figures 7.3 to 7.6. AS/NZS 3010 further identifies requirements for the connection of alternative supply.

Before you start work on an electrical installation, check for alternative supplies and the integrity of the PEN conductor as part of your risk assessment.