Main Switches in CT metered installations

This information bulletin has been developed to assist electrical contractors and electrical workers in the Northern Territory to be compliant when installing Current Transformer (CT) Metering Cubicles with an additional Main Switchboard.

A CT is a type of instrument transformer used in electrical systems. Its primary purpose is to measure alternating current by stepping down the current to a lower value that can be safely measured by kilowatt hour meters.

This bulletin applies to:

• Large installations requiring CT Metering.

Main switchboards attached to network operator CT Metering cubicles require a main switch to be fitted. This allows for isolation of the main switchboard, irrespective of whether a current limited main switch (meter Isolator) is installed within the CT Metering cubicle to meet the supply authority’s requirements for connection of supply.

Clause 2.3.3.2 AS/NZS 3000:2018 Wiring Rules states:

The supply to every electrical installation shall be controlled on the main switchboard by a main switch or switches that control the whole of the electrical installation. Property owners need to be made aware that the meter isolator which is a circuit breaker remains their property regardless of what conditions the network operator has mandated in terms of the overall capacity and current settings for the meter isolator.

The capacity and current settings will be provided by the network operator, this will represent the maximum demand of the premises.

Meter isolators must be lockable, and depending on the number of CT metered power supplies on site more than one meter isolator may be required to allow disconnection of supply to each consumer.

Image: Diagram showing main switch installed in a CT metered installation