G107A - Cable Selection (Protection Devices)

What is the maximum permissible height above the ground floor or platform for a main switch?

Answer explanation

Q.2.3.3.4

Main switches shall be readily accessible and not more than TWO metres above the ground

What is the term used in AS/NZS 3000 to describe the protection device for which a circuit is designed?

Is an isolation switch required within a unit of a multiple domestic complex, if the main switchboard is normally locked?

• Yes

• No

The magnetic mechanism of a thermal-magnetic circuit breaker:

• operates by heating a bi-metal strip

• operates on long time overloads only

• has a time response suitable for overload protection

• has a time response suitable for short-circuit protection

Answer explanation

In thermal magnetic circuit breakers, there are two ways the flow of current might be detected and disrupted. In the event of a short circuit, the electromagnet rapidly trips the switch. If the overload is smaller, over a longer period of time, the bimetallic strips will instead eventually react to trip the switch.

The term ‘discrimination’ as applied to circuit protection means:

• the circuit protection on the main switchboard should be the first to operate

• the fuse or circuit breaker with the highest fault current rating should operate first.

• the circuit protective device closest to the fault or over-current should operate before any upstream devices.

• the circuit protection should be able to distinguish between a short circuit to earth and a short circuit between live conductors

Answer explanation

AS 3000 Clause 2.5.7.2.1

Co-ordination of protective devices

What is the minimum tripping current required to ensure that a 20 A Type D circuit breaker operates instantaneously?

Answer explanation

Type D CB = 12.5 x Rated Current

12.5 x 20 = 250 A

The current causing effective operation of a HRC fuse under overload conditions in conventional time is:

Answer explanation

If a cable is protected by a H.R.C. fuse, the cable must be de-rated by a factor of 0.9

What AS 3000 Clause number is used for "Protection against Overload"

A 230 V circuit is wired in cable with a current carrying capacity of 32 A and supplies a maximum demand of 20 A. Determine the maximum size HRC fuse suitable to protect this circuit.

A 230 V circuit is wired in cable with a current carrying capacity of 32 A and supplies a maximum demand of 20 A. Determine the maximum size circuit breaker suitable to protect this circuit.