06/05/2026
⚡COMPLIANCE ALERT: generating sets⚡
Let's jump right in!
1. Fault current availability.
The gen-set, depending on its size, may or may not have adequate fault current to trip protective devices under short circuit conditions. As per clause 2.6.2 of AS/NZS 3010:2017, all outgoing circuits must be protected against short circuit - unless you meet the exception to the above clause.
The easiest way is to measure the resistance of the consumer mains all the way through the windings of the gen-set. In other words:
1. Ensure the gen-set is OFF
2. Close all isolating switches on the gen-set
3. Measure the resistance between phases, and from phases to neutral, at the main switchboard
4. Measure the resistance of the protective earthing conductor from the main switchboard to the gen-set frame.
The resistance values obtained can then be plugged into Ohm's Law to determine the prospective fault current available for both phase-to-phase and phase-to-neutral faults, as well as phase-to-earth faults.
The resultant current value should be no less than the instantaneous tripping current of the protective device at the origin of the consumer mains at the gen-set. For example:
1. Your highest loop resistance measurement (say using the protective earthing conductor) is 0.3ohms
2. As per ohm's law, the lowest available fault current is 230/0.3=~765A
3. Your gen-set overcurrent device is a B63 circuit breaker, which has an instantaneous tripping current of 63*4=252A
4. Therefore, the consumer mains are protected against short circuit.
You can also re-test this live using an EFLI tester, or an NST.
You _can_ also calculate the prospective fault current available if provided with the internal impedance of the gen-set (e.g. 10%) using the following formula:
PFC=(S*100)/(693*Z)
Where S is the VA of the generator and Z is the internal impedance of the gen-set as a percentage. 693 is for three-phase gen-sets.
But my recommendation? Measure the resistances dead, and retest live using an EFLI tester or NST.
2. Neutral switching.
IF the gen-set is the sole supply to the electrical installation, you SHALL NOT switch the neutral at the gen-set isolating device (2.6.3 exception 2). Otherwise, you MUST switch the neutral at the gen-set.
The first scenario almost always applies when gen-sets are used to supply construction sites prior to grid supply availability.
In alternative or supplementary supply situations, the following applies:
1. The gen-set isolator MUST switch the neutral
2. Where hard-wired, the changeover device and main switch should not switch the gen-set neutral
3. Where plug-in, the changeover device MUST switch the gen-set neutral.
Thankfully, AS/NZS 3010:2017 has clear and useful figures to explain the various connection scenarios.
And remember: in all cases, gen-sets and their supplies are prescribed work and require inspection.
Got any questions? Ring Text or Email .
