Where Does Short Circuit Protection Have to Go in Your Sydney Switchboard?

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Where does short circuit protection have to go in your Sydney switchboard? AS/NZS 3000 requires a device protecting against short circuit current at the origin of every circuit and at every point where the current carrying capacity of the wiring drops, with narrow, clearly defined exceptions that allow it to sit elsewhere or, in a few specific cases, be left out altogether. Getting the placement wrong does not always look wrong on the day a switchboard is energised, which is exactly why this clause matters.

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1. Start by working out the fault current, not the device

Before anything gets specified, AS/NZS 3000 requires the prospective short circuit current at every relevant point of the installation to be determined, either by calculation or by measurement. This figure, not a guess based on the size of the switchboard or the supply, is what sets the minimum breaking capacity every protective device downstream actually needs. Our guide to prospective short circuit current under SIRNSW covers the distributor side figures that feed directly into this calculation on a Sydney property.

2. Breaking capacity: the basic rule

Once the fault current is known, the standard's basic requirement is simple: a short circuit protective device's breaking capacity must be not less than the prospective short circuit current at the point where it is installed. A device rated below that figure at its own location is not doing the job the standard expects of it, whatever its current rating for normal load happens to be.

3. The coordinated backup exception and the let through energy check

There is a recognised exception to the basic breaking capacity rule. A device with a lower breaking capacity is allowed if another device with the necessary breaking capacity sits on the supply side, provided the two devices are coordinated so the energy that gets through during a fault does not exceed what the downstream device and the conductors it protects can withstand without damage. This is checked using the standard's own adiabatic equation, which weighs the conductor's cross sectional area and a material dependent factor against the fault current and clearing time. It is also why a device substituted during a repair needs genuine like for like coordination data, not just a similar looking current rating, a point covered in more depth in our guide to circuit breaker discrimination and selectivity.

4. Position: the origin, and every capacity reduction

The default position rule is equally direct. A device providing short circuit protection has to be installed at the origin of every circuit, and at each point where a reduction occurs in the current carrying capacity of the conductors, for example where a heavier cable steps down to a lighter one partway along a run. This is the rule that a protective device at the switchboard alone does not automatically satisfy if a downstream cable size change happens without its own point of protection.

5. The three metre alternative position

AS/NZS 3000 allows some practical flexibility rather than forcing a device at every single physical joint. Where the conductor between a reduction in cross sectional area and the protective device does not exceed 3 metres in length, is protected mechanically or otherwise so the risk of short circuit is kept to a minimum, and is installed to minimise fire or other danger, the device can sit downstream of that reduction rather than exactly at it. This is a genuine, bounded exception rather than a general licence to run reduced cable wherever convenient, and all three conditions have to be met together, not just the length figure.

6. The upstream device alternative

A second alternative lets the protective device stay on the supply side of a reduction in conductor size, rather than being relocated downstream at all, provided its own operating characteristic still protects the smaller conductor against short circuit. In practice this is verified by comparing the short circuit current level immediately before the downstream branch device against the performance characteristics of the device already installed upstream, confirming it would clear a fault on the smaller conductor before damage occurs.

7. When short circuit protection can be left out altogether

The omission conditions in AS/NZS 3000 are deliberately narrow. Short circuit protection must be left out where unexpected opening of the circuit would create a danger greater than the short circuit itself, which applies in specific safety critical circuits rather than as a general excuse to skip protection. Certain consumer mains built to the standard's own dedicated clause do not require it either. The remaining case covers conductors connecting a generator, transformer, rectifier or battery to its own switchboard, where protection may be omitted provided the wiring is run to minimise the risk of a short circuit, kept clear of flammable material, and the remaining circuit's short circuit protection sits on the associated switchboard instead. None of these conditions are a reason to assume protection is unnecessary anywhere else in an installation.

Why it's Level 2 ASP work

Verifying fault levels and protective device coordination at the point of supply, including the interaction between a property's own protection and the distributor's upstream service protective device, sits squarely within Level 2 ASP scope, particularly on a switchboard upgrade or a job that changes the available fault current at the main board.

How High Demand Electrical helps

High Demand Electrical checks short circuit protective device placement and coordination as a standard part of every switchboard upgrade and defect rectification, not something assumed correct because a board looks tidy. Servicing homes and businesses across Sydney's West, South West and Sutherland Shire, we regularly find protection that was adequate when first installed but has been outpaced by a later supply upgrade raising available fault current at the board. For a shared switchboard arrangement, where protective device coordination between separate occupancies adds another layer of complexity, HD Inner West's guide to shared switchboards in inner west semis and terraces covers that specific scenario in detail.

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