What Is Discrimination Between Circuit Breakers and Why Does It Matter in Sydney Switchboards?

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What is discrimination between circuit breakers, and why does it matter in a Sydney switchboard? Discrimination, also called selectivity, means a switchboard's protective devices are chosen so that only the breaker closest to a fault trips, rather than an upstream breaker tripping as well and cutting power to circuits that were never actually faulty. AS/NZS 3000 sets out specific rules for achieving it, and getting it wrong on a commercial or multi tenancy board means one small fault can black out far more of a building than it should.

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1. The basic idea behind discrimination

Picture a switchboard with a main switch feeding several submain breakers, each of which in turn feeds a number of final circuit breakers. If a fault develops on one final circuit, discrimination means only that one breaker should trip, isolating just the faulty circuit while every other circuit on the board, including everything fed through the same submain and main switch, continues operating normally. Without discrimination, a fault on a single power point circuit could trip the submain feeding an entire floor, or in the worst case the main switch itself, for a problem that only ever needed one small breaker to act.

2. Where this sits in AS/NZS 3000

Clause 2.5.7 requires an electrical installation to be designed so that it provides a reliable supply, dividing the installation into appropriate circuits and selecting protective devices with suitable discrimination so that the loss of supply from any single fault is minimised. The standard distinguishes between discrimination, where only the downstream device should operate, and backup or cascading protection, which relies on the characteristics of two devices working in series and allows for some shared clearing of a fault. Both concepts matter, but they are not the same thing, and a board designed only with backup protection in mind will not necessarily achieve genuine discrimination.

3. How discrimination between two circuit breakers is actually assessed

AS/NZS 3000 gives different coordination rules depending on the rating of the downstream breaker. For a downstream breaker rated 800 amps or higher, discrimination has to be confirmed through a proper coordination study using the manufacturer's own time current curve data, since standard ratios are not considered reliable enough at that scale. For a downstream breaker rated between 250 and 800 amps, discrimination between the overload curves is generally deemed to be achieved if the upstream breaker's overload setting is at least 1.5 times the downstream breaker's rating. For smaller downstream breakers under 250 amps, the same 1.5 times ratio applies between the two breakers directly, for example a 63 amp upstream breaker discriminating adequately with a 40 amp downstream breaker, provided both are on comparable curve types.

4. Fuses and mixed fuse and breaker combinations

Where HRC fuses are used instead of or alongside circuit breakers, the standard sets out its own ratios. Discrimination between two fuses is generally deemed to be achieved when the upstream fuse is rated at least 1.6 times the downstream fuse for overload conditions, and at least twice the downstream fuse for short circuit conditions. Where a fuse sits upstream of a circuit breaker, discrimination has to be confirmed between the breaker's overload and instantaneous settings and the fuse's own time current curve, which is a more involved comparison than the simple ratio used for two breakers of the same type.

5. Why discrimination need not apply to every fault level

AS/NZS 3000 recognises that trying to achieve perfect discrimination at every possible fault current is not realistic, and it does not require discrimination above what is called the arcing fault current, deemed to sit somewhere between 30% and 60% of the prospective short circuit current at that point in the installation. Above that level, both the upstream and downstream devices may reasonably be expected to operate, and the standard treats that as an acceptable outcome rather than a design failure. This is also why the Service and Installation Rules of NSW separately require main switchboards to be rated to withstand fault currents of up to 10kA in typical suburban areas and 25kA in commercial and industrial zones, since the switchboard and its enclosure need to survive a fault event even where full discrimination is not achievable at the highest fault levels. Our guide to SIRNSW prospective short circuit current protection covers that fault current withstand requirement in more detail.

6. Why it's Level 2 ASP work on the service side

Discrimination between an installation's own internal protective devices is general electrical design work rather than Level 2 ASP territory, but the picture changes at the point where the installation meets the network's own service protective device, typically the service fuse. The electricity distributor needs to be consulted for discrimination between an installation's main switch and the network's service fuse, since the curves and settings of that upstream device are the distributor's information to provide, not something an electrician can assume. A Level 2 ASP is often best placed to coordinate that conversation where consumer mains, service fuses or metering equipment are involved as part of a broader switchboard upgrade or power supply upgrade.

7. Where discrimination problems show up in practice

The most common place this becomes a real world issue is in multi tenancy commercial buildings, light industrial sites and larger residential developments, where a board was expanded or modified over time without anyone checking that the newly added breakers still discriminated properly with the existing upstream devices. A tenancy fitout that adds submain breakers without reference to the main switchboard's existing settings is a classic way for a building to end up with poor discrimination, and it is exactly the kind of issue a commercial switchboard upgrade or an electrical safety inspection should pick up before it causes an unnecessary building wide outage.

How High Demand Electrical helps

We are one of Sydney's most trusted Level 2 ASPs, and we check discrimination between protective devices as a standard part of any commercial switchboard design or upgrade we carry out across Sydney's West, South West and Sutherland Shire. If your building has had tenancy fitouts, submain additions or equipment changes over the years, we can review whether your existing breakers and fuses still coordinate properly, and where they do not, correct the settings or devices involved rather than leaving the building exposed to unnecessary building wide outages from a single small fault. For cafe and restaurant fitouts in Sydney's inner west dealing with multiple circuits on a shared board, HD Inner West's guide to cafe and restaurant fitout switchboards is a useful companion read on how this plays out in a working commercial kitchen.

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