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Switchboard Coordination

We’re not talking about whether your switchboard can juggle or kick a footy.

Are you aware that it’s now compulsory under AS/NSZ 3000:2018 to undertake coordination to make sure discrimination is achieved for an electrical system? There are exceptions for circuits less then 80A. Refer to AS/NSZ 3000:2018 Amendment 1 for detail.

Discrimination (selectivity) guarantees that supply remains reliable by coordinating two or more devices so that the device which is closest to the fault is the one that trips. Successful discrimination means only those circuits which are exposed to a fault will be cleared while supply on other critical loads is retained.

The two types of common trip units for adjustable circuit breakers are electronic and thermal magnetic. A graphical representation allows calculation of how fast the Short Circuit Protective Device will trip if exposed to a certain current, and how this will vary depending on the breaker’s settings.

Discrimination is frequently determined through a time current selectivity analysis. This involves plotting the time current typical curves of the manufacturers upstream and downstream devices and altering the breaker’s settings to observe its performance under fault conditions, making sure curves don’t overlap.

If the selectivity data of breakers in a particular series cannot be obtained from the manufacturer, discrimination may be determined by observing when the downstream device’s let through energy is lower than that the energy needed for unlatching the upstream device.

Consideration of both cascading protection (backup) and discrimination is required to comply with AS/NZS 3000:2018.

Cascading involves using a protective device to back up another device in order to interrupt a fault current so both devices will continue with the required circuit protection operation. It is a cost-effective solution which allows devices to be used with a lower fault rating than the installed potential fault levels, saving money and space.

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