Protection Settings

27th February 2026

The importance and benefits of ensuring correct discrimination between multiple HV protective devices were recently highlighted at a site with an HV private network consisting of 3 substations supplied via a ring circuit configuration with a normally open point approximately mid-way following an HV cable fault.

In this scenario, the customer HV circuit breaker feeding that part of the ring should detect the fault and operate, resulting in only the substations on that section of the ring losing supply, with any other substations beyond the normally open point remaining energised. However, in this instance, the customer circuit breaker did not discriminate with the DNO metering circuit breaker, with the DNO circuit breaker operating first, resulting in a total loss of supply to the site, causing even more major disruption to the business than necessary.

Following engagement with the DNO, sectionalising the fault and restoring supplies, the discrimination issue was investigated, and it soon became apparent that there were insufficient time delays between protection devices. Using in-house protection software, we conducted a protection study to calculate new HV protection settings that provide discrimination, enabling correct device sequencing under fault conditions.

As part of the process of applying the settings on site, we also conducted secondary injection testing of the devices to verify tripping times and confirm that the circuit breaker would physically open in the event of a fault.

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