Mixed circuit protection

Why choose MCP

The industry challenge

The number of circuits comprising a mix of overhead line and underground cable sections is increasing due to a range of factors; from the rise in extreme weather, to the pressure of critical new loads.

The decision of whether to allow or block auto-reclose on a mixed circuit in response to a fault comes down to a particular operator’s policy.

At the extremes, an operator may decide to either:

• Never auto-reclose on a mixed circuit, to remove the risk of reclosing on a permanent cable fault; or
• Always auto-reclose, to take the risk that the fault is a transient one in the overhead line.

Both of these approaches carry with them significant dangers.

If never allowing auto-reclose, fault-finding must be carried out after the breaker opens, which is laborious and time-consuming. This in turn causes disruption to supply, and risks reputational damage to the operator, as well as penalties for lost customer minutes.

If always allowing auto-reclose, the operator risks reclosing onto a permanent cable fault (perhaps multiple times), causing significant damage to local assets. In severe cases, the damage can be catastrophic, exposing risk to life as well as fatal damage to critical assets in the surrounding network, leading to further outage time.

In reality, an operator will rely on a policy that lies somewhere between these two options. For instance, permitting auto-reclose on circuits where the cable is a small percentage of the total line length. However, a reliance on this policy does nothing to mitigate the risks and assumptions of either approach.