Fault passage indicators on MV cable networks — finding a buried fault without digging blind

The Murray-bridge and TDR guides cover finding the exact point of a fault once the faulty cable section is already known. A fault passage indicator (FPI) addresses an earlier question on a medium-voltage cable network: which section of a ring or radial feeder did the fault current actually flow through? On an underground cable network, a fault cannot be seen or heard like an overhead-line fault, so without an FPI, crews may need to open and test multiple joints or switching points along a long feeder before finding the faulted section at all.

What an FPI does — and does not do

An FPI is a clip-on or integrated sensor mounted around a cable (or busbar) at a switching point, joint, or ring main unit, that detects whether fault current has passed through that point and shows a local visual indication (a flag or LED) — and, on communicating types, sends that indication to a control centre. An FPI is purely an indication device: it does not trip anything and does not replace the network's protection scheme (directional/overcurrent relays and breakers still clear the fault); it only tells the crew, after the fault has already been cleared elsewhere, which sections of the network the fault current travelled through.

Two fault types, two detection principles

Overcurrent (short-circuit) FPIs detect current above a set threshold combined with the loss and (usually) later restoration of voltage, distinguishing an actual through-fault from normal load current or an inrush event.
Earth-fault FPIs detect the residual (zero-sequence) current characteristic of an earth fault. On a solidly or low-impedance earthed network this current is comparatively large and easy to detect; on a compensated (Petersen-coil) network, where the earth-fault current is intentionally limited by design, sensitive or directional detection is needed — see the wattmetric transient earth-fault detection guide for how that specific network type is handled.

Non-directional versus directional

Non-directional FPIs simply flag that current above the threshold has flowed, regardless of direction — adequate on a simple radial feeder where fault current can only flow one way.
Directional FPIs add a voltage reference so the device can distinguish whether the fault current flowed towards or away from the monitored point. This matters on a network that can be operated as a ring or with normally-open points that may be switched, because the same physical point can see fault current from either direction depending on the current switching state — a non-directional indication alone would be ambiguous in that case.

Self-powered and self-resetting

Most FPIs are self-powered: a current transformer (or Rogowski coil — see the Rogowski coil guide) around the monitored cable both senses the fault current and supplies the small electronics and indicator, so no external power feed is needed at the mounting point. Indicators also reset automatically — after a set time delay, or after detecting that normal load current has resumed flowing (indicating the breaker has successfully reclosed) — specifically to distinguish a genuine persistent fault from a transient event such as cold-load pickup or an inrush current, which would otherwise leave a false indication after a routine switching operation.

Practical relevance

By narrowing the search from an entire feeder down to the section(s) between consecutive FPIs that did and did not register the fault current, restoration crews can prioritise which joints, ring main units or cable sections to inspect first — directly reducing the time a network section stays out of service after a fault, before any precise fault-location technique (Murray bridge, TDR, or a surge/arc reflection method) is applied to pinpoint the exact fault location within that narrowed section.

Common mistakes

Expecting an FPI to trip or clear the fault — it is an indication device only; the network's protection relays and breakers are what actually clear the fault.
Using non-directional FPIs on a network that can be switched into a ring configuration — without directional discrimination, an indication can be ambiguous about which side of a switching point the fault is actually on.
Applying a standard earth-fault FPI threshold on a compensated (Petersen-coil) network without adjustment — the intentionally limited earth-fault current on such a network requires the more sensitive or transient/wattmetric detection principle, not a threshold set for a solidly earthed network.
Not accounting for the automatic reset time when interpreting an indication hours after a fault — an indicator that has already self-reset following a successful reclose will no longer show the fault, which is expected behaviour, not a sign the indicator malfunctioned.