Sentinel Earth Fault Indication is a system used to identify the location of an earth fault in a network, for the purpose of reduced outage time, reduced operating costs and improved customer service by restoring the healthy portions of the network before repairing the faulted section.
The Earth Fault Indicator (EFI) also known as Fault Passage Indicators or Ground Fault Indicators can be applied to radial or openly operated ringed networks.
The Sentinel Earth Fault Indicator range consists of various models and can be catagorised as follows.
During most fault conditions, a large magnitude of fault current is present on the system from the feeding point to the fault location. Earth Fault Indicators installed at various points on the system, sense this condition and acknowledge by means of an indication (LED, Flag, Remote LED, etc).
The diagram below shows a typical underground medium voltage distribution network. The network consists of two feeders, which form a ring with a normally open point half way round. Each Ring Main Unit has an EFI installed on the outgoing HV cable.
Consider a fault at a position between switches 3 and 4, fed from an HV circuit breaker at A. EFI’s between A and the fault will detect fault current in the cable and trigger. The rest of the EFI’s will remain untriggered. When field-staff investigate any substation site they will immediately know whether the fault lies upstream or downstream from that position.
In the example below, the switches at 3 can be opened and then the circuit breaker at A closed, restoring all points up to substation 3. In addition, the switch at 4 can be opened and the NOP switch closed, restoring supplies to substations 4 and 5 via feedback from circuit breaker B. This will leave only the faulty cable section between stations 3 and 4 isolated, but with all the customers reconnected, and all done without risk of closing onto a known fault condition.
Earth faults are by far the most frequent of all faults in an electrical power system, and this is especially true for cable networks. An earth fault occurs when there is insufficient insulation between a phase conductor and earth. For cable networks this usually occurs when there is a breakdown in the insulation between the phase conductor and the conductive screen which encircles the insulated phase conductor and this is usually very difficult to find.
Under normal system conditions the three phase load currents summate vectorially to zero. An earth fault is detected by looking for an imbalance in the residual current of the system due to fault current flowing to earth. Earth faults in overhead powerlines are usually detected by installing a current transformer (CT) on each phase conductor and then connecting the CT’s in parallel with each other to yield the residual earth fault current. For a cable system the residual current can be detected more conveniently by installing a single current transformer around all three phase conductors. This is referred to as the core-balance current
sensing method.
The Sentinel range of EFI’s have been designed for reliable, convenient and costeffective earth fault indication. The Sentinel EFI’s use a low-cost split-core current sensor which is very easily installed onto underground cable systems or aerial bundled conductor systems without the need to disconnect conductor terminations. Models are available in a range of power supply options, including mains powered, battery powered and load current powered. Primary indication can be either a flashing LED or an electromechanical flag. Reset options can be healthy supply return or timer reset. Additional options include a vandal-proof remote indicator and an auxiliary relay output contact for telecontrol applications.
