Fault indicators are used to locate earth-faults or short-circuits in power distribution networks. The aim of using fault indicators is to have a reliable, cost effi cient and fast indication of the fault location in the network.

In open ring systems, the damaged line section can be disconnected from the network, as soon as a fault is located. While the service team repairs the damaged cable, the rest of the network can be powered up. In this way, the down-time of the network can be minimized to increase the quality of the infrastructure. 
But also in radial distribution systems, fault indicators help to locate the faulty cable quickly and cost efficiently without the need of harmful switching operations to locate the fault.

BASIC IDEA OF FAULT CURRENT MEASUREMENT

Every fault indicator, earth-fault and short-circuit indicators, monitors the network constantly. As soon as a fault current higher than the trip value is detected, the fault will be indicated.
To avoid wrong indications, most models of fault indicators are analysing the measured fault signal with the help of a microcontroller. Wrong indications caused by peaks, e.g. due to switching operations, will be prevented (setting of response delay).

THE OPEN-RING NETWORK

Normally, the open-ring network of switching substations is supplied by an input via two circuit-breakers. To allow proper fault detection, there is one ring main unit with an open switch somewhere in the ring network. To achieve the most accurate and fast localisation of the fault, there should be at least one indicator in every ring main unit. Due to the fact that most of our indicators can be retrofitted, the number of indicators can be easily extended step by step.

FAULT SITUATION IN AN OPEN-RING NETWORK

In case there is a fault in one of the cables between the circuit-breaker and the ring main units, a fault current will be measured by the indicators. All the indicators between the circuit-breaker and the fault will trigger and indicate a fault passage. 
The circuit-breaker of the faulty line will switch off the power of the complete line in order to avoid, any additional damage to the cables and the ring main units. An investigation of the ring by checking the triggered fault indicators will result in the location of the faulty line section. The fault location is between the last triggered indicator and the fi rst non-triggered indicator. 
When the faulty cable section is localised, the defective cable will be cut off from the rest of the ring through the ring main units before and after the fault.

RESTORED SITUATION OF AN OPEN RING NETWORK

After isolating the faulty cable section through the Ring Main Units before and after the fault, the open ring network can be re-energised. In this way, the power-down time for all customers can be minimized and the service team can repair the faulty cable.
Depending on type and configuration the triggered indicators will reset in standby mode by time reset, manual reset, by recovering operating current or recovering operating voltage of the monitored network. The normal service situation is now restored.