Earth continuity protection is provided using pilot earth loop or earth continuity monitoring. The ECM3 relay monitors a low voltage DC signal applied between the pilot and earth of the cable and, by measuring this signal, determines the resistance of the Pilot-Earth loop.
Unlike its predecessor, the ECM2, the ECM3’s earth continuity trip point can be reduced, restricting the earth resistance and maintaining touch potential compliance. The pilot resistance trip point is adjustable in steps of 5Ω from 15Ω to 90Ω. Setting the trip threshold closer to the pilot resistance ensures that the maximum touch voltage is kept within an acceptable range. The maximum loop resistance setting is 90Ω.
The ECM3 implements earth continuity protection using a resistance dependent trip characteristic, whereby the speed of trip actuation is related to the magnitude of the fault. In this manner, a very high impedance fault in the earth return path (e.g. cable open circuit) will result in a very fast trip, and a fault where the earth return path gradually increases over the trip setting will result in a slower trip actuation. The substantial advantage of this method is that sources of pilot noise, such as cable slip rings and non-symmetrical cables, have a significantly reduced impact on the ECM3. This dynamic trip time response distinguishes the ECM3 from other current generation EC relays by providing far superior noise immunity.
The relay is housed in a DIN rail mount enclosure. The user interface consists of three indication lamps (R/Y/G LED’s) and a selector switch for the setting of the series pilot resistance.
The ECM3 relay performs the following functions:
1. Ensures earth return path exist and is below the resistance set point selected to maintain touch potential limitations.
2. Provides an interlock relay to interface in the start sequence of the cables control circuit ensuring pilot circuit healthy prior to start.
3. Provides one set of voltage free contacts for indication.
The ECM3 has been designed to provide earth continuity protection for cables containing pilot cores. In practical applications, environmental factors often prevent an earth continuity protection system from complying with both requirements of AS/NZS 2081 and AS/NZS 4871. Affecting factors can include coupled noise onto the pilot of the cable, variation in the resistance of the pilot cable due to temperature, and the overall length of the cable. The introduction of the dynamic trip response characteristic allows an electrical protection system incorporating an ECM3 to maintain a much lower touch voltage under earth fault conditions without any degradation of reliability or the introduction of nuisance tripping.
Ampcontrol has developed the ECM3 – EC protection relay, which is designed with AS/NZS 2081 (2011) and AS/NZS 4871.1 (2012) in mind, to achieve superior touch potential compliance and noise immunity of an EC protected system than any current generation EC relay.
