Detail EARTH FAULT RELAY MCAG AREVA / ALSTOM
The MCAG relay, used with a stabilising resistor, is
designed for applications where sensitive settings with
stability on heavy through faults are required, and is
recommended for balanced and restricted earth fault,
bus-zone and certain forms of differential protection for
generators, auto-transformers, reactors and motors.
The relay operates as a high impedance unit protection
The relay is an attracted armature unit of simple and
robust construction. The operating coil of this unit is
connected in series with a small choke and capacitor,
forming a series resonant circuit. These components
are energised from an auto-transformer which is tapped
to provide seven current settings.
Due to the simple electromechanical construction, the
detection element, and the output contacts are one and
the same device. Operation is therefore fast, and highly
The relay circuit, tuned to the supply frequency, rejects
the harmonics produced by current transformer
The total impedance of the relay and series stabilising
resistor is usually low enough to prevent the current
transformers developing voltages over 2kV during
maximum internal faults, but in some applications a
non-linear resistor is required to limit this voltage.
Types MCAG 14 and MCAG 34 relays are single and
triple pole, respectively.