Voltage relays (CV, CVD, CVQ)

CV voltage relays

Low voltage protection

A low voltage condition will prevent motors from reaching their rated speed on starting or cause them to lose speed and draw heavy overload current. Where continuous operation is essential, such as station auxiliary service or continusous manufacturing processes, the CV relay is used for alarm purpose only

Voltage transfer schemes

When alternate sources of power exist, such as double-ended transformer units, the CV relay can be applied to initiate switching of the station bus to the alternate source

Timing applications

The CV relay has an accurate operating time characteristic over a wide range of time dial positions and minimum operating voltages. It is frequently used where specific time intervals are required after application of normal voltage or the increase or decrease from normal voltage.

Inductor motor undervoltage protection

During transistory faults, such as lightning strokes, and overhead line may be momentarily de-energized while the fault arc is being extinguished. During this perioud it is desirable to prevent disconnection of the motor load, and realize maximum benefit from gast reclosing schemes. Time delay CV relays are also used to keep the motor load in service

CV-8 harmonic filter
Harmonically restrained CV-8 relay provides excellent ground fault detection of generator startor windings. It is relatively insensitive to third harmonic (180 Hz) voltage, and will not operate for third harmonic load unbalance which normally flows in the generator neutral.


The CVD relay is used in control circuits to initiate switching when the line voltage increases or decreases beyond a predetermined value.


The type CVQ relay provides instantaneous and time delay detection of negative sequence overvoltage as well as responding with time delay to phase-to-phase undervoltage.

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