• A protective relay device that detects failure in one phase of a three-phase supply (single phasing) and immediately trips the motor to prevent damage. The absence of one phase causes excessive currents in the remaining phases, leading to heating and risk of motor winding failure Why It’s Essential • Single phasing leads to unbalanced currents, triggering negative-sequence currents that reduce torque, cause heating, and may cause motor stalling or failure • Motors running on two phases can overheat quickly even under normal loads. • Phase reversal (swap of supply phases) can result in reverse motor rotation—risky for equipment and safety. Types of Single Phase Preventers 1. Mechanical (Bimetallic / Magnetic) Type Uses bimetal strips or magnetic discs to detect phase asymmetry and physically open the no voltage coil circuit. It’s slow and less reliable—mostly obsolete. 2. Voltage Sensing Type Monitors supply voltage imbalances—especially negative sequence voltage. Uses RC networks or filters to detect abnormal sequences and activates relay. 3. Current Sensing Type (Negative Sequence Relay) Detects negative sequence current from CTs on each phase. Filters isolate the negative sequence component. If exceeding a preset threshold, a trip signal is sent. Applications and Use Cases • Used widely in three-phase induction motor protection, especially in industrial setups. • Often integrates into motor control panels and PLC systems for automated shutdown responses. Features and Added Protections • Many modern modules also include reverse phasing, over/under voltage, voltage imbalance, timing delays, auto reset, and LED fault indicators. • These relays are suitable for monitoring incoming 3-phase supply from Mains • Fixed/adjustable unbalance settings(customer Specific also can be provided) • Fixed/adjustable under voltage/over-voltage settings (customer Specific also can be provided) • Fixed/adjustable trip delays • Built-in or external power supply • Resetting Auto / Manual / Remote • Output contacts : 1 CO or 2 CO • Choice of enclosures (DIN-Rail, Flush, Plug-in) • Use of SMD Technology • User-friendly LED indications Examples from Indian Manufacturers • Selfguard’s SPP offers protection against missing phase, reverse phase, voltage imbalance, with microcontroller-based delay timers and indicators. • AT India Circuits SPP: compact panel mountable module with settings for voltage unbalance, reverse phasing, delay, LED status, and auto reset. • EAPL PMD 02 and SPPT: DIN rail devices with auto reset, LED indicators, trip delay specs, and voltage/frequency monitoring Benefits • Enhanced motor lifespan by preventing thermal damage from phase faults. • Safer operation by avoiding erratic motor behavior due to reverse rotation or imbalance. • Better diagnostics with LED status and fault indicators. • Automation ready with PLC signal integration and adjustable timing. SPECIFICATION:- Rated Voltage 415 V , 3 Ph. , 50 Hz Auxiliary Supply* 110V AC/220-240 AC/380-415 AC(ON SELECT MODELS) Operating Voltage Range 225 TO 500 AC/70 TO 110% OF Aux. Supply Voltage/190 TO 456V Power Consumption 5VA (max) at Rated Voltage (Transformer based Supply) Relay Contact Rating 5A at 250V AC (Resistive) Relay Output Rating 1NO/1CO/2NO/3NO(Model specific) Unbalance Trip Voltage Setting 36 to 42V / 30 to 60V (Variable)/40 or 60V(Model Specific) Reset Gap(hysterisis) 6 To 9V(Auto reset in auto start models) Time Trip Delay 3 Sec(±1sec) ON Delay time 30(±5sec) or variable upto 5 mins Pause time 60±5 msec(Star-Delta Timer) Under Voltage Setting 190 ± 5V(Hysterisis- 6 to 10 V)