Phase Sequence Relay Pakistan: Loss & Reverse
Phase Sequence Relay Pakistan — Phase Loss, Imbalance & Reverse Protection Guide
Updated: May 2026 · CNC Electric Pakistan
Phase Sequence Relay — Pakistan Quick Answer (2026)
A phase sequence relay monitors 3-phase supply for phase loss, reversed sequence (R-Y-B vs R-B-Y), voltage imbalance, and under/over voltage — then drops the downstream contactor if any fault is detected. Standard protection for 3-phase motors, lifts, pumps, factories. Pakistan 2026 prices: basic DIN-rail YDC-3SM from Rs.3,500; multi-function with adjustable thresholds Rs.5,200; industrial with phase reversal alarm output Rs.7,800.
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What a Phase Sequence Relay Detects
A phase sequence relay continuously samples voltage on all three phases (R, Y, B) and trips its output relay when ANY of these faults occur:
1. Phase Loss (Single Phasing)
One of the three phases goes missing — broken fuse, blown WAPDA pole fuse, damaged cable. Single-phasing a 3-phase motor causes it to draw heavily on the remaining phases, overheating winding and burning out in minutes. The relay detects within 100 ms and drops the contactor before damage.
2. Reverse Phase Sequence
The phase rotation is R-Y-B instead of expected, or vice versa. Reversed sequence spins motors backwards — devastating for lifts (cabin moves wrong direction), pumps (cavitation, no flow), conveyors (product spills), drills (workpiece flies off). Common after WAPDA pole work where electricians reconnect phases incorrectly.
3. Voltage Imbalance
One phase is sagging while others are normal — heavy unbalanced load on the WAPDA grid. Imbalance above ~5% causes motors to draw excess current on the heavy phase and overheat. The relay detects imbalance above the threshold and trips.
4. Under-Voltage / Over-Voltage
Most relays include adjustable upper and lower voltage thresholds (typically 380 V ±15%) so they trip during WAPDA sags or surges that would damage equipment.
5. Phase Reversal in Real Time (after a transfer)
After every WAPDA-to-generator or generator-to-WAPDA transfer via an ATS, phase sequence MAY have changed. The relay re-verifies on every restoration and trips if reversed.
Pakistani Real-World Scenarios Where This Matters
Textile Mill Loom Failure
Punjab textile mills run hundreds of 3-phase looms. One WAPDA pole loses a phase — single-phasing every loom in the mill simultaneously. Without phase sequence protection, in 5 minutes 200+ motors are smoke. With it, the contactors drop in 100 ms and operators stop the looms safely.
Hospital Lift Reverse
WAPDA maintenance reconnects pole wires after rain damage. Phase sequence comes back reversed. Without phase protection, the next lift call sends the cabin DOWN when passengers want UP — emergency stop saves no one. With protection, the lift refuses to operate until sequence is corrected.
Borewell Pump Burn-Out
5 HP borewell pump runs all night. At 3 AM one phase drops. Pump draws 2× rated current on the remaining phases. By morning the motor is burnt, replacement cost Rs.45,000 + 2 days of no water. With phase protection (Rs.3,500), the contactor drops within 100 ms — pump shuts off before damage.
Hotel HVAC Compressor
20 ton central chiller compressor. WAPDA-to-genset transfer brings genset online with phases R-Y-B but WAPDA was wired R-B-Y. When transfer happens back to WAPDA, sequence reverses. Compressor restart spins backwards, cracks the housing — Rs.480,000 in damage. Phase relay prevents the restart.
CNC Phase Sequence Relay Models — Pakistan 2026
YDC-3SM (Basic DIN-Rail)
Standard 2-module DIN-rail relay. Monitors phase loss, sequence reversal, voltage above 470 V (fixed). One SPDT relay output, 5 A 250 V AC contact rating — drives a contactor coil directly.
- CNC YDC-3SM — Rs.3,500
- Typical install: 3-phase pump, motor, small factory load
YDC-PMR (Adjustable Multi-Function)
Adjustable thresholds for over-voltage (380-450 V), under-voltage (320-380 V), imbalance (5-25%), and trip delay (0.1-10 sec). Two SPDT relay outputs — one for general fault, one for sequence reversal alarm.
- CNC YDC-PMR — Rs.5,200
- Typical install: commercial lift, large pump, hospital HVAC, manufacturing line
YDC-IPR (Industrial with PT Connection)
For high-voltage panels where direct 380 V connection isn't suitable — uses voltage transformers (PTs) for sensing. Phase rotation indicator LEDs on the front. Modbus RTU output for SCADA / BMS integration.
- CNC YDC-IPR — Rs.7,800
- Typical install: industrial substation, MCC main, hospital main switchboard
Wiring a Phase Sequence Relay — DOL Motor Starter
Standard wiring with a contactor-controlled motor:
- 3-phase supply R + Y + B + N (if needed) into the phase relay's voltage-sensing terminals
- Relay output contact (NO) wired in series with the contactor coil control circuit (between the START push button and the coil A1 terminal)
- STOP push button (NC) upstream of the START button as usual
- Result: contactor coil only energises if the phase relay output is closed (no fault). If phase loss or sequence reversal occurs, the relay opens its output contact, breaking the coil circuit — contactor drops, motor stops.
Common Installation Mistakes
- Trying to wire the phase relay output directly to the motor. The relay output is for low-current control — driving a contactor coil. The motor itself must be switched by the contactor.
- Skipping the phase relay on "small" 3-phase loads. Even small 3-phase pumps burn out from single-phasing. A Rs.3,500 relay prevents Rs.45,000 motor replacement.
- Setting trip delay too short. 0.1 sec delay trips on every WAPDA millisecond flicker. Use 1-3 sec for normal motors, 100 ms for sensitive equipment.
- Ignoring the imbalance threshold. Default may be 25% (very loose). Tighten to 5-8% for motor protection in Pakistani conditions where WAPDA imbalance is common.
- Mounting the relay outside its operating temperature range. CNC YDC series rated -25 to +55 °C. Outdoor enclosures in Pakistani summer can hit 70 °C internally — use a fan-vented enclosure.
- Bypassing the relay during troubleshooting and forgetting to restore it. Common: tech bypasses relay to test motor manually, forgets to reconnect, motor runs unprotected for months until next failure.
Frequently Asked Questions
What does a phase sequence relay do?
It monitors 3-phase supply for phase loss (one phase missing), reversed sequence (R-Y-B vs R-B-Y), voltage imbalance, and under/over voltage. When any fault is detected, the relay's output contact opens — typically dropping the downstream contactor and stopping the motor before damage.
What is the price of a phase sequence relay in Pakistan?
CNC phase sequence relays Pakistan 2026: basic DIN-rail YDC-3SM from Rs.3,500; multi-function adjustable YDC-PMR Rs.5,200; industrial with PT input + Modbus YDC-IPR Rs.7,800.
Do I need a phase relay on a small 3-phase pump?
Yes. Even a 2 HP 3-phase pump can burn out from single-phasing within minutes — motor replacement Rs.15,000+. A Rs.3,500 phase relay prevents that. Standard practice on every 3-phase motor in Pakistani panels.
What happens if I run a 3-phase motor on reversed sequence?
The motor spins backwards. Consequences depend on the load: lift cabin moves wrong direction; pump cavitates with no flow; conveyor reverses dropping product; CNC machine cuts the wrong direction destroying the workpiece. Phase relay prevents the contactor from closing in reversed sequence.
Can I use a phase relay with a contactor?
That's exactly how they're used. The relay output (a low-current NO contact) wires in series with the contactor coil. Normal operation: relay closed, contactor energised, motor runs. Fault: relay opens, contactor drops, motor stops.
How does it detect phase loss?
The relay's internal electronics compare the three line-to-line voltages. When one phase is missing, the L1-L2, L1-L3, or L2-L3 voltage reads near zero. The relay logic detects this within 50-100 ms and opens the output contact.
Where should I install the phase relay?
In the motor control panel or DB box, upstream of the contactor. Its voltage-sensing terminals tap the 3-phase supply (after any upstream MCB / MCCB), and its output contact wires into the contactor coil control circuit. Standard DIN-rail mount.
Protect every 3-phase motor for Rs.3,500
Phase sequence relay prevents motor burnout from single-phasing or reversal. Tell us your motor size and we’ll spec the right relay.