3 Phase Voltage Monitoring Relay Series MG21DH 1 C/O GIC L&T SM175 208-480 VAC

Specifications

• Operating Voltage Range: 208 VAC to 480 VAC (3-Phase, 3-Wire).
• System Frequency: 50/60 Hz.
• Protection Functions: Phase Loss (Single Phasing), Phase Sequence Reversal, Phase Unbalance.
• Output Relay: 1 C/O (SPDT - Single Pole Double Throw).
• Contact Rating: 5A @ 250 VAC / 28 VDC (Resistive Load).
• Mounting: Standard 35mm DIN Rail Mount.
• Dimensions: 17.5mm Width (Space-saving slim design).
• Indicators: LED for Power ON and Fault Status.
• Operating Temperature: -15°C to +60°C.

Applications

• Industrial Motor Control Centers (MCC): Protecting three-phase induction motors from single-phasing burns.
• HVAC Systems: Preventing reverse rotation of expensive scroll compressors.
• Conveyor Systems: Ensuring synchronized, correct-direction movement across manufacturing lines.
• Submersible Pump Panels: Safeguarding agricultural and municipal water pumps against grid fluctuations.
• Elevator & Hoist Controls: Critical safety interlocks to prevent mechanical failure due to power issues.

Unlock New Capabilities

• Past Projects: If you have previously built basic direct-on-line (DOL) starters using simple contactors and push buttons, you know they lack intelligent grid sensing.
• The Upgrade: Adding this MG21DH Relay to your panel unlocks Automated Grid Intelligence. Instead of relying on manual observation, your panel will automatically disable the main contactor the millisecond the power utility drops a phase or wires are swapped.
• Effort Level: Intermediate to Advanced. Requires a solid understanding of 3-phase AC wiring, contactor control logic, and strict electrical safety protocols.

Virtual Technician & Wiring Guide

• ⚠️ HIGH VOLTAGE WARNING: This device connects directly to lethal 3-phase mains voltage (up to 480V). Ensure all power is completely isolated and locked out before installation. Installation must be performed by a qualified electrician.

Wiring Pinout:

• L1, L2, L3: Connect directly to the incoming 3-phase power lines (sensing inputs).
• 15 (Common): Connect your control circuit voltage here (e.g., from a control transformer or L1).
• 18 (Normally Open - NO): Connect this to your Contactor Coil (A1). This contact closes only when all 3 phases are healthy and in the correct sequence.
• 16 (Normally Closed - NC): Optional. Connect to a fault indicator lamp or alarm system.

Package Includes:

• 1x 3 Phase Voltage Monitoring Relay Series MG21DH 1 C/O GIC L&T SM175 208-480 VAC

Frequently Asked Questions (FAQ)

• What is the primary function of the GIC SM175 MG21DH relay?
  It monitors 3-phase power lines and triggers a relay if it detects phase loss, phase sequence reversal, or severe voltage unbalance, protecting downstream equipment.
• Can this relay protect against overvoltage or undervoltage?
  No, the MG21DH series specifically focuses on Phase Sequence, Phase Loss, and Unbalance. If you need strict Over/Under voltage limits, you require a different model in the GIC series.
• Does it require a separate 24V or 230V auxiliary power supply?
  No, this relay is self-powered from the measuring inputs (L1, L2, L3).
• What does the "1 C/O" mean?
  It stands for 1 Change Over contact. It provides one Common (15), one Normally Open (18), and one Normally Closed (16) terminal for your control circuit.
• How do I wire this to stop my motor during a fault?
  You must wire your motor contactor's coil circuit in series through terminals 15 and 18. If a fault occurs, the relay opens contact 18, dropping out the contactor and stopping the motor.
• Will this relay work on a single-phase 230V system?
  No, this specific model (MG21DH) is strictly designed for 3-Phase, 3-Wire systems (208-480 VAC).
• Does the relay require a Neutral wire connection?
  No, the MG21DH is a 3-wire device and monitors the phase-to-phase voltages directly without needing a neutral reference.
• What happens if the input wires are connected in the wrong order (e.g., L1, L3, L2)?
  The relay will detect a Phase Sequence error. The fault LED will indicate the issue, and the output relay (15-18) will not close, preventing the motor from starting in reverse.
• How sensitive is the phase unbalance protection?
  While exact trip points depend on the specific datasheet revision, GIC relays typically trip when the voltage unbalance exceeds a set asymmetrical threshold, protecting motors from overheating.
• What is the difference between GIC and L&T branding for the SM175?
  GIC (General Industrial Controls) is the original equipment manufacturer (OEM). L&T often brands and distributes these exact GIC relays under their own catalog. The internals and specifications are identical.
• What is the maximum voltage I can put through the relay contacts (15, 16, 18)?
  The control contacts are rated for 5 Amps at 250 VAC or 28 VDC (resistive load). Do not run the main motor current through these contacts; they are for control signals only.
• Can this relay be used with Variable Frequency Drives (VFDs)?
  It is highly recommended to install this relay on the line side (input) of the VFD to protect the drive. Do not install it on the load side between the VFD and the motor, as the PWM signals will cause erratic behavior.
• Does this relay auto-reset after a fault clears?
  Yes, once the 3-phase power returns to normal parameters, the relay will automatically reset and close the 15-18 contact, allowing the system to restart.
• What size wire should I use for the L1, L2, L3 terminals?
  Typically, 1 sq mm or 1.5 sq mm flexible panel wire with crimped bootlace ferrules is ideal for the sensing inputs.
• Is there an adjustable time delay for tripping?
  The standard MG21DH has a fixed, rapid response time to ensure immediate protection. It does not typically feature user-adjustable time delay knobs.