Understanding IDMT Relay Wiring Schemes with Practical Circuit Examples

Start with a precise coordination curve selection: IEC 60255’s standard inverse (SI), very inverse (VI), or extremely inverse (EI) characteristics demand careful matching to system fault levels. SI curves suit distribution networks with moderate fault currents, while VI or EI curves are critical for industrial grids where high short-circuit currents risk equipment damage. Include a minimum pickup setting of 110%–130% of maximum load current to avoid nuisance tripping during transient overloads. Verify time-current characteristics using manufacturer data sheets–deviations above 5% from calculated values require recalibration.

Use copper-clad terminals for all signal connections to prevent thermal drift in low-current circuits. A typical bench test setup involves a variable DC source (0–250V), precision shunt resistor (0.1% tolerance), and a dual-channel oscilloscope: one channel monitors input current, the other traces the operated contact timing. Logarithmic scaling on test results simplifies curve validation against expected trip times. For field installation, ground the metal chassis directly to the switchgear earthing bar with a minimum 16mm² copper conductor–longer runs increase transient response errors.

Wire the auxiliary control module in series with the trip coil to ensure fail-safe operation. Bypass capacitors (0.1µF, 250VAC) across coil terminals absorb voltage spikes from inductive kickback. For digital variants, isolate analog inputs with opto-couplers (CTR >80%) to block common-mode noise above 2kV. Store relay firmware updates on a checksum-verified SD card–corrupted uploads can lock the device in an undefined state. Always cross-reference instantaneous pickup settings with upstream breaker interrupting ratings to prevent catastrophic failure during close-in faults.

Install a dedicated test switch between the protective device and the breaker trip circuit. This allows safe isolation during maintenance without de-energizing the entire panel. For Co-9 characteristics, trip times should not exceed 10ms at 20x pickup current–deviations indicate worn contact points or misaligned dashpot mechanisms. Replace internal batteries every 3–4 years; lithium-thionyl chloride cells (ER14505) maintain stability across –40°C to 85°C. Document every coordination study with single-line diagrams showing CT ratios, pickup thresholds, and calculated time margins–missing data voids warranty coverage.

Inverse Definite Minimum Time Protection Scheme Wiring Guide

Start by sourcing an IEC 60255-3 compliant protective device with adjustable time-current characteristics–common models include the ABB REF615 or Siemens 7SJ80. Ensure the unit supports at least three selectable curves (standard inverse, very inverse, and extremely inverse) to match the application’s coordination needs. Precision in curve selection directly impacts fault clearance efficiency.

Wire the current transformers (CTs) with a secondary rating of 1A or 5A to the protection gear’s input terminals, adhering to the polarity markings. Reverse polarity introduces false tripping or failure to operate. Use twisted, shielded conductors for CT wiring to minimize electromagnetic interference, especially in installations near high-voltage equipment.

Connect the trip coil of the circuit breaker to the output contacts of the protective gear. For reliability, employ Class A1 or A2 contacts rated for 10A at 250V AC/DC. Bypass temporary connections with a 100ms delay to prevent nuisance tripping during transient conditions. Verify the breaker’s coil voltage compatibility–mismatches risk coil burnout.

Calibrate the time multiplier setting (TMS) between 0.1 and 1.2 for optimal coordination. Lower values (0.1–0.3) suit downstream devices like motors, while higher settings (0.8–1.2) protect upstream transformers or feeders. Always cross-reference with adjacent protective gear curves to avoid overlapping zones where both might trip unnecessarily.

Test the setup using a primary injection kit or secondary test set. Inject 2× the nominal current and measure the trip time against the selected curve. For a TMS of 0.5 on a standard inverse curve, expect ~3.2 seconds at 2× pickup and ~0.8 seconds at 10× pickup. Discrepancies exceeding ±5% require recalibration or replacement of the protective device.

Integrate a locking mechanism for the TMS dial to prevent accidental adjustment. In environments with vibration (e.g., near generators), secure the dial with non-conductive silicone or epoxy. Document the final settings, curve type, and CT ratios in a universally accessible format–this accelerates troubleshooting during faults.

For auxiliary power, use a dedicated 110V or 220V DC supply with battery backup. AC-powered units risk dropout during system disturbances, negating inverse-time benefits. Include a fuse or miniature circuit breaker rated at 1.5× the maximum consumption of the protective gear to isolate faults without damaging internal components.

Key Components of an Inverse Definite Minimum Time Protection System and Their Functions

Select a time-overcurrent element with inverse characteristic curves (IEC or ANSI standard: SI, VI, EI, or LTI) to match fault clearance requirements. IEC 60255-3 specifies four curve types; choose VI (very inverse) for feeder protection where fault current drops rapidly with distance, ensuring faster tripping at lower multiples of pickup. Avoid LTI (long-time inverse) for motor protection–its gradual slope causes unnecessary delays near pickup.

Core Functional Blocks

• Current sensing module: Transformer cores (5A/1A secondary) must handle 20× rated primary for 1 second without saturation. Use toroidal cores for accuracy below 10% error at 10× pickup. Connect burden resistors ≤0.5Ω to prevent secondary voltage distortion during faults.
• Time-current curve selector: Dip switches or rotary encoders configure curve settings; position 1–16 typically sets time multiplier (TMS: 0.05–1.0). Verify minimum pickup values: 50–200% of load current, adjusted via potentiometers or DIP settings.
• Output tripping contact: DC-rated contacts handle 10A@250V for breaker coils. Use gold-plated silver-cadmium oxide contacts for ≤5ms bounce time. Include flyback diodes (1N4007) across coils to suppress voltage spikes exceeding 1.5kV.

Incorporate a definite-time delay override for severe faults–set at 200ms for currents ≥20× pickup. This overrides inverse curves, preventing thermal damage to conductors or transformers. Test override functionality with primary injection at 2×, 5×, and 10× pickup using a 0.2 class accuracy current source.

Ancillary Protection Elements

1. Undervoltage supervision: AC/DC monitoring relays (27) block tripping below 85% nominal voltage. Install separate VTs (69V secondary) to prevent nuisance trips during transient voltage dips. Set time delay ≥100ms to ignore capacitor switching.
2. Directional element (67): Polarizing voltage input (3V secondary) lags current by 30°–60° for phase faults. Ensure voltage transformer burdens ≤2VA to maintain angle accuracy. Use residual connection for ground faults with zero-sequence current.
3. Thermal overload mimic (49):

Model conductor heating via RTD inputs or current-squared-time integration. Set trip threshold at 120% continuous rating, alarm at 105%. Validate cooling curves match NEMA MG-1 motor class standards.

Install transient voltage suppression (TVS) diodes across semiconductor components–1.5KE200A for 170V clamping. Separate analog/digital grounds using 1Ω isolation resistors to prevent false zero-crossing detection. Power stabilizer ICs (e.g., LM7815) maintain 15V±1% for ±1% pickup accuracy across –20°C to +70°C. Test environmental drift with thermal chambers before field deployment.

Step-by-Step Wiring Process for Inverse-Time Overcurrent Devices in Protection Systems

Begin by verifying the secondary current specifications of the protection device match the transformer’s output–typically 1A or 5A. Confirm the ratio aligns with the primary system’s nominal load to avoid false tripping or insensitivity. Use shielded twisted-pair cables for connections longer than 10 meters to minimize electromagnetic interference. Label each conductor at both ends with heat-shrink tubing or permanent markers, noting phase designation and function (e.g., “A-CT IN,” “A-CT OUT”).

Mount the time-overcurrent unit on a non-combustible DIN rail or panel, ensuring clearances of at least 50mm from adjacent components. Connect the current transformer (CT) secondary leads to the designated input terminals, observing polarity: the marked end of the CT (usually denoted with a dot or stripe) must correspond to the terminal labeled “P1” or “K.” Cross-verify with a multimeter in continuity mode to ensure no open circuits or shorts before energizing. For three-phase systems, maintain consistent phase rotation (ABC) across all CT connections.

Phase	CT Terminal Pair	Wiring Color (Standard)	Torque Specification (Nm)
A	P1/P2	Red/Black	1.2
B	P1/P2	Yellow/Black	1.2
C	P1/P2	Blue/Black	1.2
Neutral	N/A	Grey/White	N/A

Route auxiliary control wiring separately from AC power cables, using conduit or cable trays for mechanical protection. Terminate the device’s trip coil to the breaker’s opening mechanism via a normally open contact; ensure the coil voltage (e.g., 110V DC, 220V AC) matches the station battery or auxiliary supply. For ground fault detection, connect the residual CT circuit to the neutral terminal, calibrating pickup settings to 10-20% of the nominal phase current. Test insulation resistance of all installed cables with a 500V megger; readings below 10MΩ warrant immediate re-inspection.

Set the inverse-time characteristic curve via dip switches or software interface, selecting the appropriate standard (IEC 60255 or ANSI/IEEE). For example, a “Very Inverse” curve (IEC) requires pickup at 1.05x nominal current and tripping at 2x in 10 seconds. Validate settings with a primary injection test at 120% of the pickup threshold–verify the breaker operates within the expected time delay (±5%). Document all parameters, including manufacturer model, firmware revision, and substation location, in the protection logbook.

Before commissioning, simulate fault conditions using a secondary current injector. Apply 3x nominal current; the device must trip the breaker within 3-5 seconds. If delays exceed specifications, recalibrate the time multiplier setting (TMS) or replace the unit if drift is detected. Finally, seal all terminal covers and conduit entries with IP54-rated glands to prevent dust or moisture ingress. Update the one-line diagram and SCADA screens to reflect the installed protection scheme.