The GEC Alsthom MCAG14C1BB0004B is a high-speed, high-impedance relay belonging to the specialized MCAG series. Designed by GEC Alsthom (a legacy brand of Alstom and AREVA), this relay is the industry standard for high-impedance differential protection of busbars, transformers, and large rotating machines. It is engineered to provide absolute stability during heavy “through-faults” (faults occurring outside the protected zone) while delivering near-instantaneous tripping for internal faults. The MCAG series is renowned for its simplicity, reliability, and immunity to CT (Current Transformer) saturation issues.

Technical Description & Parameters

The MCAG14C1BB0004B operates on the circulating current principle, where the secondary circuits of all CTs in the protected zone are connected in parallel to a common high-impedance bus.

• Model Number: MCAG14C1BB0004B
• Relay Type: Single-Pole (MCAG14) High-Impedance Differential.
• Operating Current (Setting): Typically adjustable from 10V to 100V or 25V to 175V (Standard voltage-calibrated settings).
• Current Rating: Rated for 1A or 5A CT secondaries (Interfaced via external Metrosils and resistors).
• Auxiliary Power (BB Code): 110-125V DC (Standard station battery supply).
• Operating Time: Extremely fast (Typically 7ms to 15ms at 3x setting).
• Contacts: Two normally open (NO) instantaneous hand-reset contacts.
• Mounting: Standard draw-out case (Size 1/2 or 4 depending on configuration).

 

Product Technical Description

The MCAG14C1BB0004B is a “Voltage-Operated” relay. Under normal conditions or external faults, the sum of currents from the CTs is zero, and the voltage across the relay is negligible. During an internal fault, the CTs force current through the high-impedance relay, causing the voltage to rise rapidly above the “Stability Voltage” (Vk). To protect the relay and CTs from high-voltage peaks during heavy internal faults, a non-linear resistor (Metrosil) is typically installed in parallel with the relay.

 

Product Datasheet Essentials

The “14” designation indicates a single-pole unit, meaning three units are required for a standard three-phase protection scheme. The “0004B” revision features gold-plated internal connectors for enhanced signal integrity and a high-visibility mechanical flag (target) that drops upon operation. The relay is housed in a dust-proof, draw-out case that automatically shorts the CT leads when the module is removed, allowing for “live” maintenance of the protection panel without de-energizing the primary equipment.

 

Country of Origin

United Kingdom / France (GEC Alsthom Protection & Control)

 

Application Fields

The MCAG series is a fundamental protection component in:

• Busbar Protection: Protecting the main busbars in high-voltage substations.
• Transformer Differential: Used as restricted earth fault (REF) protection for star-connected windings.
• Generator Protection: Providing high-speed differential protection for the stator windings.
• Motor Protection: Protecting large synchronous motors against internal phase-to-phase and phase-to-ground faults.

 

Product Usage Instructions

1. Installation: Mount the relay in the panel. Ensure the external stabilizing resistor (R) and Metrosil are correctly sized according to the CT’s knee-point voltage (Vk).
2. Stability Calculation: Calculate the required setting voltage (Vs​) to ensure the relay does not trip during the maximum possible through-fault current when one CT is fully saturated.
3. Wiring: Connect the parallel CT bus to the relay input terminals. Ensure all CTs in the group have the same ratio and similar magnetization characteristics.
4. Testing: Perform a “Secondary Injection Test” to verify the pickup voltage. Conduct a “Primary Stability Test” (if possible) to confirm that the CT polarities are correct and the relay remains stable under load.
5. Maintenance: Periodically check the Metrosil for signs of overheating and ensure the mechanical flag resets cleanly after a test operation.

 

Common Questions & Answers (Q&A)

Q: Why do I need a Metrosil with the MCAG14C1BB0004B? A: During a massive internal fault, the CTs can generate several thousand volts. The Metrosil acts as a voltage limiter, “clamping” the voltage to a safe level to prevent damage to the relay insulation and the CT secondary wiring.

Q: Can this relay be used for Restricted Earth Fault (REF) protection? A: Yes, the MCAG14 is the most common relay used for REF protection in transformers to detect ground faults near the neutral point where standard differential relays may lack sensitivity.

Q: What is the significance of the “Knee-Point Voltage” (Vk) of the CT? A: For a high-impedance scheme to work, the CTs must have a Vk that is at least twice the relay setting voltage (Vs​). This ensures the CTs don’t saturate before the relay has a chance to operate during an internal fault.

 

Explore More Protection & Control Parts

For expert support and to source the MCAG14C1BB0004B along with other legacy GEC Alsthom and Alstom components, visit our online technical catalog.

 

Recommended Similar Models

1. MCAG34 – A three-pole version of the high-impedance relay for compact phase protection.
2. MFAC – A legacy GEC Measurements high-impedance relay often found in older plants.
3. MiCOM P841 – A modern numerical busbar protection and terminal device.
4. METI – A definite-time overcurrent relay often used for backup in the same lineups.
5. KCGG140 – A multi-function Alstom relay for comprehensive feeder management.