The GEC Measurements CAG34AF0089A is a triple-pole (three-element) electromechanical circulating current relay, part of the widely used Midos and legacy GEC protection ranges. It is designed for high-impedance differential protection, specifically used to detect internal faults in generators, transformers, reactors, and busbars. By operating on the high-impedance principle, the CAG34 ensures exceptional stability during heavy through-fault conditions and CT saturation, only triggering when an internal imbalance is detected.

Product Technical Description

The CAG34 relay consists of three identical attracted-armature elements housed in a single case. Each element is connected in a series resonant circuit (tuned to the rated supply frequency) to reject harmonics often produced by current transformer saturation. The “AF” in the model code typically denotes the specific configuration of the current settings and contact types. It is used in conjunction with an external stabilizing resistor to set the operating voltage threshold. The unit is housed in a standard Midos 1D vertical draw-out case, which includes automatic CT shorting switches to allow the relay to be withdrawn safely while the primary system is energized.

 

Product Datasheet & Parameters

• Model Number: CAG34AF0089A
• Relay Type: Triple-Pole Circulating Current (High-Impedance Differential)
• Application: Generator, Transformer, and Busbar Protection
• Current Rating (In): 1A or 5A (Model dependent; typically 5A for industrial applications)
• Setting Range: 10–40% or 20–80% of rated current (in seven equal steps)
• Operating Time: 25ms at 5 times the current setting
• Frequency: 50 Hz (Standard)
• Contacts: 2 Normally Open (N/O) self-resetting contacts per element
• Operation Indicator: Hand-reset mechanical flag (Yes)
• Burden: 0.9 VA to 1.0 VA at current setting
• Case Size: 1D vertical draw-out case
• Mounting: Flush mounting

 

Country of Origin

This protective relay was manufactured by GEC Measurements (later Alstom/Areva/GE) in the United Kingdom (Stafford, UK).

 

Application Fields

• Restricted Earth Fault (REF) Protection: Providing sensitive earth fault detection for transformer and generator windings.
• Bus-Zone Protection: Serving as the primary protection for substation busbars.
• Generator Differential Protection: Monitoring for phase-to-phase and phase-to-ground faults within the machine.
• Autotransformer & Reactor Protection: High-speed clearing of internal winding faults.
• Transverse Differential Protection: Used for parallel feeder or multi-winding generator schemes.

 

Product Usage Instructions

The CAG34 must be installed in a vertical orientation to ensure the mechanical flag and armature function correctly. Each of the three phases must be wired through its respective relay element in a circulating current loop. It is mandatory to use the relay with the specified external stabilizing resistor (typically 50 ohms for 5A systems or 200 ohms for 1A systems) to prevent mal-operation during external faults. During commissioning, “primary injection” tests should be performed to verify the stability of the high-impedance loop. When the relay is withdrawn from its case, the internal shorting links will automatically bridge the CT secondaries, but manual verification is recommended for safety.

 

Common Questions & Answers (Q&A)

Q: Why is a stabilizing resistor required for this relay? A: High-impedance schemes rely on the resistor to ensure the relay only sees a voltage high enough to operate during internal faults. During external faults, the resistor forces the unbalanced current to flow through the saturated CT rather than the relay.

Q: Can this relay detect DC faults? A: No. The internal series resonant circuit is tuned to the rated AC frequency (50Hz/60Hz) and is designed to reject DC components and harmonics.

Q: What should I do if the mechanical flag is visible? A: A visible flag indicates the relay has operated. Before resetting the flag, the protected equipment must be inspected for internal faults. Once the fault is cleared, press the manual reset button on the front of the relay.

Q: Does the relay require an external DC auxiliary supply? A: No, the CAG34 is a self-powered electromechanical relay that derives its operating energy directly from the CT secondary circuit.

 

Similar Model Recommendations

1. GEC CAG14 (Single-pole version)
2. Alstom MCAG34 (Modern equivalent version)
3. GEC CAG34AF58A (Alternative setting range)
4. GE Grid Solutions P44x (Modern numerical distance/differential)
5. GEC CAG37 (Wider setting range version)