The connecting piece in a relay protection device serves as the interface between the relay and the circuit breaker, ensuring proper signal transmission for tripping, supervision, and control.

Function of the Connecting Piece

In a relay protection system, the connecting piece—often referred to as a master relay or interface relay—acts as the intermediary between the protective relay and the breaker trip coil. Its main functions include:

• Signal consolidation: It receives multiple commands from protective relays and converts them into a single trip signal for the breaker coil, ensuring coordinated operation in HT or LT panels .
• Isolation: It electrically isolates the sensitive relay circuitry from the high-power breaker circuits, protecting the relay from voltage spikes or faults .
• Trip circuit supervision: It works with trip circuit supervision relays (e.g., numerical 96 S) to continuously monitor the health of the breaker trip circuit and provide alarms if the circuit is not healthy .

Types and Components

The connecting piece can take several forms depending on the system design:

• Master relay: Used in HT panels to aggregate signals from multiple relays and control the breaker trip coil .
• Trip circuit supervision relay: Monitors the integrity of the trip circuit and ensures the breaker can operate when required .
• Interface relays in numerical systems: In modern multifunction relays, the connecting piece may be integrated within the relay itself, providing protection, logic, and metering in a single package .

Wiring and Installation

The connecting piece is typically wired between the secondary of current transformers (CTs) and the breaker trip coil. Key considerations include:

• Ensuring correct ferrule numbering and terminal connections for clarity and maintenance .
• Using proper insulation and color coding in multicore cables to prevent miswiring .
• Following manufacturer guidelines for mechanical and electrical installation, especially for IEDs (Intelligent Electronic Devices) that may include integrated connecting interfaces .

Importance in Protection Systems

The connecting piece is critical for:

• Reliable operation: Ensures that the breaker trips promptly during faults, preventing damage to equipment .
• Coordination: Allows multiple relays to operate in a coordinated manner without conflicting signals .
• Safety: Protects both personnel and equipment by isolating high-voltage circuits from sensitive relay electronics . In summary, the connecting piece is an essential component in relay protection systems, providing signal aggregation, isolation, and supervision to ensure safe and reliable operation of circuit breakers and the overall power system.