What Is an LVC in Medium-Voltage Switchgear?
The low-voltage compartment is where protection, control, measurement and project interfaces become one engineered secondary system.
In medium-voltage switchgear, the low-voltage compartment—commonly shortened to LVC—houses the secondary equipment used to monitor, protect and control the primary circuit. Although it does not carry the main power current, its engineering directly affects availability, selectivity, safety and the ability to operate the switchgear correctly.
What an LVC typically contains
Protection relays, trip-circuit supervision and auxiliary tripping devices.
Local/remote selection, control switches, interposing relays and control logic.
Meters, transducers and interfaces from CTs, VTs or sensors.
Network switches, protocol gateways and interfaces to station-control systems.
Terminal blocks for inter-panel, customer and field connections.
MCBs, fuses, DC/DC converters and supervised distribution circuits.
Start with functions, not devices
A reliable LVC design begins with the operating philosophy and protection concept. Define what must happen during normal operation, a system fault, loss of auxiliary supply, maintenance and local testing. Only after the functions and interfaces are clear should the detailed device selection and wiring begin.
This avoids a common failure mode: selecting relays, meters and switches before confirming that the required binary inputs, outputs, analog inputs, communication ports and control voltages are available.
Interface boundaries must be explicit
Every signal should have a defined owner, source, destination and physical interface. Important boundaries include inter-panel wiring, remote-control commands, SCADA communication, external trip signals, CT and VT secondary circuits, auxiliary supply and customer terminal points.
Wiring design is more than connection
Wire type, cross-section, color, ferrule convention, terminal technology and routing influence manufacturability and maintenance. Sensitive measurement or communication circuits may need segregation from switching and coil circuits. CT secondary circuits require particular attention because an open circuit can create dangerous voltage.
Protection and trip circuits
The trip path should be easy to trace from protection function to relay output, test switch or isolation point, interposing devices, breaker coil and return supply. Supervision requirements must be defined for the breaker states that matter. Designers should also verify the coil burden, voltage-drop margin and the behavior of every series contact at the minimum operating voltage.
Design for testing and future changes
A good LVC provides safe test access, clear terminal groups, readable device references and enough space for wiring and heat dissipation. Spare terminals and selected spare relay I/O can be valuable, but they should be specified intentionally instead of assumed.
Engineering documentation should include schematics, terminal diagrams, equipment lists, wire and cable information, network architecture and a clear functional description. The routine test and FAT should then verify the same functions and interfaces defined during design.