What Is Digital Switchgear?

A working definition

The term is used loosely, so it helps to be specific. Digital switchgear is a switchgear assembly, built to the same assembly standards as conventional gear (UL 1558 and IEEE C37.20.1 for low-voltage power circuit breaker switchgear, IEEE C37.20.2 for metal-clad), that integrates four elements: sensors for current, voltage, temperature and other conditions; IEDs for protection, control and metering; a communications network carrying measurements, status and commands; and a data layer supporting monitoring, diagnostics and often a digital twin of the equipment.

The breakers, bus and interrupting ratings do not change. What changes is how the assembly measures itself, how it is wired, and how much of its state is available outside the equipment room. A lineup with a relay and a Modbus link is not digital switchgear in any meaningful sense; a lineup designed so that every protection, control and monitoring signal is acquired once and shared over a standardized network is.

Sensors in place of conventional instrument transformers

Conventional CTs and VTs deliver 5 A and 120 V secondaries over copper to each relay and meter. Low-power instrument transformers (LPITs) replace them with Rogowski coils for current and resistive or capacitive dividers for voltage, with low-level analog outputs or, in fully digital designs, conversion at a merging unit into sampled values on the network.

LPITs do not saturate under fault current, have a wide dynamic range, take less space and remove the open-CT-secondary hazard. They also change engineering practice: there is no CT burden calculation, relay inputs must match the sensor type, and accuracy classes and verification methods differ from conventional metering-class transformers.

IEC 61850 station bus and process bus

IEC 61850 is the communications framework most digital switchgear is built around. At the station bus level, relays and controllers exchange status and measurements using MMS and pass high-speed signals such as breaker failure, interlocking and arc-flash trips using GOOSE messaging, replacing much of the point-to-point control wiring between cubicles.

Where applicable, a process bus extends the network to the primary equipment: merging units publish sampled values from the sensors and breaker IEDs carry trip and status signals, so the relay's connection to the switchgear becomes a fiber link rather than a wire bundle. Process bus depends on precise time synchronization and careful network design, and many installations adopt station bus first and process bus only where the benefits justify the added engineering.

What you gain

Data that used to require a technician at the front of the gear (breaker position, trip unit settings, temperatures, metering, event records) is available to operators, engineers and maintenance systems. Remote operation, including racking and switching from outside the arc-flash boundary, is a natural extension of the same architecture.

• Reduced control wiring and terminal count, with configuration replacing much of the hard wiring
• Continuous monitoring of thermal, mechanical and electrical condition built into the assembly
• Remote operation and remote racking support, reducing time spent in front of energized equipment
• Consistent, time-stamped data for SCADA, historians and predictive maintenance
• Protection and interlocking logic that can be modified through configuration rather than rewiring

Realistic caveats

A networked assembly is an attack surface, and the cybersecurity program (segmentation, access control, hardening and logging) has to exist before the gear is energized, not after. Electronics and firmware have shorter support lives than bus and breakers; an assembly designed for decades of service will see several generations of IEDs, so the specification should address spare parts, firmware support and how replacements are validated.

Skills are the third caveat. Testing sensor-based protection needs test sets that publish sampled values or drive LPIT inputs, and commissioning engineers must understand configuration files and GOOSE subscriptions as well as time-current curves. Verification is different, not easier. Protective functions remain governed by approved protection, control and safety procedures regardless of how the signals travel.

Retrofit versus new

New lineups are the cleanest path, because sensors, IEDs and the network can be designed together and tested at the factory. Retrofit is realistic in stages: relays with IEC 61850 capability and a station bus during a protection upgrade, temperature and partial discharge sensors in existing compartments, and remote racking and operation on existing breakers. Replacing conventional CTs with LPITs in an existing assembly is rarely worthwhile on its own.

A useful test for any project is whether the digital features change how the equipment is operated and maintained. If operators will act on the data, maintenance will use the trends and engineers will use the event records, the investment tends to justify itself; if not, a well-built conventional lineup with good relays is the better choice.