This is the fourth post in our LV Panel Engineering Series. The earlier posts cover the standards, construction, and enclosure design. This post looks at what actually sits inside the panel: the devices that switch power, control operation, and measure what’s happening on the system.
Open any Main LV Board or MCC, and you’ll find three broad groups of equipment working together. Switching devices carry and interrupt current. Control devices manage how circuits operate. Measuring devices tell you what’s actually going on. This post covers all three.
Switching Devices
These devices carry the load current and interrupt it when needed — either for normal operation or during a fault.
| Device | Typical use | Key selection factors |
|---|---|---|
| ACB (Air Circuit Breaker) | Main incomers, bus-tie, generator breakers (630A–6300A) | Breaking capacity (Icu/Ics), short-time withstand (Icw), fixed or drawout mounting, trip unit type |
| MCCB (Moulded Case Circuit Breaker) | Sub-main feeders, distribution outgoing ways (16A–1600A) | Breaking capacity, thermal-magnetic or electronic trip |
| MPCB (Motor Protection Circuit Breaker) | Combined short-circuit and overload protection for motors | Coordination with the downstream contactor |
| Switch-disconnector | Manual isolation upstream of a circuit | Making/breaking capacity, lockable in OFF |
| Fuse-switch unit | High-discrimination fault protection | Fuse category (gG/aM), coordination with upstream/downstream devices |
| Contactor | Motor starting and stopping, switching of resistive loads | Utilization category (AC-3 for motors, AC-1 for resistive loads), coil voltage |
| Motor starter | DOL, star-delta, soft starter, or VFD | Starting method matched to motor size and starting torque |
| ATS (Automatic Transfer Switch) | Switching between normal and standby supply | Transfer time, break-before-make vs. closed transition |
An ACB handles the highest currents and usually sits at the incomer or bus-tie position. An MCCB handles the mid-range feeders. Below that, contactors and motor starters do the actual switching work for individual loads.
Control Devices
Control devices manage how the panel actually operates — locally, or as part of a wider scheme.
- Auxiliary relays and timers build the interlocking logic between breakers and starters.
- Protection relays monitor incomers and bus-tie breakers for overcurrent and earth fault conditions. On a substation auxiliary system, these relays usually report to the same SCADA platform as the main GIS bay protection, often over Modbus or IEC 61850.
- PLCs handle sequenced or interlocked operation — cooling water pumps, HVAC systems, or any load that needs coordinated start/stop logic.
- Pushbuttons, selector switches, and indicator lamps give local control at each starter door.
- Control transformers step the LV bus voltage down to a control voltage (typically 110/230V AC), individually fused and isolated from the power circuit.
- Battery chargers and DC supply units feed any DC-fed protection or control circuits — common where a board also supplies the station battery system.
Measuring & Monitoring Devices
These devices tell you what’s happening on the system, and feed that information to operators or to SCADA.
- Multifunction power meters measure voltage, current, power, energy, and power quality on incomers and major feeders, usually reporting via Modbus to the site’s energy management system.
- CTs and VTs step measurement signals down to safe levels. Metering CTs use accuracy class 0.5 or 1; protection CTs use class 5P or 10P. Burden must match the actual connected load, including cable run length, or accuracy suffers.
- Analog or digital ammeters and voltmeters cover simpler boards that don’t justify a full multifunction meter.
- Insulation monitoring devices (IMDs) are mandatory on IT-earthed systems, where a first insulation fault doesn’t trip the circuit and needs continuous monitoring instead.
- Earth fault indicators monitor residual current on TN-S systems feeding sensitive loads.
- Thermal sensors watch busbar joints and cable terminations for overheating — sometimes paired with infrared viewing windows, so a thermal camera can inspect live joints without opening the compartment.
- SCADA gateways pull all of this data together and send it to the substation control system over one communication link.
How These Groups Work Together
None of these devices operate in isolation. A protection relay senses a fault and trips an ACB. A contactor switches a motor, while its MPCB protects the circuit feeding it. A multifunction meter reports the same current a CT is measuring, and a SCADA gateway carries that reading off-site. Selecting the right device for each role — and making sure they’re properly coordinated — is what turns a panel from a box of parts into a working protection and control system.
Up next in this series: Internal Arc Protection in LV Panels, covering IAC classification and the active arc-detection systems that protect people and equipment when something goes wrong inside the panel.