- Low Voltage Switchgear Definition: Low voltage switchgear is defined as electrical switchgear rated up to 1kV, including protective devices like circuit breakers and fuses.
- Components of LV Switchgear: LV switchgear includes devices such as circuit breakers, isolators, and earth leakage circuit breakers to protect the system.
- Incomer Function: The incomer feeds power to the bus, withstands abnormal currents briefly, and interrupts fault currents.
- Sub-Incomer Role: Sub-incomers supply power to feeder buses with fewer interlocks, using ACBs and MCCBs for protection.
- Feeder Types and Protection: Different feeders (motor, industrial machinery, lighting) require specific protective devices like MCCBs and earth leakage circuit breakers.
Under IEC 60947-1, Electrical switchgear for circuits not exceeding 1,000 V AC or 1,500 V DC falls within the low voltage switchgear scope. LV switchgear can include circuit breakers, switches, electrical isolators, HRC fuses, devices in the earth leakage circuit breaker family, miniature circuit breakers (MCB) and moulded-case circuit breakers. An assembled distribution board must also meet the applicable assembly and installation rules. LV switchgear switches, isolates and protects low-voltage circuits.
Incomer
The incomer connects the source of electrical power to the main bus. Its switchgear provides the main switching and protective function for the assembly. The selected switchgear must carry normal load, withstand the specified short-time current when coordination requires a delay and interrupt the prospective fault current at its installation point. Interlocks must match the source and bus-coupler operating scheme. Designers often use air circuit breakers for high-current incomers because a modern low-voltage air circuit breaker can provide these functions:
- Rated current suitable for the main bus.
- Breaking capacity above the calculated prospective short-circuit current.
- Short-time withstand capability for selective coordination.
- Adjustable protection, isolation and maintainable or withdrawable construction where specified.
Compared with smaller breaker types, air circuit breakers can require more panel space and cost. Their trip delay is adjustable rather than inherently long. Use them in low voltage switchgear when the verified current ratings, coordination functions and maintenance needs justify that choice.
Sub – Incomer
A sub-incomer is the next distribution level in an LV Distribution board. It takes power from the main bus and supplies a section or feeder bus. Its switching and protection must suit the downstream load, prospective fault level and required selectivity.
- A rated current and breaking capacity that match the supplied bus section.
- Only the interlocks required by the actual source and bus arrangement.
Depending on rating and coordination, sub-incomers may use ACBs (Air Circuit Breakers), moulded-case circuit breakers or switch-fuse units. The device must be selected from the verified assembly design rather than from its position alone.
Feeders
Outgoing feeders connect the bus to loads such as motors, lighting, process equipment, air-conditioning equipment and transformer cooling systems. A feeder may use a fuse-switch, circuit breaker, motor starter or combined protective device. Selection depends on cable capacity, load behaviour, fault level, isolation needs and coordination.
- Motor Feeder
- A motor feeder needs coordinated overload and short-circuit protection. The motor-protection scheme may also need phase-loss, imbalance, stall or locked-rotor functions.
- Industrial Machinery Load Feeder
- Industrial equipment such as ovens and an electroplating bath needs protection chosen for its load current, inrush, control method and fault risk.
- Lighting Load Feeder
- Lighting feeders need overcurrent protection and, where installation rules require it, residual-current protection for fault or additional shock protection.
In an LV switchgear system, electrical fuses and circuit breakers provide overload and short-circuit protection. Residual-current devices serve a different purpose: they detect an imbalance between live conductors. A device rated at no more than 30 mA is commonly used as additional shock protection where the applicable wiring rules require it, but it does not replace basic protection, earthing or overcurrent protection. Trip thresholds and times depend on the device type and governing rules.
The diagram shows one example distribution scheme. The main incomer is supplied from the low-voltage side of an electrical transformer. An electrical isolator and an MCCB, which is omitted from the drawing, connect it to the incomer bus. Two sub-incomers supply separate feeder-bus sections. Their interlock with the bus coupler prevents unintended paralleling of the sources: with the coupler closed, only one incomer can close; with the coupler open, both incomers can supply their separate sections. Each outgoing circuit then uses protection suited to its load. The illustrated motor feeder combines a thermal overload device with short-circuit protection. The heater uses a switch-fuse unit, while the lighting and air-conditioning circuits use a miniature circuit breaker with the shown upstream protection. This is one basic low voltage switchgear arrangement, not a universal design.
