The main components of switchgear include:

1. Circuit Breaker: A device that ensures control and protection of the power grid. It can generate, withstand, and interrupt load currents as well as fault currents, including short-circuit currents.

2. Switch: Load switches and isolating switches, which have rated current load connection and disconnection capabilities.

3. Contactor: Used to open and close circuits that require these actions during normal operation, especially for specific tasks like medium-voltage street lighting and industrial motors.

4. Isolating Switch and Grounding Switch: An isolating switch is used to separate two energized and independent circuits without affecting their insulation levels, typically in ring networks or open points. It’s used to disconnect part of the installation from the power supply, offering better performance than other switches. An isolating switch is not a safety device. A grounding switch is a dedicated device used to ground conductors reliably, ensuring safe access to the conductors. They may have rated short-circuit closing currents to withstand errors during operation, such as closing on an energized conductor.

In addition to the above switchgear, the most direct and effective short-circuit protection is achieved through fuses.

Medium Voltage Current-Limiting Fuses

Current-limiting fuses are mainly used to protect loads like transformers and motors. They are devices that, when the current exceeds a certain threshold for a sufficiently long period, fuse one or more specially designed and proportioned components to open the circuit. These fuses may struggle to clear intermediate current values (i.e., currents exceeding the rated value by 6 to 10 times), which is why they are often used in conjunction with other switching devices.

The working principle of current-limiting fuses differs somewhat from mechanical switches. A fuse has a current-carrying element, typically made of copper or silver, whose geometry is strictly controlled to ensure that when the element melts, it melts at multiple points along its length, generating a series of arcs. The current-carrying element of the fuse is surrounded by silica sand. When exposed to the arc, the sand transforms into a high-insulation form of glass. Because there are multiple arcs in the fuse, the voltage generated by the arcs is quite high. This significantly reduces the fault current magnitude and prevents the arc from reigniting after the current reaches zero due to the high arc voltage.

Fuse-Contactor Combinations

Fuses combined with contactors (F-C) are used for motor control. In an F-C circuit, the high-voltage current-limiting fuse serves as a backup fuse. When the circuit current is equal to or less than the rated interrupting current of the vacuum contactor, the integrated protection device operates, cutting off the current via the contactor. If the circuit current exceeds the set current of the integrated protection device or if the vacuum contactor fails to operate, the fuse will act.

Short-circuit protection is achieved through fuses, typically selected with a higher rated current than the motor to avoid the effects of starting currents, but without providing overload protection simultaneously. Therefore, an inverse-time or time-delay relay is needed to protect against overloads. The contactor, current transformers, cables, motor, and other equipment in the circuit may suffer damage from prolonged overloads or excess energy beyond their tolerance limits.

Motor protection against overcurrent caused by overload, single-phase operation, rotor locking, and repeated starts is realized through inverse-time or time-delay relays acting on the contactor. When the fault current between phases or phase-to-ground is lower than the interrupting current of the contactor, protection is provided by the relay. When the fault current exceeds the interrupting current of the contactor up to the maximum withstand fault current, the fuse provides protection.

Fuse-Switch Combination Devices

Fuses combined with load switches are mainly used for transformer protection. A typical application is in a ring main unit, where a load switch combined with a fuse protects the transformer outgoing line, and an SF6 load switch allows for compact, maintenance-free solutions.

In the case of a mid-mounted trolley solution, the fuse + load switch combination can be used with a mid-mounted switchgear cabinet, allowing for easy maintenance and fuse replacement. Transformer protection uses combination devices, supplemented by relays for secondary protection. When there is overload or overcurrent, the relay protection triggers the switch to trip, while short circuits are cleared by the fuse and fuse switch.

Transformer Fault Protection

When a fault, such as a short circuit, occurs inside a transformer, the short-circuit arc can cause the oil to vaporize. As the short circuit continues, pressure increases, eventually causing the oil tank to explode. To prevent the oil tank from bursting, the fault must be cleared within 20ms. The breaker’s clearing time consists of the relay protection time, the inherent tripping time, and the arc quenching time. Typically, the total clearing time is no less than 60ms, which may not effectively protect the transformer. Current-limiting fuses, however, have a fast-break function and can clear the fault within 10ms, providing effective protection for the transformer.