Glossary of Electrical Switchgear Terms
A comprehensive reference guide for medium and low voltage switchgear terminology.
A
AIS (Air Insulated Switchgear)
Switchgear that uses air as the primary insulation medium between live parts. Common in medium voltage applications up to 52kV.
Altitude
The height above sea level. Higher altitudes have lower air density, which affects the insulation strength and cooling of electrical equipment.
Ambient Temperature
The surrounding air temperature around electrical equipment. Critical parameter for determining rated current and thermal performance.
Arc Flash
A dangerous electrical explosion caused by fault conditions. Arc flash hazard analysis is required for workplace safety compliance.
Automatic Transfer Switch (ATS)
A device that automatically switches power supply from primary source to backup source during outages.
B
Box Substation
Also known as compact substation. A pre-engineered, pre-assembled enclosure containing transformer, switchgear, and protection equipment.
Breaking Capacity
The maximum fault current that a circuit breaker can safely interrupt. Expressed in kA (kiloamperes).
Busbar
A conductive bar used to distribute electrical power within switchgear panels. Typically made of copper or aluminum.
C
Circuit Breaker
An automatic switch designed to protect electrical circuits from damage caused by overload or short circuit. Can be manually operated.
Contactor
An electrically controlled switch used for switching a power circuit. Designed for frequent operation, typically used for motor control.
Current Transformer (CT)
A device that produces an alternating current proportional to the current in its primary winding. Used for measurement and protection.
D
Differential Protection
A protection scheme that compares current entering and leaving a protected zone. Detects internal faults quickly.
Disconnector
Also called isolator. A switch that provides isolation of electrical equipment for maintenance. Does not break load current.
Draw-out Switchgear
Switchgear where breakers can be physically withdrawn from the cubicle for maintenance without removing panel wiring.
E
Earthing Switch
Also called grounding switch. A switch that connects equipment to ground for safety during maintenance.
Enclosure
The outer shell of switchgear that provides mechanical protection and contains arc gases during faults.
F
Fixed Panel
Switchgear where the circuit breaker and other components are permanently installed and cannot be withdrawn.
Fuse
A protective device that contains a metal strip which melts when current exceeds safe levels, breaking the circuit.
G
GIS (Gas Insulated Switchgear)
Switchgear using sulfur hexafluoride (SF6) gas as insulation. Compact design for high voltage applications.
Ground Fault
An unintended connection between electrical system and ground. Also called earth fault.
I
IEC Standards
International Electrotechnical Commission standards governing electrical equipment. Key standards: IEC 62271 for HV, IEC 61439 for LV.
Impulse Withstand Voltage
The maximum voltage that equipment can withstand from lightning surges. Expressed in kV.
Interlock
Mechanical or electrical device that prevents incorrect operating sequences, such as opening a breaker before closing grounding switch.
IP Rating
Ingress Protection marking. Defines level of protection against solids and liquids (e.g., IP54, IP65).
K
KYN
A type designation for metal-clad withdrawable switchgear according to Chinese standards GB. Common models: KYN28A, KYN61.
L
Load Break Switch
A switch capable of making and breaking load current. Does not have fault current interruption capability.
LV (Low Voltage)
Electrical systems operating below 1000V AC. Common distribution voltage is 400V (0.4kV).
M
MCB (Miniature Circuit Breaker)
A thermal-magnetic circuit breaker for low current applications. Common in domestic and commercial installations.
MCCB (Molded Case Circuit Breaker)
A circuit breaker housed in a molded plastic case. Used for higher current ratings than MCBs, typically up to 1600A.
Metal-Clad
Switchgear with metal enclosures that provide segregation between all functional units and compartments.
MV (Medium Voltage)
Electrical systems typically from 1kV to 35kV. Common voltages: 12kV, 24kV, 40.5kV.
O
OEM (Original Equipment Manufacturer)
A company that manufactures products based on specifications provided by another company for rebranding.
Overcurrent Protection
Protection against currents exceeding the rated value. Includes overload and short-circuit protection.
P
Power Factor Correction
Improving power factor by adding capacitors. Reduces reactive current and avoids utility penalties.
Protection Relay
A device that detects abnormal conditions and trips circuit breakers to isolate faults.
PT (Potential Transformer)
Also called voltage transformer (VT). Steps down voltage for measurement and protection equipment.
R
Rated Current
The maximum current that equipment can carry continuously under specified conditions without exceeding temperature limits.
Rated Voltage
The voltage level for which equipment is designed to operate. Determines insulation level and clearances.
RMU (Ring Main Unit)
A type of distribution switchgear used in ring main configurations. Typically includes load break switches and fuses/breakers.
S
SCADA
Supervisory Control and Data Acquisition. Remote monitoring and control system for electrical substations.
SF6 (Sulfur Hexafluoride)
An insulating gas with excellent dielectric properties used in high voltage switchgear. Has high global warming potential.
Short Circuit
An unintended low-resistance connection between two points in an electrical circuit, causing high fault currents.
Surge Arrester
Also called lightning arrester. A device that limits voltage transients caused by lightning or switching.
T
Three-Position Switch
A switch with three positions: Close, Open, and Earth (Ground). Common in RMU applications.
Type Test
Tests performed to verify that switchgear meets all requirements of applicable standards. Conducted on prototype equipment.
V
VCB (Vacuum Circuit Breaker)
A circuit breaker where the arc interruption occurs in a vacuum chamber. Low maintenance and long life.
Voltage Transformer (VT)
Also called potential transformer (PT). Steps down high voltage for metering and protection equipment.
W
Withdrawable Breaker
A circuit breaker that can be removed (withdrawn) from its cubicle for inspection and maintenance without disconnecting panel wiring.
Withstand Current
The maximum short-circuit current that equipment can endure thermally and mechanically without damage.