Switchgear generally refers to the switching devices and their combinations, with the related control, measuring, protective and regulating equipment. The switchgear equipment basic purpose is switching and interrupting currents either under normal or abnormal operating conditions. Modern power systems, power lines and other apparatus operate at high voltages and carry large currents. When a short circuit occurs on the system, heavy current flowing through the equipment may cause considerable damage. To interrupt such heavy fault currents, we need switchgear; in this case, circuit breakers are employed.

Switchgear Terminology and Standards

Switchgear Terminology

In the next sections we look at various types of switchgear employed in different applications:

Circuit breaker

This is a mechanical switching device, capable of making, carrying and breaking currents under normal circuit conditions, and additional making, carrying for a specified time and breaking currents under specified abnormal circuit conditions for instance in case of short circuit. It is designed in such way that it can be operated manually (or by remote control) under normal conditions and automatically under fault conditions.

A circuit breaker is intended to switch both load and short circuit currents. Unlike a fused device, it enables supplies to be swiftly restored after operation on short circuit and is the most costly form of switchgear. It is not primarily intended for frequent operation however vacuum and SF₆ breakers are more suited to load switching duties than older switchgear types.

Moulded Case Circuit Breaker (MCCB)

A circuit breaker having a supporting housing moulded insulating materials forming an integral part of the circuit breaker.

Contactor  

This is a mechanical switching device having only one position of rest, operated otherwise than by hand; capable of making, carrying and breaking currents under normal circuit conditions including operating overload conditions.

A contactor is designed for frequent operations however it has a limited short current carrying and switching capability. For this reason, it is often backed up by fuses or a circuit breaker.

Disconnector

This is a mechanical switching device which provides, in the open position, an isolating distance in accordance with the specified requirements.

A disconnector is intended to open or close a circuit under negligible current conditions or when there is no significant voltage change across the terminals of each of its poles. It is capable of carrying rated current under normal conditions and short circuit through currents for a specified time.

Disconnector is also referred to as a no-load isolator and in this regard it can be used as follows:

• Off-circuit isolator: capable of switching ‘dead (non-energized) circuits only.
• No-load isolator: capable of switching under ‘no load’ (negligible current flow) conditions only.

A disconnector has a very limited current switching capability and is not intended for frequent use or for breaking full load current.

Switch

A mechanical switching device capable of making, carrying and breaking current under normal circuit conditions; this may include overload and short-term, short circuit current conditions. Generally a switch is used for switching loads but is not appropriate for frequent operation. Switches may be manual or motor operated and have a short circuit current-making capability but no breaking capability and thus must be used in combination with a short circuit interruptions device normally fuses. Where the fuse and switch are in combination in series, the unit is referred to as a switch fuse; where the fuse forms part of the moving contact of the switch, it is called a fuse switch.

Fuse Switch

As aforementioned, this is a switch in which a fuse link or a fuse carrier with a fuse link forms the moving contact.

Earthing Switch

This is a mechanical switching device for earthing parts of a circuit. It is capable of withstanding for a specified period current under abnormal conditions such as those of a short circuit, but not required to carry current under normal circuit conditions. An earthing switch may have a short circuit making

capacity, either to act as a “fault thrower” at the end, for example, of a long distribution feeder or to provide for inadvertent operation of a live circuit to earth.

IEC Switchgear Standards

Some of the IEC standards about switchgear are covered in the table below:

IEC Standard	Title	Notes
60038	IEC standard voltages.	Applies to AC transmission and distribution with standard frequencies of 50 and 60 Hz and nominal voltages above 100 V.
60050-441	International electrotechnical vocabulary; switchgear, controlgear and fuses.
60059	IEC standard current ratings.
60060	High-voltage test techniques.	Various parts, covering definitions, test requirements and measuring systems.
60296	Fluids for electrotechnical applications – unused mineral insulating oils for transformers and switchgear.
60364-5-53	Electrical installations of buildings – part 5-53: Selection and erection of electrical equipment – isolation, switching and control.	Low voltages less than 1 kV ac.
60376	Specification of technical grade sulphur hexafluoride (SF6) for use in electrical equipment.	Properties, methods and tests for new and unused SF6.
60427	Synthetic testing of high-voltage alternating current circuit breakers.	Applies to circuit breakers covered by IEC 62271-100. General rules for circuit breaker testing.
60466	AC insulation-enclosed switchgear and controlgear for rated voltages above 1 1 kV and up to and including 38 kV.
60470	High-voltage alternating current contactors and contactor-based motor starters.	Includes conditions for compliance with operation, behavior and dialectic properties and associated tests. Applicable to vacuum contactors.
60480	Guide to the checking of sulphur hexafluoride (SF6) taken from electrical equipment.	Checks for the condition of SF6.
60529	Degrees of protection provided by enclosures.	IP categorization.
60694 (supersedes IEC 1208)	Common clauses for high-voltage switchgear and controlgear standards.	Essential cross reference when reading IEC 62271.
60859	Cable connection for gas insulated metal-enclosed switchgear for rated voltages of 72.5 kV and above.	Complements and amends other switchgear and cable termination standards.
60947 (supersedes IEC 157)	Low-voltage switchgear and controlgear.	Applies to equipment intended to be connected to circuits operating at rated voltages less than 1 kV. Standard is broken down into several parts.
61958	High-voltage prefabricated switchgear and controlgear assemblies – voltage presence indicating systems.
62271-100 (supersedes IEC 56)	High-voltage switchgear and controlgear – high-voltage alternating current circuit breakers.	Applicable to indoor and outdoor circuit breaker installations up to and including 60 Hz on systems having voltages above 1000 V. Covers operating devices and auxiliary equipment.
62271-102 (supersedes IEC 129, IEC 1128 and IEC 1129)	High-voltage switchgear and controlgear – alternating current disconnectors and earthing switches.	Indoor and outdoor installations. Also covers operating devices and auxiliary equipment.
62271-105 (supersedes IEC 420)	High-voltage switchgear and controlgear, alternating current switch fuse combinations.	Applies to combinations for use on three phase distribution systems at rated voltages between 1 and 52 kV. Functional assemblies of switches including switch disconnectors and current limiting fuses thus able to interrupt: Load breaks up to rated breaking current. Overcurrents up to rated short circuit breaking current of the combination by which automatic interruption is initiated.
62271-107 (supersedes IEC 420)	High-voltage switchgear and controlgear; alternating current fused circuit-switchers for rated voltages above 1 kV up to and including 52 kV.
62271-200 (supersedes IEC 298)	High-voltage switchgear and controlgear. AC metal-enclosed switchgear and controlgear for rated voltages above 1 kV and up to and including 52 kV.	Service conditions, applicable terms and rated characteristics. Rules for design and construction, type and routine tests. General information on selection of devices, tenders, transport, erection and maintenance. Supplements IEC 60694.
62271-203 (supersedes IEC 517)	High-voltage switchgear and controlgear gas insulated metal-enclosed switchgear for rated voltages above 52 kV.	Covers the requirements for SF6 switchgear.

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