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Understanding switchgear01 September 2007Craig Diack-Evans takes us back to basics on the subject of switchgear, its various forms and functions, and reminds us about the latest regulations and standards that govern the selection and installation of this essential equipmentWhat is switchgear? Simply put, it is a collection of electrical equipment connected together to perform certain functions within an electrical system. One official definition is: 'Any switching and interrupting devices in combination with their associated control, regulating, metering, and protective devices'.
Switching and interrupting devices include disconnectors (also known as 'isolators'), which provide safe isolation of a circuit, and fuses, as the melting element of a fuse is used to 'break' an electrical circuit when the current exceeds the rated value of the fuse.
Also, load-breaking switches are used to open/close loaded circuits under normal fault-free conditions. Circuit breakers are also used, for control, protection and isolation; and to open/close loaded circuits under normal conditions and under fault conditions.
A remote control switch - a switch that can be remotely operated by another device such as a push button - is also featured; as is a contactor, which is a solenoid-operated device, designed to carry out numerous open/close operations.
Switchgear provides electrical protection, ensuring protection of circuit elements against the effects of short-circuit, protection of persons in the event of failures, and protection of equipment connected to the system. Switchgear also provides safe isolation of equipment, as it separates a circuit or equipment from the rest of the system, so that maintenance can be carried out safely.
Local or remote switching is also provided, with control allowing system operators to modify a loaded electrical system at any given moment, according to requirements. Switchgear is also responsible for distribution of electrical energy. However, only circuit-breakers can provide all of these basic functions.
Switchgear is located at any point where isolation, protection and control is required. For example, it can be located at generators, motors, transformers, substations and so on. It can be high voltage or low voltage. Typically, in substations, switchgear will be utilised on both the high and low voltage sides of power transformers. Low voltage switchgear is normally located within a metal cubicle structure in a building. High voltage (>66kV) switchgear is traditionally installed outdoors, insulated by air (AIS) - but more recently the trend has been towards gas insulated switchgear (GIS), installed indoors using a fraction of the space required for AIS.
Switchgear is usually selected by experienced circuit designers with the aim of ensuring satisfactory performance, compatibility with other switchgear in the electrical system and conformity with applicable regulations. Understanding the performance and function of each individual circuit is critical to making the right choice, and sometimes designers choose to use software simulations of circuit performance to explore possible configurations. Switchgear with multiple functions, whilst initially more costly (more components/materials and increased test/design costs), reduces installation costs and is considered to provide the best solution.
The Low-Voltage Directive (LVD) is applicable in Europe (including the UK) and this requires all electrical equipment to be safe in its intended use. As low-voltage switchgear has a basic safety function it must not only be safe to use, but must also be capable of performing its safety-related functions when called upon to do so - in effect, a double responsibility. There are no applicable regulations for high voltage equipment.
Potential hazards listed in the LVD include direct and indirect contact with live parts (shock), dangerous temperatures, arcs or radiation, and overloading. Additionally, other dangers could result from both insulation and mechanical failures, expected environmental conditions, and non-electrical dangers caused by the assembly.
There is, of course, the proviso that an assembly is used in the application for which it was made, and that it is properly installed and maintained. Significantly, the Directive declares that its safety requirements are deemed to be met by electrical equipment that satisfies the safety provisions of harmonised standards. Whilst there are several routes to demonstrating compliance with the LVD, the most widely used is via conformance to appropriate Mandated Standards.
Assemblies conforming to IEC 60439-1 are deemed to satisfy the essential safety requirements of the LVD and can be CE-Marked accordingly. From January 1997, all new assemblies intended for use in Europe had to be marked thus. In the United Kingdom, the 'Electricity at Work Regulations 1989', further enforces the requirements for equipment to operate in a safe manner. There are similar regulations in most developed countries.
For low voltage industrial applications, the most applicable switchgear standard is IEC 60439 titled 'Low-voltage switchgear and controlgear' with additional parts covering the following:
Part 1: Assemblies
Part 2: Distribution boards
Part 3: Assemblies for construction sites
Part 5: Assemblies for power distribution in public networks
'Built-in' components are generally covered by IEC 60947, also titled 'Low-voltage switchgear and controlgear'. Key sections include:
Part 1: General rules
Part 2: Circuit breakers
Part 3: Switches, disconnectors, switch-disconnectors and fuse combination units
Part 4: Contactors and motor starters
Part 5: Control-circuit devices and switching elements
Similarly, for high voltage, the new IEC 62271-200 covers Metal enclosed switchgear and controlgear and IEC 62271 the built-in components, including:
Part 100: Alternating-current circuit breakers
Part 102: Alternating current disconnectors and earthing switches
Part 105: Alternating current switch-fuse combinations
There are three different ways of demonstrating conformity of switchgear to these standards. The first option is to obtain an official mark of conformity issued by a Certification Body. This is often referred to as a certification mark or approval mark (and requires complete compliance testing and inspection of the manufacturing, subsequent inspection and test processes at the place of manufacture). This option is most suitable for products made in large quantities.
The second way is to obtain a certificate of conformity issued by a Certification Body. This is also referred to as a type-test certificate (less than complete compliance testing). This option is more suitable for larger products that are made in smaller quantities. It is also possible to make a self-declaration of conformity (made by the manufacturer).
The most common choice is the type-test certificate and this can typically include verification of temperature-rise, dielectric/impulse, short-circuit, effectiveness of the protective circuit, clearances and creepage distances, mechanical operation, degree of protection, internal arcing tests and so on.
If you are seeking a Type-Test Certificate, the author's advice is to contact a third-party testing and certification body that specialises in type-test certification. They will be able to conduct the appropriate tests on your behalf to meet your needs.
Craig Diack-Evans is a consultant with ASTA BEAB, now part of Intertek. Contact Details and Archive...Most Viewed Articles... |
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