Switchgear technology - Withdrawable switchgear – the Return
The popularity of withdrawable switchgear declined considerably in the UK during the
last ten years, but has recently been making a comeback. However, its rise in popularity has raised concerns about compliance with new IEC regulations. Steve Goldspink, Siemens Transmission and Distribution, outlines the new regulations and tells the untold story of the return of withdrawable switchgear to the UK market
For the last decade, the UK switchgear industry has focused on fixed pattern switchgear. This being despite the fact its alternative - withdrawable switchgear - has a reputation for being, where required, much easier and safer to repair and maintain; particularly in process orientated environments where power outages can be extremely costly. In recent times however, withdrawable switchgear has made a comeback as the industry has begun to rediscover, understand and appreciate its benefits.
This new switchgear, now available, is not vastly different technology but changes in technical requirements of users based on operational safety, service continuity and maintenance needs have driven subtle change.
In order to keep abreast with these changes, the International Electrotechnical Commission (IEC), a leading global organisation that prepares and publishes international standards for all electrical and electronic related technologies, has developed a new standard for medium voltage switchgear - IEC 62271-200.
IEC 62271-200 is a standard for AC metal-enclosed switchgear and control gear for rated voltages above 1 kV and up to and including 52kV. There are a number of differences between the previous and new standard. Contrary to its predecessor, IEC62271-200 no longer classifies switchgear according to design features but on the basis of functional characteristics. It demands a detailed description of the characteristics concerning the aspects of service continuity, maintainability and safety classifications, which are all of prime importance to the user.
Introduced in 2003, it is a legal requirement for all switchgear built after 1 February 2007 to satisfy this standard. IEC 62271-200 supersedes the previous standard for medium voltage switchgear - IEC 60298 and aims to remove some of the ambiguities in this standard by means of classifying switchgear.
An increasing concern of mine, however, is a number of medium voltage switchgear manufacturers in the power transmission and distribution industry may be deterred from putting their equipment through the appropriate testing procedures to ensure it meets this standard, due to the significant investment required in doing so. Whilst accurate statistics are almost impossible to source, it is almost certainly the case that some switchgear being sold new into the UK market has not been tested to the new standards, and is therefore unlikely to be fully compliant with the new standard. In short, the ultimate benefit of the new IEC 62271-200 standard is to provide improved classifications regarding the levels of operator safety, service continuity and maintainability - features that should not be overlooked.
The new switchgear utilises well established, high availability maintenance-free vacuum circuit breaker technology, with operating cycles far exceeding the normal number, meaning frequent access to the circuit-breaker is no longer an ongoing concern for the switchgear operators. Additionally, the vacuum arc-quenching principle in modern switchgear is technologically superior to other arc-quenching principles currently employed within the industry.
Furthermore, in line with the latest requirements of the new standard, to ensure maximum operational and functional safety, the circuit breaker and earth switches are fully type tested inside the appropriate switchgear panel and not as a standalone device to ensure all functional influences are taken into account.
One particular area where safety is of critical importance is internal arc tested equipment, an issue which is gaining widespread awareness across the world due to operator safety concerns. An internal arc is a high resistance arc fault within the switchgear enclosure due to disruptive discharges between phases or phase to earth. Internal arcs can be created by a variety of causes including insulation failures, functional faults of devices and even negligence during routine operation and maintenance. The result however can be catastrophic, in that the arc reacts explosively with the surrounding atmosphere, causing a rapid increase in temperature and pressure which, if uncontrolled, can be extremely hazardous to people in the immediate area.
With the old IEC standard, much room was left for different ways of carrying out the test and interpreting the results. Although still an optional test in IEC 62271-200, the new standard gives clear guidance on how to perform the internal arcing test and defines the acceptance criteria. More specifically, test conditions are defined and are no longer subject to agreement between the equipment manufacturer and the test laboratory. Ultimately, internal arc classification is only granted if all criteria are met.
In addition, when an internal arc classification is selected, all five internal arc criteria must be fulfilled without exception. Firstly, covers and doors on the switchgear must remain closed, with limited deformations accepted. Secondly, no fragmentation of enclosure must occur, with zero projection of small parts above 60g in weight. There must also be no holes in the accessible sides of the switchgear up to a height of two metres and the horizontal and vertical indicators used in testing must not ignite due to the effect of hot gases. Finally, the enclosure must remain connected to its earthing connections at all times.
The key now must be for manufacturers of switchgear equipment to take their responsibilities seriously and ensure all new switchgear being produced goes through the appropriate testing where required, and is re-classified in line with the new IEC standard.
This is even more important given there is a general skills shortage facing the industry. We are now in a situation where more and more purchasers of switchgear equipment are either outsourcing their operations because of a lack of skilled engineers, or being forced to employ a smaller team of engineers to carry out the work. Both of these factors make health and safety even more important in order to protect those engineers operating and maintaining switchgear equipment.
My advice to end-users is to demand the switchgear they are purchasing meets the latest testing criteria and all switchgear used has been classified in accordance with IEC 62271-200. This is the only way that users of switchgear in industrial applications can have the peace of mind of being able to select products which utilise the latest developments in technology, safety, service continuity and reliability.
Withdrawable switchgear is making a welcome return to the UK market with process industries benefiting from easier maintenance and repair, which in turn can result in much reduced downtime. However, any move to withdrawable switchgear should only be contemplated when maximum operator safety can be guaranteed. In effect this means selecting equipment type-tested according to IEC 62271-200.
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