Taming arc flash in LV switchboards
EU statistics reveal that an estimated 400 to 500 arc flash accidents occur in the UK each year, typically resulting in four or five deaths. Colin McAhren of Eaton’s Electrical Sector believes that new technology means many, if not all, of these accidents and deaths are now preventable.
An arc flash accident in a switchboard occurs when a large electrical current passes through ionised air and gasses. Such accidents can be triggered in many ways, but examples are when a metallic tool is dropped across live busbars during maintenance or when a circuit breaker fails during a switching operation. Statistics show that the majority of arc flash accidents relate to LV installations.
The effects of arc flash are dramatic. Almost instantaneously, the temperature rises by around 20,000 ºC in the vicinity of the arc. This vaporises copper conductors and, since the volume of the vapour is 67,000 times that of the metal, an explosion results. Secondary effects include the expulsion of molten globules of copper, an intensely bright flash of light and the generation of pressure waves.
Any person in the vicinity of an arc flash accident is at high risk of being injured or killed. It’s clear, therefore, that every effort must be made to eliminate these accidents or at least to minimise their effects. The best approach is to avoid the conditions under which arc flash accidents are most likely to occur, ideally by never working on live equipment. Sometimes, however, live working cannot be avoided and there is always the possibility of “isolated” systems being made live accidentally.
A solution often proposed is to equip maintenance workers with personal protective equipment (PPE). Given the range of hazards associated with arc flash accidents, a risk assessment is likely to indicate that the PPE should include a full-body protective suit, substantial gauntlets and a helmet with an integral face shield. This may provide adequate protection, but it doesn’t make it easy for the wearer to work on electrical installations!
Another possibility is to design the switchboard so that arc flash accidents cannot occur, by incorporating features such as insulated arc-free busbar assemblies. Switchboards in Eaton’s Capitole 40 arc-free range do, in fact, take this approach, but they are not an appropriate choice in every application. They are, for example, considerably larger than equivalent switchboards of conventional design.
Fortunately, there is now another solution that, despite its novelty, has already been well proven in practice. This is based on quenching the arc before it can develop its destructive potential. The time scales are very short – the pressure peak is reached just 10 ms after the arc is initiated, and the temperature peak is reached just 5 ms later, so quenching must take place in 5 ms or less.
No circuit breaker can clear a fault this fast, so a more radical approach is needed. That is to place a bolted short-circuit across the supply within milliseconds of an arc being detected. What happens then is that the energy that would otherwise feed the arc is diverted to the short circuit, so that the arc never has a chance to develop. The upstream circuit breaker will subsequently clear the short circuit fault in about 50 ms.
Devices that can place what is, in effect, a bolted short circuit across the supply within around 2 ms of being triggered are now available, and are central to the operation of Eaton’s Arcon arc control system for switchboards. These short circuiting devices must, of course, be used in conjunction with a trigger capable of reacting to an arc in its very earliest stages.
This can be implemented using a photo-detector comprising a flexible fibre optic cable that is routed through all the areas of the switchboard where an arc fault might occur. The cable detects light over its entire length and, when it sees the characteristic light of an arc, it sends a signal to a logic module that also monitors current. If the photo-detector signal is accompanied by a rapid increase in current, the module triggers the short-circuiting device instantly, and the arc is suppressed.
This arrangement has proved to be reliable and effective. Not only does it virtually eliminate the risk of arc flash injuries, it also minimises damage to the switchboard, allowing it to be returned to service rapidly and cost-effectively. Further, the arc control system adds little to the size of the switchboard and allows the use of conventional busbars and components in its construction.
Arc flash is a significant hazard in LV installations. As we have seen, there are a number of ways this hazard can be addressed, but most have significant drawbacks or limitations. The latest arc control systems are however exceptions, as they provide solutions that are dependable, convenient and effective.
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