Arc flash accidents are not nearly as rare as is often imagined, and the hazards they create are severe. In wind turbines, these hazards are further magnified. Here, Stuart Greenwood of Eaton’s Electrical Sector explains why and examines ways in which the perils of arc flash accidents can be minimised
Almost everyone who works with electricity has heard of arc flash accidents and many know that such accidents can be extremely dangerous. Ask them, however, how common these accidents are and most will say that they are exceptionally rare. Unfortunately, that’s not a particularly accurate assessment. In fact, EU statistics show that between eight and ten arc flash accidents occur in the UK every week.
Clearly, arc flash is not a phenomenon that anyone can afford to ignore, least of all as we shall see, those who work on wind turbines. But what exactly is an arc flash accident and how do such accidents occur?
In simple terms, an arc flash accident happens when a large electrical current passes through ionised air and gasses. These accidents can be triggered in many ways, but examples are when a conductive tool is dropped across live busbars while maintenance is being performed, or when a circuit breaker fails during a switching operation. Statistics show that arc flash accidents most commonly occur in LV systems.
Arc flash accidents produce dramatic results. Almost instantaneously, the temperature near the arc increases by around 20,000ºC. This vaporises copper conductors and, since the volume of the vapour produced is around 67,000 times that of the metal, a violent explosion results. Secondary effects include the expulsion of globules of molten copper, an intensely bright flash of light and the generation of pressure waves.
It’s easy to see that when such an accident occurs, there is a very high risk that any person in the vicinity will be severely injured or even killed. Further, the equipment in which the fault occurs will usually be damaged beyond repair.
Now consider the situation in a wind turbine. The immediate effects of the arc flash accident are unchanged, but dealing with the consequences is much more difficult. For example, getting medical treatment to a badly injured person is no trivial matter if they are working in a nacelle tens of metres above ground or sea level. And rescuing the injured person is likely to prove equally challenging, especially as many wind farms are in remote locations or even offshore.
The consequences of equipment damage are also exacerbated. While replacing a damaged switchboard in building is no easy task, replacing switchgear in a turbine nacelle is even more difficult and even more costly. And it’s also necessary to think about the loss in revenue resulting from the wind turbine’s inability to generate electricity until repairs have been carried out – which could be weeks or even months.
With all of this in mind, it’s unsurprising that arc flash accidents are attracting a lot of attention not only from electrical engineers, but also from health and safety professionals. But what can realistically be done to minimise the occurrence and impact of these events?
The first step is to try to eliminate the conditions under which arc flash accidents can occur, ideally by not working on live equipment. Indeed, Regulation 14 of the Electricity at Work Regulations makes it clear that live working should never be accepted as the norm, and that it must only be sanctioned under specific conditions, one of which is that suitable precautions must be taken to prevent injury.
It’s worth bearing in mind, however, that arc flash accidents are not limited to situations where live working is being carried out intentionally. An arguably greater risk relates to those situations where an “isolated” system is accidentally made live while it is being worked on.
In truth, there is no 100% certain way of completely eliminating arc flash accidents, although the risk can be greatly reduced by specifying switchgear that features insulated arc-free busbar assemblies. Unfortunately, such switchgear is larger than conventional equipment and is, therefore, unlikely to find favour in wind turbine applications.
One way of minimising risks is to provide those working on electrical equipment with suitable personal protective equipment (PPE). Indeed, the Protective Equipment at Work Regulations 1992 make it mandatory for employers to provide PPE if a risk assessment indicates that it is needed. The same regulations make it very clear, however, that the use of PPE is a last resort because it does not control the problem at source, and it protects only the wearer.
In the case of arc flash, these are serious limitations, as PPE does nothing to prevent equipment damage, nor does it provide any protection for persons who may be in the vicinity when an arc flash accident happens but are not wearing PPE. The conclusion has to be, therefore, that PPE makes a valuable contribution to minimising arc flash hazards, but on its own, it is not enough.
So what can be done? An increasingly popular and effective option is the use of electrical/electronic systems that quench the arc produced in an arc flash accident so fast that there is no time for it to unleash its destructive power. The time scales involved are very short – the pressure peak is reached just 10mS after the arc is initiated and the temperature peak is reached 5mS later. To be effective, therefore, quenching must take place in 5mS or less.
No circuit breaker can clear a fault that fast, so a different approach is used: a bolted short-circuit is placed across the supply within milliseconds of the arc being initiated. This effectively ‘robs’ the arc of the energy it needs to develop and become dangerous. The upstream circuit breaker will subsequently trip and clear the fault within about 50mS.
Devices that can place what is, in effect, a bolted fault across the supply within as little as 2mS of being triggered are now available, and these devices form the basis of an effective, affordable and dependable arc fault suppression system for LV switchgear.
They are used in conjunction with a high-speed detector that reacts to the arc in its very earliest stages. In the Eaton Arcon system, for example, which has recently been confirmed by the IPH testing institute in Berlin as an effective choice for use in wind turbine applications, the detector comprises a photoelectric system with a flexible fibre-optic cable that can be routed through all of the areas of the switchgear where an arc fault could develop.
This cable detects light over its entire length and responds to the characteristic light of an arc by sending a signal to a logic module that also monitors current. Only if the photodetector signal is accompanied by a rapid increase in current is the short-circuiting device triggered - an arrangement that ensures reliable response to real faults with freedom from false operation.
No method of preventing arc flash accidents can ever be described as totally effective, but suppression systems of the type described come much closer than any alternative approach and have the outstanding benefits of protecting everyone in the vicinity, not just the persons wearing PPE. In addition, they virtually eliminate equipment damage - in most cases, switchgear fitted with this type of suppression system require little or no remedial work before it is returned to service after an arc fault event.
These benefits are important in every application, and in wind turbine applications they might well be considered indispensible. The number of wind power installations will undoubtedly continue to grow at a rapid rate, and arc fault suppression is, without doubt, an important tool for enhancing their reliability and the safety of those who work on them.