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Electrical Safety Offshore

2 November 2011

In the oil and gas sector, safety, reliability and efficiency are key concerns. Of these, safety is, of course, paramount. Offshore safety, however, is not simply about protecting the environment; effective protection of personnel and equipment is also crucial. In electrical installations specifically, one of the most serious hazards is arc flash. Eaton has, however, developed technologies and philosophies that allow this hazard to be tamed.

Thankfully, arc flash accidents, which happen when a large electrical current passes through ionised air, are rare. When they do occur, however, the consequences are invariably serious, disruptive and costly. In an arc flash accident, the temperature in the vicinity of the arc rises almost instantly to around 20,000ºC. Copper conductors are vaporised, leading to an explosion that expels molten globules of copper.

The equipment in which the arc flash occurs is invariably severely damaged or even destroyed and anyone in the vicinity of the event is at high risk of injury or even death. The direct costs are high but in the offshore sector in particular, the indirect costs of lost production can be enormous.

Arc flash accidents can be triggered in many ways, for example when a circuit breaker fails during a switching operation, or when insulation suffers a catastrophic breakdown. In reality, the risk of arc flash accidents can never be eliminated entirely. Eaton has, however, made substantial investments of time and money over many years into finding ways in which this risk can be greatly reduced. As a result, the company now holds a well-established position as a world leader in the field, and offers more than 40 products and solutions for arc-flash prevention and control.

Eaton has adopted the practice of designing its switchgear from the outset to minimise arc-flash hazards. This has led it to adopt moulded insulation for its MV products, which allows excellent control to be achieved over electric fields, thereby greatly reducing the susceptibility of the insulation to breakdown. The company also makes extensive use of vacuum switching elements in its MV switchgear. As these are not susceptible to the effects of contaminants and moisture, and because they require no maintenance, they virtually eliminate the risk of failures during switching operations.

A further important benefit of using moulded insulation and vacuum interrupters is that the switchgear has minimal environmental impact during its life and also at the time of its eventual disposal. This contrasts strongly with switchgear that uses SF6 as an insulating agent, as this gas is a well-known agent of environmental damage.

For low-voltage systems, Eaton has developed its Arcon system, which adopts an active approach to minimising the effects of arc-flash accidents. Arcon places bolted short-circuit across the supply within milliseconds of an arc being detected. The energy that would otherwise feed the arc is therefore diverted to the short circuit, so that the arc never has a chance to develop. The upstream circuit breaker will subsequently interrupt the supply in about 50ms.

Arcon uses a device that puts the bolted short in place within around 2ms of being triggered, and a detection system that is capable of reacting to an arc in its very earliest stages. The detection system uses a photodetector 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 flash of an arc, it sends a signal to a logic module that also monitors current. If the photodetector signal is accompanied by a rapid increase in current, the module triggers the short-circuiting device, and the arc is suppressed.

This arrangement is 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.

As was made clear earlier, however, even the best design and most innovative technology cannot entirely eliminate the risk of arc-flash accidents. Because of this, Eaton offers a comprehensive assessment service for electrical installations, designed to ensure that all major risks, including arc flash, are properly assessed and that appropriate protective measures are put in place.

The company’s arc-flash hazard assessments are carried out by experts, and can be expected to produce recommendations covering arc-flash boundaries, safe working distances, practical methods of hazard reduction, personal protective equipment (PPE), and safe working practices.

Electrical equipment is at the heart of every offshore operation and, provided it is specified and installed correctly, there is a high probability that it will operate reliably and safely throughout its life. As we have seen, however, there are some hazards, such as arc-flash accidents, that can never be completely eliminated.

Nevertheless, Eaton, with its wide experience of the issues involved and of the special challenges posed by offshore applications, can reduce the risks to a minimum, thereby providing the best possible protection for personnel, plant and productivity.

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