The energy released in an arc flash incident is directly proportional to arcing time. Even a few milliseconds improvement may shift hazard levels and PPE requirements to lower categories. For arcing times
of 40 ms or less there is typically no personal injury or switchgear damage, while arcing times of 500 ms or more will cause serious personal injury and major damage
Arc flash is not a new phenomenon, but interest and concern about the dangers associated with arc flash events have increased dramatically in recent years. This is largely due to new guidelines and standards put forward by the various international trade and safety bodies such as the Institute of Electrical and Electronic Engineers (IEEE) and the US Occupational Safety and Health Administration (OSHA).
Dedicated arc flash protection
An arcing fault instantaneously releases large amounts of radiant light and thermal energy. This intense light is used in arc flash detection relays to achieve faster operating times than is possible with conventional relaying.
Arc flash detection systems are stand-alone protection systems and do not need to be coordinated with existing protection systems. Therefore, it is not necessary to delay tripping for coordination with other protection.
Arc flash relays have been around since the early 1990s. However, first generation arc flash protection used only single-point lens sensors. In this type of system, one or more lens sensors are located in each high-voltage compartment where a potential arc flash might occur.
From around 2000 onwards, ABB introduced the new generation REA series of arc flash detection relays. This system uses a radically different type of light sensor: a long unclad fibre-optic sensor that can absorb light throughout its length.
There are several advantages to the REA’s fibre-optic sensor technology. First, it dramatically reduces the cost of installation. A single optical fibre sensor can be as long as 60 metres, typically covering the same protection zone associated with conventional bus differential protection, but at much lower cost than lens sensors. Second, any concerns about shadows from internal structures that might block the direct exposure to an arc flash are eliminated. Third, if the fibre sensor is configured in a loop, it can provide regular self-checking of the sensor’s integrity and continuity, and generate an alarm if a problem is detected.
In normal operation, both light and over-current must be present simultaneously for tripping to occur. Detection of an intense light alone will not result in a trip unless the system is intentionally set to operate that way (which is not recommended).
Over-current settings allow different current threshold levels for phase and ground fault currents. High-speed insulated gate bipolar transistors (IGBTs) are used to provide two fully trip-rated outputs rather than relatively slow conventional dry contacts. The overall tripping time is a miniscule 2.5 ms.
Many thousands of REA systems utilizing long fibre-optic sensor technology have been installed worldwide. And there have been at least two documented cases where the REA concept has prevented serious damage to medium voltage switchgear following an arc flash incident.
Optical arc flash relays are ideally suited to modern vacuum and SF6 breaker technologies where the fault interruption takes place inside a sealed container. However, with proper precautions, an optical relay may also be applied to air magnetic breakers.
DNV GL, the leading marine certification body, recently gave its approval to ABB’s REA 10 arc flash monitoring systems for ship and offshore electrical systems.
There has been growing awareness of the risks posed by arc flash, particularly in the oil and gas sector, in response to new guidelines and standards developed by various international trade and safety bodies including the IEEE (Institute of Electrical and Electronic Engineers), the US Occupational Safety and Health Administration (OSHA) and Lloyds Register.
Offshore operators are now required to carry out studies to identify risk from arc flash and put protective measures in place. One approach is to supply and renew specialist PPE on an annual basis, but operators could instead install ABB’s REA 10 system, which eliminates risk by reducing tripping times.
Boosting safety at South Humber Bank
Centrica Energy has improved its HSE (Health, Safety and Environment) performance at its South Humber Bank power station by retrofitting REA 10 arc flash protection systems to the site’s medium-voltage power distribution network.
The combined cycle gas turbine plant in north-east Lincolnshire has an output of almost 1.3 MW. By retrofitting the site’s existing ABB medium-voltage switchgear with REA 10 arc flash protection relays, Centrica has minimised the risk of arc flash incidence from the 11 kV distribution network.