With increasing attention for health and safety, the awareness is growing about hazards posed by arc-faults within electrical switchgear. Switchgear can be found in most factories, hospitals and other comparable installations and is used to control the flow of electrical power through the electrical distribution system. It allows a selected part (or parts) of an electrical system to be isolated so that work can be carried out safely, and also it works automatically to isolate equipment when faults happen. It is sometimes necessary to carry out work on, or close to a switchboard when it is energised (i.e. electrically “live”). To carry out such work safely, it is important to be able to assess the level of risk associated with the work.
An arc-flash is an explosive release of energy which happens during a fault in a low or medium voltage switchboard. Such events are rare, but they can pose a real risk of injury or death to anyone working close by. These risks include:
- temperatures hot enough to melt or vaporize metallic conductors,
- bright light,
- extremely loud noise and
- a sudden increase in pressure, which can result in burns, loss of sight and hearing, and in the most extreme cases, death.
Recent changes in standards require risk assessment for electrical installations. There are, however, only US specific guidelines or standards on how to quantify the risk posed by arc-flash events.
Challenge
Ma’aden Aluminium is a Saudi Arabia based aluminium producer and supplies high quality primary and rolled product solutions to customers in local and international markets. DNV, with the still well-known strong KEMA legacy, was selected to perform a failure investigation and a protection coordination study.
We were asked to build a (ETAP) model of Ma’aden’s power distribution system and carry out an arc flash study for all medium voltage and the main low voltage switchboards. What is the current safety level and what measures can be taken to improve?
Approach
A common misconception is that arc-flash studies deal with the management of the risk using personal protective equipment. This is not the case, either in the standards themselves or in practice. The preferred mitigation strategies are to remove the risk by avoiding live working altogether (where possible), reduce the severity (e.g. faster relay settings, use of additional relays working on both light and over-current) or reducing the exposure (find safer ways of operating or maintaining the equipment, at a distance). It is important to understand the risk level when determining how it can best be managed.
With the model of the power distribution system, we calculated short-circuit current and the operating times of protection devices. This enabled us to determine the energy released if an arc-flash should occur (all in line with standard IEEE1584).
We found that at some areas energy levels were higher than desired. We advised new protection settings which reduced risk to personnel at the expense of technical considerations. We provided a workshop session to Ma’aden Aluminium personnel to explain the results and agree on pragmatic and realistic risk-minimisation strategies where needed.
In this case, Ma’aden Aluminium safety procedures mandated that the clothing to be worn by all staff on site gave an basic arc-flash protection level., It was agreed that changes should be made to the settings of the protection relays so they operated faster, thus reducing the arc-flash energy to a level below the protection given by the standard clothing worn. This solution meant that nothing changed for the operation staff, but it did introduce a chance that upstream relays could operate before downstream relays, resulting in too much equipment being switched out after a fault. These areas were well documented, so that following such an incident, the power could be quickly restored.
Additionally, labels were provided to be installed on the front panel of all switchgear which indicated the potential arc-flash energy, and the date on which this was last assessed.
Value to the customer
The risk-minimisation measures give greater protection to Ma’aden Aluminium personnel, whilst minimising impact on day-to-day activities (like changing a bulb). Close interaction between experts from DNV and Ma’aden Aluminium meant that solutions were discussed and agreed together, before implementation. This has been perceived as very valuable.
Risk-assessment of arc-flash events should be carried out, whenever there is a major change in the installation, or every five years. Ma’aden Aluminium now has a repeatable method for evaluating risk from arc-flash events.
