Feature Arc Flash Minimising the dangers


As Simon Raglione-Hall, technical manager at Clydesdale explains, electrical arc flash events occur on a daily basis in the UK, but many electrical workers are still unaware of the dangers that they are regularly exposed to


can be to replace high risk electrical installations, implementing arc flash detection and control, re-formatting electrical supplies to lower potential incident energy and distancing work- ers from potential arc sources. Calderwood continued, “A customer operating offshore drilling platforms carried out flash risk assessment and found some very serious arc risks. They successfully reduced all current and future risks to sub 8 cal/cm2


levels.” Protective equipment T


he consequences of an arc flash event can be disastrous and pos- sibly fatal. However, these risks can be mitigated by a risk assess- ment. The key elements to an arc flash risk assessment (as required by EU law) are listed below.


Investigation and education It is important that the risks of arc flash are understood. Initial investigation involves identifying all live electrical work - however insignificant it may seem - finding the voltage and current, the distance to workers at risk and the characteristics of electrical supply. As a result arc flash risks can be quantified. Clydesdale provide a number of tools that can assist with this process such as its heat flux calculator. Calculation guid- ance is also given and continually updated in IEEE 1584 to ensure best international practice. Traditionally, this energy is expressed in calories per square centimetre (cal/cm2


) with a recent


move towards Joules. Clydesdale’s Andy Calderwood com- mented, “People are surprised by the rel- ative level of their risks. The perception before assessment is usually that their high voltage work carries the risk of arc flash and that day to day, low voltage tasks are risk-free. After assessment they are often stunned that work they carried out without concern was actually among the higher risks.”


Risk reduction Tasks and associated incident energies should be reviewed to reduce them to minimal levels. The best sort of arc flash protection is to remove the risk altogether by not working live. Possible strategies of risk reduction


10


Above and below right: while industry is


increasingly aware of the risks


associated with arc flash and the need for reducing its effects, many electrical workers are still unaware that they are regularly exposed to potential electric arcs at life


threatening levels


Remote operation devices (ROD) and personal protective equipment (PPE) can play a key role, further reducing the risk to workers. For example, Clydesdale’s ROD kit allows many types of switchgear that require human operation to be mechanically operated from several metres away. Calderwood continued, “We don’t want someone to carry out initial arc flash assessment and then just place their guys in suitably rated PPE. We can supply clothing up to 100 cal/cm2 rating but the risks to workers from an arc that big are more physical than thermal. In the US, NFPA 70E basi- cally tells you not to work with risks greater than 40 cal/cm2


.” Communication


Once policies and procedures are in place they should be recorded, clearly documented and communicated to those at risk. Guidance on what workers can and can’t do and why, should always be given. The better the educa- tion and information, the more lives could be saved.


PPE selection


The most readily available and com- monly known source of arc flash pro- tection is clothing. However, it should only be specified once all arc risks have been assessed and reduced to their lowest practical level.


Although it is important to provide protection to body and limbs, special attention should be given to protecting hands and face as these are the body parts that are typically closest to an arc and are exposed to the highest energy. The face and head are the most easily injured but are the hardest to heal. Arc flash clothing is most easily spec-


APRIL 2013 Electrical Engineering


ified using the overall classification method ‘Arc Rating’. This is usually expressed as a sub-method, Arc Thermal Performance Value (ATPV), given in cal/cm2


. The basic requirement


is to ensure a garment’s ATPV is higher than the incident energy identified in arc flash assessment. Testing of PPE gar- ments to determine ATPV is covered by EN 61482-1-1 in Europe and is carried out using an ‘open’ or unconstrained Arc between two electrodes.


An alternative method is the garment ‘Class’ system covered by the EN 61482- 1-2 using a ‘box’ arc directed at a gar- ment to simulate a fault in a cabinet or enclosure. Test results are quite coarse but give either Class 1 or 2 protection - 1 being a very basic level of protection whereas 2 is relatively high. Amazingly, there are no European standards available for arc flash testing gloves and face protection for both ATPV and Class. Although this is being addressed, manufacturers must currently ensure their products demonstrate required protection using the above tests or derivatives. Further consideration should also be given to wearability, particularly when using garments based on artificial fibres which don’t breathe as well as those based on natural fibres. A manufacturer’s ability to support the technical aspects of the entire arc flash process is also pivotal to a successful implementation. Arc flash PPE garments or equip- ment should be produced in accor- dance with EU PPE Directives. Arc flash PPE is ‘Category 3’, meaning its failure could be fatal. Therefore, prod- ucts must be tested and assessed by external, ‘Notified’ bodies to ensure correct construction.


Clydesdale www.clydesdale.net T: 01234 855 855


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