Arc flash studies get a lot of engineers and managers very excited and opinionated; but once you get pass the initial discussions, they are often poorly understood. There is very much a tendency to over-simplify arc flash assessments, or to take the output data from a model and take it as an absolute result with no interpretation. I have tried to pull together various thoughts on the subject from discussions over the last few years, and hope they are of interest / use.
First, let’s think about the easy things we understand well initially. The mechanism for an arc flash occurring is well understood, and the amount of energy released during an event is (being a bit simplistic) a function of the fault level in kA and the protection clearance times – so a high fault current with a fast-clearing time can be less incident energy than a lower fault current with a slow clearing time. This is why it is essential to have correctly set protection relays, or other suitable detection systems.
Second, when could an arc flash event occur? Roughly speaking you have two main cases 1) Entering a substation to look at something or carry out a visual inspection or 2) Carry out maintenance work within the substation. What does this mean though? In the first case, if you are only inside the substation the risk is low then arc flash incidents can and do occur due to degradation and partial discharge causing faults, but most switchroom and not occupied normally. This means that most faults occur during maintenance and switching. NFPA 70E has some guidance on when you do and don’t need to wear PPE – but this is not quite as clear as it first seems.
Third, legally, where do employers stand on the need for an arc flash study? Important caveat – I am not a lawyer so happy to be proved wrong here – but it is very much in a grey area. In simple, terms the UK primary legislation and HSE guidance place a requirement on an employer and asset owner to ensure it is safe to work on and operate electrically – this is conventionally met by carrying out fault studies, protection coordination studies and risk assessments. But is there a requirement for an arc flash study? Maybe! The problem is the IEEE 1584 standard has no legal basis in the UK, and many of its assumptions don’t really hold that well for the UK market. But as an assessment tool it can give an idea of the level of risk faced by an operator and give an indication of what PPE is needed. If an engineer working on a system suffers arc flash injuries these can be fatal, or life changing, so the excuse for not carrying out a proper risk assessment and suitable PPE is difficult to justify.
Next, let’s think about some of trickier grey areas. These are areas, where carrying out an arc flash study is possible, but faces some problems.
1) Is an arc flash assessment needed for switchgear that is classified as arc proof / resistant? This is an interesting issue – many switchgear designs have a level of arc containment, such as AFLR for 1s. The key thing here is to understand what the arc containment really means and how it was tested. If the arc flash containment is based on the all the compartment doors being closed then how much use is it if someone is maintain the switchgear and has the doors and compartments open. Such an arc proof design may therefore give a false sense of security or minimise an already low risk.
2) Can you do an arc flash study for 20kV or 33kV switchgear? The answer here is sort of, The IEEE 1584 standard, is specifically limited to switchgear rated up to 15kV – once you go above this voltage, the standard is not valid. There are some hypothetical approaches – the most common being the ‘Lee method’, but this tends to result in a very high values of incident energy and require PPE levels. Perhaps the answer at this level is that live switching shouldn’t be carried out at all – and everything should be done remotely or on a dead system.
3) How do you assess an arc flash study for LV switchgear with different compartment sizes. This is probably the most complex one to address. The arc flash assessment is based on determining cubicle sizes, dimensions, electrode spacing and working distances – these can be determined to some extent on HV switchgear – but on a complex multi-panel LV switchboard there can be multiple cases and configurations all within the same switchboard – thus all posing different risks. In theory you would therefore need an assessment for each cubicle type you are working on. You then also have the problem of how to assess older switchgear – which is often higher risk. Switchgear dimension drawings showing individual cubicle spacings, and electrode spacings are not easily available – so we move into the realms of having to make assumptions.
Lastly, lets throw in a bit of a curve ball and say why we perhaps shouldn’t be carrying out arc flash studies – or at least be treating them with a lot more caution. IEEE 1584, which is the calculation method for arc flash, is a very good standard and has been comprehensively revised several times. But, it is based on North American design practices for switchgear and operation practices, which don’t correlate well to the IEC / UK designs and practices.
These are very different to IEC switchgear for both HV and LV. At a fundamental level busbar spacings and containment spacings are considerably different. Considering LV specifically, IEC design is based on forms of separation – which can go a big way to reducing an arc flash risk, and HV switchgear in IEC design uses substantially different racking systems. So, this is where things get complicated – how do you tackle these projects? Do we leave the default assumption set for busbar dimensions and compartment spacings used in USA design or do we start trying to specify specific values by obtaining detail dimensions drawings from manufacturers (tricky and time consuming). How do we know what is important and what is not so important?
To some of these points, particularly the last one – there isn’t really a clear answer. The IEEE 1584 method isn’t directly applicable to the UK / IEC market, but it is currently the only method available that for these markets and it provides a way of determining risks and required levels of PPE. It is certainly not perfect, and may contain a large amount of engineering assumptions, that could push the results one direction or another, but fundamentally by carrying some level of arc flash risk assessment for a site you are providing operatives with a level of safety they may not have otherwise – and that can only be a good thing. It is just important to make sure that owners and operators realise that the IEEE 1584 method does have some issues that mean it can’t be directly used within the UK / IEC without some solid engineering assessment and interpretation.
If you would like to know more, please get in touch and we can talk through your project requirements.
- Calculation of Incident Energy Levels
- Determination of PPE required
- IEEE 1584 analysis method up to 15kV
- Use of the ‘Lee Method’ above 15kV
- Identification of problems with protection settings
- DC Arc flash calculations are also possible