A number of organisations across the UK recommend a standard earthing design when building smaller scale substations. The key driver behind these is one of cost. Nobody wants to pay more than they have to. So, the question is — does installing a standard earthing design meet your obligations under the Electricity at Work regulations?
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The New ENA earthing standards have finally arrived after a very lengthy consultation process. For those of you not too familiar with this standard, it is used to calculate the earth grid impedance, earth potential rise and associated touch and step voltages, during a phase-earth fault on an HV electrical network. The update is long overdue, and whilst the older standards were very good, they were rather dated in some areas. This results in a much more streamlined and practical standard, with some helpful new criteria and assessment procedures.
Whilst the latter approach is always best for larger projects, it is often not viable for smaller 11kV sites. We will add some further detail about the standards when we have chance, so please check back soon!
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At a practical level, what does this mean for earthing designers, operators and ICPs? There are a few key major differences: Lots of updated pictures, and scenarios. There is a very useful practical flow chart for deciding on the design route that must be followed see below.
Global earthing is now an accepted approach — this will be very useful for smaller substations in urban environments, and may avoid the need for detailed CDEGS modelling if certain criteria are met. Practically the new criteria are very similar to the old guidelines though! The safe touch and step voltages have been updated and revised for a variety of different protection clearing times.
The step voltage limits are now much higher, and tolerable step limits are much less likely to be a be problem unless it is a sensitive site. More practical guidance is given in terms of separation between HV earthing systems and other buried services and buildings.
ENA TS 41-24 and S34 – Earthing Standard For Earth Potential Rise Calculations
Your Standard Earthing Design – Are They Valid Anymore?