What are the Earthing Safety Calculations?

Safety Calculations – you’re overdue an update!

If 1-ohm is still in circulation on your documentation then this section applies to you.

Even though the Safety Calculations for determining a System Earth (Ground) has shifted away from an arbitrary Resistance figure, usually <1 ohm.  It’s still all too common to see this outdated measure of installed success appear on client / employer requirement specifications(ER’s) as part of the Earthing Design Safety Calculations.

IEC based EN 50522 Earthing of power installations exceeding 1 kV a.c. was implemented in 2010 and supersedes BS 7354 (1990) Code of practice for design of high-voltage open-terminal stations.  BS 7430 Earthing Protection, was similarly updated in 2011.

Probabilistic Approach

The first really big step-change is the recognition that electrical system safety is probabilistic in nature.  What do I mean?

For many years, engineers have used a deterministic approach to allowable step and touch voltages. Based on recognised international standards such as IEEE-80 / BS 7354 / BS 7430. The weakness with the ‘deterministic’ approach was that a constant safety criteria was applied across all sites, regardless of the physical context of that site. For example, an industrial site has very different safety context to a school or swimming pool which may have children with bare feet and possibly wet bodies.

Or, another example is a T&D (transmission & distribution) manned substation in the middle of a urban setting may have very different risk characteristics compared to an unmanned DNO (distribution network operator) substation in a remote hard to reach rural setting.

This is where the ‘Probabilistic’ approach, using well-understood risk management procedures to assess individual site risks (likelihood and consequence) have been introduced. This approach considers the fibrillation risk (from the step or touch voltage) and the coincidence risk (from the fault frequency and circulation of people on or near the site). Thanks to Charles Dalziel’s work, a target fatality / survivability factor is provided (IEC 60479-1) which is relative to various societal risks within a general population.

Safety Calculations – Applied Research

The societal risks mentioned above relate to a typical population of the ‘general public’.  The population is made up of people with a massive array of physical (and mental) variables. From gender, age, size, shape, BMI levels, mental endurance, etc.  Some of the population sampled may carry a range of additional medical conditions, which could increase their susceptibility to a body current / stress voltage.

The latest standards attempt to take this survivability factor into account.  There are subtle differences between the IEEE and IEC standards when it comes to the calculation of body current. But in all cases, it’s important to understand, that in this context ‘probability’ means that not everyone is predicted to survive.  The design question then follows for compliant safety calculations …

“What survival rate do your Safety Calculations allow for? “

Accepting that statistically, it is not reasonably practicable to design for 100% survival. What do you consider to be an ‘acceptable’ casualty number (deaths)?  5%… 30%… 50%?

It’s a sombering thought. Even taking onboard the highest survival rate for your Earthing arrangement. For ease of Safety Calculations, if a facility has 100 people passing through it or nearby it. Statistically, the probability is that for a compliant design at least 5 of those 100 people may not make it home.  Indeed, some of the initial survivor’s injuries may have life-shortening and / or life-changing consequences that do not appear in the initial mortality figure!  And this assumes a scenario where the Earthing is considered ‘safety compliant’, technically secure and in good condition. Imagine what the casualty number might be for a non-compliant design?

Safety Calculations – Ignorance is bliss

Electrical Power System related faults and surges are known to occur frequently across networks.  Whether switching, lightning or a fault / failure.  Rise of Earth Potential events are a reality and their frequency is being measured and monitored by responsible operators.  Usually, a well-designed Earthing / grounding system means an EPR (earth potential rise) event passes unnoticed.  This can foster a false sense of security to the point of contempt. But make no mistake, the risk is very real and the consequences of a poor or insufficient design can be absolute.

It’s only recently within the past decade or so, that Earthing Risk is firmly back on the safety agenda as advances in technology, such as the visual software tools, make understanding the risk easier to identify across multiple disciplines, not just for the benefit of the Electrical Engineer.

Safety Calculations Automation

Technology based automation is entering into many facets of our day-to-day life.  For example, from driverless cars to the wifi-enabled expresso coffee maker in our office. The latest Safety Calculations for Earthing / grounding design is no different.

EN 50522 – Earthing above 1kV recognises the scale, extent and complexity of Earthing safety calculations exceed what most reasonable engineers would attempt by manual calculation. And any manual calculation involves using simplified equations (at best) which can lead to inaccurate scenario safety calculation and therefore insufficient design with all the consequences (risk) mentioned above.

The most recent updates in the standards, recognises the advances in FEA (finite element analysis) software tools and their contribution to a better-understood and safer world.  FEA tools run by competent engineers, like CDEGS allow vast amounts of data to be processed in safety computations, which, without a limitless reserve of cash, in many cases were previously un-studiable.

Cautionary note – Bill Gates (founder of Microsoft) better understood that Technology was not going to be the ‘silver bullet’ to all our problems, when he said:

The first rule of any technology used in a business is that automation applied to an efficient operation will magnify the efficiency. The second is that automation applied to an inefficient operation will magnify the inefficiency. (Bill Gates)

The automation of Safety Calculations relies on technically secure ‘inputs’ AND the competency level of those using it.  Bill Gates understood the limitations of his software.  It is always advisable that you check the limitations of yours (see this article for a comparison) together with the skillset of those who are using it.

So, in summary. The standards have changed and been in force for a number of years and the old 1-ohm rule has gone!  If you have this requirement referenced in one of your own Employer’s Requirements file.  It’s time to update.

The King is dead, long live the King! (wikipedia)

The 1-ohm rule might be dead but its successor most definitely is not. As a top 1% accredited specialist earthing consultant, GreyMatters is always happy to assist with policy development including Employer’s Requirement specifications.