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Optimising Earthing System Design and Management Through the Responsible Application of Quantified Risk Analysis (QRA)

When it comes to electricity and earthing, risk management has been a maligned subject. Quantitative Risk Analysis (QRA) can provide insight into the consequence and likelihood of project or asset risk and assist in decisions about what constitutes responsible management. QRA is a systematic risk assessment approach used to quantify the risks associated with the operation of an engineering process. It is a valuable tool to support the understanding of exposure of risk to employees, the public, the environment, and company assets.

Risk management should usually begin with the hierarchy of controls. Under common and statute law we are required to eliminate exposure to hazards, so far as is reasonably practicable. And where elimination is not possible, to reduce the associated risk so far as is reasonably practicable. Electricity is fundamentally hazardous and due to its incredible utility, is unlikely to be eliminated.

As electrical engineers it is our job to offer adequate safety, protect plant and equipment, and keep everything operational. Thankfully, electrical safety in Australia has improved over recent decades and the industry has progressed with the introduction of “working live” restrictions, test before you touch and LV rescue training and kits. There have also been developments in earthing and that is what this article will focus on.

Safety Criteria – What has changed, what is changing & why?

In the late 1940s and early 1950s, American Charles Dalziel did much of the early research on the effects of current on the human body. He used volunteers in his experiments but for safety reasons, only used low levels of current and predicted fatality based on animal and historical data. Dalziel determined a level of electrical current that was unlikely to be fatal.

  • Dalziel equation (0.5% probability of fibrillation)

0.03 < t < 3 only

  • Where

t  = current duration in seconds

k = 0.116 for 50kg body weight

k = 0.157 for 70kg body weight

Dalziel turned the ‘fibrillation current’ into an allowable touch voltage using a body impedance of 1000Ohms. These calculations featured in IEEE80 – 1986, the Guide for Safety in AC Substation Grounding, and his equations are still used in IEEE80 today. In Australia AS 2067 (Substations and high voltage installations exceeding 1 kV a.c.) was introduced in 1984 using criteria from IEEE80.

Austrian born; Gottfried Biegelmeier was another early influencer on electrical safety in Australia. His contribution to the international standard IEC 479-1 – 1994, which provides guidance on the effects of shock current on human beings and livestock, has been profound. His research presented variation in tolerable current limits and body impedance across the population in the form of probability distributions. He also showed body impedance varies with applied voltage, which was a significant difference to Dalziel’s position.

As European standards progressed, so did Australian standards. In 2008 AS2067 was revised, and the safety criteria change from IEEE 80s straight line to IEC61936s Z curves (as shown below).

Australia then got a new set of earthing safety criteria in AS 2067 but at the time, Australian Standards were not automatically followed by all power utilities, and some continued to follow the IEEE 80 criteria, partly perhaps because they weren’t revised in the ENA Substation Earthing Guide EG1 (which has since been updated). With these two criteria defining a razor edge of acceptable or unacceptable, and being therefore in conflict, something had to be done to determine what was actually ‘safe’. What we are trying to avoid is ventricular fibrillation, where the current driven through the heart disrupts its normal rhythm and blood circulation effectively stops.

Fortunately, IEC 61936 provides the data required to calculate exactly how likely ventricular fibrillation is and therefore how hazardous criteria are (for any given exposure or clearing time).

So how safe are they?

           =  Probability heart enters fibrillation

                =  f(Vapplied, Rseries, contact configuration, fault duration)

  • Use probability distribution data from IEC 60479 for
    • Body impedance
    • Current sensitivity
  • Conduct Monte Carlo or similar to determine Pfibrillation

The results of this calculation were surprising for some and showed that none of the criteria provided a negligible risk of death, in fact for some criteria and clearing times the probability of fibrillation is as high as 20%.

It should now be clearly understood that all commonly used safety criteria ‘allow’ touch voltages that are hazardous. So how can they be safe? They can only be deemed ‘safe’ by considering the (usually) very low probability of someone being in a touch voltage situation at the precise time a hazardous touch voltage is also present, that is, when a fault is present. This chance is defined as the probability of coincidence, and it is calculated as a Poisson distribution. When the probability of coincidence and the probability of fibrillation are known, the risk can be calculated. This is Quantified Risk Analysis (QRA) applied to earthing safety.

  • Pcoincidence =  Probability of exposure to hazardous potential

=  f(fault frequency, fault duration, contact frequency, contact duration)

  • Work, Health and Safety (WHS) regulations require asset owners to demonstrate due diligence in eliminating or reducing risk, so far as is reasonably practicable (SFAIRP). Although there have been claims in Australia about how this outcome of ‘all reasonably practicable controls’ should be achieved, both AS 2067-2016 and the British Health and Safety Executive provide clear guidance that following an as low as is reasonably practicable (ALARP) processes is equivalent to SFAIRP. After common and expected controls are implemented, is the residual risk above a level considered unacceptable or intolerable (with due consideration to societal value)?
  • Are there further controls that can be applied and if there are what is the comparison between the cost of those controls and the further reduction in risk?
  • Is the residual risk level, after implementing common and expected controls, at or below a level considered acceptable, tolerable, or negligible?

What are recognised tolerable risk targets?

Many references including AS2067 provide quantified targets for intolerable and tolerable risk.

The use of QRA in earthing risk management provides new support for the following end cases: We shouldn’t spend endless time or money lowering touch voltages below a magic number unless the risk reduction justifies the cost. Similarly, we shouldn’t leave hazardous voltages without further treatment if the cost of further treatment is justified by the risk reduction.

There is another application for QRA which is equally, or perhaps even more, valuable. Consider the situation where further controls are required, provided they are reasonably practicable, but you have several different controls to consider and choose between. QRA lets you determine the risk reduction offered by each control available. When compared to the cost of each option, an optimised design can be produced. This can help answer questions like ‘should we use more crushed rock, or should we add more grid conductors?’ or ‘should we improve protection or improve the earthing system impedance?’.

The international groups Cigre and CIRED released the Technical Brochure TB749 on substation earthing design optimisation through the application of quantified risk analysis (QRA) in 2018. This guide was produced by the joint working group B3:35 which had 22 members from 19 countries including practising earthing engineers from utilities/consultancies and academia.

The application of QRA in earthing designs provides 5 key outcomes

  1. Reduction of waste where traditional approaches produce overly conservative requirements
  2. Reduction of ‘risk of fatality’ from earthing related (indirect) electric shock where such reduction is justified
  3. Provide a measure of risk that allows broad comparison and understanding by non-specialists
  4. Understanding the contribution of each hazard source and therefore the most cost-effective risk control
  5. Provision of a staged implementation – which fits into existing processes (using predefined V/t curves or QRA of specific location or class of hazard)

In conclusion, when considering your earthing system responsibilities, you should ask yourself:

  • Why would or should we change our approach to earthing system design and management?
  • How do we quantify the risk associated with earthing related indirect electric shock?
  • How much time and money should we spend to determine and reduce earthing related risks?
  • Where should we focus our time and money to better manage earthing related risks?

Safearth consultant: Stephen Palmer, Managing Director, Safearth

For more information please call 1800 327 844