This writing is a case study about when GreyMatters was asked to analyse a Rail Track Feeder Station in Europe. This Feeder Station had already undergone a preliminary Electrical Earthing Design Study. The principle constructor was concerned that the previous analysis had uncovered some strange numbers. These numbers, raising more questions than it had answered.
So, following a review of the previous electrical earthing design study, the potential sources of error and identified as:
- An inaccurate soil model (>10% RMS error)
- An incorrect fault level (100% error)
- Ignoring the contribution of the rebar mat
- Overlooking the conductive Steel fence
- Touch voltage hazards identified, yet left unmitigated
- Wrongfully using CDEGS MALT version on a rail system which contains long parallelisms
- Ignoring the impact of the Rail Bonds
Exploring the Problems
Mistake 1 – Soil Model
Observing that Electrical Earthing is not a precise science. It relies on the interface between two components, 1) the human-made (electrical system) with 2) the natural element, e.g. the geology of the site.
The human-made can be well defined. Its behaviour easily predicted and calculated. Whereas, nature has a way of throwing curved balls (unpredictability) which can make ‘making sense’ of this interface a lot more challenging.
Some of the best minds in the business have tried to simplify this interface into elegant equation(s), some of which feature in the legacy Electrical Earthing standards, such as BS 7453, IEEE Std80, BS7430 but even these equations rely on assumptions to simplify the mathematics into an advanced, yet manageable suite of equations.
The Soil Model, the foundational cornerstone and critical point ALL subsequent parts of the Electrical Earthing System Design thus built upon and 100% reliant on for their validity. This principle means, elimination of error at this stage is of absolute capital importance. In this case, studied, the stated error % reported was greater than 10% – this is an unacceptably high error as a starting point.
Mistake 2 – Fault Level
The supplying authority or asset owner should accurately calculate fault levels. These days, usually done by circuit theory based computation, but the manual calculation is also practical.
For the case study, using a fault level twice the magnitude of the real situation and reported. This fault level was probably due to the incorrect data being provided or assumed. The net effect was to overstate any resulting EPR by 100% immediately, which would have led to significantly installing more buried copper than necessary.
Mistake 3 – Impedance Contributions
Rail, rebar, and bonded equipment can all provide valuable contributions to the overall impedance of an Electrical Earthing electrode. As well as controlling the surface voltages and providing possible parallel Earth-return paths back to the source of the fault.
A challenge with a manual calculation of multiple returns is that without the use of high-end FEA tools, such as CDEGS, the complexity when considering multiple layer soil structures can defeat even the most accomplished of mathematicians. The various standards do not help much either and only provide simple calculations or indicative split or reduction factors for guidance, but applying these without considering the full context, is not a good idea.
Mistake 4 – Spotting the risk but not doing anything about it!
Since the mid-2000s, there’s been a paradigm shift away from manually calculating final Earth resistances of earth mats to understanding and ‘controlling’ how a human being interacts with a system in fault. The standards now set limits on the step and touch voltages during fault conditions.
Touch voltages relatively simple to explain, simply a voltage caused by a potential difference between one thing you can touch and another.
Step voltages are simply the potential difference between your feet. By looking at the human’s physical ‘points-of-contact’ or interaction with an energise system and controlling this interface to stay BELOW permissible limits.
The IEC defines permissible limits in a set of plot curves that map out the survivability of a typical population. In the UK, when considering earthing system design we use the IEC C1 curve, for which 95% of people will survive an electric shock below the curve – this implies 5% will not survive, so it’s essential to understand the risk is probabilistic.
In the rail environment, a different standard applies in the EU, which uses different calculations to set the touch voltage limits. Unfortunately, in this case, the previous earthing design study identified the magnitude of these touch/step voltages but offered no advice or measures to lower them to within the permissible limits – so, effectively the design (as presented) remained in a state that could kill or injure anyone touching or in the immediate proximity of the site.
Mistake 5 – Software Selection
In this case, the previous report carried out by others using the CDEGS ‘MALT’ version.
Unsurprisingly, the creators of CDEGS, SES Technologies, recommend that mating the correct version/module to the site-specific context. The less capable versions, MALT and MALZ rely on several simplifying assumptions the can lead to a significant underestimation of EPR, sometimes greater than 50%. Below are a couple of the key points from the CDEGS versions to be aware of:
- MALT (CDEGS) assumes that the entire earth mat is at the same voltage, which is very dangerous to consider, particularly on more significant sites with low soil resistivity.
- MALZ (CDEGS) does allow for voltage drops along conductors but does not account for inductive or capacitive effects between conductors.
- HIFREQ (CDEGS) – directly solves Maxwell’s equations to give you the full picture of the performance of your earthing system, including both above and below ground elements. We cover this in more detail on our blog, here [link].
Unfortunately, the graphics engine is the same for all versions, so the output plots and reports look the same, but the results may not be! This scenario is especially real for the more extensive system and where there is significant distance(s) of parallel elements, for example, a rail network and associated systems.
Following the steps taken to correct the aforementioned findings.
Getting the foundation right
First, GreyMatters attended site to perform a series of soil resistivity (Wenner) soundings using high accuracy equipment. And, capable of injecting ten times the standard signal current to that of other accepted instrumentation. By using a far more robust signal, it is possible to dominate other potential sources of in-ground noise. Thus, ensuring an accurate interpretation of the returning signal. Besides, higher signal energy is capable of reliably sounding to deeper depths whatever the geology to provide the subsequent Earthing calculations are using the optimum data-set. This data-set is particularly crucial for the electrically more extensive electrode system.
You can find out more about this in our blog series on soil resistivity testing.
Using high signal injection equipment, GreyMatters was able to develop a more accurate soil model by reducing the error from ~12% down to less than 5% using CDEGS RESAP. Having the key foundational piece of data down to less than 5% which means the subsequent design and calculations are all based on a robust, reliable, accurate data which is crucial when human lives/safety are at stake.
Representing the Real World
Finite Element Analysis (FEA) simulation techniques rely on building meaningful representation(s) of the real physical world in a virtual environment. This analysis can involve above ground elements as well as the buried. Indeed, for this case an accurate geometric model built by GreyMatters research engineers including several kilometres of track, the OLE (overhead line equipment) structures, cabling, switchgear, transformers and the earth mat so that the impact from the track bonds, the fence and rebar examined against their positive contribution to the EPR of the site.
Accounting for the impedance contributions and other parallel paths not only better represents ‘real life’ but avoids over-engineering the electrode system by utilising the full mix of accepted elements.
An often ignored part of any earthing system design is the civil engineers, e.g. more specifically, the foundations. Not often recognising that concrete is porous, so when wet has a low resistivity. As a result, rebar can be used to great effect to reduce earth resistance without significant additional cost. This fact by itself had the most significant impact on the touch voltage hazards, together with introducing further surface voltage control and fence bonds.
When modelling systems with above ground components, it is critical to use the HIFREQ module, as MALT and MALZ ignore all above ground structures, while HIFREQ calculates the interactions between ALL the elements (Maxwell’s equations).
GreyMatters’ final design reduced the site touch voltages from more than 3500 V to below the permissible safety threshold of 645 V across the entire site, as well as lowering the EPR by a factor of 10, from more than 9000 V to less than 900 V without introducing significant additional copper.
- Rubbish in = Rubbish out. Robust, accurate, correct Data must be used at the outset to develop a precise model
- High-end soil resistivity testing equipment capable of dominating any in-ground noise. Understanding the soil model is critical to accurate earth design, and inaccuracy here is dangerous.
- Ignoring the contribution parts of the above-ground network can play when modelling the impact of rail bonds. It’s important not to assume something is going to improve the situation – requiring the proof that any additional bonding makes the site safer.
- Designing a safe environment without introducing additional costs. By modelling existing structures, it was possible to use the existing equipment to reduce the hazards on-site without requiring new construction, which more than paid for the cost of the study work.
Further reading: The 10 Commandments of Electrical Earthing Design
If you believe your current Electrical Earthing System Design needs a rethink from some new GreyMatter, then get in touch, and we can discuss your challenge.