Let’s not beat around the bush … An unplanned release of Electricity can and does kill!
Power systems are dangerous and proper Electrical Earthing and bonding are necessary to prevent unplanned current passing through personnel, critical equipment and other nearby metallic objects. But be careful of simply bonding everything back to an electrode and considering the job done.
Although Electrical Earthing system design can be a huge subject let’s take a look at some common electrical Earthing/grounding mistakes that can adversely affect your Electrical Earthing design.
The IEC has developed some strict requirements for professional electrical Earthing system design. So, always consult an electrical Earthing professional to help ensure the safety of personnel and the protection of critical equipment at your site.
7 Common Electrical Earthing Design Mistakes
- Lustful over-simplification
- Gluttony of sand
- Greed for land
- Slothliness of legacy methods
- Wrath of errors
- Instrument of Envy
- Misplaced Pride
#1 – Lust for the over-simplification
There are times when simplifying a problem makes perfect sense and can be useful. Usually, this is applicable when the problem is easily understood, low complexity and not prone to errors. Equally, there are times when simplifying is the worst thing one can do, because an error in the initial simplification can be multiplied many times over as it is progresses through the process until a 50% error at one end can reach many 100’s of % error – Soil structure is one such example.
According to EN 50522-2010, an electrical “Earth” is defined as:
“A conductor or group of conductors in intimate contact with, and providing an electrical contact to earth.”
So by it’s very definition, it’s paramount to get ‘intimate’ with the soil structure of your electrical arthing system design, as this is the very medium in which the energy from a fault or lightning strike will flow on its return journey back to its source.
So the first Electrical Earthing Sin is the lust of the over-worked/under resourced Engineer to over-simplify (for convenience) the soil structure by assuming a single layer 100 Ω.m or at best a two-layer soil structure.
Unfortunately, mother-nature does not make things easy for Electrical Earthing Designer – Soils are a product of a natural geological formation process and will vary according to what and how the area was formed using what materials, for example, sediment deposits, pressurised movements, molten rock, etc.
Each soil material layer will have its own unique electrical signature – for electrical earthing, this is measured and quantified by electrical ‘resistivity’. So knowing how a soil layer will perform when loaded with an electrical fault is a foundational requirement for a safe design – to ‘assume’ an arbitrary value based on Miller Box Soil data or similar can introduce massive errors in the subsequent safety calculations from the outset.
So, always take good soil resistivity testing method – such as the Wenner 4-point resistivity measurements using instruments that can inject high signal current/voltage to ensure good accuracy. Avoid lustful thoughts to over-simplify.
Sin #2 – Gluttony of Sand
On recent sites, I’ve seen backfilling of conductor electrodes at both ends of the spectrum – one using sand, the other using stone (both imported).
What’s the problem?
Electrical Earthing successfully achieves it’s purpose in life through the control of surface potentials – this is done through the careful geometric arrangement of electrode conductors AND by the quality of the electrical earthing connection (of the man-made bits) achieved with the Earth itself.
The quality of this interface or connection to Earth is largely reliant on the immediate volume of soil which surrounds the conductor(s). In turn, the connection quality is dependent on the surface contact and the resistivity of this immediate volume. For example, let’s take a look at the jar of stone below – what’s going to happen if an earth electrode is backfilled using this material?
Ignoring the resistivity of the rock for a moment – the size of rocks (granularity) shown below will be relying on “point-contact” for its interface. This leads to a really HIGH contact resistance, which is a POOR choice for a covering backfill for an earth electrode/conductor because of the low contact surface area as a result of the point-contact.
The situation can be improved massively, by introducing a finer material of compatible low resistivity to infill the voids and provide a fully contacted surface area with the conductor(s).
WARNING! When faced with a rock backfill as above – do NOT be tempted to use sand or stone dust to infill the voids. The infill must be sufficiently low in resistivity to improve the connection … not destroy it.
Unfortunately, due to the abundance (gluttony) of sand as a construction material it’s all to common to see sand used as a backfill material for electrical earthing systems – this does NOT work!
Dry, silica sand has an extremely high resistivity (x10^18 Ωm) so one can almost consider the material an insulator.
This means coating a ‘conductor’ in an insulation covering because it is cheap, plentiful and convenient is not a great idea when the objective is to achieve a ‘conductive’ relationship with the surrounding ground.
Therefore, Sin #2 is the Gluttonous use of sand!
Part 2 to follow next week.
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FEATURED IMAGE: by Mihiru93
Inspired by the THE SEVEN DEADLY SINS OF ELECTRICAL Earthing DESIGN whitepaper published by E&S Grounding Solutions
Ian is a Principal Consultant at GreyMatters, with 26 years experience solving HV earthing, EMC, and lightning problems for clients worldwide. When he’s not busy studying problems and designing solutions, you can find him mountain biking, sailing and racing motorbikes in the summer. In the winter he tends to head off to the mountains chasing the snow with friends and family. Ian holds a Master’s Degree, and Degrees in both Mechanical and Electrical disciplines, and is one of the top 1% accredited CDEGS consultants and advisor to international utility companies, data-centre and infrastructure developers globally.