Electrical Earthing Design
What are the important Safety Calculations for Earthing Design?
Safety Calculations are essential to a 'Safe' Earthing Design. Fundamentally, the electrical earth system has the prime purpose (in safety terms) of ensuring that the biggest amount of people (population) will survive the effects of a fault, if exposed. This is done by safety calculations and software tools like CDEGS.
The effects of a fault on humans is probabilistic in nature. This one fact means it's almost impossible to say with 100% certainty that a general human population will survive every fault. Why? Because within any given population there will be those individuals that can take more ... and those than can withstand less of the harmful effects, e.g. those with pacemaker or medical condition. So, a safe design is defined in terms of reaching a given level of safety. Where not all may survive but within a general population, most will. A balance of probability against what is practicably constructible.
The international standards, such as, IEEE-std 80 and IEC 50522 take this onboard. Setting minimum levels / limits for an Earthing system to achieve before it can be considered 'safe' or within permissible levels. Demonstrating that a design will achieve 95% survivability (C1 - IEC 60479) if exposed to a fault, is considered 'safe'.
So, what are the important Safety Calculations?
Human safety calculations of note are:
- Touch voltage or touch potential calculations (the two terms 'voltage' or 'potential' are the same thing).
- Step voltage (potential) calculations
These TWO safety calculations are the prime indicators for determining whether an earthing system design has met the necessary safety standard. In order to calculate the touch and step potentials for a site. It is necessary to calculate the EPR (earth potential rise) or GPR (ground potential rise) as a result of a fault. The EPR in turn is reliant of determining the Soil Resistivity and structure of the geology that the earthing system sits in.
Additionally, Ampacities of the conductors is an important calculation. Earthing System Design needs conductors that can cope with the size of likely fault.
Can I do these Safety Calculation by hand (manually)?
It is possible to do manual calculations. WARNING: hand safety calculations are notoriously inaccurate! You should only use as a guide or sense-check purposes. The final design should be accurate. And developed using accepted / validated Finite Element Analysis simulation software tools like CDEGS. After-all, lives depend on getting this right! An inaccurate or erroneous safety calculation can kill.
What's an interference study and why is it needed?
High Voltage (HV) lines and other sources of HV can have a significant influencing effect (through the electromagnetic induction) on other nearby metallic or conductive items. The bigger the Voltage, the bigger the induced affect on surrounding items.
So, imagine if you are the person responsible for a section of railway track, or fenceline, or pipeline that shares its space with a 275kV overhead transmission line (OHL) ... you would reasonably like to know HOW MUCH voltage is being induced into your item and whether or not it's at a level that might harm, or worst still, kill someone and in what conditions this might occur, right?
This is what the interference study can uncover for you. We can model the real life context in 3D and simulate the field intensities and their paths of induction, so that you get a visual/numeric model of what is really happening and where the risks exist ... and what measures can be taken to mitigate to internationally recognised safe limits.
You can learn more, with examples, in the resources section.
What's involved in a simulation?
When first exposed to CDEGS studies, it's easy to underestimate the scale of the task and what is required in order to create a virtual representation of the real world system.
When simulating an electrical power system fault or interference, the closer to the real world one gets, the better the study's quality can be. For example, on the higher voltages, given, the return current must return to its 'source' (Kirchoff's law), and given the towers themselves can form an important part of the return circuit; it may be necessary to plot and factor-in all the towers (pylons) leading to the site's electrical feed point... typically this can be many 10's of km of line.
What is CDEGS and do all versions perform the same?
CDEGS - Current Distribution Electromagnetic fields Grounding and Soil structure analysis.
CDEGS, is the recognised as the industry's best of breed software tool for Earthing System Design. But be aware, there are many 'versions' of CDEGS to choose from depending on your application or concerns that you want to examine and depending on your budget - NOT ALL VERSIONS ARE THE SAME!
An interesting analogy is drawn from cars. If all you need is a basic means of transport to get you from your house to the local shops to pick up the food for the week. Then you might consider something cheap, economical and with enough boot spare to take the bags of food. This type of car might cost £5,000-8,000 for example.
If you need a means of transport that ensures you get the lap record at the Nurburgring race track (sub 6:48 minutes). Then you should consider a totally different car to the one above, right? Unfortunately, the cost may also be totally different to the above too! ... closer to £100,000 mark.
CDEGS is no different. If you have a small, simple earthing/grounding arrangement, then you can use a £5,000 version of CDEGS. Assuming the asset is not mission critical and doesn't need a high degree of accuracy.
The networked version of CDEGS is at the other end of the spectrum. Something like Hi-FREQ. This version is more accurate than any other; and because of electromagnetic theory drives the computational engine. This version of CDEGS can account for all the interactions, e.g. capacitive, inductive as well as the resistive and conductive. This capability allows you to study large scale, complex systems and contributions from other conductive influences; to give you the most accurate real-world representation currently available.
The electromagnetic version of CDEGS is also capable of studying effects above ground as well as below. Circuit theory limits the study scope of MALT / MALZ versions of CDEGS to below-ground studies only.
CDEGS performance is only half the story for Earthing Design
A matched version of CDEGS to the application is only half the story. The other half is ensuring the software tool is being operated by someone who is formally trained and certified to use CDEGS.
Using the car analogy again. ANYONE can buy a car. But that does not mean you can legally drive it on the public highways. For that you need a 'driving license', insurance and the car needs to be taxed / certified safe in many countries BEFORE you can drive it on the roads.
Compliant Earthing Designs follow the same process. The specialist Earthing consultant will have bought a license for an appropriate version of the software. But the software doesn't drive itself, right?!
Engineering software tools are generally complex. They have many characteristics / properties - metaphoric levers, dials, and controls to master before you can drive it. Familiarity with Faraday's law is also something that is all too often overlooked. Therefore, the specialist shall demonstrate competence by passing a 'driving test' for the software. In the form of sitting and passing a formal exam to qualify them (accreditation) to use it. This exam will test first principles of electrical theory, including Faraday's law amongst others.
User competence is covered in more detail here
How important is it to model the Soil Resistivity?
It is really, really, really important! Is the simple answer.
And here's a brief why ...
Current carrying cables (buried ones), are insulated to prevent 'leakage', amongst other things, so that the electricity you've just spent money buying or generating can reach its destination, do it's job (load) and isn't wasted or depleted to the point where it can't perform the job intended.
Well in earth conductor terms, the soil is its insulation layer and there are certain conditions, e.g. a fault (short circuit - a problem!), where you want these buried conductors and the surrounding soil to facilitate the 'leakage' to ground, in order for this fault current to dissipate and return to its source. And it does this by flowing the faulted current in to the soil that the conductors are in contact with ...
So, it's pretty apparent that understanding what kind of a job the soil will do when subjected to a fault, is pivotal, i.e. will it help the flow, or will it hinder?
For example, if the surrounding soil is supportive of current flow (e.g. low resistivity) then the path back to the source is going to be easy and the Rise in Earth potential (GRP, EPR or RoEP - abbreviations of the same thing) as a result is likely to be low.
Let's compare this to the scenario with a soil that is not very conductive and will cause a high resistance to the flow. In other words the Soil Resistivity is high.
In this case, the higher resistance will create an increase in the Rise of Earth potential, and this is where the danger lies. If during a fault, the RoEP rises above to what the human body can take, and somebody has the misfortune to be in the wrong place at the wrong time, then there is the very real danger that someone will get hurt or worse.
The problem is, EVERY safety earth design calculation relies on an accurate understanding of the soil resistivity.
Get this wrong and every subsequent safety calculation will be wrong! Because all the calculations that go into a safe design, e.g. fault distribution, impedances, touch and step potential, ERP, reach potentials, etc. ... rely on accurate soil resistivity testing
Just to make things more interesting, soil is made up of more than 2 layers ... typically 4-6 layers, and each one of these layers will perform differently (electrically speaking). So, soil resistivity is a foundational piece of understanding with capital importance.
The above is a very simplistic explanation - this is a big topic with many variables that can influence outcomes, which can be explored in our Soil Resistivity archives section.