Impressed voltages and interference from electromagnetic fields are increasingly becoming a hot topic for safety professionals and engineers alike. Hugh looks at understanding the risks of EMFs, when you need to do an EMF study and how XGSLab does the heavy lifting for you when doing EMF studies. Watch a replay of the webinar here.
Ian: Hello, and welcome.
Thank you all for coming here today. For those of you who don’t know me, my name is Ian Griffiths from GreyMatters, I am the principal consultant here. And I’d like to start things off with a quote or something that kind of resonated through the past week. One that came to mind was this, it was beautifully done. “Nearly everything awesome, takes longer than you think, get started now and don’t worry about the clock.” Think that’s a massive mantra to live by.
So, in today’s webinar, we are looking at EMFs, presented by my good colleague, Hugh Wren. This presentation acts as a trigger for those questions to feed through. So, we’re going to look at the risks of EMF, electromagnetic forces and alike.
EMFs and Impressed Voltages
Hugh: Thanks for the introduction Ian. As Ian says, I’m going to be giving a quick introduction to EMFs and impressed voltages.
We are going to look at what an EMF is and why they matter, the risks of EMFs, what impressed voltages are. Then we’re going to have a quick look at how to model impressed voltage and how that affects your site and its context. Then we’re going to review everything we’ve covered and answer questions at the end.
Every energised conductor is going to have an electrical and a magnetic field associated with it. Which is a product of the voltage and the current flowing through the conductor. When you have a high voltage transmission line, those high voltages and high currents mean that we’ve got to pay extra care when we’re working in, near and around those high voltage transmission lines.
Today we’re going to look at some of the hazards associated with impressed voltage. Look at the modeling tools we can use to assess the magnitude of that hazard. Therefore design mitigations to reduce the magnitude of that hazard.
What is an EMF?
Fundamentally, it’s the consequence of the voltage and currents impressed upon a conductor. It’s comprised of an electric field and a magnetic field. This is either static, which tends to be associated with a DC conductor. Or time variant associated with an AC conductor.
What we’re going to look at today is the consequences of power frequency electromagnetic fields. So that’s in the 50 or 60 hertz region. We can also look at the electrical magnetic fields associated with lightning or anything else like radio up to about 10 megahertz in XGSLab.
Why do EMFs matter?
Well, if you remember a previous webinar we presented where we talked about modeling electrical magnetic fields. We talked about the safe thresholds. If that’s of interest, you can visit our webinar library and see that previous presentation. We’re going to look at today is one of the major consequences of these electromagnetic fields. What we call impressed voltage.
What is an impressed voltage?
Fundamentally an impressed voltage is the consequence of a not very efficient transformer. Where we have our overhead power line as effectively the primary winding of our transformer and the fence or the earthing system or the overhead line conductors as the secondary winding of the transformer. As with a transformer, the longer the length of parallel cables. Obviously in a transformer we wind those into a coil so that they’re more space efficient. The better that inductive coupling is going to be. So fundamentally, that electrical and magnetic field is going to induce inductively.
Potentially in certain scenarios, impress capacitively, voltage and current on to structures like fences etc. Where you have significant parallelisms with overhead lines. It certainly tends to be where that parallelism extends for perhaps 100 meters or more, where it starts to become an issue. It’s dependent upon the voltage and current that those lines are loaded to.
How XGSLab can help you understand impressed voltages
How do we assess that? Well, using XGSA FD, we can build a 3D model of our overhead power lines and we can build a model of our substation earthing system, our independently earthed fence, and we can load up that overhead line with whatever its design operational loadings are going to be and directly measure what that impressed voltage, what the induction, what the capacitive coupling is. Thereby determine what the hazard to life is going to be.
Why do we earth substation fences?
Here’s the first example we are going to look at. This is where we have a substation fence that isn’t earthed. That’s running parallel to a single circuit 132KV overhead line.
You can see that there’s an unearthed fence is all energized to about 84/85 volts. You can see if you look at the key in the maximum box, it says 84.94. So that’s the maximum voltage on our fence. Because this fence isn’t earthed at all, it’s something like a mesh fence on wooden poles. We’re making a very bad capacitor where our overhead line is one plate of the capacitor, the dielectric in the middle is the air, the other place to the capacitor is our fence. And so, every time someone touches that fence, they’re going to effectively earth the capacitor and then there’ll be a capacitive discharge through their body down to earth and that can be quite hazardous.
If we earth that fence, we just add a handful of earth rods. We can see that the voltage on the fence goes from 80volts to 0.4 and that’s from seven earth rods. Which hardly costs us very much money at all.
Assess and manage
And so, we can very quickly using our modeling tools, assess and manage the coupling and the impressed voltage and therefore determine the magnitude of hazard that this perhaps cause to our staff and third parties.
What can sometimes happen is that someone puts up a fence next to your substation, that’s not within your control at all. A farmer has built a fence next to your substation, and that’s his fence, his asset, you don’t have any control over it. Well, this fence is only 200 or 300 meters long. But you can see the because of the length of that parallelism. It’s now energizes up to over 300 volts, which is really quite a significant voltage.
The standard for this area says that the maximum static voltage on a fence should be kept to less than 60 volts. And so, you see here that we’re significantly above that safe threshold. Because the substation fence is earthed, if they connected their fence to the substation fence. Even though from an earthing point of view, we might say that this is problematic in terms of creating a transferred voltage hazard, we can see that it pulls down the voltage in the fence to a figure of about 4.5volts. And you can see that there’s now a voltage gradient along the fence where the highest voltage is at the far end.
That’s a very quick introduction into the sort of modeling you can do in XGSLab to understand impressed voltages. We can see that even relatively short fences of only 100 meters can be impressed up to quite high voltages. As I said, BS EN 50443 gives us a maximum safe voltage, which is about 60 volts under normal operating conditions. It also gives us some thresholds for volt conditions.
What’s quite interesting is that in the scenarios we looked at today, the measures we commonly put in place already. Adding a handful of earth rods to our substation fence are already sufficient to manage the impressed voltage. But by using a modeling tool, not only can we prove that before we break ground, but when we look at more challenging scenarios with longer parallelisms and higher load currents in the fence. We can quickly and easily design effective mitigation that works on a project and site specific basis, instead of relying on the simplification and assumption and just assuming that it will be alright.
So that’s a very quick introduction to what an EMF is and why they matter, the risks of EMF, what impressed voltages are and how to the module and impressed voltage. We’re going to open to questions now.
Ian: Thank you for that Hugh. Exactly right, we’re going to open the forum now to the floor and answer any questions you may have.
Can you talk a bit about the distinction between inductive and capacitive impressed voltages?
There are two scenarios that we looked at and presented. One with the unearthed fence line, so this fence unearthed and so that is the capacitive buildup of voltage. And then once we added the earth rods to the fence, we then switch over to more inductive effects.
EMF is a whole topic, it’s becoming more and more prevalent. It’s something that we’re being asked to understand and answer questions to more and more. It can be perceived as a bit of a black art. Because magnetic fields and electrical fields is something that we can’t see. It’s very kind of space dependent as well. So, modeling tools, like the XGSLab tool can uncover, quantify, and just take the uncertainty off the table. So that you can establish whether there is a risk or there isn’t. Which from a client perspective is something that is needed.
Everyone’s got an obligation and a duty of care to not only the people within the confines of a site, but also outside the confines. So, somebody that is interacting with the fence line, for example, the client will need to know that they’re not being subjected to any hazard voltage. As Hugh said, there is a voltage threshold of 60volts, working voltage that we need to try and keep within.
Would this modeling tool also be used for modeling the impact of overhead line on long pipe routes underground, for example oil or gas pipes of 50k long or more?
Yes, is the definitive answer. So, it’s anything conductive, we’ve talked about conductive fences, but it could equally be a metallic buried substernal pipe that shares the same corridor as the overhead line. It’s either inducing or impressing that voltage onto that pipeline. In that kind of scenario, then what you might get is maintenance staff go out to maintain the valves or take some readings and if there aren’t any measures to kind of mitigate the fields that have been impressed on the pipeline itself, then they could be subjected to touch voltage when sort of cranking open the valves.
Daniele Cuccarollo did a lovely piece for us last year on Bringing Your Design to Life and shows that corridor of the overhead line (integrating with Google Maps), with a pipeline and the induced voltage that was sitting on it. It also looks at the integration with XGS tool as well as Google Earth. If you missed this webinar, watch a replay here.
What is the regulation regarding the 60volt limit? Is this for induced voltage or any form of transferred or exposed voltage?
The 60volt nominal figure covers the impressed and induced. It doesn’t care where the source of that voltage is, it’s picked up in BS EN 50443, it’s just that human touch voltage limit.
Great questions thank you very much for that.
Thank you again and we’ll look forward to seeing everybody next month. If you’re interested in oil refinery and the pipeline question, then next month’s webinar is going to be an absolute Corker. We’re looking at bypass conductors with a guest presenter, it’s going to be something exciting to watch. We will keep you posted on that. Until then, thanks very much and goodbye.
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