Why am I getting a hot site?

Hello, and welcome this month’s topic, Ian answers the question Why am I getting a hot site? You can find the webinar replay here.

Now for those that know me, I like to start these sessions with a thought of a concept that I’ve come across over the week. This is another one from James Clear, one of my favourite authors, which is, simple is nearly always better. But if it’s going to be complicated then made sure the problem is worth the complexity.

What we will be looking at in this webinar

Now talking of complexity. In this webinar, I’m going to do is to try and simplify and take a high level look at why you might be getting a hot site. If this is your first time that you’ve come across, or maybe you come across the term thrown at you hot site before and you’re wondering what it means. This is kind of a session to demystify what that means and the implications of whether a site is hot or not. It’s also useful, I think, to note that the hot/cold concept I’m going to be talking about is very much UK/Ireland centric. So, if you’re viewing this from another country, then there may be subtle changes and thresholds and terminologies as well, that you need to be aware of. This is very much from a UK perspective.

So first of all, I’ll be talking about defining the hot site. What does the hot site even mean? Then we’re going to go into stage two, which is the key ingredients if you like that make up the hot site. So, you can look very quickly at the constituent parts and the influence that they might have on the end result. And then thirdly, the downstream the hazards. If you like the implications, the impacts of having a hot site. It’s another thing to realize that you can have a hot site that is safe. We’ll go into a little bit more detail later. And finally, then we’ll have our usual Q&A session, where we’ll open the questions to the floor. Put your questions in the text chat field. We will vacuum those up and answer those once the presentation is finished. So that’s the that’s the plan.

Defining a hot site

Let’s look at what is a hot site and I take the definition from the UK version of ENA S36, which is classification of hot sites and it looks something like this. This is point 3.6, a hot site is where the ROEP underneath what conditions can exceed 430 volts RMS. The limit of 430 volts may be extended to 650 volts for electrical or electricity sites with only high reliability circuits.

Now, we’re going to unpack that little sentence there. It’s also interesting to note that they state that the term cold site is often used to describe a site where the ROEP and if you’re wondering this point. What is ROEP, that’s rise of earth potential does not exceed two values here 430 volts or 650 volts under earth fault conditions. Let’s unpick this definition.

What is rise of earth potential?

Firstly, mentioned before ROEP rise of earth potential, what is that? You can think of it as a torch shining on a wall or surface. The energy that we’re talking about with the light torch is light energy. But for electricity, obviously, the energy we’re talking about is electrical energy. What you can see, when you shine a torch on a surface, like a wall the beam is concentrated at the epicenter of the center, and then it starts to decay, the further you go out until the shadows take over again. And this is very much like what happens on the electrical side of things. And we’ll come back to this in a minute. So that’s our ROEP.

The next bit from the sentence is the 430 volts. It’s saying if the rise of earth potential is above 430 volts, it’s classified as a hot site. There’s also another number in that, the 650 volts. But that only applies if your protection systems can clear the fault in under 200 milliseconds. So, point two of a second. If the protection system is confirmed. Being able to clear it within point two of a second, then you can use the 650 volt as threshold from when it becomes a hot site instead of the 430. Otherwise, for all other circumstances, if you can’t meet that 200-millisecond clearance time, you need to take the 430 volts as the threshold. Anything above it is hot, anything below it is cold.

So, hot site, it does give you this feeling that you’re going to get burnt if you get it wrong. So, after today, hopefully you won’t feel that way, so much, anyway.

What does ROEP look like?

This is what rise earth potential looks like when it’s modelled, and especially software, like XGS labs. The three terms, you’ll frequently hear is ROEP, rise as earth potential. EPR earth potential rise or GPR ground potential rise means the same thing. Zooming in a little bit, here. We’ve got our Epi Centre, we’ve cranked on the torch here to simulate the earth fault. And what we’re seeing is the contours are being deflected, depending on certain elements, and the set elements. It will depend largely on what is buried in the ground. What conductive structures are buried in the ground, I should say. And that influences our surface voltages. How the contours shape themselves, through soil conduction, and the like.

We’ve described and defined what a hot site is. We’ve also covered earth potential rise with potential ground potential rise, same thing. And what it might look like if we model it in software, that you might see in a report that lands on your desk.

Moving on now, what are the key factors? What are the ingredients that make a site hot or cold? We’ve covered the definition of a hot site i.e., in normal circumstances with normal protection. If the rise of earth potential goes above 430 volts, it will be classified as a hot site. We’ve also covered what riser of earth potential is or looks like at least in terms of when we model it with specialist software.

Key factors

Now, we’re going to look at the key factors or the ingredients that go into making a hot site hot. The key factors, we’re going to focus on the four main ones for the purposes of this webinar today.

First being, probably the most important one is geology. This is without stating the obvious all your earthing arrangements are going to be buried, hopefully, in the medium, soil medium, which we’re calling the geology in this case, so pretty important.

Second, we’ve got fault level. Now you can think of fault level going back to the analogy of the torch of the torch. Putting it on high beam or really cranking up the light intensity. You know, the fault level is a key ingredient if it’s low, not too much to worry about. If it’s high, then you know, the higher it gets, the more of a problem you may have. So, fault level is number two on the list.

Number three deployable real estate. You can imagine. If you’re in a city, then the available land to deploy your earthing arrangements is going to be probably far less than if it was a greenfield site somewhere in some rural location, way beyond. So deployable real estate can have a significant effect on whether your site ends up being hot or cold. There are some other factors when you come to city, kind of invert environments, that will save for another webinar. But just try and keep things simple for the for the moment. Just think of it in isolation.

The fourth and final is the clearance time. And referring to the torch analogy, if it’s on for a long time, then you can imagine that the amount of energy that that surface has to absorb. It’s going to increase with the longer time it’s being subjected to that light. In our case is replacing it with electrical energy.

These four factors they combine if you like, to give us our site context.


The first ingredient that we’ll look at more closely is geology. Geology is a fundamental ingredient to making sure that your earthing system is effective at discharging an earth fault safely.

We need to understand how it’s made up the layers. For example, this is a typical kind of model of the soil model. Theoretically, ideally where you’ve got the solid geology and then the strata above. But it’s understanding the thicknesses and the relativity or the ability to conduct through that soil that is important to understand.

The key thing about geology is that it is almost infinitely variable. Which means that for each site, it will be different to the next. Each site will have its own geological signature. And when we’re talking earthing, that means that its ability to handle an earth fault is also unique to that particular site. Geology is a very complex beast. As we can see by this, just this geological map of the UK. Each of these colours represents a different type of solid geology with fault lines, etc. And they will have its own unique signature of resistivity and ability to conduct the electrons through on their journey back to the source.

Fault level

So, fault level. As we said, fault level is like having a torch shining a torch onto a wall. If you increase the brightness that comes out the torch, what’s going to happen? Well, your lit areas are going to increase massively. And that’s pretty much what happens with the electrical terms.

Deployable real estate

And finally, the deployable real estate. Now, I like to think of this using another analogy is that of a bucket. Now we use the metric system, you imagine you’ve got a 10-litre bucket. Two gallons, for old money’s sake. It’s full of water now that can take 10 litres, if you put more into that bucket, what’s going to happen to the water? It’s going to spill out across the edges. And that’s similar to what you know what happens in an earth fault.

You can imagine if you’ve got a site that’s very small, and it’s very we and you’re relying pretty much exclusively on Soil conduction to take your earth fault, then that’s really going to struggle if it’s over that 10-litre mark, if you like. Whereas if your bucket is much bigger, if your site is much bigger, say 100 litre bucket, and you’re now only subjecting it to the 10 litres of water from previous, then it’s going to be able to absorb much more energy. Consequently, the levels are suppressed with more land that we can recruit. I hope that makes sense and I hope the analogies are useful.

What are the hazards?

We’re going to look at now moving on to the third part of the webinar, which is the hazards. If you can see, we’ve got two substations here, and these could be kilometers apart but one of the key hazards is that of transfer potential and that’s a particular concern with the hot site classification.

It’s trying to categorize those sites that have this hazard and what is it? It’s basically when you have a fault in this substation here, that there is this earth, potential rise will flow and transfer across the kilometers, say to another site.

I know it says substation here, or station one here, but it could be anything and more usually. This is where the hubs hot site classification really came into its own was with telecoms engineers you can imagine. Poor Joe over here is minding his own business repairing our line that’s just gone down, don’t know why it’s gone down. He’s at the exchange two kilometers away from our site. We see a fault and because the lines feeding this exchange are conductive i.e., their copper, then the death potential rise is transferred across the distance and Joe sees the effects when he starts messing around with his connectors.

This has very serious impacts in in the telecoms industry. Which is why the ITU the international telecoms union, set the scene and we’re interested to get a register of all those sites that are going to be classified as hot, so they can protect their workers that went to the exchanges, did their work and weren’t affected. So, transferred voltage is one of the key hazards of a hot site.

Touch potential hazard

There’s another potential hazard when a site is hot, which is that of the touch potential hazard. And we’re going to take a look at that in a bit more detail now.

So, in this site, here, we’ve got a fence and we’ve got looks like a slab or in slab mesh, with an electrode around the outside and some rods, these dots. And we’ve got a leg rod group, which shoots out northwest and there’s been a fault in the maybe there’s been this is a cabin maybe and there’s been a fault there.

We can see that there’s a rise of earth potential. Because of the buried leg, that manipulates the amount of energy in its direction. There may have been like a school or some third party here that we’re trying to prevent the earth potential rise from spilling into that area. We drag it in the opposite direction. This is called counter poison earthing, that’s a topic for another day.

What we’re seeing here is that it’s reasonably what I would call EQI potential. If we take this as the legend of the above, we can see that there’s an EPR of maximum APR of 303 volts, that’s the red bit over there. So, within 20 volts, you’ve got the purple. There’s 20-volt difference, if you like from each colour, each colour contour is a 20-volt difference. 20 volts is not that much of a difference. There is unlikely to be a touch hazard, because 20 volts is going to take a lot a lot more to create that flow and cause our heart to go into fibrillation.

Is the hot site safe?

This you can see from these safe areas plot that anything that is in green means that it’s both below the threshold of what a human can withstand. It’s safe in permissible threshold terms. And that’s both in in this case, because it’s green. We’ve got the UTE, which is the touch and UTS, which is the step it’s safe in both touch and step voltage terms. And we’ve done that by deploying the rods and the electrode and bonding the mesh as well. This is what the steps that we can do to mitigate the hazards from a hot site.

You can get a hot site, something that that has an EPR higher than 430 or 650 volts. But as long as you provide an accurate potential plane within the hot site. When you go into the hot site, it’s still within permissible levels, it’s still considered a safe site. What you’ve got to be really careful of is transferring that voltage that high voltage out of the area, inadvertently, so that’s where you need to gap fences, need to isolate conductive tenant telephone lines, for example, anything conductive would need to be isolated so it prevents the energy from propagating out further.

What we covered in this webinar – Why am I getting a hot site

I hope that’s given you a high-level intuitive understanding of what a hot site is, and potentially what you can do, those four factors that you can manipulate, to do something about the hot site that you may have. If you remember them, the geology, fault level, clearance time, and the amount of land that you’ve got available with those four factors, you can start to do something about your hot site.

Watch the next webinar in the Hot Site series here – Why is a hot site a problem?

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