Introduction
When we carry out an earthing study for a site, one of the key questions we try and answer quickly are ‘Is the site a Hot Zone?’ and ‘Is the site a high EPR Zone?’. Back in the olden days (Pre 2020), these questions in the UK regulatory framework really meant the same thing.
Recapping quickly on earlier posts, an Earth Potential Rise (EPR), is the magnitude of the voltage rise that occurs on a substation during a phase-earth fault. The EPR value is in turn what determines the touch and step voltages on the system and the risk to personnel.
- High EPR and Low EPR are terms that are used in earthing standards and relate to the magnitude of the EPR in relation to the allowable touch voltage. They refer to different things.
- In the ‘old days’ (pre 2018) Hot sites were ones that triggered detail design and analysis. Now it is a High EPR site (one where the EPR is > 2x the touch voltage) that triggers detail design and analysis.
- ‘Hot’ and ‘Cold’ sites refer to the ITU classification of sites and has specific thresholds of 430V for sites without fast protection (i.e. distribution networks) and 650V for sites with fast protection (i.e. transmission networks)
- With the latest changes to the standards a few years ago, it is now possible to have a Low EPR site that is also Hot, and a High EPR site that is Cold – which can be somewhat confusing!
In modern analysis, if the EPR is above a certain threshold value of 2x the permissible touch voltage, then additional design and control measures are required. The important thing is that a high EPR is not necessarily unsafe, it just needs more careful management and specialist design.
Touch Voltage Limits
In practical earthing designs we almost always focus on touch voltage limits as these are far more onerous and usually the defining factor in any design. The limits are based on the voltage a person can touch without entering fibrillation (i.e. having a heart attack). The limit depends on the type of surface the person is standing on and the shock duration. Looking at some typical cases:
- For a basic, old simple unit substation without any dedicated earth fault protection – with a slow fault clearance time of 3s. If a person was barefoot, a limit of 60V would apply. The classification for a High EPR site would be 2x 60V = 120V
- For a typical modern 6.6kV / 11kV unit substation, with standard IDMT protection and a 1s fault clearance time, then a typical person with shoes on then a limit of 233V is applied. The classification for a High EPR site would be 2x 233V = 466V
- For larger 33kV and above outdoor open terminal switchgear – a protection clearance time of around 0.5s would be usual, and a light layer of stone chippings then a limit of 650V would apply. The classification for a High EPR site would be 2x 650V = 1300V
- For transmission substations with fast fault clearance of <0.2s, and a thick layer of stone chippings then a limit of 2064V would apply. The classification for a High EPR site would be 2x 2064V = 4128V
| Soil Conditions | Maximum Touch Voltage (V) for Protection Clearance Times | Maximum Step Voltage (V) for Protection Clearance Times | ||||||
| 0.2 s | 0.5 s | 1 s | 3s | 0.2 s | 0.5 s | 1 s | 3s | |
| Bare Feet | 407 | 166 | 80 | 60 | 17077 | 6044 | 2249 | 1503 |
| Shoes on Soil / Concrete | 1570 | 578 | 233 | 162 | N/A | N/A | 17571 | 11727 |
| Shoes on 75 mm stone chippings | 1773 | 650 | 259 | 180 | N/A | N/A | 20253 | 13517 |
| Shoes on 150 mm stone chippings | 2064 | 753 | 298 | 205 | N/A | N/A | 24083 | 16074 |
In some cases, we can get even higher tolerance and use asphalt / tarmac to increase the values to much larger ones.
As noted above, step voltage limits are almost never a problem for most plants. This is because the values are so high. There are however a few occasions when they need looking at a bit more carefully:
- Camp sites, residential areas and swimming pools – these are because there could be people who are barefoot
- Livestock areas – any livestock can be very vulnerable, and are sensitive to a step voltage as low as 50V.
A typical touch voltage profile is shown below. There are few points that are not always obvious. On the right hand part of the plot, there are some ‘blue’ areas which indicate a touch voltage that is modest high, supposing our touch voltage limit was 650V, these areas indicate a problem – but is it a real problem? On this site they are laydown areas, and a car park with nothing else there – so there is nothing metallic to touch and get a shock from. Similarly, we can see the outside of the main plot rises to a high value – this is a because of the way the software calculations the touch voltage (distance to the earth conductor) – there is nothing out here bonded to the earthing system, so there is no real risk!
What happens if a site is Hot, but Low EPR?
A common scenario is that after performing the earthing design a site is classed as Hot (as per ITU), but Low EPR. This tends to happen often on substations at 33kV and above. Taking the 650V touch limit from earlier, the High EPR classification would be 1300V. If we calculate an EPR of 1 kV, then the site is technically Low EPR, but ‘Hot’ as per the ITU classification.
This means we don’t need to do a very detailed analysis of the substation (although we always would for safety). We would however need to produce a Hot Zone contour and notify BT / OpenReach.
What happens if a site is High EPR and Cold?
This is very unusual but it can happen. Consider a camp site with a small substation or pole mounted transformer nearby, that is only protected by HV fuses. A 3s protection time could be present, and the chance of people near the substation being barefoot the risk is significant. Taking the 60V touch limit from earlier, the High EPR classification would be 120V. This is below the ITU ‘Hot’ classification, so the site is High EPR and Cold.
In this case we would do a detailed analysis of the touch (and step) voltages in the area, but there is no need to produce Hot Zone contours of liase with BT / Open Reach.
What happens if the site is High EPR and Hot?
These are sites where specialist earthing design is needed and tends to occur on most larger substations at 33kV and above and on rural isolated areas, that are supplied via overhead lines with limited earth return paths.
In these sites, it is necessary to carry out a detail design of the substation, to assess the touch voltages throughout the substation area to check that all contact points are within the permissible touch voltage limits. Sometimes a High EPR site can be made into Low EPR, by adding in a lot more earthing conductor, but often this is not economical or practical. Instead earthing conductor is added strategically around the site to control the touch voltage profiles. Care needs to be taken with incoming / outgoing supplies and auxiliary services to the site to make sure that the High EPR is not exported to a remote location.
What happens if the High EPR / Hot Contour extends beyond the site boundaries?
This is where things can start to get more complex and challenging. Within a substation, the EPR and associated touch voltages can be managed and contained through design practices. But if the substation has a very high EPR its zone of influence can extend many hundreds of meters. The impact depends on the surrounding area, sites located in arable areas, will present a very low practical risk, but sites in industrial or residential areas need to be considered if they can induce a touch voltage on nearby systems.
Assessment of these sites is always site specific, and dependent on the magnitude and extent of the zone of influence and what it is affecting. Mitigation measures can include trying to lower the resistance of the earthing system, to reduce the zone extent, adding additional earthing in to help shape the contour away from the 3rd party areas and in some cases, adding specific control measures to 3rd party systems like TT earthing.
A contour plot below, shows some typical results for a new BESS site. The higher value contours, extend beyond the site, but there is nothing within the sphere of influence to be affected. The 650V contour extends a good distance from the site and can be seen impact another site to the NW – however this is actually another substation and so the problem can be managed and contained. There are no other residences in the area, so there is no specific risk. A final check would be to make sure the surrounding fields are arable land and not used for livestock farming.
Summary
As has been shown above, Hot Zones and High EPR sites are related, but different phenomena. The fact that a site is High EPR does not mean it is unsafe, just that additional measures need to be considered and a more detailed assessment carried out. An important consideration for these sites is understanding the locational aspects and any site specific risks. If large contours extend from the site, what can they interact with and are any mitigation measures needed?
If you would like to know more, please get in touch.