• Skip to main content
  • Skip to primary sidebar
  • Skip to footer

GreyMatters

Electrical Earthing System Design & Soil Resistivity Testing

  • Home
  • About
    • Contact Us
  • Blog
  • Earthing Services
    • Earthing Design Services
    • Lightning Protection Design
  • Earthing Training Courses
  • XGSLab
    • XGSLab Services
    • XGSLab Online Demo
    • XGSLab Sales
  • Members Academy
Home » Ask the Experts Electrical Earthing Blog » Myth Busting – 5 reasons why a hot-site does not mean it’s a dangerous site!

12/01/2016 By Ian Leave a Comment

Myth Busting – 5 reasons why a hot-site does not mean it’s a dangerous site!

What is ‘HOT-ZONE’ or ‘Hot-Site’?

There are so many misconceptions surrounding the whole concept of a‘Hot-Site’ and ‘Hot-Zoning’ for Electrical Earthing Designs.  In this article, I attempt to simplify what ‘Hot Zone’ (or Hot Site) means in Power System terms AND crucially, explain what a ‘Hot Zone’ is not.

By the end of this article, you should hopefully understand that just because a site classified as a ‘Hot Site’. It does not mean it is necessarily unsafe.

Hot Site Basics – Earth Potential Rise (EPR)

If you know how an Earth Potential Rise (EPR) or Rise of Earth Potential (RoEP) occurs, then you can skip this bit.  If not read on.

When a part of an Electrical Power System suffers a fault, it usually means there’s a short circuit somewhere.  Often the short circuit can occur between one of the ‘phase conductors’ and the Earth. (To keep things simple, that’s the scenario we’re going to stick with here)

Take a minute and watch the video below. 26 seconds in, one of the phase conductors shown detaching itself from the supporting structure and dropping to the ground.  What happens next is a representation of the Earth Potential Rise process, e.g. the energy inside the phase conductor released into the soil, and a rise in the surface voltage is created locally where the conductor dropped, which over time expands out across a given distance [In the video is it shown by colour contours bleeding/expanding across the site].

If you prefer a more physical analogy, it’s a bit like dropping a pebble into a pool of calm water. With the water analogy, the pebble has a store of kinetic energy (mass x velocity = KE). Which transfers into the water, creating ripples.

The more KE, the bigger the magnitude of the ripples. If we consider these ripples to equate to the electrical voltage contours of Earth Potential Rise (EPR). Then these will expand from the point of impact and extend out.

However, as the contours extend, so their magnitude decays with distance as the energy descends into the soil on its return journey back to its source (Earth Return). Therefore, most of the energy released will be present (momentarily at least) at the epicentre of the fault.

So, having explained a little bit about how and why the Earth Potential Rise takes place. We can move on to defining the term ‘Hot Zone’…

‘HOT-ZONE’ – What does it mean?

The Hot Zone or Electrical Hot Earth defined as the Earth Potential Rise (EPR) which extends in a radius from any bonded metalwork to the site’s Electrical Earth Electrode System or Main Earthing System (MES) that exceeds a magic number of volts. This ‘magical’ number of volts determined by the relevant national authority for each country. It will vary from country to country – but the principle remains once the EPR magnitude exceeds the threshold voltage, the sites now defined as being ‘Hot’.

The extent and shape of this ‘hot’ surface-voltage contour known as the ‘Hot Zone’.  In the States, this Hot threshold set at 300 V.  But for the IEC based countries, the picture gets a little more confusing…

Hot Zone - Hot Site

EA TS 36 – when is a site Hot or Cold?

For European and IEC based countries, the magnitude used to define this ‘Hot’ threshold subjected to the site’s protection settings (clearance times).

For example, a site that can clear a given fault reliably and quickly (e.g. within 200 ms) can behave a higher EPR before it falls into the classification of being ‘Hot’ than say, another site that clears its faults in much longer times, say, 1 second for argument’s sake.

The concept of defining ‘high reliability’ vs ‘normal reliability’ is hugely relevant.  After all, a high-reliability site will be able to restrict the amount of energy released before isolation much more efficiently compared with those sites that do not have the same level of control or protection.

This level of protection is why the high-reliability sites have ‘Hot’ voltage threshold than the regular reliability sites. Given the resulting hazard from touch and step voltages are proportionally lower for sites with a quicker clearance time (protection).

In the UK, the Hot Zone historically measured as anywhere within 100 m of the boundary of the high voltage compound at a Hot Site.  However, since 2007 it is allowable to use the Energy Networks Association (ENA) Recommendation S34 – A Guide for Assessing the Rise of Earth Potential at Substation Sites, to calculate the Hot Zone.  This guideline currently defined as a contour line marking where the Rise of Earth Potential (ROEP) exceeds 430V for regular reliability power lines or 650V for high-reliability lines.

So, you’ve got a site that’s classified ‘Hot Site’ – now what?

hot zone

Don’t panic!  The Hot/Cold classification is simply part of a formal Risk Management process that allows the asset owner to register the site as a ‘hot site’ and inform other parties that a hazard ‘may’ exist – A sort of red flag or indicator.

If a site defined as a Hot Site, it’s ‘Zone’ (extent) will also need to be calculated.  Software tools like CDEGS widely accepted as the preferred method/tool of choice to establish the Hot Zone contour(s).

In some circumstances (such as when a ‘cold’ site upgraded to ‘hot’ status), the Hot Zone may take in residential or commercial property which is not within the property of the site’s owner.  In these cases, an assessment needs to establish if the Hot Zone EPR exceeds a second ‘actionable’ level.

The second ‘actionable’ level defined as the contour line marking where the Rise of Earth Potential (ROEP) that exceeds 1,150 V for standard reliability protection and 1,700 V for high-reliability systems, confused?

Don’t be, Noise regulations in the EU follow a similar approach.  For example, the Noise at Work Regs has a lower (80 dB) and upper exposure action value (85 dB) for the lower level, guidance that’s all it needs; it’s when reaching the higher threshold that controls are mandatory, and the Hot Zone principle adopts a similar methodology as below table lays out.

 Normal ReliabilityHigh ReliabilityWhat to do
First level EPR value430 V650 VEnter site details on the national register
Second level EPR Value1,150 V1,700 VAction Controls/mitigation

What does ‘Hot’ NOT mean?

It is a common misconception to think that just because a site has an EPR that exceeds the first level (430 V or 650 V), as Energy Networks Association (ENA) Recommendation S34 that the site is unsafe to enter or to work inside this zone.  This recommendation, by itself, is not the case.

When a substation or site classified as ‘hot’ within the first action level, it is providing a management trigger for Risk Management measures to be considered… It’s when it exceeds the SECOND action level that then controls measures required to mitigate.

Let me repeat that! – Just because a site has reached level 1, the ‘Hot’ classification does not mean that mitigation measures are automatically required.  Since 2007 under S34, mitigation required when the second action level gets exceeded.

What do Hot Zone control measures look like?

Control Measures may include several actions depending on configuration, and these include (not exhaustive):

  • Appropriately designed Earthing arrangements to satisfy permissible touch & step safety voltage thresholds
  • Ensure sufficient maintenance and inspection
  • Isolation of incoming/out-going telecommunications cabling
  • Informing 3rd parties as appropriate

Maybe we will explore the control measures in a subsequent blog.

This principle is probably one of the more misunderstood concepts within Electrical Earthing Design, so reach out via live chat or contact us to access a bit more GreyMatter on the subject and we can demystify a little more.

Filed Under: Earthing System Design Tagged With: BS7430, Earth Potenial Rise, Electrical Earthing System Design, EPR, Hot Site, Hot Zone, Rise of Earth Potential, ROEP

Electrical Earthing Training – Free Trial

Do you want to understand more about Electrical Earthing System Design – Greymatters Academy is our Earthing Training site take a look or access your free trial here.

Recent Posts:

Ian getting ready

We take a look at soil resistivity and provide practical advice on what is ground soil resistivity, why we measure it and these common mistakes when Soil Resistivity Testing:

Soil Resistivity Testing Methods Wenner 4 Probe Test

Soil Resistivity Testing Methods a popular post. Originally published in 2013 and now updated. A Wenner probe test is a geotechnical investigation method used to determine the electrical resistivity of the soil. Soil resistivity testing can be carried out using different methods. The Wenner 4 Probe test is one of the most common soil resistivity […]

Schlumberger Array

The Schlumberger array is a geotechnical investigation method which determines the electrical resistivity of the soil. This is similar to the Wenner probe test, but it uses multiple current electrodes rather than just two. This allows for a more detailed and accurate measurement of soil resistivity. Soil resistivity testing carried out using different methods, this […]

Driven Rod Method

The driven rod test is a geotechnical investigation method which determines the resistance of soil to the penetration of a rod or other penetrometer.  The Soil resistivity is determined using different Soil Resistivity Testing methods. However the Driven Rod, described below is the last of the three most popular Soil Resistivity Testing methods we use […]

2022 webinar - the highlights

Knowing in today’s world it’s all about time. We’ve summarise hours of technical content into this 20-minute bite-sized session, including the key highlights from the 2022 webinar series. If you missed it, watch the webinar replay here. In this webinar we cover:

About Ian

This post is written by Ian Griffiths, Principal Engineer at GreyMatters, an Earthing & Lightning Consultant of 28 years, one of the top 1% accredited CDEGS and XGSLab consultants and professional advisor to international utility companies, data centre and infrastructure developers.

Reader Interactions

Leave a Reply Cancel reply

You must be logged in to post a comment.

Primary Sidebar

GreyMatters Accreditations

GreyMatters Accreditations

Online Earthing Courses

Free Trial Learn More

Free Online Course – Learn Ohms Law in 10 minutes or less!

Learn Ohms Law

Blog Categories

  • Acadamy (2)
  • Anti-Theft (1)
  • CDEGS (6)
  • Earth Testing (6)
  • Earthing Designs (7)
  • Earthing System Design (16)
  • Earthing Systems (5)
  • Electrical Earthing (38)
  • Energy Modelling Software (1)
  • General Post (10)
  • Grounding (1)
  • Lightning (11)
  • Lightning Protection Design (16)
  • Lightning Strike (1)
  • Renewable Energy (2)
  • Soil Resistivity (18)
  • Soil Resistivity Testing (10)
  • Solar (4)
  • XGSLab Updates (3)

Tag Cloud

BS7430 BS EN 50522 CDEGS data centres earth electrode Earthing earthing design earthing protection earthing standards Earthing system design earthing systems earth potential rise earth systems Electrical Earthing System Design Electromagnetic Field Theory electromagnetic interference EMI EN 50522 EPR Finite Element Analysis Finite Element Analysis Software greymatters Grounding HV Earthing HV Earthing Protection lightning lightning danger Lightning Protection lightning strikes Renewable Energy Rise of Earth Potential risk management ROEP Soil Resistivity Soil Resistivity Methods Soil Resistivity Testing Soil Resistivity Testing 10 Common Mistakes Soil Resistivity Testing Methods soil structure Step Potential Surge Protection Tag Archive - Lightning Strike Touch Potential what to do in a thunderstorm what to do when lightning strikes

Footer

  • Terms
  • Privacy
  • Cookies
GREYMATTERS PART OF THE GROUP OF COMPANIES © 2023