An EFW Plant Case Study: Examining Low and High Voltage Earthing Protection

In the context of electrical earthing or grounding. Earthing Protection defined as the Protective Earthing of electrical installations at low voltage, i.e. <1,000V AC as described in Electrical Earthing Standards BS 7430.

So, in this case, study. We take a look at a few challenges we came across while designing the HV Earthing and Lightning Protection Systems. For a 9MW Energy from Waste Plant in Europe. 

Case Study Low Voltage & High Voltage Protective Earthing – At an EFW Plant

Design Considerations: 

• Combined High Voltage HV and Low Voltage LV
• Finite volumes of differing geology affecting earth resistivity
• High soil conduction due to low resistivity geology
• Frequent foot-traffic by the public
• Moderately high Fault Levels

The design intent for the 9MW EFW plant was to provide a buried combined HV and LV global earth arrangement. Additionally, that would grade out all hazardous surface potentials across the entire site. The rationale behind this. Also, the site had a relatively high HV feed-in fault level. Additionally, the facility enjoys frequent visits from the public (foot-traffic) across a wide area of the site. As they go about their business getting rid of the weekly trash.

Therefore, these findings meant doing a detailed earthing study and assessment of the Rise of Earth Potential. To model what happens from an imported HV fault. Thus, ensuring the downstream electrical earthing protection is adequately provisioned. As a result to cope so the general public remains unaffected.

Earth Electrode Design to BS EN 62305

So, once the main electrode had been designed to keep touch potential and step potentials to within permissible levels. Therefore, the above-ground electrical earthing protection (protective earthing) designed. Thus, maintaining equipotential between structures, and equipment sufficiently to handle fault conditions and/or a lightning attachment (BS EN 62305).

Therefore, the ampacity of the bonding conductors had to be correctly sized. In order to handle the full lightning current. This current, characterised in BS EN 62305. Also to be able to cope with the relevant portion of fault current predicted in certain areas. And without exceeding thermal limits.

Additionally, the remaining electrical earthing protection not going to be subjected to full lightning current or primary faults. And, bonded to BS 7671. (See What’s the Difference Between Earthing and Bonding)

So the protective earthing procedure looked something like:

High Voltage Earthing Study to BS EN 50522

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Lightning Protection to BS EN 62305

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Protective Earthing to BS 7430

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Earthing Protection to BS 7671

Key benefit

So, one of the main benefits with a global earthing system is tying everything to the same reference (earth) creating a quasi equipotential surface. Therefore, adding future extensions to, or adding equipment a far simpler task to manage, without having to remember if a particular piece of equipment has to stay on separate earth or not, i.e. designing out the potential for human error in the future – future-proofing.

Approving the design-work (compliance)

The final piece in the overall design is the Validation and Verification (V&V). This consists of measuring the impedance of the overall earthing system when it reaches a point of practical completion and comparing the measured result again the predicted design outcome.

So, in a perfect world, for a design that spans nearly 350m across the diagonal dimension, the test leads would normally extend out to at least 1,000m to escape the electrical influence of the electrode under test (see the previous blog). In a corner of West London, that was going to be a problem.

In addition, a previous test using conventional earth tester with only 2.5W signal strength had suffered badly from a build-up in contact resistance at the probe surface. Due largely to a drying out of the top layer of soil. This caused inconsistent and unreliable measurements that were not initially usable. So, to overcome this, the team used their considerable knowledge (top 1% minds) and drove the probes a little deeper and applied a multi-probe array approach as shown in the image.

This approach is particularly useful in the longer lead deployments to increase the contact influence of the potential probe and make picking up returning signal more effective when the surface layer is problematic.

Project success consists of doing lots of little things right, in the right order. So I want you to succeed by having less trouble achieving the approval, ‘the win’.

CDEGS HI-FREQ Modelling

Once effectively measuring the impedance next the real-world test configuration is replicated and modelled in the software environment using the top 1% version of CDEGS, called HI-FREQ.

This particular premium version of CDEGS allows the modelling of above-ground elements, such as the fall-of-potential earth test above. (Fyi, there are many versions of CDEGS and this post covers their selection.)

The methodology is to calibrate the virtual model with the measured result. Then one can simulate a fully deployed fall-of-potential earth test without any of the limitations found in the physical world, such as motorways, walls, rail tracks, etc. This means the deployed earth test can theoretically be of infinite length, eg, 1km, 2km, 10km … so you can really make sure the results calculated escape the electrode influence – eliminating doubt! And help make better, more informed decisions going forward.

Reference: BS 7430 Code of practice for protective earthing of electrical installations