Continuing the theme for lightning design. I was having a technical discussion with a delegate, during a seminar in Germany. The question popped up regarding Lightning design on an AD plant (AD = anaerobic digestion). This site, with long-buried cable-runs, between plant areas. The delegates question, “What about the buried cables?”
What do we do about Lightning protection for buried cables in Lightning design?”
The question was not aimed at surge protection, but at physical protection of the cables from a direct lightning strike. And, before you say what? Yes, buried cables are at risk from direct strike attachments.
What’s the problem?
So, consider this. The lightning stroke itself will have travelled a considerable distance through the air. Also, in the order of 1-2 km a dielectric medium. Therefore, the resistivity of air is approximately 1.3 – 3.3 x 10^14 Ωm depending on moisture levels (humidity, etc).
So, do we think a few hundred millimetres of soil cover (which incidentally, is far more conductive than air), is going to prevent a lightning attachment to the cable? Well, maybe yes, maybe no.
Clearly, the cable construction will play a part in a potential strike attachment. But lightning has been known to overcome the cable insulation of a few millimetres AND the soil cover combined. The actual risk may be acceptably small for most facilities, but what if you had the cables travelling in a Hazardous Zone or feeding a mission-critical facility, such as, a large-scale data-centre, nuclear power plant, telemetry on a highway or similar, then de-risking may be a no-brainer.
How to protect buried cables – Lightning Design
Lightning protection for buried cables follows the same principles as Lightning protection above ground. See the image below. Applying the angle of protection method, one can see the buried conductor shown performs the same protective function as a catenary wire or Earthwire on an Overhead Powerline (OHL). Imagine the cables are structures, and these structures are above ground so the soil doesn’t exist.
All we need to do is exercise a little imagination and imagine the cables are above-ground structures like the phase conductors on an OHL and ignore the soil.
This is what Lightning protection for buried cables looks like. If you’ve got a project or query you’d like us to look at, why not drop us a line on LiveChat?
Engage with us…
- Earthing Design Services & Lightning Protection Design – If high voltage power systems and lightning are causing you concern why not see how we can help with a quick ‘live chat’ below as a start.
- XGSLab – A complete software tool for the simulation of Power, Earthing, Grounding and Lightning Systems, get in touch to request a free demo.
- Get Certified – Start your journey to become certified in Power Systems Earthing & Design.