Lightning design for buried cables - really?

Continuing the theme for lightning design. I was having a technical discussion during a seminar in Germany. The question popped up regarding Lightning design on an AD plant (AD = anaerobic digestion) with long cable buried runs between plant areas - "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?

Consider this. The lightning stroke itself will have travelled a considerable distance through the air, in the order of 1-2 km a dielectric medium. The resistivity of air is approximately 1.3 - 3.3 x 10^14 Ωm depending on moisture levels (humidity, etc).

lightning design

Courtesy of Dehn

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 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?

 


Ian Griffiths SelfieIan Griffiths

This post is written by Ian Griffiths, Principal Engineer at GreyMatters, an Earthing & Lightning Consultant of 27 years, one of the top 1% accredited CDEGS consultants and professional advisor to international utility companies, data centre and infrastructure developers.

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