Graphite rods and lightning

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Graphite rods and lightning

Published on Wednesday, 22 December 2010 18:07

Written by Daemon de Chaeney

Ed: The Brown Hornet posted an interesting question in the yakass forum a few days ago, asking about the veracity behind claims that graphite rods are likely conductors of electricity, and whether or not they are really a risk hazard in electrical storms. Not being much of an electrician myself I really didn't know the answer to that one (though I have found myself wondering if I was about to find out the hard way several times) so I read Daemon's response with interest. I couldn't tell you if his comments are 100% accurate but I'm willing to guess he's pretty close to the mark - certainly sounds like he knows what he's talking about. It's such a well detailed reply that I felt it worth adding to the article archives to enshrine it for future reference.

Graphite in fishing rods is actually carbon fibre. It differs from actual graphite in that though its building blocks have the same structure as graphite’s mono-atomic graphene sheet, unlike graphite, these sheets are not stacked on each other in a crystalline molecular bond organised structure (graphite is made of graphene sheets stacked by weak statistically electrostatic van der waals bonds).

In the turbostratic carbon fibre used for fishing rods, the graphene sheets are stuck together by being crumpled and tangled amongst each other (the same as plastic polymer). True graphitic carbon fibre is the same stuff but heat treated to a few thousand degrees, and this makes it less elastic but with higher tensile strength. Turbostratic graphite is more elastic with lower tensile strength.

Irrespective of all this, graphite is a semi-metal and a moderate conductor, not a good conductor. Carbon fibre is usually slightly less conductive again. This is because the conductivity is mediated by an electron cloud that can move across an individual graphene sheet, but not between sheets, with electrostatic mediation between sheets (which is better in the more organised crystalline graphite).

However, when it comes to lightning, other effects come in to play. Importantly, at high voltage, electricity is conducted over the surface of a conductor in preference to passing through it. Under these conditions dodgy conductors like trees, people, cows, plastic insulation and carbon fibre act as fine conductors indeed. Still not as good as an iron pole, but they’ll do the job enough to blow you to heaven. Interestingly, this is why some people survive a lighting strike: Most of the current passes over the outside of your body, burning you and generally sodding you up, but not passing through you where it would flash boil your guts and make you explode or otherwise end up fully FUBAR.

When lightning strikes, what is happening is that there’s an imbalance in electrostatic potential between the ground and the wet cloudy (or rarely even dry and clear) air. An electrostatic potential builds in the air. This encourages an electrostatic potential in the ground. Next, an electrostatic ionised path extends from the ground to the air, and then the potential is neutralised as the current flows along this path between the ground and the air; a lightning strike. This all goes on at a goodly pace, after the initial imbalances are established. This first bit might be fast or it might be slow.

The place the lightning strikes is marked by the build up of the ground charge imbalance. This will occur on the locally best conducting object. If it builds up relatively slowly, this will tend also to be the highest best conducting path. If it builds up very fast, a lower good conduction path may take precedence over a taller slightly less conductive path, if it’s in a preferred location with respect to the building charge.

So, whether you CF rod is going to ground the lightning will depend on a mob of factors:

Is it the best conducting path, which includes you, your shoes, its grip etc.
Are you wearing good insulating shoes like rubber gum boots or crap ones like leather soled boots?
Are there surface modifiers of HV conductivity? Are you, your rod grip and the ground wet or covered in sun screen containing ionic solid particles?

Are the taller things around you likely to be better surface conductors? Wet trees without rubber boots may be better. Wet trees may be worse than you holding a CF rod if you have no shoes on and are well earthed by a water path like standing on the wet beach sand.

All bets can be off if the next tall object, even an iron pole, is a bit of a distance off and the charge builds very quickly: you come into the nearest object rule and you may well cop it.

The obvious conclusion to all this is that there are so many variables you just can’t be sure, and it’s the right conclusion. You aren’t attracting the lightning per se, but you may be at increased risk of becoming the locally most favourable path for it to take, of it’s going to happen in your locality anyway.

From a risk analysis perspective, holding a long moderately conducting CF rod puts you at more risk than holding nothing, everything else being equal. The other environmental risks are too complex to predict an outcome in advance. Variables like cork or rubber grips, or shoe type strongly depend on their ultra high voltage conductivity, which is very hard to find out, but it’s worth remembering that some types of purpose built plastic insulation don’t insulate HV surface current, especially over short (read less than several metres) distances.

I would suggest that you are definitely at elevated risk compared to not holding a CF rod. You are at elevated risk compared to holding a glass rod. You are at elevated risk compared to holding a wooden stick. The risk is not elevated to the fatal 95th percentile. It’s not a death sentence, it’s just an elevated risk, and probably still lower than being splattered on the highway like a cane toad caught in a procession of geriatric gypsy motor homes.

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