Electricians often debate how big their bonds are, where they should place them and how. Are they financial things you’d find in a stock market or is it about something that should be confined to the bedroom? James Eade gets his clamps out and prepares for action……
Everyone will have seen birds sitting on power lines, but they don’t get electrocuted and die, or in the case of the high voltage lines, vaporised in a puff of charred feathers. The reasoning is not, as more than one school physics exam paper answer has stated, that the birds have rubber feet, but because there is no potential difference across the bird. You never see a bird with a foot on each wire, or spreading their wings and touching an adjacent line. If you look at YouTube though, you can see the results of squirrels climbing from one line to another, but it’s not for the squeamish and illustrates the point…
This leads us onto the fundamental requirement for getting an electric shock – the body must be at two different potentials so that the voltage difference can drive current through the body. As we saw in the last article, it’s not so much the voltage that does the damage, but the current flowing through your soft tissues, which causes the water in the flesh to boil. The voltage is needed to drive the current around the body and the higher the voltage, the easier it is. For example, if you went and touched the positive and negative terminals of the 3.3V battery on your phone, you wouldn’t get much of a shock, in fact you more than likely won’t notice a thing. At mains voltage (230V) it is much easier to push current around and only ten-thousandths of an amp (0.01 Amps) can do you damage at this higher voltage. The socket on the walls in your home can, in theory, deliver up to 32 amps – i.e. 32,000 times more current than can kill you.
So this is the first principle; we need a potential difference. The second part of this story is the Earth. As we saw in the last article, Earth is used as a conductor with a potential of zero volts and is more often than not used to help in detecting faults. All well and good, but of course any old bit of metal in contact with the Earth is therefore at the same potential as Earth, and this becomes a problem when the metalwork is within the confines of our electrical installation. A simple, common example is as follows: Imagine you have a metal kitchen sink with metal taps. This is connected to copper pipes which are in turn connected to the incoming metal water main to the house. The water main, being buried in the ground, is at the same potential as Earth and so it follows that the kitchen sinks and taps are too as the pipes all form one long electrical conductor. Don’t get too excited about having plastic pipes though, the impure water you get out of a tap is conductive to a degree too and some underground water mains are still cast-iron.
Now, imagine there is a fault with your kettle and you have one hand on the sink and you reach out to pick up the kettle to fill it. If the kettle is at a higher potential than Earth caused by a fault, you are now the short circuit between the kettle and the sink and will hence receive an electric shock. The sink and taps are known as ‘extraneous-conductive-parts’ as they are external to the electrical installation but introduce the potential of Earth and can therefore increase the risk of an electric shock. The metal case of the kettle is an ‘exposed-conductive-part’ which is a protective measure for when basic insulation fails. As we noted, you need a potential difference to get a shock. If there is no potential difference, there is no risk of shock and so we aim to create a zone in which all metalwork has the same potential – i.e. an ‘equipotential zone’. To do that, we connect all metalwork that may introduce the potential of Earth, i.e. the extraneous-conductive-parts, to the metal work we use as protection around electrical equipment, the exposed-conductive-parts, and connect all of that to the mass of Earth with an Earth electrode.
Would you put a bond between the stage to the roof legs, or not?
Taking the kettle example, if the taps and sink are connected to the case of the kettle, it’ll all rise to the same potential in the event of a fault and, in theory, you won’t get a shock – or at least the risk is greatly reduced. That is why your kitchen sink and taps are connected to the main earthing terminal in the consumer unit – when you get home have a look under the sink and you should see the green/yellow wires connecting them, they will probably also be on, or around, the central heating boiler or cylinder too. In old electrical parlance, this method of protection used to be called ‘earthed equipotential bonding and automatic disconnection of supply’, or EEBADS. The connection between the extraneous-conductive-part and the main earthing terminal (MET) is a protective bonding conductor and is often colloquially, and wrongly, called an earth bond.
In events, especially outdoors, all manner of things need to be assessed to see whether they constitute an extraneous-conductive-part or not. Structures like stages, delay or front-of-house towers, camera gantries, marquee frames, seating structures etc. all need to be considered. It is *not* to say that all such things must have protective bonding, but considering that they are all likely to be in good contact with the ground and therefore at Earth potential, they ought to be checked. The key thing is that someone must be able to touch a faulty appliance and the metalwork in question at the same time. If they can’t, then the metalwork is nothing more than conductive bit of metal, like a patio table and chairs in a field or a fence around a duck pond.
How many potentially extraneous-conductive-parts can you count?Protective bonding is often not done properly, one tends to see the two extremes; either it is not done at all, or every scaffolding joint has a protective bond across it, which is usually over-the-top. And if you disagree with that, as some do, read BS 7430 ‘Code of Practice for Earthing’.
As a sobering reminder of the importance of ensuring adequate protective bonding, I would have regaled a tale of a friend who called one Christmas in a panic because the guitarist of a band he was working with had just been carted off in an ambulance following an electric shock. Poor protective bonding had allowed a wiring fault to go undetected on the stage, until the point when the guitarist grabbed the mic which was cabled back to the front-of-house desk, itself on a separate supply. The guitarist unfortunately detected the fault, but he wasn’t so easily re-set like an RCD is.
Instead, a recent similar incident was posted here on YouTube that demonstrates it a lot better. It’s a short clip in which a singer holding a mic at one potential grabs a truss leg of the lighting grid which is clearly at another. Watch the singers left arm and remember what I said about electricity being a potent source of energy, and why it needs protecting against.
Finally the usual caution: This article is designed to help foster an understanding of the particular subject in a light-hearted fashion. Electrical safety is of course no laughing matter, as the above YouTube link shows, so if you don’t understand, are not sure or can’t make your mind up, seek expert help. Both BS 7671 and BS 7909 provide the right advice and guidance.
James Eade is a Chartered Engineer with over 20 years experience working in the event industry, including theatre, festivals, tours and corporate events. He advises several trade associations and represents the industry on various British Standard Committees, including those for BS 7671 and BS 7909. James is also the author of the guidebook on temporary power published by the Institute of Engineering Technology. For more information visit www.eade.uk.com