Well, both are waterproof so it does not matter so much from that point of view. Both can be comfortable to handle and both can offer adequate protection, if used correctly, too. The question is, protection for what, exactly? Chartered Engineer James Eade explains all……

People rarely pay any attention to the cables used to deliver power. A cursory check to ensure there are no chaffs, scrapes or bare copper poking out would satisfy some; but even then most don’t bother. When was the last time you actually checked the whole length of the extension lead for your garden tools, your laptop power supply or the vacuum cleaner? All get relatively arduous use and are subject to damage. Even repeated bad coiling, as often happens with the vacuum cleaner cable, causes the insulation to crack over time.

At events, cables have an even harder time. They are regularly trodden on, driven over, stretched and trapped; in temperature extremes from the heat of the summer sun to the ice of the winter. Needless to say they need to be pretty robust and cared for as a result, but it helps to know which cables are better than others and that is largely a function of the material the insulation is made of.

Cable insulation falls into two categories. One is ‘thermosetting’ insulation that typically consists of rubber compounds that get harder, and brittle, when heated but are flexible under most conditions. The other types are ‘thermoplastics’ which deform and melt when hot and go brittle when cold. Most readily available extension leads are made from PVC, which is thermoplastic, as are the cables that come with appliances, from laptops to toasters.

There is a British Standard covering temporary power for events called BS 7909 and it is quite explicit in requiring only rubber insulated cables with an insulation rating of 450V single phase / 750 V three phase (typically expressed as 450/750V) to be used outdoors.

The prime reason for this is one of mechanical protection. In the cold, PVC cables get brittle whereas in the warm they get very soft. In both cases detrimental damage to the insulation can easily be caused if, for example, a cable is trapped in a flightcase lid or driven over by a stage truck. The outer insulation of PVC cables is actually quite thin too.

Rubber cables remain pliable when cold, robust when warm and do not exhibit the same problems. Those rated at 450/750 V, commonly referred to by the designation ‘H07’, have thicker insulation and thus more mechanical protection. And that is why BS 7909 requires H07 rubber types to be used – the most common is ‘H07-RNF’ and that reference can be seen stamped on the flex itself. Most PVC cables are H05-VVF or may carry the older designation ‘BS 6500’ or ‘3183Y’ and that too will – or at least should – be stamped or embossed on the cable.

There is no specific requirement for PVC cables to have any special resistance to abrasion or mechanical damage above what is ‘normal’ and neither do they have to have good low temperature performance – the minimum required by the standard they are manufactured to is +5°C. They come in many different colours – white, black, orange, yellow, blue etc. and are generally made to the same standard. Some manufacturers may put in extra plasticiser to give the cables a better low temperature performance, but it is not a requirement, and more plasticiser can mean less mechanical resilience as a trade-off.

However there is an ‘arctic’ grade PVC type of flex coloured yellow. It is commonly used on construction sites in 110 V reduced-low-voltage (RLV) systems and can also be found powering traffic lights. It is only rated for use at 110V,o it can’t be used on ordinary low voltage (230/400V systems). It is made to British Standard BS 7919 (Table 44) – the whole concept of RLV is a British thing; hence there is no European standard for it. So the only true ‘arctic’ cable is not suited to anything other than 110V RLV systems. There is no Standard for 230V ‘arctic’ cables and blue PVC cables at an event are ordinary PVC cables coloured blue with no special protection. They may as well be white or black.

Midas recently questioned the application of this rule to festoon lighting – something in abundance at many outdoor events. The company rightly pointed out that H07 festoon cable is rarer than hen’s teeth. Festoon cables have special requirements to ensure compatibility with the bulb holders and generally the cables are thermoplastic or light-duty rubber, neither of which technically allows compliance with BS 7909 for outdoor use. As a result, this has been tabled for amendment in the next version of BS 7909 and it is anticipated that an exception will be made for such cables. In the meantime it may be an acceptable deviation from the Standard as long as the equivalent level of safety,that would be achieved by using H07, is met by the designer.

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