Our guest blogger today is James Eade, tutor on our new Temporary Electrical Systems (Understanding BS 7909) short course…
“To answer that question, it is probably easier to look at where problems lie and explore why current accepted practice is not necessarily best practice. Generally the industry has a good reported safety record as far as electrical injuries are concerned but few can honestly say that they have not had an electric shock or do not know of someone who has. Equally most will have seen burnt-out electrical equipment, the cause of which is as likely to be bad design as it is component failure. In most cases it is purely good luck that these incidents do not result in injury and typically embarrassment over the (often elementary) mistake by the recipient has precluded them being reported appropriately.
The industry itself has developed good and robust electrical distribution equipment capable of withstanding the arduous conditions that touring puts it through while facilitating rapid set up in a ‘plug and play’ fashion. This though has led to a level of complacency which has increased the risk of shock and fire with regard to the design of such systems. For example, cables and connectors are considered as being capable of delivering their rated load – e.g. a 63A single phase cable/connector assembly can deliver a maximum current of 63A, which is not always the case.
RCDs and other circuit protective devices in distribution equipment are often considered a ‘panacea’ in many cases for fault and shock protection, but an incorrectly designed or deployed system may render such protection void or give rise to ‘nuisance’ tripping. The plethora of electronic equipment has also given rise to issues with the design and operation of temporary systems. Large lighting rigs have few simple incandescent loads these days with the bulk of the lighting rig being made up of moving lights or LEDs with Switch Mode Power Supplies (SMPS), with attendant problems in correctly setting up RCD protection caused by leakage currents as well as power quality issues such as high harmonic currents. Harmonic neutral currents give rise to overheating and again most techs will have experienced, or know of, and event where the neutral has failed.
As you probably know there is a growing impetus behind ensuring the electrical safety of events brought about largely by the introduction of the revised BS 7909 – Code of Practice for temporary electrical systems at events. While broadcasters (in particular Sky and the BBC) have been working hard at ensuring their OBs and general shoots (and hence subcontractors) work in accordance with the standard, the events industry at large is slow to catch up.
We know there is nothing new in that, and as an industry we have been keeping our head low and just getting on with life – thankfully without significant electrical incident, or at least publicly so. The culture though of ‘plug and play’ mains distro equipment leads us into a false sense of security as people tend not to think about what they are actually doing – a recent example of this was an event that I went to inspect that had (amongst other things) a long 32A cable feeding a couple of lights in a building. The load was only in the order of around 10A, but given the length of the cable had one of the lights suffered a complete short circuit and started burning, the circuit breakers would have taken several minutes to trip by which time the light could have set fire to the carpet. Also the voltage at the light was about 180V so perhaps unsurprisingly the lighting company had already replaced one ballast that had died.
BS 7671 has, in Part 1, some fundamental principles and amongst others states that an electrical system should not cause a fire or electric shock. The rest of the standard shows the reader how to achieve that. Also BS 7671 is not for houses and commercial installations as is often believed, it is for all electrical installations with a few exceptions such as mines or boats. It makes no distinction between a temporary and a permanent installation in that respect and the fundamental requirements are pretty basic aims regardless of the nature of the actual system.
Another area widely misunderstood is that of earthing temporary systems – it is often not known when it is necessary to put a spike down or when to join the earths of different supplies together such as when operating generator fed equipment inside a building for example. Even the concept of bonding metalwork such as stages to the main earthing terminal of the supply is often misunderstood. For the most part though, poor earthing practices just result in equipment failure or interference problems such as erratic DMX, hum on audio systems or fuzzy video pictures. However depending on circumstances, it is foreseeable (and not unknown) for all exposed metalwork to reach mains potential.
While BS 7909 is evidently not law and not a ‘requirement’, compliance with the Electricity at Work Regulations is and in the event of an electrical incident such as a shock or fire, 7909 may well be cited in the aftermath along with the IET Wiring Requirements (BS 7671), which references BS 7909 for event power distribution systems. There is a common misconception that site electrics should be done to BS 7909, not BS 7671. It is not an ‘either/or’ case – systems should be designed in accordance with 7671, and deployed, managed and tested in accordance with 7909. Both standards cross reference each other in that respect. And dare it be said, learning how to apply them might require a bit of a culture change within our industry.”
Written by Backstage Academy
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