Moving story

Portable electrical equipment is very useful, but when regularly transported from place to place it suffers far greater physical abuse than stationary items. The result, naturally, is a far greater level of risk.

The Health and Safety Executive, as a result of the Electricity at Work Regulations, recommends that construction site equipment should be initially tested every three months. The requirement for equipment to be ‘maintained so as to prevent danger’ goes beyond such simple portable appliances, and this article describes possibilities for larger transportable systems.

Electricity is treated as an essential part of modern living, so even in canal boats and camp sites, the user demands a source of power that can be used like ‘mains electricity in a house’. But the environment is totally different and so, therefore, are the risks.

The main differences between a mobile installation and a fixed one are mechanical abuse, movement, and earthing. Movement on wires can cause fatigue and fracturing of conductors. More rapid movement, such as vibration, can cause screws to come loose. Earthing for a mobile installation is uncertain, and in some cases, particularly when using small generators, may be completely lacking. The associated concept of an equipotential zone may be difficult to implement, which may mean a PME terminal is not available.

Such considerations have been behind the inclusion of caravans as a special location in the Wiring Regulations (BS7671:1992), and various guidance notes from the Health and Safety Executive. These cover the possibility of a generator supplying an ‘unearthed’ installation (See box, “Tent pole tips”).

In some situations, the complete transportable or temporary structure is larger than those that the regulations directly cover. Examples include fairs, circuses, pop festivals, and mobile bases for military and disaster relief purposes. In broad terms, there are three design methodologies:

• treat each site as a new installation, and design to suit that site, with many components being discarded when the event moves on;

• divide and conquer – use small, prewired units, which are treated as individual items, and may have their own independent generator. Caravan sites and fairground rides are common examples of this methodology;

• use a modular system that can be reused, and adapted to each variant. This is only possible where similar arrangements are repetitively used, and is likely to use a large number of plugs and sockets. Mobile lighting rigs for pop concerts are an example of this methodology.

Holy order

On a more personal note, my main experience has been with a large marquee owned by a church. A modular system has been used, and has given satisfactory performance for over ten years of touring the UK.

The requirements for this application have included good artificial lighting to allow use at night, with emergency lighting, external security lighting, and suitable supplies for various special purpose caravans, sanitary accommodation, and catering facilities, as well as supplies for an audio system, and stage and effects lighting.

The initial design concept, developed when the marquee had just three main poles, was to use fluorescent luminaires permanently attached to beams that could be suspended beneath the ridge of the marquee. These also acted as cable ducts, with flying leads to plugs at one end, and sockets at the other. The low cost option was taken of using 13A BS 1363 plugs and sockets, and ordinary 2·4 m 125 W fluorescent fittings, that have proved to be adequate for the environment.

A total of six independent circuits were included in each beam, and some additional outlets were provided which allowed a choice of two circuits for each luminaire, which had separate plugs.

This basic structure met most internal distribution and lighting requirements while the marquee increased in size by 350%, with the initial two lighting beams becoming 12 beams (see figure 1).

External security lighting was provided by 500 W tungsten halogen floodlights mounted on top of the main marquee poles, with the feed coming from the lighting beams, which also provided power for other minor requirements, such as battery chargers, within the marquee.

Maintained exit signs were permanently mounted on boards that could be fixed above the doorways, and daisy-chained, with waterproof connectors via a cable running tround the eaves of the marquee.

Separate circuits were used. This has the backing of the Guide to Health, Safety and Welfare at Pop Concerts and Similar Events, as it allows repairs to be undertaken more easily, and may avoid an event being terminated due to a fault on the lighting circuit.

As a result of the marquee growing in size, the total power distribution became more complex, often using two sources of power, one for overnight, to give security lighting and similar low power loads, the other during events which required all the lighting, and so a far greater demand. Often, the low power source was a supply from the mains, via a park pavilion, but the main supply came from a generator. This helped the noise level at night, but required a safe, quick method of switching loads between power sources after dusk.

Multiple power sources

With no synchronisation between a generator and the mains, the generator can run at a slightly different frequency (such as 51 Hz), giving a voltage difference of 460 V rms at the difference frequency (1 Hz in this example). Therefore, extra care must be taken, as this is greater than standard supply voltage, but still within the official definition of low voltage (see figure 2). BS7671 therefore requires an insulation test voltage of 1000 V, with a pass level of 0·5 m, that applies between separate circuits that may be supplied from different sources.

Switching between these circuits requires the devices to withstand this 460 V rms, which limits the choice to suitably rated contactors and some rotary cam switches. These were used in a distribution box with two inlets and several outlets.

Three individual 32 A feeds were taken to a further distribution point. This fed the individual circuits in the lighting beams, with 10 A mcbs in that distribution point being used to control the lighting. This 10 A rating was selected to ensure minimal likelihood of any 13 A plug fuses in the beams failing before the mcb tripped, with each circuit having up to five 2·4 m twin fittings (a nominal lighting load of 1250 W, but 2256 VA, 9·4 A). Only one of the three feeds to the distribution point was switchable between the two supplies, which allowed limited lighting during the night.

Testing times

For such an installation, safety is important, and it may not easily fit into existing categories. It may be considered a portable appliance, coming under the scope of such documents as Maintaining Portable and Transportable Electrical Equipment (HS(G)107), or as an installation, subject to BS 7671 requirements for wiring installations.

This topic is covered in chapter 13 of the Wiring Regulations, on ‘Fundamental requirements for safety’. This highlights:

• good workmanship;

• good materials;

• a reduction in danger by suitable construction, installation, protection, maintenance, inspection and testing;

• good design practice, including cable sizing and overcurrent protection;

• precautions against earth leakage.

The correct design and initial construction of a modular system therefore goes a long way towards these requirements. However, by itself, this is not adequate.

User visual checks, that are an important part of portable appliance maintenance, apply here, and the erection of the equipment is considered a skilled task. Given the continual moving of the installation, damage is likely, and so maintenance and repair is ongoing, to cope with that damage as soon as possible.

The subject of precautions against earth leakage requires closer attention. In almost all applications of this type, the protection devices will be 30 mA rcds. However, they should not be regarded as fit and forget items, as they can ‘fail to danger’. They are also only half of the solution to Earthed equipotential bonding and automatic disconnection of supply (413-02), giving the automatic disconnection, but not the bonding. The earth bond is generally a weak link in any portable appliance, as it can become disconnected, removing the safety it offers, without being detected by the user. In the case of our marquee, the earth loop impedance at the furthest points of each circuit was measured to ensure the ECC was complete, and all rcd devices were tested at the 30 mA level. A regular formal inspection and test should also be done.

A modular electrical installation is able to offer a cost-effective solution to the electrical requirements of a mobile structure. But care is needed to ensure that the high safety standards required are maintained, which requires skilled labour. A well-designed system can have a long effective life, and be versatile enough to cope with significant changes.