|
Merlin Industries were given the contract to install a new Electrical System, AV System and Data Network in a “New Build” straw bale house in Orkney.
This was the first Straw Bale house we had worked on, so a considerable amount of time an effort was put into preparing for the installation and gathering as much information as possible on any special requirements for Straw Bale construction. A great source of information came from the clients Nick & Barbara Morrison and from the “Information Guide to Straw Bale Construction” by Amazon Nails (available at www.strawbalefutures.org.uk).
The design of the electrical system was based on meeting BS:7671 (17th Edition), Scottish Building Regulations and incorporating additional features to guard against Fire and Rodent damage. Whilst there is still much debate about Fire Risk and Rodent damage in Straw Bale construction, there are a couple of points to consider in the electrical design.
Thermal Effects
Clearly straw bales in a compacted environment and rendered in lime mortar present a low combustible risk due to lack of Oxygen. Ever tried to burn a phone book in one go? From the electrical side the issue is the good thermal properties (U value) of the straw compared with traditional (modern) building materials and insulation products. The thermal effects on cables buried within the bale walls is somewhat of an unknown. BS:7671 (17th Edition) does not allow for such properties in its calculations, which are based on traditional (modern) building materials and insulation products.
The requirement here is to provide some form of barrier between cables and the Straw Bales to mitigate the thermal effects.
Rodent Effects
There is much debate concerning rodents in a Straw Bale house. Clearly the densely packed straw bale sealed within a mortar barrier is not an ideal environment. However, what gets overlooked are the voids between bales and around the timbers, floors and joists. Given the ample supply of dry warm bedding material i.e. straw, these would make ideal nesting locations for rodents. For some strange reason rodents love PVC covered electrical cables, and thus the issue for the electrical installation.
The requirement here is to provide some form of mechanical barrier to prevent rodent damage and ultimately fire / shock hazard due to damaged cables.
We reviewed several wiring solutions which would resolve both of the issues above in one go and decided on plastic conduit and galvanised steel box trunking. This combination provided the best all round solution in terms of cost, design and had the benefit of allowing the electrical installation to be modified in the future with minimal disruption.
After review, we rejected Steel Wired Armour cable (SWA) as too costly and inflexible, Mineral Insulated Copper Cable (MICC) as too costly, inflexible and having possible corrosion issues with the lime mortar, and steel conduit also as too costly, inflexible and having possible corrosion issues with the lime mortar.
The overall design of the electrical systems was based on a central “spine” of galvanised steel box trunking running in the void created between the roof and the roof plate. From this spine, individual circuits were created using a combination of 25mm & 20mm plastic conduit. The conduit was installed in roof and floor voids and channelled into the straw bales for connection to accessory points such as light switches and socket outlets.
 A special technique for channelling the straw bale was developed with help from our clients. We used a 110mm angle grinder fitted with a diamond cutter blade. The course blades of the cutter created a shredding action on the straw and we were able to easily and accurately cut channels and holes for accessory boxes. When cutting the straw we fully recommend the use of Personal Protective Equipment (PPE). Our operatives used gloves, goggles, particle dust masks and head protection. Cutting the straw in this way creates a huge amount of dust and chaff. The angle grinder needs to be kept clear of chaff, otherwise it will overheat. An airline to keep it clear is recommended if there is a lot of cutting required.
Fitting the accessory boxes to the straw was relatively easy. We adapted the method of wedges from the “Information Guide to Straw Bale Construction” by adding a piece of scrap plywood over the head of the wedge to create a flat surface for mounting. Wedges of 300mm depth were used with a 200mm x 100mm flat plate. The wedge was hammered into the bale using a lump hammer and the flat plate fitted over the head to give the correct mounting point for the accessory box. By employing this technique, the location of the wedge is not critical as the adjustment is made when fitting the mounting plate. Wedges tend to drift off course easily when hammered in so its can be quite difficult to maintain accuracy. Deep accessory boxes of 35mm and 47mm depth were used to facilitate cables running through a box to another accessory point and to help with the final fix.
Cable runs for the AV Network and Data Network were made in a separate system employing 25mm plastic conduit, meeting the requirements of BS:7671 (17th Edition) for segregation of Band I and Band II circuits. Accessory boxes were mounted in the same way as the electrical accessories.
 Final cable runs to the Distribution board and Communications cabinet were made in 75mm x 75mm and 50mm x 50mm galvanised steel box trunking. The trunking was installed within a utility cupboard so we used surface mounting for both systems.
By combining trunking and conduit, both systems would be easily adaptable in the future. An extra circuit or accessory could easily be added by pulling a new set of cables through the existing trunking or conduit without having to disturb plastered internal walls.
Updates will be added after final fix
|
 "Click for larger image")
 "Click for larger image")
 "Click for larger image")
