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QU E ST I ON
We are about to install a PV system array on the zinc roof covering on a domestic house. Are there any particular requirements that must be met in such cases? ANSWE R
We continue with our series of answers to some of the more frequently asked questions put to our Technical Helpline Q UE STIO N
What is meant by the term ‘premature collapse’ in Regulation 521.10.202?
Are plastic wall plugs an acceptable fixing method for the support of wiring systems to prevent premature collapse?
Q
A N SW E R
Yes. Properly selected and It is not practical to try installed plastic wall plugs can to define ‘premature be used to support cables, collapse’ in this context wiring systems and the like. purely in terms of time elapsed from the time a fire starts. The intention of Regulation 521.10.202 is to permit fire officers to safely navigate a building without fear of entanglement. The expectation is not for the cable supports to remain effective indefinitely, as conditions could reach a point where fire officers are no longer able to operate effectively in a property that is on fire. At that point the condition of the wiring system is irrelevant. Experience of past fires has shown that, in some cases, non-metallic cable supports have only failed after exposure to high temperatures for a considerable period of time, and certainly after the route was no longer viable. In other cases, the failure has occurred more rapidly. Cables have been known to fall almost instantly when a door was opened from an area in which a fire was burning onto a corridor, because so doing exposed the lids of plastic trunking in the corridor to the hot products of combustion. The resultant collapse of the wiring system caused an obstruction to those trying to enter or leave the building when, in all other respects, the route should still have been viable. Note 1 to Regulation 521.10.202 clarifies that the purpose of the requirement is to prevent wiring systems falling across access and egress routes, hindering evacuation and firefighting activities. Notes 2 and 4 give examples of measures that are deemed to meet the intent of the requirement. Note 3 clarifies that the requirement precludes the use of non-metallic cable clips or cable ties as the sole means of support where cables are clipped directly to exposed surfaces or suspended under cable tray, and the use of non-metallic cable trunking as the sole means of support for the cables contained therein.
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Structural integrity Section 4.3.11 of the MCS/ECA Guide to the Installation of Photovoltaic Systems advises that where a PV array mounting system is secured to the metal roof cladding, the adequacy of the roof covering to transfer all additional loads back to the supporting structure of the building should be verified. This verification should also include consideration of all elements of the roof construction that could be affected by the additional loading. It should also be verified that the weather-resistance of the roof covering has not been affected adversely by the installation work. Earthing Earthing is only necessary where the protective measure of automatic disconnection of supply (ADS) is employed and the equipment being connected is of Class I construction and so reliant on a connection with earth for reasons of safety. As the DC side of a PV system is a current-limiting generating set, the protective measure ADS is rarely used. More typically, the DC side of the installation is constructed to meet the requirements of an installation using the protective measure of double or reinforced insulation, and so protective earthing between the PV modules or frame and the main earthing terminal is not required. It should be noted that a Class I inverter will need to be earthed on the AC side of the PV system for which the protective measure is ADS. It may also be necessary to earth one or other pole of the DC side of the inverter for functional reasons, as recommended by the manufacturer of the modules and/or inverter. Protective equipotential bonding It is unlikely that the metal roof covering will qualify as an extraneousconductive-part, if only on account of
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