UV effects

We know about the importance of covering up in the sun because of the harmful effects of UV radiation on our skin. Prolonged exposure to UV from some lighting installations can have a damaging effect on certain electric cables, but are architects and designers correctly specifying suitable products to minimise it, asks Glynn Stainthorpe, business development manager of the British Approvals Service for Cables (BASEC).

BASEC has recently seen cases where insulation on some low smoke halogen free (LSHF) cabling close to a light source has become prematurely brittle, suffered cracks and in some cases fallen away exposing the conductor beneath as little as five years after installation in a non-domestic situation. Such effects are not limited to UK installations as we are aware of similar problems in Germany and Scandinavia.

The finger was initially pointed at heat being the cause, but the temperatures measured in the installations were not high enough to produce this sort of behaviour by themselves, in fact, the exposure of the cable to a combination of heat and UV is possibly a more significant reason for the dramatic degradation. We are currently testing for this but other factors could also be at play.

Where the installation is in a public building which requires lights on 24-7 the effect of UV exposure could be accelerated, but another significant factor could be that LSHF cable had been specified for its performance in a potential fire situation, rather than for the application as a whole. Previously, these applications would have utilised materials that have a higher tolerance of UV exposure, like PVC, but which conversely produce smoke and other toxic products in a fire situation.

The cases of degradation we have seen were within public buildings and involved modular lighting units installed around five or six years ago, although the UV effect might not be exclusively affecting modular systems. The cracked wiring was often found during routine maintenance and required remedial work or replacement of the affected cables.

Meanwhile we are continuing preliminary tests at our laboratory facility in Milton Keynes. So what is happening?

BASEC is now working with the Lighting Industry Association (LIA) on this issue. Additionally, we are seeking a test requirement for combined UV and heat degradation on cables to be introduced, possibly BS EN 50289-4-17:2011 which is for communications cables, or a similar test. Such a testing requirement needs to be included within appropriate cable standards.

Currently the standard answer to UV exposure is to put a black sheath on a cable as carbon black absorbs UV well and provides some protection from damage, but in most lighting applications the requirement is for white cables. However, this approach is only a partial solution as it does not remedy the issue of core insulation cracking where the sheath is stripped back for termination. A long term solution could be to manufacture cables to existing standards and build in a UV test for both cores and sheath, with yet to be defined pass and fail parameters.

At the present time an amendment to the UK Wiring Regulations BS 7671 is being considered, which would require consideration of heat and UV effects when specifying cable for lighting applications. If this goes through, cable suppliers are soon going to have to provide reassurance to users on this aspect.

In the meantime and until a suitable standard is published, architects and designers, contractors and electrical professionals should comply with regulation 522.11 of the IET Wiring Regulations (BS 7671:2008) which states:

• Where significant solar radiation (AN2) or ultraviolet radiation is experienced or expected, a wiring system suitable for the conditions shall be selected and erected or adequate shielding shall be provided. Special precautions may need to be taken for equipment subject to ionising radiation.

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