Power demand of road lighting

This project investigated the charging wattage of un-metered street lighting, and found that neither of the current methods used in the UK to predict the power demand of street lighting is accurate. Public lighting in the UK is powered from the electricity supply system without metering. Charges for electricity are based on the agreed power demand values for different lamp and ballast combinations and a set of agreed hours of use. The consumption ratings, formerly presented in an appendix to Balancing and Settlement Code (BSCP) 520 are now found in Un-metered Supplies Operational Information.

The original schedule of consumption ratings, the appendices to BSCP520 were determined from the power demands for lamps and control gear reported in manufacturer’s literature. Following suspicion that these rates underestimated the true power demand, two field studies were carried out by the Electricity Association’s Load research Group, who installed meters and data loggers inside a sample of in-use lighting columns using 35 SOX lamps and 70W SON lamps. The field studies found that the average measured power demand was greater than that identified the schedule, an increase of 20% for the power demand of 35W SOX lamps, and these results are now the imposed charging rates.

A review of power demands was carried out by Steve Fotios in collaboration with Peter Boyce (Independent Consultant) and Richard Forster (Independent Consultant) for the UK Lighting Board of the Roads Liaison Group. It was found that the original schedule of charging rates as determined from manufacturers literature underestimated the real power demand because these data do not account for lamp ageing, failure of power factor correction capacitor, mismatch between lamp and ballast, and increase in supply voltage.

The field surveys of power demand carried out by the Electricity Association installed meters and data loggers inside a sample of in-use lighting columns. A common problem with road lighting is that the power factor correction capacitors fail, typically after only 12 months, they are not replaced until bulk lamp changes typically every three years, and hence the lighting is running at a low power factor in the region of 0.3 to 0.4. A report from Collins & Gross (Accuracy of electrical measurements on discharge lamp circuits. J. Illum. Eng. Soc., 1991, 20, 56-68) shows that at low power factor the measurement of power consumption is unreliable, over-estimating the power demand, unless very high quality power analysers are used. Thus, until proven otherwise, it must be considered that the field survey over-estimates the actual power demand.

It was thus concluded that neither of the original power demand schedule nor the field measurements which are used in the current power demand schedule are accurate: the former under-estimates, and the latter over-estimates, the true power demand.

Publications

• Fotios SA, Boyce PR, and Forster R, The power demand of discharge lamps used in public lighting, The Lighting Journal, 2005; 70(2); 32-37. (Awarded best article printed in the ILE Lighting Journal in the past year, December 2005).

• Fotios SA, Boyce PR and Forster R, The power demand of discharge lamps used in public lighting, proc. Local Authority Lighting 2004: Best practice = best value?, The 8th Annual Lighting Conference & Exhibition, 30th November 2004, Business Design Centre, London, Surveyor/ILE.

• Fotios SA, Boyce PR and Forster R, The power demand of discharge lamps used in public lighting, final report submitted to the Department for Transport, October 2004.