Quote:
"Any part of an electric fence which is installed along a public path or highway shall be identified by warning plates securely fastened to the fence posts or firmly clamped to the fence wires at intervals of approximately 10 metres. The warning signs shall be at least 100mm x 200mm. The background colour of both sides shall be yellow. The inscription shall be black and shall be the substance of TAKE CARE - ELECTRIC FENCE. The inscription shall be indelible, inscribed on both sides and have a height of at least 25mm."
Source (which contains a great deal more information):
http://www.electricfence-online.co.uk/ishop/10 47/shopscr20.html#15
Chris