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If I were you I wouldn't ask here, no disrespect but you will get half a dozen different and conflicting answers .HSE will have the right answers. Here is a start;https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0ahUKEwiXlNXgypHOAhXhLcAKHdEnCeIQFggcMAA&url=http%3A%2F%2Fwebcommunities.hse.gov.uk%2Fconnect.ti%2Fworkheight%2Fview%3FobjectId%3D12723&usg=AFQjCNHyk_NMD9kQznv2bmsoLmRi1stQ5A