Also it could be the fan wiring shorting out. I had this and when i looked at the fuse box the space for the rad fan fuse was melted.
This was the tread:
https://passionford.com/forum/ford-sierra-sapphire-rs500-cosworth/271986-traking-down-probelm-with-the-fan.html
I got a replacement fan wiring loom and it works great now!!
heres a link to it on ebay if you need one:
http://urlant.com//yvwjmu