Wiring problem?

Hi, I've got an Ideal Icos HE24, about 3 yrs old which has had problems for a while - ie thermistor fault/failing to light/overheating etc and was advised that it was either the board or thermistor. Last week I changed both. Yesterday it was dead - the wiring loom for all the sensors had burnt out. The fuse on the spare and board had also gone. I've ordered a new loom but wondered what could cause the melt down? There also seems to be no heat sheild for this wiring which seems strange. Any ideas?