The insurance company has the right to exclude from the claim the part that actually failed in the first place and caused the fire to start, because this part was damaged by its own breakdown, not by the fire. However they have to show which part it was - they can't just say 'the wiring failed and caused the fire so none of the wiring is covered'. You need to find out exactly what they are looking to exclude from the claim and why - from your question I don't think they have made that clear so you need to ask to see the engineer's report on the cause of the fire, and get them to tell you what's covered and what's not.