Hawk Fog Light fuse blowing.

[Halfa1200]

I have had these fitted for just over a year and now the fuse (15 amp),fitted between the battery and the rely has started blowing every time I switch the ignition on. Originally one of the bulbs blew so I replaced it and then discovered that the 15 amp fuse had blown also. I have fitted a different switch and mounted it next to the instruments so it is sheltered from the weather. Alas the 15 amp fuse is still blowing every time I turn the ignition switch on with the fog lights switched off. The 2 amp fuse between the relay and switch has not blown. I have checked the wiring for any chaffing and there is no sign of any damage.
The relay case can be separated and I have sprayed the relay with electrical cleaner. This clicks once when turned on with the fog lights turned off.

Any advice would be appreciated.

Halfa1200.