Text in force at CS25-28: from CS25-0, effective 17 Oct 2003.
Each fuel tank filler connection must prevent the entrance of fuel into any part of the aeroplane other than the tank itself. In addition –
(a) Reserved (b) Each recessed filler connection that can retain any appreciable quantity of fuel must have a drain that discharges clear of each part of the aeroplane; (c) Each filler cap must provide a fuel-tight seal; and (d) Each fuel filling point must have a provision for electrically bonding the aeroplane to ground fuelling equipment.
ED Decision 2003/002/RM · Source