While aircrafts are under servicing on ground, the external dedicated earth wires are connected to the aircraft to avoid static hares effect to human beings. How much resistance value has to be maintained?.Is there any standards available for the same.
For the US Navy, according to this instruction:
The maximum resistance allowable for the cable (MS27574) itself shall not exceed 0.5 ohms.
The cable’s operational (in use) maximum resistance shall not exceed 10 ohms.
The difference in values corresponds to the loss of quality in the electrical contact at both ends of the wire (clips). Contacts are allowed to be as resistive as about 5 ohms. For comparison hand to foot impedance is about 500 ohms at 100 Hz, meaning the current will be 50x lower in the body than in the wire in case the aircraft is not at the potential of the ground.
You'll find a similar requirement of maximum 10 ohms in: TO 00-25-172, Ground Servicing of Aircraft and Static Grounding/Bonding from USAF.
In small aircraft, a refueling bond wire is typically clipped to the exhaust pipe, the other end of the wire presumably connects to earth ground somewhere. This dissipates any static electricity on a metal airplane and also any static that may build up due to fuel flowing thru the fueling hose. I suspect the resistance is in the tens of ohms range or higher depending on what kind of deposits arae built up on the exhaust pipe.
You said military - they are larger aircraft, but still subject to the same phenomonem.