The fixed ones in the a/c, do they activate on contact with water or only due to a high impact force? E.g., if an aircraft broke up mid-air does that mean the ELT would not activate?
I believe they only operate with high impact force (the manufacturer's notes don't mention water activation... only "crash situation"). I don't have any values for the g forces required, but I'd imagine they would need a respectable jolt to operate them (certainly higher than bumps sustained during aircraft ground handling). The sensor may be oriented in a particular plane/axis, so a mid-air breakup may not trigger it.
The ELT transmitter unit is mounted to the stringers on the upper fuselage and a short coaxial cable runs to the external antenna. The ELT has an internal battery pack.
Someone mentioned EK aircraft having a manual switch in the flight deck.
Yes, there is a guarded switch on the cockpit overhead panel for both activating the ELT and cancelling a triggered ELT. You can't disable the unit with this switch, only reset it if it has been triggered. You may see this switch in cockpit photographs. The switch can either have a black plastic guard or a red/orange flap to keep it (the switch) in the armed position.
Rgds.
NSEU