How much damage does a 240V AC supply do to a human?

It is not the voltage which kills you, it is the current. For example, if you get a shock from static electricity, the voltage involved can be in the millions of Volts range (mega-Volts), but the momentary current is millionths of Amperes (micro-Amps). Makes you jump, but that’s all. On the other hand, the 240 volts AC of the domestic mains supply can easily kill under the wrong circumstances, for example when the skin is damp.

The voltage drives the current through the body, but it cannot drive more current than is available from the source, and a certain minimum voltage is needed to get the current through the barrier of the skin. The epidermis (dead skin at the surface) is quite a good insulator, and around 50 V DC or 30 V AC is required in order to feel the effects when the skin is dry. Also, the skin can limit the current from higher voltages. However, because the skin contains a lot of salt, and salt solution is a good conductor, wet skin “allows” electrical current into the body more easily.

Depending on the route that it takes through the body, a current in the range 10 thousandths of an Amp (10 milli-Amps) can kill by interferring with the rhthym of the heart. Larger currents burn along their tracks, and can kill by damaging organs and the nervous system.

So, pretty unpredicatable. Any source that can deliver more than a few 10s of milli-Amps or so at a voltage of a few 10s Volts, should be considered as dangerous.

I have personally received many 240 volt shocks over the years, so Techno Alien is wrong.

AC relays are specially designed using electromagnetic "slugging", so they don't chatter. The slugging slows down the mechanical movement of the armature, using induction damping, so it's a bit slower in operating. This stops chattering. So you should really use the right relay for the job. If that's impossible, I'd use a rectifier circuit to turn the a.c. into d.c. But you'll need a smoothing capacitor -- plus a dropper resistor or the relay coil will overheat, as the capacitor will charge to the peak of the a.c. voltage (339V).

death for sure.there wont be a chance at survival.