poss problem is a bad connection inside the pull switch probably on the neutral so when the shower is off the power light is ok on the switch but as soon as a load is called for by switching on the shower you are losing the electric supply at the bad connection at the switch,or maybe at the terminal block inside the shower unit, BEWARE TURN OFF THE ELECTRIC SUPPLY AT THE FUSEBOARD OR BETTER STILL GET AN ELECTRICIAN IN FOR SAFTEY ONLY
CLIFF DAVIES, October 2007