Hi, we've recently installed a timeguard NTT03 timeswitch to a water heater that was always on, but the fuse keeps blowing after maybe an hour or so and I'm not sure why, do this connections look about correct to you? thanks in advance!
Are these immersion heater elements ,within an unvented cylinder by any chance ? I suspect you had two seperate switches ,one for each element ,only allowing one to be used ,but not both simultaneously. You have now connected both elements to come on at the same time ,which would cause a 13 amp fuse to blow.
As @terrymac says, I assume you have a hot water cylinder with two immersion heaters. If that’s the set up it’s quite possible that the bottom heater doesn’t need a time clock because it is supplied from an off-peak electric supply and the top immersion was intended to be left turned off unless you ran out of hot water and wanted a daytime boost. So it’s probably the wrong timer and you have messed up the wiring. If it has two immersion heaters with one supplied by an E7 Off peak supply take the time clock out and reconnect the bottom immersion with a 20 amp switch, then reconnect the top immersion using an immersion countdown timer https://www.toolstation.com/2-hour-electronic-boost-timer/p66796 If you have two immersion heaters, but not an E7 supply, leave the bottom heater connected to the timer and install a countdown timer to the top.