Hi Alice If your shower has a 30 Amp fuse then you must have and older style consumer unit (fusebox). However...if the original shower was about 7-7.5 killoWatts then this would have been adequate at that time. I would also suspect that the shower cable from the fuseboard feeding the shower would only be 6mm cable. But if it was replaced about a year ago by a higher spec shower, say 8.5 killoWatts or higher, It could be that the cable was not changed and should be at least 10mm. Also the fuse size should be higher (at least 40 Amps) and you also need RCD protection. If you have all this, it could be loose connections in the pull-switch or a broken pull-switch. This would cause fuses to blow or trip. If in doubt, get a local electrician to have a look. Log a call on here and get up to 3 quotes.