What year did it come in that you needed an isolation switch for an electric shower?

Difficult to give a specific date, but its always been a requirement since electric showers first started to be produced, so probably the early 1970's or before. All fixed electrical equipment such as showers, cookers, hobs, ovens, heaters etc, always need an isolator switch, so that the equipment can be turned off when not in use and to make them safe to work on for cleaning and servicing. If you have had an electric shower installed in the last 40 years and no isolator was fitted, then its been installed incorrectly and was not compliant with the regulations in force at that time, let alone BS7671:2008. - What has provoked the question anyway?