Professional Indemnity Insurance protects a business from financial claims where a business has been found to cause a loss to someone by provision of poor advice or professional services. If as an Electrician you are engaged in provision of advice for example, checking someone's work and writing a report, then yes you should have professional indemnity insurance. This is because if you provide incorrect advice (via a faulty report) then you can be sued for losses caused by your report. In most cases though, if you are changing switches and fixing light fittings you work isn't likely to be considered as advisory in nature in which case you probably wouldn't need to have professional indemnity insurance but the best way to know is to discuss with a qualified insurance broker.