What does the law say?
The law says you must maintain electrical equipment if it can cause you danger; you should decide what maintenance is needed based on the risk of danger from the appliance.
The law says you have a responsibility to maintain any equipment used by employees that is provided by you, leased (e.g. a photocopier or water tower) or provided by a contractor. Equipment brought on to site and used by the contractor (e.g. a builder or disco company) must be maintained by them.
The law requires you to periodically check the equipment to see if there is any maintenance needed; how you do this depdends on the type of equipment but this is where the “PAT” test usually comes into effect… all electrical appliances need checking in one way or another; some will need an appliance test using a test machine but all will need appropriate user checks and visual inspections.
The appliance or portable appliance test will need conducting periodically, not necessarily every year whilst the user check and visual inspection will need doing more often.