All electrical equipment used in, or associated with, places of work is required to meet certain standards of Health and Safety regulations (HSE, PUWER, EAW).
The scope extends from the largest distribution systems, down to the smallest piece of electrical equipment such as a hairdryer, a kettle, a telephone, or a PC.

There are four circumstances where employers are obliged to maintain and inspect equipment:
1. Where appliances are used by employees.
2. Where the public may use appliances in establishments such as hospitals, schools, hotels, shops etc.
3. Where appliances are supplied or hired.
4. Where appliances are repaired or serviced.
(Health and Safety at Work act 1974)
Basically speaking, everything that exists on a commercial premises that can be moved or unplugged must "be checked regularly and maintained... to prevent danger", (Electricity At Work Regulations 1989). The only practical way to do this is by portable appliance testing. Periods of official testing will be established by Three Pin, according to the environment and usage of the equipment under test. These times vary between three months and four years, with the typical period being one year between tests.