Complex sentence: When people work with electrical appliances, health and safety regulations say that the appliance must be; the interval between tests depending on both the type of appliance and the environment it is used in.
Split into simpler sentences: When people work with electrical appliances, health and safety regulations say that the appliance must be safe. Many types of equipment require testing at regular intervals to ensure continual safety; the interval between tests depending on both the type of appliance and the environment it is used in.