Electrical Testing - How often is it required?
The required frequency of fixed wire or periodic testing is set by the latest Electrical Standards (currently BS7671 18th Edition) and takes into account the installation's use as well the operating environment which predicts the types of external influences to which it is subjected.
For instance, testing in a commercial office is typically every five years whereas testing in a leisure complex is typically every three years whilst a spa or swimming pool would require annual inspection. This is because an electrical installation exposed to a wet environment will most likely deteriorate quicker than one in a dry environment.
Larger companies or those with multiple sites may choose to test part of the site at a time (eg 20% per year for a 5 year frequency, or 33% per year for a 3 year frequency) in order to spread the cost and minimise disruption. As long as the entire installation is tested within the required testing interval, it is ok to spread the testing in this way.