Event organisers, contractors and others using electrical equipment must do all that is reasonably practicable to ensure that electrical installations and equipment at an event are properly selected, installed and maintained so as not to cause death or injury.
Installation of electrical supplies must be carried out by a competent qualified electrician. Your contractor needs to make sure that:
A certificate should be obtained from the contractors as a record that the electrics have been installed safely.
For full and comprehensive guidance: Health and Safety Executive