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by the NFCC

Three-phase high-voltage systems


Large industrial or commercial

Where the demand for power is high, electricity companies may supply power at high voltage (typically 11kV) to a substation on the consumer's premises. For a medium-size consumer, a transformer may be installed at this point to supply a normal three-phase low-voltage installation, that is a three-phase four-wire busbar with separate sub-circuits connected to it.

Consumers may require a high-voltage supply at remote points on the premises. In this case, the electricity company will install a high-voltage meter circuit breaker and connect this to the consumer's high-voltage busbar.

The consumer installs their own high-voltage switchgear to control each high-voltage substation. Transformer stations are connected to the ends of the sub-mains to give a three-phase low-voltage system.

Because the operation of high-voltage switchgear should only be carried out by properly trained and competent personnel, the system will be provided with an emergency trip switch to isolate the consumer's high-voltage system from the electricity company's distribution system.

The emergency trip switch for this type of system can differ depending on the size of installation. This level of information should be recorded in the Site-Specific Risk Information gathered by fire and rescue services on site visits prior to incidents.

Hazards (for further information refer to National Operational Guidance: Utilities and fuel)

  • Electrocution (especially if there is water at the electrical intakes)
  • Damage to switchgear can make isolation difficult - this will require specialist assistance

References and further reading