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UK Mains Supply Voltages

The electrical supply that we connect to is supplied from a generating station or renewable power source through the National Grid. As the electricity moves through the grid its voltage is reduced through a series of sub-stations and eventually into the three phase and single phase power supplies we use.

The International Electrotechnical Commission groups the voltages (in IEC 60038) into the following categories:

  • Low Voltage (LV) up to 1000Vac
  • Medium Voltage (MV) from 1000V to 35kVac
  • High Voltage (HV) from 35 to 230kVac
  • Extremely High Voltage (-) above 230kVac

The electrical supply voltage is generated at a high level for a number of reasons including transmission losses within the National Grid distribution system.

Single and Three Phase Mains Supply Voltages

In 1995, electricity supply voltages in Western Europe were harmonised to 230Vac single phase and 400Vac three phase.

In the UK, the traditional single phase mains power supply voltage was 240Vac and in Europe the supply voltage was 220Vac. This has been harmonised to 230Vac +10% to -6%.

The traditional three phase mains power supply voltage was 415Vac and in Europe the supply voltage was 380Vac. This has been harmonised to 400Vac +10% to -6%.

The mains power supply voltage can vary dependent upon the time of day, local load and distribution arrangements.

A high mains power supply voltage can reduce equipment reliability. A low mains power supply voltage can lead to erratic server operation and potential crashes if the voltage levels falls outside the input voltage window of the switch mode power supply.

Uninterruptible power supplies provide a means to mitigate these risks and protect server room and datacentre environments from mains power supply problems.