Substations are where electricity lines are connected and switched and where the voltage is changed by transformers.  They range from the very large to the very small - see below for a guide.  But in nearly all cases, the highest field  is usually produced by the lines and cables supplying the substation and not by the equipment inside the substation itself. If the substation itself produces a field outside its perimeter, it usually falls away over the first few metres.

We give more detail of the fields from different types of substations.  In each case click on the link for more details of the field it produces.

National Grid substations

photo of national grid substationThese are large substations where 400 kV and 275 kV lines are switched and electricity is transformed down to the next voltage, 132 kV.

more details


Sealing End Compounds

photo of sealing end compoundA sealing end compound is where an overhead line joins on to an underground cable.  It looks a bit like a small substation, surrounded by a metal fence, but does not contain any transformers or switchgear.

more details


Intermediate substations

photo of intermediate voltage substationThese are like National Grid substations but smaller and transform electricity between 132 kV, 33 kV and 11 kV.  They produce fields in a similar way to a National Grid substation, only generally at a lower level reflecting the lower voltages and smaller sizes.
more details


Final distribution substations

These transform the electricity from (usually) 11 kV to the voltage used in the home, 230 V.  See more details of the magnetic field they produce.  They come in different types:

photo of indoor substationsub-built-in-h150Indoor - built into a small building which may look like a garage, or as part of a bigger building.  The clue is the safety sign on the door.  This is the one type of substation which can sometimes give markedly raised magnetic fields beyond its own confines.

photo of cabinet type substationCabinet - built into a fully-enclosed cabinet.
photo of outdoors substationOutdoors - the separate components are mounted on the ground and enclosed by a metal or wooden fence.
photo of pole mounted substationPole-mounted - the transformer is raised above ground on a wood pole.

Numbers of substations in the UK

The following table gives numbers of substations in the UK published in 2011:

Substation Type

Typical Voltage Transformation Levels

Approximate number nationally

Typical Size

Typical Number of Customers Supplied

GridGrid Supply Point400kV to 132kV380250m by 250m200,000/500,000
Bulk Supply Point132kV to 33kV1,00075m by 75m50,000/125,000
Primary33kV to 11kV4,80025m by 25m5,000/30,000
Distribution11kV to 400/230V230,0004m by 5m1/500

In addition there were 349,000 pole-mounted 11 kV/400 V transformers.

source: ENA Adaptation to Climate Change First Round Report under the ARP, Electricity Transmission and Distribution Network Companies, 23 April 2011


To avoid confusion:

photo of warning signSubstations and pylons carry warning notices warning of the dangers of electrocution - this is nothing to do with EMFs.