Substation Earthing Installation & Testing

What we do

Substation earthing is a different job from lightning protection, even though the two share materials and techniques. It normally comes to us as a set of approved drawings from the client or their consulting engineer, and what is wanted is careful installation, honest testing and records that stand up afterwards.

We install earth grids and mats, driven rods and deep electrodes, horizontal tape runs, and the bonding that ties the structure, the enclosures and the equipment into the same system. Where the design allows it we make use of structural steel and piling, which is usually the lowest impedance connection available on site and the cheapest to make while the ground is still open.

Standards we work to

BS 7430. Code of practice for protective earthing of electrical installations, including soil resistivity measurement.

BS EN 50522. Earthing of power installations above 1 kV.

ENA TS 41-24 (2018). Design, installation, testing and maintenance of main earthing systems in substations, including touch and step voltage limits.

BS 7671. For the low voltage installation and the interface with it.

BS EN IEC 62305. Where lightning protection and the earthing system have to be coordinated.

Touch and step voltage

The point of a substation earthing system is not simply to get current into the ground. It is to keep the voltage differences a person could bridge, standing on the ground or touching a structure, inside safe limits for as long as the fault lasts.

Those limits are set out in ENA TS 41-24 and depend on the protection clearance time, which is why the earthing design cannot be separated from the protection settings. It also means an earthing system that was adequate can stop being adequate if the protection is changed.

Talk to us about a site

Testing and records

Total earth electrode impedance is measured using the AC fall of potential method. ENA TS 41-24 rules out the 61.8 per cent rule and the slope method for that measurement, so we do not use them. Soil resistivity is measured to BS 7430, and continuity and bonding are verified across the grid and to the equipment.

You get the measured values, the method and the conditions they were taken in, and the as installed drawings. On a buried system that record is the only evidence that will exist once the ground is closed, so it is worth having it done properly.

Common questions

What is substation earthing?

Substation earthing is the buried system of electrodes, conductors and bonding that gives fault current and lightning current a controlled path into the ground, and holds touch and step voltages at the site within safe limits while it happens. It typically combines an earth grid or mat, rods or piles, and bonding to the structure and equipment.

What are the two types of earthing in a substation?

Substation earthing is normally described as system earthing and equipment earthing. System earthing connects the neutral point of the electrical system to earth to control voltages during a fault. Equipment earthing bonds the metalwork, enclosures and structures so exposed parts cannot reach a dangerous potential.

What standards apply to substation earthing in the UK?

BS 7430 is the code of practice for protective earthing of electrical installations and covers soil resistivity measurement. BS EN 50522 covers earthing of power installations above 1 kV. ENA TS 41-24, in its 2018 issue, sets out design, installation, testing and maintenance requirements for main earthing systems in substations, including the touch and step voltage limits, which depend on protection clearance time.

How is a substation earthing system tested?

Total earth electrode impedance is measured using the AC fall of potential method. ENA TS 41-24 specifically excludes the 61.8 per cent rule and the slope method for this measurement. Soil resistivity is measured in accordance with BS 7430. Continuity and bonding are verified across the grid and to equipment.

How can substation earthing be improved?

Options depend on why it is falling short. Extending the grid, adding rods or deep driven electrodes to reach lower resistivity strata, adding horizontal tape, using earth enhancing backfill, and bonding to available structural steel or piling all reduce impedance. Which is appropriate comes out of measured soil resistivity rather than from a rule of thumb.

Do you design the earthing or install to our design?

Usually the second. Substation earthing normally reaches us as a set of approved drawings from the client or their consultant, and our job is to install it correctly, test it, and hand back records that prove it. Where you need design input as well we can look at that, but installation to plan is the service most clients want from us.

Substation earthing