How do I know if my home has a TT earthing system?

Look for a green-and-yellow cable running from the consumer unit down to an earth rod in the ground, usually outside near the meter. TT properties do not have the earth terminal connected to the incoming neutral. Your Distribution Network Operator can also confirm the supply type.

Yes, when correctly installed. TT systems rely on an earth electrode and require full RCD protection on every circuit because the earth fault loop impedance is typically much higher than on TN systems. A correctly installed TT system with RCBOs throughout is very safe.

Why do rural properties often have TT earthing?

Rural areas served by overhead power lines often have TT supplies because overhead line networks do not provide a metallic earth return path. The electricity supplier provides line and neutral only, so the installation has to create its own earth via an electrode.

Can I add a socket in a TT property without calling an electrician?

Like-for-like socket replacements are the same as on any other system -- isolate, photograph, transfer wires, check earth, restore. Adding new circuits or spurs in a TT property should involve an electrician, particularly to confirm the earth electrode reading is still within limits and the new circuit has adequate RCD protection.

Understand TT Earthing in UK Homes

Helpful video reference. John Ward (jwflame) -- an electrician based in Poole, Dorset, with over 500 educational electrical videos -- covers the key considerations of a TT supply installation in "TT Installations". John is well-known in the UK electrical industry for explaining the regulations clearly and without shortcuts. This video is a good companion to this guide if you want the technical detail behind the overview here.

Before you investigate your earthing. Do not disconnect or modify any earthing conductors yourself. The green-and-yellow bonding and earthing cables carry no current during normal operation but are critical to safety under fault conditions. If you want to check your earthing system, ask an electrician who has a multifunction tester to measure the earth electrode resistance and earth fault loop impedance. Guessing is not an option here.

1. The three UK earthing systems

UK domestic properties are supplied with one of three earthing arrangements:

TN-C-S (PME): The most common type in towns and cities. The incoming neutral cable also serves as the earth, and the electricity supplier provides an earthing terminal at the cut-out. Main bonding is particularly important here because a broken supply neutral can push dangerous voltages onto your metalwork.
TN-S: Older system, mainly in areas where steel-armoured supply cables are still in the ground. Earth comes from the outer sheath of the supply cable. Becoming less common as networks are upgraded.
TT: No earth provided by the electricity supplier at all. The installation must create its own earth by driving a copper rod into the ground. Common in rural areas served by overhead lines, and in some older urban areas with lead-covered supply cables.

2. How to identify a TT installation

The clearest sign is a green-and-yellow cable running from the consumer unit to an earth rod outside, usually to a small inspection pit or directly into the ground near the meter position. There will be no earth terminal on the electricity supplier's cut-out.

If you are not sure, your Distribution Network Operator (the company that owns the lines and cables, not your energy supplier) can tell you what type of supply your property has. In east Kent that is UKPN (UK Power Networks).

A qualified electrician can also measure your earth fault loop impedance (Ze) at the origin. A TN-C-S system typically reads below 0.35 ohms. TT systems are much higher -- often 20 to 200 ohms -- because the current has to travel through the ground to return to the source.

3. The earth electrode

The earth electrode in a TT system is most often a copper-clad steel rod, typically 1.2 metres long, driven vertically into the ground. Its resistance to earth must be low enough that RCDs will trip fast enough to be safe. The target set by BS 7671 is that the product of the electrode resistance and the RCD operating current must be less than 50 volts.

In practice, with a 30 mA RCD, the electrode resistance needs to be below about 1,667 ohms -- but in reality a resistance above a few hundred ohms is a sign the electrode needs attention or a second rod needs to be added. An electrician with an earth electrode tester can measure this properly.

Electrode resistance increases during dry weather when the soil moisture drops. If your installation has a history of nuisance RCD tripping, particularly in summer, a high earth electrode resistance could be a contributing factor.

4. RCD protection requirements for TT

Because the earth fault loop impedance in TT systems is high, overcurrent devices (MCBs) alone cannot be relied on to clear faults quickly enough. BS 7671 requires that all circuits in a TT installation are protected by a residual current device. In a modern installation this typically means an RCBO on every circuit, or RCDs in the consumer unit covering all circuits in groups.

An older TT property with a split-load consumer unit where some circuits have no RCD protection at all has a Code 2 defect under an EICR. This is not immediately dangerous in the way a Code 1 is, but it does need remedying.

5. Main and supplementary bonding in TT systems

Main bonding -- the green-and-yellow conductors connecting gas and water services to the main earthing terminal -- is required in all UK domestic installations, TT or otherwise. In a TT property this bonding connects to the earth electrode via the main earthing terminal.

Supplementary bonding in bathrooms may be omitted if all circuits supplying the bathroom are protected by a 30 mA RCD and the main bonding is intact. An electrician can confirm which approach is correct for your installation.

Stop and call an electrician if: your consumer unit has no RCDs covering TT circuits, the earth electrode cable has been cut or corroded, your RCDs are tripping repeatedly for no obvious reason (a high electrode resistance can cause spurious tripping), or your EICR lists earthing-related Code 1 or Code 2 items that have not been addressed.

When to call us

Richard carries out EICRs and earthing checks on TT properties across east Kent. If you have bought a rural property, are getting a consumer unit upgrade, or simply want to understand whether your earthing system is up to current standards, a test and inspection report will give you a clear picture. Quotes are free.

Not sure about your earthing system?

Richard can test your earth electrode resistance and earth fault loop impedance and explain what the readings mean. Honest assessment, no upselling.

Book an earthing check

Understand TT earthing in UK homes