Understand the three UK earthing systems (TT, TN-S and TN-C-S)

Helpful video reference. John Ward (channel: jwflame) covers the three UK earthing types -- TT, TN-S and TN-C-S -- in his video "Types of Earthing System for Electricity Supplies (UK)". John is a qualified electrician based in Dorset with one of the most thorough UK electrical tutorial channels available. His explanation of how PME earthing works is particularly clear.

1. Why earthing exists

Every electrical installation needs a path for fault current to flow back to the source when something goes wrong -- a cable rubs through insulation, an element fails, a live wire contacts metalwork. That path is the earth conductor.

Without a good earth, a fault could make the metal body of an appliance or the water pipes live at full mains voltage. Touch them and you become the return path. The earth conductor, combined with an RCD or RCBO, makes sure the protective device operates quickly enough to prevent a lethal shock.

The earthing system is the arrangement that provides this return path from your installation all the way back to the source of supply.

2. TN-C-S (PME) -- the most common UK system

TN-C-S stands for Terra Neutral Combined-Separate. It is better known in the UK as PME: protective multiple earthing.

In a PME supply, the neutral and earth conductors are combined into a single conductor (called the PEN conductor) throughout the distributor's underground network. They are only separated at or near your property -- typically inside the meter cupboard, where the combined conductor splits into a separate neutral terminal and an earth terminal.

The main advantage is that the earth impedance is very low, because there are multiple earthing points along the distributor's network. This means protective devices trip quickly in a fault.

The potential downside: if the neutral conductor in the network fails (a broken joint, corrosion, a dig-through), the PEN conductor can rise above earth potential. Any metalwork connected to the PME earth can become live relative to the actual ground. This is why PME earth is not taken to outbuildings, garden rooms, hot tubs or swimming pools -- a local earth electrode (TT arrangement) is used instead for those locations.

How to spot PME at the meter: look for an earth block or busbar inside the meter enclosure with a label along the lines of "PEN" or "Main Earthing Terminal." The earth block is connected directly to the incoming neutral.

3. TN-S -- separate earth from the substation

TN-S supplies have entirely separate neutral and earth conductors running from the substation to the property. The earth conductor is usually the metal sheathing of the old lead-covered underground cable, or in some cases a separate conductor within the supply cable.

TN-S is found mainly in older urban areas that have not had their supply cables upgraded. Pre-1970s lead-sheathed cables are the giveaway. The system is being phased out as ageing cables are replaced, but plenty of UK homes are still on TN-S supplies.

The limitation of TN-S is that the earth impedance can be higher than PME because the earth path relies on the cable sheathing. If the sheathing corrodes, the earth quality degrades and EICRs often flag high Zs readings as a result.

Identifying TN-S at the meter: look for a separate green-and-yellow conductor connecting the sheathing of the supply cable to the earthing terminal, rather than the neutral block.

4. TT -- earth rod only

TT stands for Terra-Terra: earth to earth. In a TT installation there is no earth connection from the distributor's network at all. The entire earth is provided by an electrode buried in the ground at the property -- usually a 1.2 m copper-clad steel rod.

TT is common in rural areas where the supply runs overhead on poles rather than underground. It is also found in some older properties and any location where the distributor cannot or does not provide an earth connection.

Because the earth path runs through the soil rather than a metallic conductor, the impedance is much higher -- typically several tens of ohms rather than the fraction of an ohm you see with PME. This means overcurrent devices alone cannot be relied on to operate fast enough in a fault. BS 7671 requires 30 mA RCD protection on every circuit in a TT installation.

EICRs on TT properties include a test of the earth electrode resistance. Values above 200 ohms are acceptable in principle, but the lower the better. An electrode in poor soil can give high readings that require additional electrodes or a longer rod.

Identifying TT: the meter cupboard will have no earth terminal connected to the supply cable or neutral. There will be a cable running outside to an earth rod in the garden or beside the property. If you are not sure, a qualified electrician can confirm.

5. Which system does my home have?

Start at the meter cupboard. Look -- do not touch any DNO equipment.

• If there is an earth block or busbar inside the enclosure labelled "MET" or "PEN" connected to the incoming neutral terminal: TN-C-S (PME).
• If there is a green-and-yellow conductor clipped to the sheathing of an old lead-covered supply cable and running to the earthing terminal: TN-S.
• If the only earth connection is a cable running outside to a rod in the ground, with nothing coming from the supply cable or neutral: TT.

Not sure? Your electricity network operator (UKPN, Western Power, Northern Powergrid, etc.) can confirm what type of supply you have at your address. An electrician carrying out an EICR will also record it on the report -- look at Box 5, "Earthing arrangements."

6. Why it matters for practical work

The earthing system affects several decisions your electrician needs to make:

• Consumer unit upgrade: the type of surge protection device (SPD) and the earthing conductor size depend on the system type.
• Outbuildings: PME earth cannot be extended to a garden room, shed, hot tub or pool. A local TT arrangement with an earth rod must be used instead.
• RCD protection: TT installations need 30 mA RCDs or RCBOs on every circuit. TN systems have more flexibility, but Amendment 2 to the 18th Edition now requires RCD protection on most new circuits regardless.
• EICR observations: a degraded TN-S earth (corroded cable sheathing) or a high-resistance TT electrode can produce C2 or FI codes on an inspection report.

When to call us

Identifying your earthing system from the meter cupboard is something any homeowner can do. Actually improving or modifying it is qualified electrician territory. Anything that touches the main earthing conductor, the DNO's cut-out, or the bonding to gas and water pipework requires test equipment and certification.

If you are in Sandwich or east Kent and need an EICR, a consumer unit upgrade, or just a plain-English explanation of what your existing report means, call Richard.