What is the Main
Earthing Terminal?
The MET is the central earthing point of every UK installation. What it does, what physically terminates there, what cable sizes BS 7671 demands plus how to identify yours during an upgrade.
The Main Earthing Terminal (MET) is a copper bar or terminal block that acts as the single earthing point of your installation. Every protective conductor in the property terminates there. The earth from the supply (or from a local earth electrode) connects on one side; every CPC from the consumer unit, every main protective bonding conductor plus any supplementary bonds connect on the other. Without a sound MET no protective device on your consumer unit can do its job. BS 7671 specifies 16mm sq minimum for the main earthing conductor on most UK domestic installs.
Three figures
worth knowing
Main Earthing Conductor
Minimum CSA for the main earthing conductor on a UK domestic install with TN-C-S supply. Larger installs require 25mm sq or above.
BS 7671 Section
Section 543 sets the cross-sectional area requirements for protective conductors based on the line conductor or earth fault current.
BS Standard
BS 951 defines the earthing clamp standard. Copper construction, marked SAFETY ELECTRICAL CONNECTION DO NOT REMOVE.
Five conductors
terminate at the MET
The MET is the meeting point of every protective conductor in your installation. Five distinct conductor types arrive there.
The earth from the supply
For TN-S plus TN-C-S systems, the earth conductor from the DNO arrives at the MET. TN-S uses the supply cable's metallic sheath. TN-C-S (PME) uses a combined earth-neutral conductor split locally to provide an earth connection on a dedicated terminal.
The earth electrode (TT only)
On a TT system there is no earth from the supply. The installation has its own earth rod driven into the ground outside the property. The conductor from the rod terminates at the MET via a BS 951 earthing clamp.
CPCs from the consumer unit
Every circuit's circuit protective conductor (CPC) terminates on the MET via the consumer unit earth bar. This is the path that carries earth fault current from any appliance fault back to the supply earth.
Main bonding to gas plus water services
Main protective bonding conductors run from the MET to the incoming gas pipe plus the incoming water pipe (within 600mm of the meter or stop tap). This brings the metallic services to the same earth potential as the installation.
Main bonding to structural metalwork
Where structural metalwork (steel beams, lightning protection downconductors, oil tank pipework) is exposed plus accessible, it must also be bonded to the MET. Less common in standard UK domestic builds. Always required where present.
The single point that holds the whole earthing system together
UK domestic earthing exists for one purpose: to provide a low-impedance return path for fault current so that the protective devices upstream can detect the fault plus disconnect the supply within the BS 7671 disconnection times. Without that return path an MCB cannot trip on an earth fault. An RCD has nothing to compare its line plus neutral currents against. The whole automatic disconnection of supply (ADS) framework collapses.
The MET is the single physical point that ties the whole system together. Every protective conductor in the installation either originates from or terminates at the MET. Three things follow from that:
- The MET must be physically robust. A copper bar with bolted connections, normally rated to handle the prospective fault current of the supply for the disconnection time without overheating.
- The MET must be accessible plus identifiable for testing. EICR engineers measure the resistance from the MET to every accessible earth point during inspection.
- The MET must be permanent plus labelled. BS 7671 requires the durable label "SAFETY ELECTRICAL CONNECTION DO NOT REMOVE" near the terminal so that it is not accidentally disturbed.
Where the MET physically lives
On most UK domestic installs the MET is one of three physical configurations:
- Inside the consumer unit. The earth bar at the base of the consumer unit doubles as the MET on most modern installs. The supply earth, all CPCs plus the main bonding conductors all terminate on this bar.
- Adjacent to the consumer unit. A separate copper bar mounted in the same enclosure or on the same backboard, fed by the supply earth plus connected to the consumer unit earth bar via a short link.
- At the meter cabinet. On older installs or where the consumer unit is at distance from the supply, the MET is a copper bar in the meter cabinet itself with the supply earth terminating directly there.
None of the three is more correct than the others. All are BS 7671 compliant. The choice is normally driven by the original install plus by where the gas plus water services enter the property.
Why MET CSA matters during a board upgrade
One of the most common reasons a consumer unit upgrade quote increases on a survey is the discovery of an undersized main earthing conductor. The MET cable from the supply earth on older installs was sometimes 6mm sq or 10mm sq which was acceptable under earlier editions of the wiring regs. Current BS 7671 requires 16mm sq minimum on a TN-C-S supply for typical UK domestic installs. Where the existing main earthing conductor is undersized, the upgrade work must include replacing it from the meter cabinet to the consumer unit. This is normally the largest single price-add discovered during a board survey.
The same applies to main bonding conductors. Older installs sometimes used 6mm sq main bonding to gas plus water. Current standard is 10mm sq for a TN-C-S supply. Upgrades typically include replacing both bonds back to the MET as part of the new install certification.
Earthing system types: TN-S, TN-C-S, TT
The MET configuration depends on which earthing system the property uses:
- TN-S. Older system. Earth provided by the cable sheath of the DNO supply. Now uncommon on new installs. Where present the supply earth conductor terminates directly at the MET.
- TN-C-S (Protective Multiple Earthing, PME). Modern UK default. The DNO supplies a combined earth-neutral conductor which is split locally. The earth side terminates at the MET. Identifiable by a green-yellow conductor between the supply head plus the consumer unit.
- TT. No earth from the supply. The property has its own earth electrode driven into the ground outside. Mostly found on rural overhead supplies. Requires 30mA RCD protection on every circuit because the earth fault loop impedance is too high for an MCB alone to operate within the disconnection time.
Three signs that
identify your MET
Open your meter cabinet or consumer unit cover. The MET will normally show one of these three identifying features.
The yellow safety label
"SAFETY ELECTRICAL CONNECTION DO NOT REMOVE." A bright yellow durable label directly attached to the terminal or the earthing clamp.
Multiple green-yellow conductors
The MET is the only point where multiple green-yellow protective conductors converge. Earth from supply, CPCs from consumer unit plus main bonding all together.
A copper bar or BS 951 clamp
Either a horizontal copper bar with multiple bolt holes or a brass clamp with the BS 951 marking. Both are designed for permanent multi-conductor termination.
Four things every homeowner
should know
Never disconnect anything from the MET
Removing a single conductor from the MET breaks the earthing system. The yellow safety label is mandatory plus serious.
Old MET cables often need replacement
Pre-2008 installs often used 6mm sq or 10mm sq for the main earthing conductor. BS 7671 now demands 16mm sq on most UK domestic installs.
Main bonding terminates here too
The MET is also where the gas plus water main bonds terminate. Loose or missing bonds normally show up at the MET first on an EICR.
TT installs have their own earth rod
If your property is on a TT supply, the MET connects to an earth rod outside. Check the rod is intact plus the BS 951 clamp is sound.
Earthing plus Bonding Upgrades in Milton Keynes
C-Lec Electrical inspects, upgrades plus certifies main earthing terminals plus main bonding conductors to BS 7671 across Milton Keynes, Bedford plus the surrounding Bedfordshire area. Full EIC plus Building Control notification on completion.
For the wider context on consumer units, RCBOs, AFDDs plus the regulations behind UK distribution boards, head back to our full guide to consumer units where every common question is answered in one place.
Back to the Consumer Units Guide
This article sits inside our complete Consumer Units knowledge base. The hub covers everything from board types plus RCBOs through to landlord requirements plus BS 7671 wiring regulations.
More on consumer
unit anatomy
To understand the bonding conductors that terminate at the MET in detail, head to what is bonding in electrical systems for the practical breakdown of main plus supplementary bonding. To understand the BS 7671 framework that drives the earthing requirements, see consumer unit wiring regulations. To understand the protective devices that depend on a sound MET to function, see what is an RCBO board. If you need an earthing or bonding upgrade in Milton Keynes or Bedford, our consumer unit upgrades service page is the fastest route to a quote.