The main earthing terminal (§542.4) — why every connection must be individually disconnectable for measurement

The guide on protective earthing (§411) and the guide on earth electrodes and foundation earthing cover the earthing arrangement of an installation as a whole. This article covers the physical collection point of that arrangement: the main earthing terminal (MET), and the specific requirement §542.4 places on it.

What the main earthing terminal is

The main earthing terminal is the point where the following conductors of an installation come together:

the earthing conductor to the earth electrode(s);
the installation's main protective conductor (PE);
the main bonding conductors to extraneous-conductive-parts (water and gas pipework, metal structural steel — see the guide on main bonding);
in a TN system: the connection to the neutral/PEN conductor at the point where the network's earthing enters the installation.

In practice this is often a copper or brass busbar with several connection points, mounted near the meter cupboard or main distribution board.

The core requirement: individually disconnectable, tool-only

§542.4 (based on IEC 60364-5-54) requires that each conductor connected to the main earthing terminal can be disconnected individually, without disturbing the other connections — and that this disconnection is only possible with a tool, not by hand. This serves two purposes at once:

Enabling measurement: to measure the resistance of the earth electrode itself (see the guide on measuring earthing resistance), the earthing conductor to the electrode must be isolatable from the rest of the installation — otherwise one does not measure the electrode resistance alone, but a parallel combination of the electrode with all other earth connections (comparable to the problem described in the guide on determining the earthing arrangement of an unknown installation).
Preventing unintended disconnection: the requirement that this can only be done with a tool prevents a connection from being accidentally (or deliberately, by an unauthorized person) loosened during regular use — a disconnected main-earthing-terminal connection can leave part of the installation without an earth connection, without this being immediately visible.

Why this is more than a mounting detail

A main earthing terminal that does not meet this requirement — for example because all conductors are secured with a single, shared bolted connection without individual disconnection capability — forces a technician performing an earth resistance measurement to either disconnect the entire main earthing terminal (risking the installation being temporarily left completely without an earth connection), or to measure with a method that does not exclude the parallel earth connections and therefore yields a misleadingly low resistance value.

Practical relevance

At commissioning and periodic inspection it should be checked whether the main earthing terminal actually provides individually disconnectable connections operable only with a tool, and whether each connection is clearly labelled (see the guide on identification and documentation of switchgear) so that during a measurement it is clear which conductor serves which function, before anything is disconnected.

Common mistakes

Securing all conductors with a single shared bolted connection without individual disconnection capability — this makes an isolated earth electrode measurement unnecessarily cumbersome or impossible without temporarily de-earthing the whole installation.
Not labelling the connections on the main earthing terminal, leaving it unclear during a measurement which conductor runs to the electrode, which to main bonding, and which to the PEN/N.
Measuring earth electrode resistance without first isolating the electrode conductor from the other earth connections on the main earthing terminal — the result is then a parallel resistance, not a true electrode resistance.
Making the main earthing terminal disconnectable by hand (for example with wing nuts) instead of only with a tool — this increases the risk of unintended disconnection during regular use.