Earthing of portable generators — floating system versus TN connection

The emergency generators guide and the load-bank test guide cover fixed, permanently installed emergency generators. A small, portable generator — for example for temporary use on a construction site or at an event — has its own earthing question, separate from the mutual-exclusion issue of fixed generators.

Floating system as the default case

A small generator (generally up to around 3 kVA, portable and not permanently installed) often operates as a floating system: the generator winding is not connected to the frame or to earth. Protection against electric shock then relies on electrical separation (comparable to §413) — a first fault between a live conductor and the frame does not create a dangerous touch voltage, and immediate disconnection is not necessary, as long as the system actually remains separated from any other earthed system.

TN configuration as an alternative

A generator can also be configured as a TN system: one pole of the generator is then connected to the installation's main earthing terminal (MET), and automatic disconnection of supply (ADS) on an earth fault is achieved because all conductive parts are connected to the MET, the MET is connected to an earth electrode, and RCDs are present to actually provide the disconnection.

RCD requirement on every output circuit

For a TN-configured generator, every circuit fed by the generator must have additional protection via an RCD with I∆n ≤ 30mA and a disconnection time of at most 40ms at 5×I∆n — analogous to the regular TN-RCD requirement, but specifically applied to the generator as its own supply source without an external earth reference.

Note: the choice between a floating system and a TN configuration depends on the generator's power rating, the number and type of connected loads, and whether the generator serves as the sole source or runs temporarily in parallel with the grid — this article gives the principle, not a definitive decision scheme for every situation.

Practical relevance

When deploying a portable generator on a construction site (see also the construction sites guide for the regular 30mA RCD requirement on construction-site circuits), it is important to know which type of earthing the generator itself applies: a floating generator that is accidentally coupled to an already-earthed system loses the protection of electrical separation without this being visible from the outside of the generator.

Common mistakes

Connecting a floating generator to an already-earthed grid or part of an installation — this undermines the electrical-separation protection without this being visually apparent.
Assuming the generator itself always has a built-in RCD — this must be verified per generator and per output circuit, not assumed.
Using a TN-configured generator without its own earth electrode or earth connection, so that the assumed ADS protection does not actually function in practice.