How should a generator-fed temporary supply be earthed?

A generator feeding a whole site or event is normally earthed as TN-S. Its star point is connected to the frame and main earthing terminal, a separate protective conductor runs to every board, and outgoing circuits are RCD-protected. An earth electrode at the generator is part of that TN-S system, and the IET calls certificates that label it TT a common mistake. A small generator can run unearthed as electrical separation, but on a UK construction site only for one item. A competent electrician designs, connects and tests it all before energising.

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3 months is the interval HSE’s HSG141 (second edition, 2023) gives for an electrically skilled person to inspect and test a construction site’s distribution system, 12 months for site offices. The earthing is part of what that test proves. Source: HSE, HSG141 Electrical safety on construction sites

Key takeaways

Topic In brief
Site or event generator TN-S: star point bonded to the frame, separate protective conductor, RCDs on outgoing circuits.
Earth electrode Part of TN-S, wherever one can be installed. Tested before energising.
PME (TN-C-S) Normally not used for a construction site supply.
Small single-item generators Electrical separation, one item only on a construction site.
Before energising Dead tests and the electrode test first, loop impedance and RCD tests once live.

What do TN-S, TN-C-S, TT and IT mean for a generator?

The letters say how the source and the installation’s metalwork connect to Earth. A generator is a source in its own right, with its own arrangement.

System How it is earthed Where it appears on temporary supplies
TN-S Star point earthed, separate neutral and protective conductors The usual site or event generator feeding several boards
TN-C-S (PME) Network combines neutral and earth in its cable DNO supplies, normally kept off construction sites
TT Installation earthed through its own local electrode A grid-fed site where PME is unsuitable
IT Source isolated from Earth or earthed through an impedance Generators with insulation monitoring, often in field use

HSE’s HSG141 says larger generators supplying a whole site "should be earthed and the outgoing circuits should be RCD-protected", so the protective devices operate in a fault.

IT works differently. Under IEC 60364, as Bender summarises it, a first insulation fault does not have to disconnect the supply, so an insulation monitoring device must report it. In October 2026 TSR’s EZG 80/4 II/TN-S PTO generator page described a switchable network: IT with active insulation monitoring in field use, TN when feeding a building, connected by a qualified electrician. The ESE 1606 DYS-GT ES ISO DI page described insulation monitoring to VDE 0100-551.

Why is the earth electrode part of TN-S?

James Eade’s IET MythBusters article (September 2020) notes that an alternator outputs 230 V whether or not it is connected to Earth. Earth is there for fault protection. When a Class I appliance fails, fault current returns to the generator through the protective conductor. Earth joins the fault path only when a live conductor touches the ground, for example through a damaged cable.

The electrode ties the generator’s star point to the general mass of Earth. Eade calls it "the first ‘T’ in TN-S, and not the second ‘T’ in TT". Mark Coles made the same point in Wiring Matters in 2011.

On resistance, the IET’s 2021 temporary power webinar said the often-quoted 20 ohm figure comes from HV networks and does not apply to LV generators. The value has to be low enough to operate the protective device on the generator output, a design calculation for the installer.

When does a generator need an earth electrode?

For a generator feeding a site or an event, plan for one. Eade writes that with an electrode "the protection will then be much more reliable, especially in detecting cable faults". The IET webinar adds a point for standby supplies: a generator generally needs its own means of earthing, because if the grid fails, its earthing may be lost too.

The IET guidance covers the awkward cases:

  • Hard standing. Where concrete or a raised position rules out an electrode, Coles described bonding accessible metalwork from other systems to the generator’s main earthing terminal.
  • Several generators together. Where their circuits are likely to be intermingled, the main earthing terminals are interconnected.
  • A nearby building. Connecting to its main earthing terminal was allowed, but not as the generator’s means of earthing unless it can be guaranteed reliable.

Without an electrode the system is "floating", which Eade prefers to call IN-S. He describes it as acceptable only with conditions, including an RCD on the generator output and short-duration use.

Can a small generator run without an earth?

Sometimes. HSG141 recommends 110 V or double-insulated (Class II) equipment on small single-phase generators, and says that used this way there may be no requirement to earth the generator. With earthed (Class I) equipment, only one item should be connected, or each circuit needs RCD protection.

This is electrical separation, common on 1 to 2 kVA sets according to Eade, and an RCD on its output serves no purpose. BS 7671 Section 704, as NICEIC summarised it in winter 2018-19, does not permit electrical separation to supply more than one item of equipment on a construction site.

The old rule that generators under 10 kVA need no earth has no clear engineering basis, Eade argues: a 2 kVA set can give an injurious shock just as a 12 kVA set can. Check the unit’s manual: the 2025 manual for TSR’s 7500i inverter generator shows an RCD and a ground terminal and tells users to consult an electrician on grounding. For hire fleets, Coles adds: never change the internal connections of hired equipment.

How do RCDs work on a generator supply?

Per the NICEIC summary, BS 7671 Section 704 requires a 30 mA RCD on every circuit supplying socket-outlets or hand-held equipment up to 32 A, and no more than 500 mA for socket-outlets above 32 A. Where a generator runs floating, Eade’s article sets the output RCD at no more than 100 mA with a time delay of no more than 0.2 seconds, selective with the 30 mA devices downstream.

TSR’s site sub-distribution board 63 A carried 30 mA and 300 mA RCDs in October 2026. HSG141 says never bypass an RCD to stop nuisance tripping, and the IET webinar adds that an RCD tripping on normal leakage "is of the wrong rating". Device type is covered in our guide to specifying temporary power distribution for a UK site.

Why is PME kept off construction sites?

PME relies on the network’s combined neutral and earth staying intact. HSG141 says it may not be suitable for a construction site, so the site needs a TN-S supply from the DNO or earth electrodes for a local earth, which is TT. Per the NICEIC summary, BS 7671 allows a PME earthing facility on a construction site only if every extraneous-conductive-part is reliably bonded, and BS 7375 says that normally cannot be guaranteed while the site keeps changing.

What changes at an event?

The code of practice. In October 2026 BSI listed BS 7909:2023+A1:2024 as current for temporary low voltage a.c. systems at entertainment events, written for organisers, hire companies and electrical contractors alike. HSE’s event guidance lists earthing and the use of generators among the things to plan. For the wider checklist, see temporary power for UK outdoor events.

Pre-energisation checklist for a generator-fed supply

HSG141 says nothing is made live until it is confirmed safe. The order follows the initial verification sequence NICEIC set out in November 2025: what the competent person does and records, so crews know what to ask for.

  • Earthing arrangement decided. TN-S, IT or separation, and where the electrode or bonding goes.
  • Generator configuration confirmed against the manufacturer’s instructions. No changes to hired equipment.
  • Continuity of protective and bonding conductors. Tested dead, first.
  • Insulation resistance and polarity. Tested dead.
  • Earth electrode. Tested before energising, with an earth electrode resistance tester rather than a loop impedance tester, per Coles.
  • Earth fault loop impedance and prospective fault current. Measured once live.
  • RCD operation. Tested once live, then by test button as the site sets.
  • Records kept. The IET webinar notes that without them there is no way of proving the system is safe.

Summary

A site or event generator is normally a TN-S source with an earth electrode wherever one fits and RCDs on outgoing circuits. PME stays off construction sites, separated small sets supply one item only, and a competent person tests it all before energising. To match generators and boards to a site or event, see TSR-Elsite’s generators and worksite power supply or get in touch: sales@tsr-elsite.fi or +358 9 4555 588.

Frequently asked questions (FAQ)

Does a construction site generator need an earth rod?

HSE’s HSG141 says a generator supplying a whole site should be earthed, with RCD-protected outgoing circuits. The IET recommends an electrode wherever one can be installed.

Is a generator with an earth electrode a TT system?

No. The IET describes the electrode at the generator as the first "T" in TN-S, because the star point is earthed and a separate protective conductor runs to the equipment.

Do I need an RCD on a generator?

On a TN-S generator supply, yes. For construction sites, BS 7671 Section 704 as NICEIC summarised it requires 30 mA protection for socket-outlets up to 32 A and no more than 500 mA above 32 A.

Can a small generator be used without earthing?

HSG141 says a small single-phase generator used with 110 V or double-insulated equipment may need no earth. With Class I equipment, connect only one item, and follow the unit’s manual.

What is an IT system on a generator?

The generator’s live parts are isolated from Earth or earthed through an impedance. A first insulation fault does not disconnect the supply, so an insulation monitoring device must report it.

What is tested before a generator supply is energised?

Continuity, insulation resistance, polarity and any earth electrode before energising, then earth fault loop impedance and RCD operation once live, done and recorded by a competent person.

Sources

TSR-Elsite. Published 23.10.2026.