Earthing under the PUE code

Author: Viacheslav Yurdyk, quality engineer at LK Energy Group.
Acceptance of equipment at the factory (quality control department) and technical supervision on electrical installation sites.

Short answer

Earthing is not «a rod in the ground» but a system: an earth electrode in the soil, a main earthing busbar in the building, protective PE conductors to every load, and equipotential bonding. The resistance limit depends on the voltage and on the neutral arrangement: in the most common case — a 0.4 kV network with a solidly earthed neutral — the earthing arrangement to which the source neutral is connected must be no more than 4 Ω [ПУЕ, п. 1.7.92].

But resistance alone guarantees nothing. The core requirement of the current Rules is not «few ohms» but fast disconnection: in a 230 V network a TN system must disconnect the faulty circuit within 0.4 seconds [таблиця 1.7.1]. That is why earthing is always checked together with the protective devices, never on its own.

What earthing consists of

Four terms make the numbers below meaningful.

Part	What it is	Where it lives
Earth electrode	what makes contact with the soil: rods, strip, natural electrodes	in the ground
Earthing conductor	connects the electrode to the building	from the ground to the intake
Main earthing busbar (MEB)	the point where everything meets: earthing conductor, PE, equipotential bonding	in or beside the intake unit
PE conductor	protective conductor to every load	throughout the installation

Earth electrodes are either natural or installed. Natural ones are what is already in the ground and suitable: buried parts of reinforced-concrete and steel supports, foundations, metal pipelines in the ground, reinforcement. Installed ones are placed there on purpose.

🔴 What may NOT serve as a natural earth electrode [п. 1.7.116]: pipelines carrying flammable liquids, flammable or explosive gases and mixtures; sewer, heating and water-supply pipes; reinforced-concrete structures with pre-stressed reinforcement. This does not mean such pipes are ignored: in installations up to 1 kV they must still be connected to the main equipotential bonding system — but as part of bonding, not as an earth electrode.

How many ohms are required

A 0.4 kV network with a solidly earthed neutral — the most common case:

What is measured	690 V	400 V	230 V
Earthing arrangement to which the source neutral is connected	2 Ω	4 Ω	8 Ω
Earth electrode to which the neutral is connected directly	15 Ω	30 Ω	60 Ω
Total resistance of all electrodes of the line PEN conductor	5 Ω	10 Ω	20 Ω
Each individual repeated earthing of PEN	15 Ω	30 Ω	60 Ω

[пп. 1.7.92 і 1.7.95]

Three more figures from the same chapter: - 30 Ω — repeated earthing of the PEN (PE) conductor at the building intake [п. 1.7.94]; - for installations with an isolated neutral the formula check may be skipped if the resistance does not exceed 4 Ω (source above 100 kV·A) or 10 Ω (up to 100 kV·A and all DC installations) [п. 1.7.97]; - in soils with a resistivity above 100 Ω·m the limits may be increased by a factor of 0.01ρ, but by no more than 10 times [п. 1.7.96].

🔑 Why «one figure for everything» is a myth. The limit depends on three things at once: the voltage, the neutral arrangement, and what exactly you are measuring — the whole arrangement or a single electrode. So the answer «it must be 4 Ω» is right about half the time, and expensively wrong the other half.

And a word about soil. Clause 1.7.96 is the Rules acknowledging that in sand or rock the same ohms are physically harder to reach. If you are told «you have 10 Ω instead of 4, this is bad», the first question should be about soil resistivity, not about the number of rods.

The point is not ohms but disconnection time

Modern shock protection is built on speed, not on resistance. Resistance is only the condition under which protection has time to operate.

Maximum permissible disconnection time for protective automatic disconnection in final circuits with an operating current up to 32 A [таблиця 1.7.1]:

Nominal voltage U₀	TN, AC	TT, AC
50 < U₀ ≤ 127 V	0.8 s	0.3 s
127 < U₀ ≤ 230 V	0.4 s	0.2 s
230 < U₀ ≤ 400 V	0.2 s	0.07 s
U₀ > 400 V	0.1 s	0.04 s

For final circuits of a TN system with an operating current above 32 A supplying fixed equipment only, this time may be increased, but to no more than 5 s and only under the conditions set out in the same clause [п. 1.7.82].

🔴 The rule broken most often: an RCD must not be used in installations with a TN-C system [п. 1.7.81]. In other words, in an old network with a combined neutral conductor you cannot simply add an RCD «for safety» — the PEN must first be split into PE and N, i.e. converted to TN-C-S. In a TN-C-S system the PE conductor is connected to the PEN on the supply side of the RCD [ibid.].

Main earthing busbar

An MEB is required in every installation up to 1 kV in which a main equipotential bonding system is provided [п. 1.7.126]. The requirements below are easy to verify by eye on your own site:

if a building has several separate intakes — an MEB is required for each intake unit; where there are built-in transformer substations — for each of them [п. 1.7.127];
material: copper or brass, steel is permitted. 🔴 aluminium MEBs are not permitted [п. 1.7.128];
the MEB cross-section must provide a conductance not lower than that of the most conductive conductor connected to it [ibid.];
the design must allow each conductor to be connected and disconnected individually, and only by means of a tool [п. 1.7.129];
the MEB may be placed inside the intake unit or separately next to it; the PE busbar of the intake unit may serve as the MEB [п. 1.7.130];
in places accessible to outsiders a separate MEB is not recommended; where it cannot be avoided — in a cabinet with a lockable door. Where access is restricted to electrical personnel, it may be left open [ibid.].

Protective PE conductors

What may be used as a PE conductor [п. 1.7.131]: conductors provided specifically for the purpose (cores of multi-core cables, conductors run in a common conduit or tray with the line conductors, permanently installed conductors); exposed conductive parts — metal sheaths and screens of cables, metal enclosures and supporting structures of assemblies and busbar trunking, metal ducts and trays (where the manufacturer's documentation allows it), metal conduits; certain extraneous conductive parts — structural steelwork of buildings, steel reinforcement of concrete, crane rails, galleries.

🔴 What is PROHIBITED as a PE conductor [п. 1.7.133]: - gas supply pipes and other pipelines carrying flammable or explosive substances; - water supply, sewer and central heating pipes; - support catenary wires of catenary wiring; - lead sheaths of cables (exception — where justified by calculation); - structural parts liable to mechanical damage in normal service; - metal hoses, insulating tubes and tubular conduits.

Minimum PE conductor cross-section [таблиця 1.7.6]:

Line conductor cross-section S, mm²	Minimum PE cross-section, mm²
S ≤ 16	S (the same)
16 < S ≤ 35	16
S > 35	S / 2

The table applies where the PE conductor is made of the same material as the line conductors; for a different material the cross-section must be equivalent in conductance [п. 1.7.137].

Two further requirements, visible on site at a glance: - PE conductors should be run in a common enclosure with the line conductors or alongside them; where shock protection is provided by overcurrent devices this requirement is mandatory [п. 1.7.136]; - conductors provided specifically as protective conductors must not be used for any other purpose [п. 1.7.131].

What earth electrodes are made of

Material [п. 1.7.117]: bare or coated black steel, stainless steel, copper. In a highly aggressive environment black steel is normally avoided — copper electrodes or steel with an electroplated copper coating are recommended.

🔴 Installed earth electrodes must not be painted [ibid.]. Paint is insulation, and an electrode's job is the opposite.

Minimum dimensions [таблиця 1.7.5]:

Material	Type	Diameter, mm	Cross-section, mm²	Wall thickness, mm	Coating, µm
Bare black steel	vertical, round	16	–	–	–
	horizontal, round	10	–	–	–
	horizontal strip	–	100	4	–
	section bar	–	100	4	–
Hot-dip galvanised steel	vertical, round	16	–	–	70
	horizontal, round	10	–	–	50
	horizontal strip	–	90	3	70
Steel with electroplated copper coating	vertical, round	14	–	–	250
	horizontal, round	10	–	–	250
Stainless steel	—	same as for hot-dip galvanised
Copper	round	12	–	–	–
	strip	–	50	2	–
	tube	20	–	2	–
	stranded rope	1.8 per wire	35	–	–

For installations above 1 kV the cross-section of horizontal electrodes is additionally selected for thermal withstand, with a permissible heating temperature of 400 °C [п. 1.7.118].

🔑 How to use this when accepting the works. These figures are exactly where money is saved invisibly: 3 mm strip instead of 4 mm, «galvanising in general» instead of 70 µm. A tape measure and the delivery note are enough to check — and it has to be done before the trench is backfilled, otherwise you will be digging it up again.

How often to check — and this is no longer PUE

🔴 Test intervals are set by PTEES, not by PUE. PUE says how much it must be; PTEES says how often this has to be confirmed. In brief:

What	How often
Visual inspection of the visible part	once a year
Resistance measurement (ordinary installations, except overhead lines)	after installation, refurbishment and repairs, but at least once every 12 years
The same in especially hazardous conditions (lifts, laundries, bathhouses, lifting machinery)	once a year
Contact transition resistances, arrangement in service up to 25 years / over 25 years	once every 12 years / once every 6 years

[ПТЕЕС, додаток 1, таблиця 25]

One figure worth remembering: the transition resistance of a contact joint must not exceed 0.1 Ω in current service. And a separate requirement that is almost never met: the earthing arrangement must also be inspected after a short circuit or lightning strikes.

More detail on the PTEES page.

Frequently asked questions

What is the resistance limit for an earthing loop?

For a 0.4 kV network with a solidly earthed neutral: the earthing arrangement to which the source neutral is connected — 2, 4 and 8 Ω for 690, 400 and 230 V respectively; the electrode to which the neutral is connected directly — 15, 30 and 60 Ω [ПУЕ, п. 1.7.92]. There is no «single figure for everything»: the value depends on the voltage, the neutral arrangement and on what exactly you measure.

Where does the figure of 4 Ω that everyone quotes come from?

It is the limit for an earthing arrangement with a connected neutral at a line voltage of 400 V — that is, for an ordinary three-phase 0.4 kV network. Hence its popularity. But for 690 V it is 2 Ω, and for 230 V it is 8 Ω [п. 1.7.92].

What if the soil is poor and 4 Ω cannot be reached?

The Rules allow for this: in soils with a resistivity above 100 Ω·m the limits may be increased by a factor of 0.01ρ, but by no more than 10 times [п. 1.7.96]. So before driving extra rods you need to know the soil resistivity.

What is an earthing arrangement?

It is the earth electrode (what sits in the soil) together with the earthing conductors that connect it to the installation. The resistance limit applies to the arrangement as a whole, not to a single rod — and these are different figures: 4 Ω for the arrangement, 30 Ω for a single electrode with a directly connected neutral [п. 1.7.92].

What is earthing resistance measured with?

With a dedicated instrument — an earth resistance tester (ammeter-voltmeter method with auxiliary electrodes, or clamp-on). What matters is not the make of the instrument but who does the measuring: under PTEES it is carried out by an accredited laboratory, and the result is recorded in a test report kept together with the equipment passports.

How often does earthing have to be measured?

For an ordinary installation — after installation, refurbishment and repairs, but at least once every 12 years; each year there is a visual inspection, not a measurement. Annual measurement applies in especially hazardous conditions and for lifting machinery [ПТЕЕС, додаток 1, таблиця 25]. This is a PTEES rule, not a PUE one.

What is the minimum size of an earth electrode?

For bare black steel: vertical round — 16 mm in diameter, horizontal round — 10 mm, strip — 100 mm² in cross-section at a thickness of 4 mm. For hot-dip galvanised steel, strip — 90 mm² at 3 mm thickness with a 70 µm coating. Copper strip — 50 mm² at 2 mm thickness [таблиця 1.7.5].

What cross-section should a protective conductor have?

Up to 16 mm² of line conductor — the same as the line conductor; from 16 to 35 mm² — 16 mm²; above 35 mm² — half the line conductor [таблиця 1.7.6]. This applies where the PE conductor is of the same material as the line conductors.

Can water or heating pipes be used for earthing?

No. Water supply, sewer and central heating pipes may not be used either as natural earth electrodes [п. 1.7.116] or as protective conductors [п. 1.7.133]. They must, however, be connected to the main equipotential bonding system in installations up to 1 kV — that is a different function.

Can an RCD be fitted in an old network?

In a TN-C system it is prohibited [п. 1.7.81]. The PEN conductor must first be split into PE and N, i.e. the system converted to TN-C-S, and only then may an RCD be fitted, with the PE connected to the PEN on the supply side of the RCD.

What is an MEB and is it mandatory?

The main earthing busbar is the point where the earthing conductor, the PE conductor and the equipotential bonding conductors meet. It is mandatory in every installation up to 1 kV with a main equipotential bonding system; with several intakes — for each intake, and where there are built-in transformer substations — for each of them [пп. 1.7.126 і 1.7.127].

What must an MEB be made of?

Copper or brass; steel is permitted. An aluminium MEB is not permitted [п. 1.7.128]. Its cross-section must provide a conductance not lower than that of the most conductive conductor connected to it.

Can earth electrodes be painted?

No. Installed earth electrodes must not be painted [п. 1.7.117] — paint acts as insulation. Corrosion protection is provided by the material and the coating: hot-dip galvanising or electroplated copper, with thicknesses specified in table 1.7.5.

Is low resistance the main thing in earthing?

No, the main thing is fast disconnection. In a 230 V network a TN system must clear the faulty circuit within 0.4 s, a TT system within 0.2 s [таблиця 1.7.1]. Earthing resistance is the condition that lets protection operate in time, not an end in itself. That is why earthing is checked together with the protective devices.

Let us check your earthing together with the switchgear

LK Energy designs and manufactures 0.4–35 kV switchgear and carries out electrical installation work. If you need an earthing arrangement calculated for your site or an existing one checked — send us a single-line diagram or a completed data sheet.

This material is provided for information only and is neither an offer nor legal advice. We do not publish the full text of the Rules — only quotations with clause numbers; obtain the official text from the source named above.

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Earthing under the Ukrainian electrical code (PUE): limits, measurements, intervals