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The conditions for combining HV and LV earthing systems are covered in BS EN 50522 (Clause 6.1) and BS EN 61936-1.
The definition of "Hot" and "Cold" with respect to telecommunications systems is addressed separately in ITU-T K.68 Telecommunication Standardization Sector of ITU recommendation, Series K: Protection Against Interference. Operator responsibilities in the management of electromagnetic interference by power systems on telecommunication systems (as referenced by BS 6701 and BS EN 50522).
There are occasions in which, despite the site being "Hot" from a telecomms perspective, BS EN 50522 recommends the interconnection of HV and LV earthing systems, because of the proximity of the LV system to the HV system. Under these conditions, affected telecommunications systems crossing hot zone boundaries are likely to require mitigation measures in accordance with BS EN 50174-3.
Whether an installation is TT, will depend on whether the DNO or iDNO believes it is safe to offer an earthing terminal. Where it is not considered safe to offer an earthing terminal, the consumer must provide their own electrode and the installation will be TT.
Otherwise, the DNO will declare whether PME conditions apply. There are situations in which the installation will, in reality, be TN-S, but PME conditions apply for other reasons.
Not necessarily. An earth electrode may be provided for three reasons:
(a) The installer has decided that the EV charging equipment must have its own TT island. In this case, separation above and below according to the IET Code of Practice for EV Charging Equipment (4th Edition) should be ensured.
(b) The installer has provided a supplementary earth electrode in accordance with Regulation 722.411.4.1 (ii) - although this is unlikely to be practicable in many installations.
(c) The EV charging equipment (EVSE, or electric vehicle supply equipment) is Mode 4, with an IT earthing arrangement on the secondary (DC) circuits connecting to the vehicle. The means of earthing for the charging point could well be the installation MET - but if the supply is PME, the installer may have selected to "island" the IT earthing system electrode.
If the earth electrode is connected to the MET (directly, or via circuit protective conductors) then it is provided to keep down the touch potential in the event of an open-PEN fault in the distribution network.
See section 14.5 of Guidance Note 5 Protection against electric shock, Regulation 722.411.4.1 (ii) of BS 7671, and Annex G to the IET Code of Practice for Electric Vehicle Charging Equipment Installation.
To change the entire property to TT would require all of the following provisions:
(a) No extraneous-conductive-parts shared with other properties on the PME network.
(b) All earth electrodes and any extraneous-conductive-parts separated below ground from any buried conductive parts connected to the PME network. The recommended separation distance where the whole installation is to be converted to TT is 10 m (see Table H1 of the 4th Edition of the IET Code of Practice for Electric Vehicle Charging Equipment Installation).
(c) No exposed- or extraneous-conductive-parts to be simultaneously accessible with exposed- or extraneous-conductive-parts of other electrical installations connected to a different means of earthing.
(d) Means of isolation and automatic disconnection to be suitable for TT.
It is therefore often not practicable, although, in rural installations, or installations in large properties, it may be possible.
Earthing systems that are simultaneously accessible should be connected at one point at least in the installation, to prevent touch voltages in the installation from causing electric shock. Protective earthing requirements take precedence over functional (including EMC) earthing requirements.
See Section 444 of BS 7671, along with BS EN 50310 (IEC 30129) and BS IEC 61000-5-2 for further information.
In industrial plant, requirements for earthing of explosive atmosphere installations should comply with BS EN 60079-14.
In filling stations, the APEA publication Design, construction, modification, maintenance and Decommissioning of Petrol Filling Stations should be followed.
If the earthing arrangement of the building(s) receiving a secondary supply is (are) not TT, this should be discussed with the DNO.
If the generator operates in parallel with the grid, the provisions of ENA Engineering Recommendation G98 or G99 would apply.
If the generator continues to supply power when grid power is lost, or as a switched alternative to the grid supply, an independent means of earthing is also required (Regulation 551.4.3.2.1). This can be connected to the main earthing terminal at the same time as the distributor's means of earthing.
It is often necessary to isolate the Neutral of the grid supply as well as the line conductors, to prevent circulating neutral currents, and ensure the protection of those working on the distribution network. See the IET Code of Practice for Electrical Energy Storage Systems (Section 9) for further details of the switching arrangements for this purpose.
Yes, but:
(a) if the generator operates in parallel with the grid, the provisions of ENA Engineering Recommendation G98 or G99 would apply.
(b) if the generator continues to supply power when grid power is lost, or as a switched alternative to the grid supply, an independent means of earthing is also required (Regulation 551.4.3.2.1).
This can be connected to the main earthing terminal at the same time as the distributor's means of earthing. It is often necessary to isolate the Neutral of the grid supply as well as the line conductors, to prevent circulating neutral currents, and ensure the protection of those working on the distribution network.
See the IET Code of Practice for Electrical Energy Storage Systems (Section 9) for further details of the switching arrangements for this purpose.
IET Code of Practice for Electric Vehicle Charging Equipment Installation, 4th Edition, and Electric Vehicle Charging Installations At Filling Stations, see our Guidance and Codes of Practice web pages for details.
There is different guidance here, that you will need to be careful to comply with.
In terms of meeting the earth electrode resistance, if the LPS can be bonded to the MET in accordance with BS EN 62305 series, then of course that would address the NOTE to 702.410.3.4.3 (ii)
However, the guidance in ENA Engineering Recommendation G12/4 (Clause 6.2.5), requires a metal grid to be installed under poolside areas and bonded to equipotential bonding. This would lead to a completely different, and additional, requirement.
The term "clean earth" is no longer used in IEC, CENELEC and British Standards.
Functional earth provides earthing for functional reasons (for example signal reference, or EMC). How functional earthing is arranged depends on the issues you might want to mitigate. Protective and functional earth can be provided via the same conductor(s) - this is termed combined protective and functional earthing. BS IEC 61000-5-2 and BS EN 50310 provide further information.
The requirements for protective earthing always take precedence.
The water around a vessel can be conductive, and this is why there are prohibitions on the use of PME in marinas and similar locations. When connected to shore connections, suitable provisions are in place in the UK to help address the phenomena of Electric Shock Drowning that may be attributable to faults in vessels.
When the vessel is afloat with no shore connection, there is typically no path back to the general mass of Earth, and therefore no current flow. Metal hulls of vessels are used similar to a protective earth reference.
See BS EN 60079-14, and also the APEA publication Design, construction, modification, maintenance and Decommissioning of Petrol Filling Stations.
It would not normally be advisable to use a PME earthing terminal for hazardous areas. Petrol filling stations are recommended to be TT.
An independent means of earthing is also required (Regulation 551.4.3.2.1). This can be connected to the main earthing terminal at the same time as the distributor's means of earthing.
It is often necessary to isolate the Neutral of the grid supply (if PME, the isolation is made after the DNO cutout) as well as the line conductors, to prevent circulating neutral currents, and ensure the protection of those working on the distribution network.
The minimum size of a CPC only applies where a separate conductor is installed outside of a cable or enclosed cable containment (i.e. conduit, trunking or similar). This is to prevent breaking the CPC by mechanical damage.
Smaller CSA may be used where the CPC is in a cable or enclosed in containment, provided it meets the criteria of either Regulation 543.1.3 (adiabatic criterion, or non-adiabatic calculation to BS 7454) or Regulation 543.1.4 (selection in accordance with Table 54.7).
Yes, but that may have to be a decision for the DNO, and the answer might depend on the arrangement of the distribution network in that area, as well as the local ground composition.
See BS EN 50310 and BS EN 61000-5-2.
The actual provisions and arrangements to be made for "instrument earths" or "signal earths" depend on whether the instruments or communication devices are single-ended (earth-referenced) or differential mode (in communications, "balanced twisted pair"). In some instrument inputs, it is recommended to connect screens or functional earth conductors to the instrument input device (PLC or other electronic product) where such conductors may be solidly earthed, capacitively earthed, or resistance earthed (and in some cases, this user-selectable).
No, diverted neutral currents can occur naturally, if the effective earth electrode resistance of extraneous-conductive-parts or supplementary earth electrodes, and those of the PME system earth electrodes, are low enough.
Unless the island mode (operating from generator only) earthing arrangement is TT, the resistance of the consumer's earth electrode provided according to Regulation 551.4.3.2.1 would not normally form part of the earth fault loop.
However, the protection study (for both ADS and fault protection) will have to be reviewed for "island mode" (operating on generator only) vs "connected mode" regardless of this, simply because the source impedance of the generator is greater than that of the grid (i.e. the overall prospective fault current is lower, and the earth fault loop impedance higher). This is a requirement of Regulations 551.4.1, 551.4.2 and 551.4.3.1.
Yes. However, it is important to select the correct type of power converting equipment, particularly any inverters, to ensure they remain functional when both the DC and AC side of the PCE are earthed to the same system. Other (automotive industry) standards may apply if the mobile/transportable unit is a vehicle, and the 24 V DC system is combined with that of the vehicle.
There are pros and cons of each of the earthing arrangements. In some circumstances, installations with TT earthing arrangements may be more susceptible to surges and other transients, and there may be issues with electromagnetic compatibility - although of course, the latter should be less of an issue in dwellings.
Another issue with TT is that, if you are outside the influence of main protective bonding, the touch voltage in a fault to earth of negligible impedance approaches the supply voltage, for the duration of the disconnection time of protective devices (potentially 2 s for a distribution circuit). This is more pertinent in installations where there are few, or no, extraneous-conductive-parts, and hence no real main protective bonding - all faults will lead to a touch voltage of a little below the supply voltage.
On the other hand, TN-S systems may be subject to the degradation, or breakage, of the supply protective conductor in the same manner as a broken CNE conductor in PME systems. This might not be detected until someone receives a shock from a faulty appliance where the protective device hasn't operated until (if protected by an RCD) the RCD operates when the user touches the appliance. Potentially, this is less of an issue if there are extensive extraneous-conductive-parts.
That of course depends on the reason for the provision of the earth grid.
There are pros and cons of each of the earthing arrangements, and each may have knock-on costs for consumers. TN-S systems may be subject to the degradation, or breakage, of the supply protective conductor in the same manner as a broken CNE conductor in PME systems. This might not be detected until someone receives a shock from a faulty appliance.
If the solar PV inverter is designed to cease generation when the grid supply is lost, then the distributor's earthing terminal may be used.
In systems that operate in island mode as well as connected mode, the system is not permitted to rely solely on the distributor's means of earthing in island mode (as this may be disconnected when the distributor is maintaining their network). A consumer electrode is required in this situation, and in addition to complying with the provisions of Section 551 of BS 7671, we would suggest the installation is designed according to the IET's guidance:
(a) IET Code of Practice for Electrical Energy Storage Systems
(b) IET Code of Practice for Grid-Connected Solar PV Systems.
It's not 100% clear what arrangement is being referred to, but we think this is describing a "Private PNB" type system, and similar to that shown in Fig 44.9 of BS 7671. Where the transformers are dedicated to the installation or are consumer-owned, PME conditions do NOT apply.
PME conditions only apply to public (DNO) supplies in accordance with the ESQCR, either from a PME distribution network, or a PNB arrangement where PME conditions are declared. These arrangements are described in ENA Engineering Recommendation G12/4.
The main earthing conductor, as a protective conductor, should be sized and selected according to Section 543 of BS 7671.
The maximum earth electrode resistance may be determined in one of the following ways:
(a) Where RCDs are used throughout the installation for automatic disconnection of supply, in accordance with Regulation 411.5.3 - but see Note 2 to Table 41.5.
(b) Where overcurrent protective devices are used in the installation, to comply with Regulation 411.5.4, noting that the earth electrode resistance forms part of the overall earth fault loop impedance.
Yes. In that case, the magnitude of diverted neutral currents is reduced. However, a voltage may still be transferred through the pipe in the event of a broken PEN conductor - depending on how poor an "earth electrode" the extraneous-conductive-parts are.
Since a lot of metal water and gas pipes have been replaced with plastic in the street, it is impossible for installers to know what sort of potential may be transferred through the extraneous-conductive-parts in the event of an open PEN conductor in the PME distribution network. Hence, it may still not be considered safe to connect the protective earth in that particular outbuilding to an electric vehicle, as the shared metallic pipe can still export touch potential to the vehicle body in the event of an open-PEN event. Separation of earthing systems as per Annex H of the IET Code of Practice for Electric Vehicle Charging Equipment Installation is recommended.
Agreed. Whether this is an issue or not depends on the particular arrangement in both the installation and the DNO distribution network.
TT is sometimes recommended by the DNO to help prevent or reduce diverted neutral currents.
The armour of the cable would have to be earthed. If the SWA is protected by an RCD in the main property, the armour could be "gapped" and insulated at the property, and earthed in the TT system at the shed, and the cable would effectively be protected as TT.
If the armour is earthed in the main property, and the shed to remain TT, then the armour would have to be gapped and isolated at the shed as shown in the presentations.
If the question is whether PME may be exported to a shed, then strictly yes, this is possible. There are two cases:
(a) There are extraneous-conductive-parts (a connected earth electrode would be an extraneous-conductive-part). In this case, the protective conductor of the armoured cable would have to be sized as a PME main protective bonding conductor, or an additional conductor run alongside it.
(b) There are no extraneous-conductive-parts. In this case, the only constraints on the size of the protective conductors are for fault protection and achieving ADS (if ADS is provided by an overcurrent protective device).
If the PNB supply is provided by then DNO, then PME conditions apply. See G12/4.
There is a requirement for DNOs to maintain their networks so that they are safe, so far as reasonably practicable.
On a periodic inspection and testing report why is the Earthing terminal not measured to see if there is an incoming ohms value?
This is only necessary for TT systems.
In TN systems (TN-S and TN-C-S), the earth electrode does not form part of the earth fault path - the return fault current should ideally travel down the supply protective earth (PE) conductor, or supply protective earth and neutral (PEN) conductor. In practice, fault current is shared through extraneous-conductive-parts, but this is known as fortuitous earthing. Hence only the Ze measurement is taken in TN systems.
The generator earth electrode is not in the earth fault path. You will distribute the earth from the generator star point with a separate neutral so the earthing method is TN-S.
The important thing is where is the vehicle being charged.
If it is being charged inside the garage you do not need the additional protective measures. If the vehicle is being charged outside then one of the measures set out in BS 7671 Amendment 1 must be adopted. You can view the amendment free on the IET website.
The more connections to the true mass of earth the better. However, if you want to use the electrode to limit touch voltage in the event of PEN failure you need very low earth resistance values. See BS 7671 722.
Yes if the two earthing systems are simultaneously accessible then yes, they need to be connected together. This could be at the TX position. 73s G8NUP.
No. The generator earth needs to be connected to the DNO earth. If the generator is 10kW or more then an earth electrode of 20Ω
The device would need to disconnect all live conductors and the incoming earth. This type of device is available for vehicle charging points on PME systems. We don’t believe this device is available for whole installations.
It would depend if each transformer has a source electrode connected to the star point. Very good idea to have an additional electrode connected to the MET. With all the METs on site connected to an electrode, you have a mini global earthing system with a level of redundancy.
Either selection using Table 54.7 or by calculation using the adiabatic equation but subject to the minimum CSA set out in Regulation 543.1 and Table 54.1.
Cable faults, cable strikes and cable thefts.
The DNOs will pay compensation and/or replace electronic equipment if the fault is in their network.
No. The supply can be disconnected but if the installation is connected to the supplier's earth diverted neutral currents will cause damage and cause all conductive parts connected to the MET will become live.
See Table 41.5 of BS 7671. Note 2. It may need to be lower depending on the tripping current of the RCD.
IET Guidance Note 8 is a very good guide to earthing and bonding. If your garden building is all wooden and has no extraneous conductive parts then you will probably be OK but the earthing should be considered as part of your design. It may be safer to run an SWA with the armour earthed at the supply end and then cut back and insulated at the far end and the MET in the consumer unit in the remote building connected to an earth electrode. This is a job for a competent qualified electrician to design.
The device would need to disconnect all live conductors and the incoming earth. This type of device is available for vehicle charging points on PME systems. We don’t believe this device is available for whole installations.
You cannot combine the earth and neutral inside the installation unless very special circumstances apply. See ESQCR regulations and 411.4.3. If you are supplying a remote building, and that building has extraneous conductive parts, then the cable earth can be used as a bonding conductor providing it meets the CSA requirements for a CPC AND bonding conductors.
If the lightning protection meets BS EN 62305 then it is highly probable that it should be bonded to the MET but you would need to seek guidance from the lightning protection designer. See Regulation 411.3.1.2.
Probably not. The application of separate earthing for HV and LV would have been for a purpose. The original designer should be consulted if you were considering doing this.
Yes as we mentioned in the Webinar foundation earthing for new housing estates would be advantageous.
The DNO.
We do know the ENA and DNO have been looking at this.
The device would need to disconnect all live conductors and the incoming earth. This type of device is available for vehicle charging points on PME systems. We don’t believe this device is available for whole installations.
Not to our knowledge.
One of our retired IET colleagues was involved in producing a risk assessment and safe system of work for this situation.
A member of the ENA sits on JPEL 64 so we are aware of the ENA policy documents and know each DNO publish their own policy documents.
On large sites and large buildings, you may find private transformers. It is not uncommon on these types of installations to have an internal HV ring serving a number of private transformers.
We are aware of this but we concentrate on LV installations as HV is out of scope for BS 7671.