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Information for electrical installers

Please contact us to discuss your requirements  if you require one of the following:

  • a new electricity connection;
  • you are making significant changes to an existing; installation - for example, adding electrical equipment or altering earthing arrangements. 

We will undertake, on request, a small number of tasks on behalf of customers and their electrical installers. The tasks as listed below are primarily for domestic connections, but may also be applied to small commercial installations that utilise whole current metering.

De-energising and re-energising the customer's installation

In some cases it is necessary for the incoming electrical supply to be de-energised to allow an electrical installer to safely work on a customer's wiring and equipment safely.  You can arrange for the supply to be de-energised and re-energised by calling the customer's Electricity Supplier.  The Electricity Supplier's name, address and telephone number is shown on the customer's electricity bill. If you have any difficulty arranging for this to be carried out please contact us.

Provision of Earthing Terminals 

We provide an earthing terminal at each new electricity connection, as long as it is safe to do so. 

We can also provide an earth terminal at existing connections, on request, if it is safe to do so, however, we charge for this service.

The type of earthing that we provide depends on design of our existing network and the characteristics of the customer’s connection. In some cases we are unable to provide an earthing terminal due to the installation type or due to particular equipment the customer is proposing to use within the installation. For some installations, special criteria need to be satisfied by the customer / installer before we can provide an earth terminal. Installations and equipment types that require careful consideration include: 

  • Camping and caravan sites / parks
  • Mobile / transportable units
  • Exhibitions, shows and stands
  • Temporary electrical installations
  • Marinas and similar installations
  • Fuel filling stations
  • Construction and demolition sites
  • Mines
  • Quarries
  • Sports pavilions
  • Swimming pools and other basins
  • Agricultural / horticultural premises
  • Locations intended for livestock
  • Railway / tramway auxiliary supplies
  • Multi-occupancy buildings
  • Communications stations
  • Street lighting / street electrical fixtures up to 500W
  • Street electrical fixtures
  • EV charging points
  • Generators
  • Metalclad buildings

 

Once we have provided an earthing terminal at an installation we will continue to maintain it for the life of the installation, as far as is practicable, unless changes are made to the customer’s installation or equipment that prevent this, on the grounds of safety. 

The customer / installer is under no obligation to use our earth terminal but they must ensure their installation meets the requirements of the relevant edition of BS7671 (IET Wiring Regulations).

Installation of an Isolation Switch

We can install a 2 pole or 4 pole isolation switch that may be operated by the customer’s electrical installer, allowing them to de-energise and re-energise the customers’ installation themselves.

Replacement of meter box door

Where an external meter box is fitted, this is owned (and should be maintained) by the customer. If a meter box door has been damaged and needs replacement, we may make a charge for this work.

Fixed charges apply to each of the above standard tasks. For a list of our current charges please click here.

Connection characteristics

The electrical characteristics of an installation have to be considered when designing a new - or modifying an existing - electrical system. The following information is often required:

System voltage

Low voltage connections (up to 1000V)

The following standard voltages apply:

  • Single phase connections; 230V (phase-neutral)
  • Two and three phase connections; 230V (phase-neutral) and 400V (phase-phase)
  • Split phase connections; 230V (phase-neutral) and 460V (phase-phase)

The voltage should remain within +10% and –6% of these figures.

High voltage connections (above 1000V)

Large commercial or industrial installations are often connected at high voltage. A number of different standard voltages are in use within Western Power Distribution’s area, including:

  • 6600V (phase-phase)
  • 11000V (phase-phase)
  • 33000V (phase-phase)
  • 66000V (phase-phase)
  • 132000V (phase-phase)

The voltage should remain within ± 6% of the nominal connection voltage, with the exception of 132000V connections where the tolerance is ± 10%.

Number of phases

Low voltage connections (up to 1000V)

The number of phases provided at low voltage connections will vary depending on the capacity of connection and the nature of our electricity network. The following arrangements are typical: 

  • Single phase
  • Two phase (also known as two phase – four wire)
  • Split phase (also known as two phase – three wire)
  • Three phase

Where more than one phase is provided load must be balanced, as far as possible, between each of the phases.

High voltage connections (above 1000V)

Three phases are provided at high voltage connections.

System frequency

The statutory limit for frequency is 50 Hertz  ± 1%.

Maximum prospective short circuit current

The maximum prospective short circuit current is used to determine the short circuit rating requirement of electrical equipment.

Low voltage connections (up to 1000V)

The following values can be assumed at low voltage connections:

  • Single phase connection; 16kA
  • Two phase, split phase or three phase connection; 25kA

High voltage connections (above 1000V)

We can provide maximum prospective short circuit current information for high voltage connections upon request.

Maximum earth fault loop impedance

The maximum earth fault loop impedance is used to calculate the available earth fault current within an installation connected at low voltage (below 1000V).

The following values are typical for low voltage installations:

Earthing type       Maximum earth fault loop impedance       
Protective multiple earthing  (TN-C-S)0.35 ohms*
Cable sheath earth / Separate continuous earth wire (TN-S)0.8 ohms*
Direct earth (TT)

Dependent on resistance of the customer’s
earth electrode. Values exceeding 100 ohms can apply.

* These values are sometimes exceeded where the connection is derived from a long overhead line network or from a small pole mounted transformer.

Type and rating of WPD’s fuse or circuit breaker

The type and rating of our fuse or circuit breaker installed at the point of connection can be provided on request. In some cases it will be necessary for us to carry out a site check, and to de-energise the supply for a short time in order to check the fuse size.

Types of earthing

Low voltage connections (up to 1000V)

We provide the following types of low voltage earthing:

Protective Multiple Earthing (PME)

PME (TN-C-S) is the most common form of earthing provided at new installations. We utilise a single conductor for the neutral and earthing functions within our network and provide a PME earth terminal at the customer’s installation. The customer’s earthing / bonding may be connected to this terminal providing the TN-C-S requirements within BS7671 (IET Wiring Regulations) and Engineering Recommendation G12 are satisfied.


Separate Neutral and Earth (SNE) derived from a Combined Neutral and Earth (CNE) Network 

SNE derived from a CNE Network (TN-C-S) is commonly provided at existing connections and sometimes at new ones. In this case we utilise a single conductor for the neutral and earthing functions within all, or part of, our network, provide a separate neutral and earth service and make an earth terminal available at the customer’s installation. The customer’s earthing / bonding may be connected to this earthing terminal providing the TN-C-S requirements within the relevant edition of BS7671 (IET Wiring Regulations) and Engineering Recommendation G12 are satisfied.

Separate Neutral and Earth (SNE) derived from a Separate Neutral and Earth (SNE) Network

SNE derived from a SNE network (TN-S) used to be common in the past, however, as networks have been modified and sections of combined neutral and earth cable installed, it is now relatively rare. Where we do maintain a separate neutral and earth conductor along the whole circuit length and within all connected services, we may provide a TN-S earth terminal at the customer’s installation. The customers earthing / bonding may be connected to this earthing terminal providing the TN-S requirements within the relevant edition of BS7671 (IET Wiring Regulations) and Engineering Recommendation G12 are satisfied.

Protective Neutral Bonding (PNB)

PNB is common in rural locations where LV connections are made at just one point on our network, typically where a pole mounted transformer is dedicated to a single customer connection. It is also used at some large connections that have a dedicated substation. In these situations the substation’s buried LV earth electrode is installed at the point on the network where the customer connection/s are made. This type of earthing is normally considered to TN-S but is classified as TN-C-S if a combined neutral / earth conductor is installed between the earth electrode and the customer’s connection, as shown in the following diagrams. The customers earthing / bonding may be connected to this earthing terminal providing the appropriate sections of BS7671 (IET Wiring Regulations) and Engineering Recommendation G12 are satisfied.


Direct Earthing

Direct Earthing (TT) is common within older properties and is also used within installations that cannot be provided with a WPD earth terminal, e.g. on the grounds of safety. 

In this case we do not make our earth terminal available to the customer. If the installation requires an earth, which is invariably the case, the customer or their installer must provide and maintain a buried earth electrode (buried earth rods and/or buried conductor) and provide their own earthing terminal. Where Direct Earthing is employed the installation normally requires a residual current device (RCD) or residual current circuit breakers with overcurrent protection (RCBOs) in order to provide adequate protection. The customer’s installation must comply with the TT requirements of BS7671 (IET Wiring Regulations).

High voltage connections (above 1000V)

Information on earthing at high voltage connections can be provided on request.