DNO service head

The service head is where the distribution network’s service cable terminates inside or at the outside of a property. It is also called the cut-out. It normally contains the distributor’s main fuse and may provide the distributor’s earth terminal.

It sits upstream of the electricity meter and consumer unit. It belongs to the network operator, so it is inspected from the outside by the electrician but is not opened, altered or uprated as part of ordinary customer-owned installation work.

The equipment boundary

At a simple domestic intake, electricity normally passes through:

1. the DNO service cable
2. the DNO cut-out and fuse
3. the electricity meter and associated metering connections
4. an isolator, where one is fitted
5. the customer’s meter tails and consumer unit

The service cable, cut-out and distributor’s earth terminal, where provided, are DNO equipment. The meter is metering equipment managed through the supplier and its meter operator. The consumer unit and downstream installation are the property owner’s responsibility.

Responsibility for tails and a separate isolator can depend on where they sit, who fitted them and the local metering arrangement. Do not assign ownership from position alone. Where work crosses the interface, the electrician should record which party must attend and what must be made ready before that visit.

Regulation 24 of the Electricity Safety, Quality and Continuity Regulations requires distributor and meter-operator equipment on consumer premises to be suitable, installed and maintained so as to prevent danger. It also requires the supply to be protected by a cut-out fuse or circuit breaker positioned as close as reasonably practicable to the supply terminals.

Do not remove the fuse without authority

The sealed fuse is not a customer isolator. The householder must not break the seal or withdraw it.

Some DNOs authorise defined groups of competent electrical contractors to remove and replace a cut-out fuse under a published procedure. Others require their own staff or an authorised meter operator to attend. Permission is DNO-specific and does not extend to tampering with the meter.

The correct route is one of:

• use a suitable installed isolator
• arrange isolation through the supplier or network operator
• follow the DNO’s express contractor procedure where the electrician qualifies for it

Working live because isolation is inconvenient is not an acceptable substitute. Old or damaged cut-outs can be particularly dangerous to disturb.

What the markings do and do not show

A modern service head may be single-phase or multiphase and may show a manufacturer, type, standard and maximum rating. The carrier or moulded enclosure rating is not necessarily the rating of the fuse actually fitted.

Do not open the carrier to find out. ENA guidance says an uncertain cut-out rating should be established by an enquiry to the DNO. If it still cannot be established, the relevant apply-to-connect process should be followed.

A three-phase-shaped cut-out does not prove that the customer has a live three-phase supply. Only one phase may be connected through to a single-phase meter. The meter, fuse arrangement and network records all need checking.

Earthing at the service head

The intake helps an electrician identify the earthing arrangement, but appearance alone is not sufficient. The installation must be tested and the DNO information checked where necessary.

• TN-S normally provides a separate distributor earth associated with the service cable.
• TN-C-S, commonly called PME, separates a combined neutral and protective conductor at the service position.
• TT uses a customer earth electrode rather than a distributor-provided earth terminal for the installation’s protective earthing.

The service head does not create a TT earth, and the presence of an earth conductor near the cut-out does not prove that it is sound. Earthing and bonding must be verified before connecting significant new equipment.

Condition checks

An installer should visually inspect accessible intake equipment before adding solar, storage, an EV charger, a heat pump or a new consumer unit. Refer the following to the network operator:

• cracks, missing covers or exposed parts
• scorching, heat damage or deformation
• unusual heat, smell, buzzing or crackling
• leaking filling compound
• corrosion, water entry or damaged service cable
• a suspected fused neutral or asbestos-containing component
• an insecure cut-out or one pulling away from its mounting
• uncertain earthing or a damaged earth terminal

Do not clean, tighten, tape or dismantle DNO equipment. If there is immediate danger, keep clear, call 105 to reach the local network operator and call the emergency services where life or property is at risk.

Age alone does not prove that a service head must be replaced. The DNO decides whether its equipment is safe, serviceable and suitable for the proposed load.

Capacity is more than the fuse carrier

The safe connection capacity depends on the complete service, including the fitted fuse, cut-out type and condition, cable, joints, ambient conditions, earthing and upstream network.

This matters when a project adds sustained load. An enclosure capable of accepting a larger fuse does not show that the cable or network can support it. The DNO must approve and perform an uprating.

The customer’s meter, tails, isolator and consumer unit must also be adequate. A DNO can decline to fit a larger fuse until customer-side or metering work is complete, but it does not become responsible for upgrading those downstream items.

Looped and shared services

A looped service uses part of the service connection to supply another property. More than one service cable at the cut-out can be an indicator, but not every unusual cable arrangement is a loop and it must not be disturbed to investigate.

Looping can affect a load increase, EV charger, heat pump, meter move or three-phase conversion. The DNO decides whether the existing arrangement can remain or must be separated. The project should not promise that unlooping is free or will take a particular time, because scope, access, land and network policy affect the work.

Information to record

Photograph the intake without removing seals or covers and record:

• the property and meter serial number
• the visible cut-out type and condition
• any visible rating, clearly labelled as a carrier or service-head marking if the fitted fuse is unconfirmed
• whether the supply appears single- or three-phase, marked as provisional until verified
• the stated and tested earthing arrangement
• signs of looping or shared intake equipment
• meter-tail, isolator and consumer-unit condition
• the DNO enquiry or job reference

That record lets the installer and DNO discuss the same equipment without treating a photograph as final proof of capacity.

Related entries

• Head fuse rating
• Looped electricity supplies
• PME earthing systems
• TT earthing arrangements
• Consumer unit capacity
• Main earth bonding