What is the difference between an isolation transformer and a step-down transformer?

An isolation transformer has a 1:1 turns ratio — the output voltage equals the input voltage. Its purpose is to provide galvanic isolation between the primary and secondary circuits, breaking any earth connection and preventing shock from a single contact with the secondary live conductor. An earth referenced IT system is commonly created using an isolation transformer. A step-down transformer has a higher primary turns count than secondary, reducing voltage (e.g., 11 kV to 400 V for a distribution transformer, or 400 V to 110 V for a site safety transformer). An autotransformer uses a single winding with a tapping point — it is smaller and cheaper but provides no galvanic isolation.

What ventilation clearances are required around a transformer?

Ventilation clearances depend on the transformer type, rating, and manufacturer's recommendations. As a general guide: dry-type transformers (AN — air natural cooled) require at least 300 mm clearance above and on all sides for natural convection cooling; clearances increase with rating (consult the manufacturer data sheet). Oil-filled transformers require similar clearances plus fire separation from combustible materials. Both types must be positioned so that cooling airflow is not obstructed by adjacent equipment, walls, or ductwork. High-voltage clearances to live parts must also be maintained in accordance with BS EN 61936-1 for voltages above 1 kV.

When must I notify the DNO before installing a transformer?

DNO notification is required in several situations: (1) connecting a new load transformer above the capacity that the existing supply agreement covers — the DNO may need to reinforce the network; (2) connecting a generator or other source of generation through a step-up transformer — Engineering Recommendation G99 applies for connections above 16 A per phase (3.68 kW single phase, 11 kW three phase); (3) installing a high-voltage (HV) transformer connected to an HV supply — a formal connection agreement is required. Even for purely secondary distribution (e.g., 400 V to 110 V site safety transformers) where no new supply is needed, check that the existing supply agreement covers the additional demand.

What are the commissioning tests for a new distribution transformer?

Commissioning tests for a distribution transformer (e.g., 11 kV/400 V) typically include: insulation resistance measurement (primary to earth, secondary to earth, primary to secondary) using a 5 kV insulation resistance tester; turns ratio test (using a transformer turns ratio meter — should be within ±0.5% of nameplate ratio); polarity/vector group test (confirming Dyn11 or other vector group is correct); winding resistance measurement (compared to factory test certificate values); no-load loss measurement; and oil dielectric strength test (oil-filled only, minimum 30 kV/2.5 mm gap to BS EN 60156). Results must be recorded and compared to the factory type test certificate.

How often does transformer oil need to be sampled and tested?

Oil-filled distribution transformers in service should have oil sampled and tested every 2–3 years under normal operating conditions, or more frequently if the transformer is loaded above 80% of its rating, installed in a polluted or humid environment, or if previous test results indicated degradation. Dissolved Gas Analysis (DGA) is the most valuable oil test — it detects thermal and electrical faults inside the transformer by identifying gases dissolved in the oil (hydrogen, methane, ethylene, acetylene). Moisture content (Karl Fischer), dielectric strength, and acid number are also measured. Oil condition testing is performed by specialist laboratories from a sample drawn while the transformer is energised (not de-energised).

What fire containment is required for an oil-filled transformer installed indoors?

Oil-filled transformers installed indoors require fire containment measures because burning oil can spread and cause structural fire damage. Requirements depend on the oil quantity and building type, but typically include: an oil retention bund (sump) capable of containing 100% of the transformer oil volume plus fire-fighting water; fire-resistant construction of the transformer chamber (typically 2 hours fire resistance); self-closing fire dampers on ventilation openings; and clearance from combustible materials. Detailed guidance is given in the IET Code of Practice for In-Service Inspection and Testing of Electrical Equipment and in BS EN 61936-1 for HV installations. Many insurance policies require sprinkler systems in transformer chambers above a threshold oil quantity.

What is a tap changer on a transformer and how is it set?

Distribution transformers are fitted with an off-load tap changer (OLTC) that allows the turns ratio to be adjusted in steps (typically ±2.5% and ±5% from nominal) to compensate for supply voltage variations. Setting the tap position adjusts the secondary voltage — selecting a higher primary tap reduces secondary voltage; selecting a lower primary tap increases secondary voltage. The tap changer is only operated when the transformer is de-energised and isolated. The correct tap position is determined by measuring the secondary no-load voltage and adjusting to achieve the target busbar voltage. Large HV/HV autotransformers use on-load tap changers (OLTC) that can adjust under load.

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Transformer Installation UK: Complete Electrical Guide

Everything UK electricians and engineers need to know about electrical transformer installation — types of transformer, ventilation and clearance requirements, oil-filled vs dry-type selection, primary and secondary protection, DNO notification, commissioning tests, and maintenance schedules.

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14 min readUpdated 2026-05-18Andrew Moore, Founder of Elec-Mate

Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.

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Key Takeaways

1Transformers must be installed with adequate ventilation clearances — oil-filled transformers require fire containment (bund or fire-resistant construction) to BS EN 61936-1 and IET Guidance Note 4.
2Oil-filled (ONAN) transformers are cheaper and more efficient for large ratings but require an oil containment bund and fire wall; dry-type (AN/AF) transformers are preferred for indoor and high-rise installations where oil is not permitted.
3Primary protection (HV fuses or HV circuit breaker) must be sized to the transformer full load current on the primary side; secondary protection must coordinate with the primary protection to ensure discrimination.
4DNO notification is required before connecting any transformer above a threshold capacity to the distribution network — typically via an Engineering Recommendation (ER) such as ER G99 for generation or ER P2 for demand.
5Commissioning tests must include insulation resistance (primary-to-secondary, primary-to-earth, secondary-to-earth), turns ratio check, polarity test, and no-load loss measurement before energisation.
6Transformer maintenance intervals depend on type and loading — oil-filled transformers require oil sampling every 1–3 years; dry-type transformers require visual inspection and cleaning annually.

01 · Industrial Guide

Types of Electrical Transformer

Transformers are static electromagnetic devices that transfer electrical energy between circuits at different voltages using electromagnetic induction. All transformer installations in the UK must comply with BS EN 61558 (small power transformers) or BS EN 60076 (power transformers above 1 kVA) and with BS 7671 or BS EN 61936-1 for the associated electrical installation. The principal types encountered in UK electrical work are as follows.

Isolation transformers (1:1) — provide galvanic isolation without changing voltage. Used for medical locations (BS 7671 Section 710), swimming pool pump rooms, IT systems requiring floating earth, and where safety isolation from the earthed supply system is needed.
Step-down transformers — reduce voltage for specific applications. Common examples: 400 V to 110 V centre-tapped for construction site tools (BS EN 60742), 11 kV to 400 V distribution transformers for LV supply, 33 kV to 11 kV for primary distribution substations.
Autotransformers — single winding with a tapping point. Smaller and cheaper than double-wound transformers for the same rating, but provide no galvanic isolation. Used in star-delta motor starters, voltage regulators, and some HVAC equipment. Not permitted where isolation is required.
Distribution transformers (HV/LV) — the most common large transformer type on UK sites. Typically 11 kV or 33 kV primary, 400 V secondary, rated from 100 kVA to 3 MVA or more. Either oil-filled or dry-type. Installed in a dedicated transformer chamber or outdoor compound.

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02 · Industrial Guide

Installation Requirements: Ventilation, Clearances, and Fire Containment

Transformer installation requirements depend primarily on the voltage class and the installation environment. Installations above 1 kV must comply with BS EN 61936-1 (Power installations exceeding 1 kV AC). LV transformer installations are covered by BS 7671. The critical installation parameters are ventilation, clearances to live parts, and fire containment for oil-filled units.

Ventilation — natural ventilation (AN) requires openings at low level (inlet) and high level (outlet) with total area sufficient for the heat dissipation. Forced ventilation (AF) uses fans to move air, allowing higher transformer loading. Ventilation openings must be protected against entry of vermin and insects without significantly restricting airflow.
Electrical clearances — minimum phase-to-earth and phase-to-phase clearances in air depend on voltage. At 11 kV, minimum clearance is 120 mm phase-to-earth, 160 mm phase-to-phase in accordance with BS EN 61936-1 Table 1 for internal installations. HV terminals must be inaccessible to unauthorised persons — enclosed in locked switchgear or in a locked room.
Fire containment (oil-filled) — the transformer chamber must be constructed with walls, floor, and ceiling of minimum 2 hours fire resistance. An oil retention sump (bund) must be provided under and around the transformer, sized to contain 100% of the transformer oil volume. Drainage from the sump should go to an oil separator, not directly to a surface water drain.
Access and safety — HV transformer chambers must be lockable, with warning notices (yellow triangular HV warning signs) on all access doors. A safety document system (permit to work) must be in place before any work is carried out near HV equipment. Earthing facilities (earthing sticks) must be provided for safely earthing isolated conductors.

03 · Industrial Guide

Oil-Filled vs Dry-Type Transformers

The choice between oil-filled and dry-type transformers is one of the most important decisions in transformer specification. Both types are available from approximately 100 kVA to several MVA. The correct choice depends on the installation environment, fire risk, maintenance capability, and lifetime cost.

Oil-filled (ONAN/ONAF) — mineral oil provides both insulation and cooling. Lower purchase cost for equivalent rating, lower no-load losses, proven long service life (30+ years with proper oil maintenance). Requires fire containment, periodic oil sampling, and disposal of condemned oil to licensed waste contractor. Preferred for outdoor substations and large indoor installations where fire containment can be provided economically.
Dry-type (AN/AF, cast resin) — epoxy resin or open-wound dry coils, cooled by air. No oil, no fire hazard, no bund required. Preferred for indoor urban buildings, hospitals, shopping centres, and high-rise buildings where oil storage is not permitted. Higher purchase cost than oil-filled equivalent. Less tolerant of overloading (no oil thermal buffer). Cast resin type has excellent moisture resistance.
Ester fluid-filled — synthetic or natural ester dielectric fluid as an alternative to mineral oil. Higher fire point than mineral oil (fire point >300°C vs ~160°C), biodegradable (natural ester), and can be installed in locations where mineral oil transformers are not permitted. Higher fluid cost than mineral oil but may eliminate the need for expensive fire containment construction.

For new indoor substation installations, dry-type or ester-filled transformers are increasingly specified to simplify planning consent and reduce the cost and complexity of fire containment. Consult with the building insurer before specifying the transformer type — some insurers have specific requirements.

04 · Industrial Guide

Primary and Secondary Protection

Transformer protection prevents damage to the transformer from overloads and faults on either the primary (HV) or secondary (LV) side. Protection must be coordinated so that faults are cleared by the closest protective device, and the primary protection does not operate unnecessarily for secondary faults that the secondary protection can clear.

HV primary protection — HV fuses (for transformers up to approximately 1 MVA) or HV circuit breakers with overcurrent and earth fault relays (for larger transformers) protect the primary winding. Fuses are sized to the transformer rated primary current, allowing for magnetising inrush (typically 10–12 times rated current for 100 ms) without blowing. Coordinate with the DNO's protection — the DNO fuse or breaker must discriminate with the customer's primary protection.
LV secondary protection — the main LV circuit breaker (ACB or MCCB) on the secondary side protects the transformer from secondary faults and overload. Sized to the transformer rated secondary current. Must have a breaking capacity sufficient to interrupt the maximum prospective short-circuit current at the LV terminals (can be very high — 50 kA or more for large transformers on a stiff HV supply).
Buchholz relay (oil-filled) — a Buchholz relay installed in the pipe between the transformer tank and the conservator detects gas accumulation (indicating internal arcing or overheating) and oil surge (indicating a serious fault). Provides both alarm (slow gas accumulation) and trip (oil surge) signals. Standard on all oil-filled transformers above approximately 500 kVA.
Winding temperature protection — winding temperature indicators (WTI) and oil temperature indicators (OTI) provide alarm and trip signals at preset temperatures (typically alarm at 100°C, trip at 115°C for oil-filled). Dry-type transformers use PT100 or thermistor sensors embedded in the windings.

05 · Industrial Guide

DNO Notification Requirements

The Distribution Network Operator (DNO) must be notified before connecting certain transformer installations to the distribution network. Failure to notify can result in the connection being refused, the installation having to be modified, or the customer being disconnected. The relevant Engineering Recommendations (ERs) are published by the Energy Networks Association (ENA).

ER G99 — applies to generation connected to the distribution network above 16 A per phase (3.68 kW single phase, 11 kW three phase). Any transformer associated with a generator (step-up transformer for export) requires G99 application and DNO approval before connection. G99 specifies protection requirements (loss of mains protection, vector shift, ROCOF) and connection procedures.
ER P2 — the DNO's security of supply standard. Large new demand connections (including large transformer installations) may require a formal capacity assessment under ER P2 to ensure the network can supply the load reliably. The DNO may require network reinforcement works, which are funded by the connecting customer.
HV supply connections — connecting a private HV/LV transformer to the DNO's HV network always requires a formal connection agreement. The DNO specifies the metering arrangement (typically at HV), protection relay settings, and earthing requirements. The site HV switchgear must meet DNO specifications.
Timescales — DNO connection agreements for HV supplies and large LV supplies typically take 3–12 months to process, depending on the network complexity. Plan well in advance and submit the application as early in the project as possible to avoid programme delays.

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06 · Industrial Guide

Commissioning Tests

A new transformer must be tested before energisation to verify it has not been damaged in transit, is correctly installed, and its protection systems are functional. Commissioning tests are carried out by the transformer manufacturer's representative or by a specialist high-voltage testing contractor. An Electrical Installation Certificate is required for the associated LV installation work.

Insulation resistance tests — using a 5 kV insulation resistance tester (for HV/LV transformers): primary to earth, secondary to earth, primary to secondary. Minimum acceptable IR values depend on the transformer voltage class and winding temperature — compare results to the factory test certificate. A significant reduction from factory values indicates moisture ingress or insulation damage.
Turns ratio test — verifies the transformation ratio matches the nameplate data and confirms the tap changer is set correctly. Carried out with a TTR (transformer turns ratio) meter applied to each phase. Result should be within ±0.5% of nameplate ratio. Incorrect ratio indicates a winding fault or incorrect tap position.
Vector group test — confirms the phase relationship between primary and secondary voltages (e.g., Dyn11 — delta primary, star secondary with neutral, 30° lag). Incorrect vector group causes circulating currents and potential damage if the transformer is paralleled with another unit.
Oil dielectric strength (oil-filled) — oil sample tested to BS EN 60156 using a standard oil test set. Minimum breakdown voltage 30 kV for a 2.5 mm gap. Low dielectric strength indicates moisture or contamination in the oil — oil must be filtered or replaced before energisation.

07 · Industrial Guide

Maintenance Schedule

Transformers are long-life assets but require planned maintenance to achieve their design life and to provide early warning of developing faults. The Electricity at Work Regulations 1989 require that electrical systems are maintained to prevent danger. The specific maintenance requirements depend on the transformer type.

Annual visual inspection — check for oil leaks (oil-filled), external damage, corrosion, termination integrity, label legibility, condition of cable boxes and glands, operation of temperature indicators, and Buchholz relay (oil-filled). Record and act on all defects found.
Oil sampling (oil-filled) — every 2–3 years — dissolved gas analysis (DGA), moisture content, dielectric strength, acid number, and interfacial tension. DGA results interpreted against IEC 60599 (Guide to the Interpretation of Dissolved and Free Gases Analysis) to identify developing faults.
Thermographic survey — every 2–3 years — infrared thermography of all LV and HV terminations identifies loose connections and overloaded conductors before they cause failures. Requires the transformer to be loaded to at least 70% of rated capacity during the survey.
Protection relay testing — every 3–5 years — secondary injection testing of overcurrent and earth fault relays to verify operating times and current settings match the protection coordination study. Buchholz relay float test. Winding temperature relay simulation test.

08 · Industrial Guide

For Electricians: Transformer Installation Certification

All fixed installation work associated with a transformer — LV connections, earth conductor sizing, cable installations, and protective device installation — requires an Electrical Installation Certificate under BS 7671. HV work on the transformer itself requires specialist HV-qualified personnel.

Complete the EIC on Site

Use the Elec-Mate EIC app to record all LV commissioning test results and complete the Electrical Installation Certificate on your phone. Record insulation resistance values, earth conductor details, prospective fault current, and protective device ratings — then export the PDF certificate before leaving site.

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Transformer Installation UK: Complete Electrical Guide

Key Takeaways

Types of Electrical Transformer

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Installation Requirements: Ventilation, Clearances, and Fire Containment

Oil-Filled vs Dry-Type Transformers

Primary and Secondary Protection

DNO Notification Requirements

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Commissioning Tests

Maintenance Schedule

For Electricians: Transformer Installation Certification

Complete the EIC on Site

Certificate transformer installations with Elec-Mate

Frequently Asked Questions About Transformer Installation

What is the difference between an isolation transformer and a step-down transformer?

What ventilation clearances are required around a transformer?

When must I notify the DNO before installing a transformer?

What are the commissioning tests for a new distribution transformer?

How often does transformer oil need to be sampled and tested?

What fire containment is required for an oil-filled transformer installed indoors?

What is a tap changer on a transformer and how is it set?

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