Question 1

What is high voltage (HV) commissioning?

Accepted Answer

High voltage commissioning is the controlled process of inspecting, testing, verifying and energising HV electrical assets — substations, switchgear, transformers, cables and protection systems — to confirm they operate safely and exactly as designed before being put into service. It covers static checks, insulation testing, protection injection testing, interlock and trip path proving, SCADA verification and finally energisation under senior authorised person control.

Question 2

What voltage range does High Voltage Commissioning Ltd cover?

Accepted Answer

From 11kV package substations up to 132kV bay commissioning, including 33kV and 66kV distribution and sub-transmission systems. Our engineers hold senior authorised person authorisation up to 132kV.

Question 3

Why use an independent commissioning engineer rather than the installer?

Accepted Answer

When the installer tests its own work, the errors that matter most tend to survive — a transposed CT, settings applied from a superseded schedule, a trip link left out. An independent commissioning engineer tests to the design, challenges the design when results disagree with it, and produces documentation that insurers, DNO connections teams and asset owners can rely on.

Question 4

Where are you based and what areas do you cover?

Accepted Answer

High Voltage Commissioning Ltd is based in Letchworth Garden City, Hertfordshire, and delivers HV commissioning, testing and maintenance services across the whole of the UK.

Question 5

What is a senior authorised person (SAP)?

Accepted Answer

A senior authorised person is an individual assessed as competent to control a high voltage system. The SAP takes charge of switching, isolation and earthing, issues safety documentation such as permits to work and sanctions for test, and controls access for anyone working on or near the HV system.

Question 6

Do you provide SAP cover on an ad hoc basis?

Accepted Answer

Yes. We supply senior authorised person cover for individual switching operations, planned shutdowns, contractor access control or as ongoing contracted support across an estate.

Question 7

What is the difference between an AP and a SAP?

Accepted Answer

An authorised person (AP) is competent to carry out defined HV work under instruction and to work under permits issued to them. A senior authorised person additionally has authority to control the system — planning and executing switching, applying earths and issuing safety documents to others.

Question 8

Do you work under our HV safety rules?

Accepted Answer

Yes. We work to the client's HV safety rules and operational procedures where they exist, and provide our own documented safety system where they do not.

Question 9

How often should HV protection relays be tested?

Accepted Answer

Common UK practice is a three to six year interval for numerical relays, with tighter intervals for electromechanical relays and for critical circuits. Testing is also required after any settings change, relay replacement, protection mis-operation or fault clearance event.

Question 10

What is the difference between secondary and primary injection testing?

Accepted Answer

Secondary injection applies simulated current and voltage directly to the relay terminals to prove its elements, settings and timing. Primary injection applies real current through the primary conductor, proving the complete circuit — CT ratio, polarity, wiring and burden — end to end. Secondary injection proves the relay; primary injection proves the installation.

Question 11

What is a protection grading or coordination study?

Accepted Answer

It is an analysis that sets each protection device so the device nearest a fault operates first, with defined time margins to upstream devices. It keeps outages local, keeps fault energy within equipment withstand ratings and satisfies the network operator's discrimination requirements.

Question 12

Do you produce protection settings the DNO will accept?

Accepted Answer

Yes. Studies and settings schedules are prepared against the relevant network operator's protection policy and known upstream settings, and issued in a format suitable for connections team review — which is what avoids late rejection and energisation delay.

Question 13

What tests are needed before energising a new transformer?

Accepted Answer

As a minimum: insulation resistance with polarisation index, winding resistance across all taps, turns ratio and vector group, magnetising current, oil quality where oil-filled (dissolved gas analysis, moisture, dielectric strength), and functional testing of every protection, temperature and cooling device through to its correct alarm or trip destination.

Question 14

Why is VLF used for HV cable testing instead of DC?

Accepted Answer

DC testing can leave trapped space charge in modern XLPE insulation and can accelerate degradation in aged cable. Very Low Frequency testing applies an AC waveform at around 0.1Hz, stressing the insulation in a way representative of service conditions while keeping test equipment practical for site use. We test HV cables up to 33kV with VLF equipment.

Question 15

What is factory acceptance testing and do we need it?

Accepted Answer

Factory acceptance testing verifies HV equipment against the project specification and approved drawings while it is still at the manufacturer. A wiring error found on the works floor costs an hour; the same error found during site commissioning can cost a programme week. For switchgear, protection panels and transformers on programme-critical schemes it is almost always worth witnessing.

Question 16

Do you supply your own test equipment?

Accepted Answer

Yes. We mobilise with calibrated primary and secondary injection sets, VLF test equipment, transformer test sets, insulation and winding resistance testers and circuit breaker analysers, all with traceable calibration certification issued in the test pack.

Question 17

Which sectors do you work with?

Accepted Answer

Data centres, independent connection providers, renewable energy producers including solar, wind and battery storage, and facilities maintenance providers managing industrial, commercial and healthcare estates.

Question 18

How far in advance should we book?

Accepted Answer

Four to six weeks allows the design and settings review to be completed properly before mobilisation. We do hold capacity for shorter-notice work including outage support, fault investigation and emergency protection testing.

Question 19

What documentation do we receive at handover?

Accepted Answer

A complete commissioning pack: signed ITP, all test records with calibrated equipment traceability, as-left settings schedules, relay configuration files, switching and energisation records, punch list close-out and input to the O&M manual.

Question 20

How is HV commissioning work priced?

Accepted Answer

Scope is priced against the asset schedule and voltage band rather than a flat day rate, because the test suite, safety controls and documentation burden change significantly between 11kV and 132kV. Quotations list activities, exclusions and expected durations so there are no assumptions to argue about later.

Question 21

Can you work on live and occupied sites?

Accepted Answer

Yes. Work on live HV sites is carried out under an agreed switching programme with senior authorised person control, permits and exclusion zones appropriate to the site, and sequenced around available outage windows.

High voltage commissioning, answered

HV commissioning basics

SAP provision & HV safety

Protection testing & studies

Transformer, cable & equipment testing

Working with us

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