14.1 Initial Verification and DC Inspection

Quick Answer: Initial verification of a solar PV installation applies BS 7671 to general inspection and testing of the electrical installation, then applies BS EN 62446-1 and the IET Code of Practice for Grid Connected Solar Photovoltaic Systems to the remainder of the PV system. DC inspection is a documented schedule covering protection against shock and insulation faults, overcurrent, earthing and bonding, lightning and overvoltage, and equipment selection and erection.

Why inspection comes before any meter

A PV array can sit at several hundred volts DC as soon as light hits the modules. That voltage is present whether or not the inverter is commissioned. Initial verification is therefore not turn-it-on-and-see-if-it-generates. It is a two-layer process that the City & Guilds 2922 handbook (v1.3) Learning Outcome 5 treats as general inspection and testing plus additional PV-system checks.

General inspection and testing of the electrical installation follows BS 7671: safe isolation, inspection of AC tails and any consumer-unit or distribution-board alterations, earthing at the origin, circuit protective conductors, RCD or RCBO function where used, dual-supply labelling, and certification of that work. Additional inspection and testing of the remainder of the PV system follows BS EN 62446-1 and the IET Code of Practice: the array, DC cables, combiners, DC isolators, surge protective devices (SPDs), module earthing or bonding where used, and the DC input side of the inverter.

Neither document replaces the other. An Electrical Installation Certificate covering the AC connection does not prove that string polarity is correct. A 62446-1 commissioning sheet does not replace BS 7671 certification of the AC installation. You complete both layers and you keep both records for handover. The 2922 unit tests whether you can apply those published documents on site; it does not replace them.

The inspection half of initial verification is a schedule of inspections, not a walk around the roof looking for anything that feels wrong. You work through defined headings the same way a BS 7671 inspector works through Regulation 642 items. Defects are recorded against items. You do not energise, and you do not treat a later open-circuit voltage reading as a substitute for a failed visual check.

DC inspection headings

General

Confirm the array layout matches the as-designed stringing: module count per string, orientation groups, and which strings land on which maximum power point tracker (MPPT). Check cable routes, bending radii, UV-stable clips, drip loops at module junctions, and that cables are not resting on abrasive tile edges. Enclosures must be closed, glands tightened, and off-cuts not left as a fire load in the loft or eaves. First-fix is the moment to catch a string that was landed on the wrong MPPT — not after the customer has lived with a poor yield for six months.

Protection against shock and insulation faults

Array voltages are frequently several hundred volts DC. Class II module construction is common, but that does not make the DC cable a touch-safe extra-low-voltage circuit. Live parts must not be accessible. Connectors must be fully mated and locking. Unused DC ports need sealing caps. Look for pinched insulation under clamps or tiles, damaged module backsheets, crushed cables at eaves, and gender-changing adapters used to hide a reversed string. Confirm DC isolators are off, locked or labelled as required by the site method statement, and that nobody can close them while you inspect.

Overcurrent

String fuses or ganged fuse holders in the combiner, cable current-carrying capacity versus string short-circuit current (Isc) and the inverter's maximum reverse current, and DC isolators that are actually DC-rated (an AC-rated switch on a PV string is a classic defect). Fuse ratings must match the module and cable data, not whatever was left in the van. Where parallel strings feed one MPPT, confirm that overcurrent protection exists where the design required it.

Earthing and bonding

Follow the design. Some arrays bond module frames and mounting rails to the main earthing terminal; some rely on Class II construction and do not earth the frames. Conductors that are installed must be continuous, labelled, and correctly terminated. Do not add an earth the design did not call for just because a rail looks metallic. Do not omit a bonding conductor that the lightning and overvoltage assessment did call for. Check that bonding clamps on rails are compatible with the rail metal and that paint or anodising has been dealt with as the manufacturer requires.

Lightning and overvoltage

Where the risk assessment and the IET CoP require SPDs, confirm they are present, of the correct type and voltage rating, on the correct side of the isolator as specified, and with the protective conductor landed. Missing SPDs when the design required them is an inspection fail, not a note to mention at handover. An AC SPD fitted on a DC bus is the wrong device. A DC SPD with the PE terminal left floating will not do its job.

Equipment selection and erection

Inverter IP rating versus location (a kitchen utility with a tumble dryer is not a ventilated plant room). DC isolator adjacent to the inverter as required. Fire-service DC isolator labelling where adopted. Module clamps of the correct type (mid or end), torque, and rail compatibility. Undersized rails, missing end clamps, and over-torqued clamps that crack frames all belong here. Labels: DC isolator, dual-supply warning, PV array on the roof, polarity marks, and circuit charts.

Inspection item versus defect

Record defects against items. A useful schedule looks like this:

Scenario: first-fix complete, inverter not yet live

The array is landed, DC cables are pulled, the combiner and DC isolator are fitted, but the inverter is on the wall with AC tails not terminated and the unit not energised. This is a normal site state and a typical exam scenario.

You can complete a thorough DC inspection: polarity of connectors, labels, IP, combiner segregation, SPD presence, module clamps, cable supports, bonding conductor routing, and isolation of the inverter DC terminals so nobody can back-feed the unit. You can often complete dead DC tests (continuity of bonding, polarity, insulation resistance with the inverter isolated) once strings are complete. You cannot treat the job as commissioned. Functional tests — inverter start-up, G98 or G99 protection, export limitation, and live isolator operation under generation — wait until the AC side is complete, notified as required, and safe to energise.

Do not borrow a temporary AC feed to spin the inverter up for a photograph. Do not measure string current by shorting at the inverter input while the inverter is still being installed. Record the inspection as first-fix DC complete, inverter not live, list outstanding AC items, and return for the functional and handover stage.

If a connector is reversed at first-fix, that is the moment to cut it out and remake it. Measuring Voc later with reversed polarity risks reverse-feeding the meter and, if anyone has closed the DC isolator onto the inverter, reverse-feeding the inverter input. Inspection is cheaper than a destroyed instrument.