C&G 2922-34 Solar PV cheat sheet

PV modules generate DC voltage whenever light falls on them; the DC side remains live when the AC isolator is open and must be treated as live.

BS 7671 treats PV as a special installation; all AC and DC work must comply with the current edition and relevant building regulations.

G98: generation up to 16 A per phase (3.68 kW single-phase) may be installed and notified to the DNO within 28 days; G99 requires prior application for larger systems.

DC isolators and protective devices must be DC-rated for system maximum Voc and Isc; AC-rated devices must not be used on PV DC circuits.

In England and Wales, Part P applies to electrical work in dwellings; notifiable work must be certified by a registered competent person or building control.

Work at height follows the hierarchy: avoid, prevent (edge protection or scaffold), then mitigate (harness, fall arrest); roof fragility and asbestos must be assessed.

RIDDOR requires reporting of dangerous electrical incidents and certain injuries; maintain emergency procedures for DC arc hazards.

String maximum Voc is calculated at the lowest expected cell temperature using the module Voc temperature coefficient; it must not exceed inverter maximum DC input or module maximum system voltage.

String Vmp at the highest expected cell temperature must remain above the inverter MPPT minimum; otherwise the inverter cannot track maximum power.

In the UK, a south-facing array at 30–40° tilt gives near-optimal annual yield; east/west orientations reduce yield and shift generation to morning and evening.

PV arrays typically add 10–20 kg/m²; roof structure, rafters and fixings must be checked for dead load plus wind and snow loading.

DC cable voltage drop should typically be kept below 1% and AC voltage drop below 3–5% to limit energy loss and maintain inverter input voltage.

Inverter DC:AC ratio is array peak DC power divided by inverter AC rating; typical UK ratios are 1.1–1.3, trading clipping against yield.

Shading analysis should quantify shade across the year; persistent shade requires module-level optimisers, micro-inverters or array relocation.

Cable current-carrying capacity must be corrected for ambient temperature, grouping and installation method; Isc and continuous current must be considered.

Module clamps must sit within the manufacturer's specified clamp zones, commonly 1/4 and 3/4 along the frame, and be torqued to the stated value.

Roof hooks and brackets should be fixed into rafters or trusses, not tile battens, using stainless steel fixings sized for wind uplift.

PV DC cable must be double-insulated, UV-resistant and rated at least 1000 V DC (often 1500 V DC), with current rating at least 1.25 × Isc.

Only connectors of the same manufacturer and type should be mated; use the correct crimp tool and verify pull-out strength to avoid overheating.

Series connection adds module voltages while current remains constant; parallel connection adds currents while voltage remains constant.

Every roof penetration must be sealed with a compatible flashing or sealant; leave a ventilation gap behind modules to limit overheating.

DC and AC circuits should be segregated; DC cable must be supported, protected from sharp edges and not installed in AC-only conduit without separation.

A lockable AC isolator must be installed adjacent to the inverter; a DC isolator is required at the array and/or inverter as specified by design.

The inverter AC circuit may require a Type A or Type B RCD depending on the inverter's DC residual current characteristics; follow manufacturer instructions and BS 7671.

The inverter must have anti-islanding protection and disconnect from the grid on loss of supply; this is verified during commissioning.

The inverter should be connected via a dedicated MCB or RCBO in the consumer unit, rated to the inverter maximum AC output current and cable capacity.

An export limitation current transformer clamp on the incoming supply measures export; the inverter or limiter reduces output to the DNO-approved limit.

A dual supply warning label must be fixed at the consumer unit and meter position to warn that the installation is energised by PV as well as the grid.

On TT systems the inverter AC circuit requires RCD protection, typically 30 mA, and main protective bonding must be verified.

Before connection, measure DC insulation resistance of each string to earth; values below 1 MΩ indicate a fault requiring investigation.

String open-circuit voltage is compared with expected Voc corrected for cell temperature and irradiance; zero or low voltage indicates open circuit, reverse polarity or failed module.

An I-V curve tracer compares actual array performance with the expected curve and can reveal shading, soiling, mismatch or failed bypass diodes.

Thermal imaging detects hot spots, faulty bypass diodes, loose connections and cell damage during commissioning and maintenance.

RCD trip tests verify trip time and current meet BS 7671; functional tests confirm anti-islanding, export limitation and monitoring.

Annual maintenance includes visual inspection, connector torque checks, DC insulation resistance, inverter error log review, cleaning and label checks.

Earth faults are located by isolating strings and testing insulation resistance to earth; the faulty string shows low resistance.