C&G 2922-34 Solar PV key terms

A UK quality assurance scheme that certifies solar PV installers and products, and is normally needed for customers to claim government incentives and for the installation to be accepted under permitted development.

The UK national wiring regulations that set the design, installation, inspection and testing standards for low-voltage electrical work, covering both the AC and DC sides of a PV system.

The Energy Networks Association engineering recommendation setting technical requirements for connecting small-scale generation to the UK distribution network, including voltage and frequency trip settings, and normally requiring notification to the DNO before energising.

A formal procedure for proving a circuit dead and keeping it that way: identify, isolate, secure with a padlock, prove the tester, test for dead, then prove the tester again.

Work on roofs, ladders or scaffolding where a fall could cause injury; it needs planning, suitable access equipment, edge protection or fall arrest, and a rescue plan before starting.

The array produces voltage whenever light falls on it, so the DC side stays live even with the inverter and isolator off; a switch only breaks the circuit downstream, it does not de-energise the modules.

The instantaneous power of sunlight falling on a surface, measured in watts per square metre (W/m²). Module output current is roughly proportional to irradiance.

The total solar energy received on a surface over a period, usually given in kilowatt-hours per square metre per day or per year, used to estimate annual energy yield for a site.

The reference conditions for module nameplate ratings: 1000 W/m² irradiance, 25 °C cell temperature and AM 1.5 spectrum, giving figures for peak power, open-circuit voltage and short-circuit current.

The rate at which a module's open-circuit voltage falls as cell temperature rises, usually a negative percentage per °C, used to work out the highest voltage expected on the coldest day.

A survey of how chimneys, trees, dormers and nearby buildings will shade the array through the year; even light shading on one module can sharply cut string output and should be mapped at design stage.

Choosing how many modules to connect in series so that cold-weather open-circuit voltage stays below the inverter's maximum input and module system voltage, while still sitting in the MPPT range.

The array's rated DC power divided by the inverter's rated AC output; slight array oversizing improves output in low light but must stay within the inverter maker's permitted limits.

The extra dead load, wind uplift and snow load that modules and mounting add to a roof; older or lightweight roofs may need checking by a competent structural person before work starts.

A diode inside a module's junction box giving current an alternative path around a shaded or faulty cell group, limiting hot spots and reducing the power otherwise lost.

The aluminium rail and hooked bracket system fixed to roof structure that carries the module clamps; rail spacing and fixing type must match the manufacturer's wind and snow load data.

Where a fixing or cable passes through the roof covering, a weathertight seal must be formed and the fixing must land on a structural member, so water cannot enter the roof space.

An above-roof frame held down by weights rather than roof fixings, used where penetration is not allowed; weight, wind uplift and protection of the membrane all need calculating.

A lockable, touch-protected DC connector used to join module leads and string cables; only matching brands should be mated and crimping must be correct to avoid resistance heating.

Flexible, double-insulated, UV and ozone resistant single-core cable rated for PV DC circuits, typically 1.5 kV, chosen so voltage drop and current rating suit the string.

Ingress Protection code showing how well an enclosure resists dust and water, such as IP65 or IP68; inverters, isolators and connectors need a rating suited to where they are fitted.

Reducing a cable's current-carrying capacity to allow for grouping with other cables, high ambient temperature, enclosure or thermal insulation, so the protective device still protects the cable.

Equipment that converts the array's DC into AC synchronised with the mains and must disconnect automatically if the grid fails or drifts outside permitted voltage and frequency limits.

A control function in the inverter that continually adjusts the array's operating voltage to extract the greatest possible power as irradiance and cell temperature change.

A function that stops the inverter exporting power when the grid supply is lost, protecting network staff and the public from lines that would otherwise stay energised.

A switch on the DC side that separates the array from the inverter for maintenance; it must be rated for DC, positioned correctly and capable of being locked off, though it does not de-energise the modules.

Type A responds to AC and pulsating DC residual current; Type B also detects smooth DC. Inverters able to inject DC fault current may need a Type B device or one approved by the manufacturer.

The increase in AC voltage along the cable between inverter and supply point caused by exported current; it must not push the voltage at the inverter beyond the connection standard's limits.

A setting or control arrangement that caps the power exported to the grid, often imposed by the network operator as a condition of connection, using metering and inverter control or a separate limiter.

Connecting module frames and mounting metalwork to the installation's earthing system so fault current has a reliable path and exposed metal cannot become live, as required by the wiring regulations.

A DC test voltage applied between array conductors and earth to check insulation integrity; a low reading in megohms points to moisture, damaged cable or an earth fault.

Plotting the array's current against voltage across its whole operating range and comparing it with the expected curve, which reveals shading, mismatch, faulty bypass diodes or degraded modules.

Measuring string open-circuit voltage and short-circuit current on site, then correcting the readings for irradiance and temperature to confirm the string matches the design and is wired correctly.

Using an infrared camera to find hot cells, joints and connectors under load; temperature anomalies indicate high resistance, loose terminations, hot spots or failing bypass diodes.

A system's actual energy yield divided by the theoretical yield of its modules at the same irradiation, expressed as a percentage; it captures losses from temperature, soiling, shading and conversion.

A localised overheated area in a module caused by a cracked, mismatched or shaded cell being forced into reverse bias; it can permanently damage the cell and laminate and is a fire risk.

A voltage-driven loss of module power caused by leakage currents between cells and frame under high system voltage, often made worse by damp heat and poor earthing arrangements.

The documents given to the customer on completion, including test results, schematics, equipment manuals, the MCS certificate, warranty details and maintenance advice.