Direct Current (DC) generated by solar photovoltaic (PV) arrays presents unique fire and safety hazards. Unlike alternating current (AC), DC does not cross zero voltage 100 times per second, meaning DC electric arcs do not self-extinguish. Attempting to switch high-voltage DC (up to 1,000 V) with standard AC switchgear causes sustained plasma arcing, switch explosion, and roof fires. Here is the technical engineering standard under BS 7671 Section 712 and BS EN 60947-3.
The Physics of DC Arcing vs AC Extinction
In standard 50 Hz AC systems, voltage and current alternate sinusoidally, passing through zero volts twice every cycle (every 10 ms). When an AC switch opens, this natural zero-crossing rapidly cools and de-ionises the air gap, quenching any miniature arc.
In a solar PV string operating at 400 V to 800 V DC, the potential difference is constant and unyielding. When contacts open or when a cable connection loosens:
- Current continues to ionise surrounding air molecules, creating a conductive plasma channel reaching temperatures exceeding 3,000°C.
- The plasma arc will effortlessly sustain across an air gap of several centimetres, vaporising copper contacts and igniting adjacent combustible roofing materials.
- The PV array cannot be turned off by opening a circuit breaker, as long as sunlight strikes the panels, the DC cables remain fully energised.
Switch Disconnector Ratings: BS EN 60947-3 (DC-PV1 vs DC-PV2)
Standard AC isolators (rated AC-21 or AC-22) must never be installed on solar DC circuits. BS 7671 Regulation 712.536.2.2 mandates dedicated DC switch-disconnectors conforming to BS EN 60947-3 with explicit utilization categories:
| Category (BS EN 60947-3) | Operational Capability | Application in Solar PV |
|---|---|---|
| DC-21A / DC-21B | Switching of purely resistive DC loads. Very limited arc-breaking capacity. | Prohibited for PV arrays where capacitive/inductive inverter filters exist. |
| DC-PV1 | Occasional switching of single solar PV strings under normal load currents. | Acceptable on single-string residential systems with zero risk of parallel reverse currents. |
| DC-PV2 | Frequent switching under heavy load, overload, and reverse fault currents from parallel strings. | Mandatory for commercial multi-string arrays and highly recommended for all modern PV installations. Features internal snap-action magnetic blowout coils. |
Cabling Standard: BS EN 50618 (H1Z2Z2-K)
Under Regulation 712.521.101, DC cables between the PV modules and the inverter must be specially constructed single-core cables conforming to BS EN 50618 (type designation H1Z2Z2-K):
- Voltage Rating: Rated for a nominal voltage of 1.5 kV DC (1,500 V) between conductors and to earth.
- Thermal Withstand: Operating temperature range from −40°C to +90°C (maximum permissible conductor temperature of 120°C for 20,000 hours).
- Environmental Durability: Double-insulated with cross-linked polymer insulation and sheath; fully UV-resistant, ozone-resistant, and Low Smoke Zero Halogen (LSOH).
Inverter RCD Selection: Regulation 712.411.3.2.1.2
Where an RCD is installed on the AC supply side of a solar PV inverter (e.g. to provide fault protection or cable protection in walls):
Mandatory Inverter RCD Rules:
Regulation 712.411.3.2.1.2 states that the RCD shall be of Type B conforming to BS EN 62423,unless one of the following exemptions applies:
- The inverter provides at least simple separation (galvanic isolation) between the DC side and the AC side via an internal isolation transformer.
- The inverter manufacturer provides a formal written declaration that the inverter construction inherently prevents DC residual currents from back-feeding into the AC electrical installation, in which case a standard Type A RCD may be used.
Firefighter Safety & Emergency Isolation (Reg 712.522)
Because PV arrays remain live in daylight even after the property's main AC switch is opened, emergency responders face severe shock risks when entering roof spaces or cutting through ceilings. BS 7671 Section 712 mandates:
- DC Cable Routing: DC cables must be routed in metal conduit or positioned clearly away from structural timbers.
- Clear Warning Signage: High-visibility warning notices ("DANGER: DUAL SUPPLY - SOLAR PV ARRAY REMAINS ENERGISED IN DAYLIGHT") must be displayed at the main intake cut-out, the consumer unit, and the inverter location.
- Array-Level Isolation: Modern installations increasingly deploy module-level rapid shutdown devices (RSDs) or micro-inverters that drop DC string voltages below 80 V within 30 seconds of AC power disconnection.