On a utility-scale solar plant, the DC collection system is thousands of connections made in the open, often by contract labour working to a deadline. Every field-made connection is a chance for a poor crimp, a mismatched connector or a reversed polarity, and DC faults are slow to find once the plant is energised. A pre-assembled harness moves most of those connections into a factory, where they are made and tested under controlled conditions.
Two ways to collect DC power
| Home-run to combiner box | Harness with branch connectors | |
|---|---|---|
| Layout | Each string runs its own pair of cables to a combiner box, where strings are fused and paralleled | Strings join a harness through branch (Y) connectors, and a shared trunk runs to the inverter or a recombiner |
| Field connections | Many terminations at the combiner box | Mostly plug-in connections; terminations pre-made |
| Cable quantity | Higher, one pair per string | Lower, strings share a trunk |
| Fusing | In the combiner box | In-line fuses in the harness branches, where required |
| Best suited to | Small and irregular arrays | Large, repetitive table layouts |
Connectors
DC connectors for PV systems are covered by IEC 62852. The most important field rule is never to mate connectors from different manufacturers, even if they look compatible. Mismatched contacts can raise contact resistance and overheat. A harness should use one connector system throughout, matched to the module connectors, with factory crimps made on the connector maker’s specified tooling.
Fusing
String overcurrent protection is required when the reverse current that parallel strings can push into a faulted string exceeds what the module can withstand. IEC 62548 sets out the rule. In general it becomes necessary once three or more strings are paralleled. In a harness design, fuses are placed in the branch legs, sized to the module’s maximum series fuse rating, and their position is shown on the drawing.
Cable
Harness cable must be rated for the DC system voltage, sunlight, heat and the ground conditions on site. Solar cable to BS EN 50618:2014, IEC 62930 and UL 4703 is rated for 1.5 kV DC and 120°C, with tinned class-5 copper conductors. Sizes, resistance and current ratings are in the solar DC cable chart, and APAR’s range is on the solar cables page.
Designing the harness
- Start from the table layout and string configuration, so branch positions and lengths match the modules exactly.
- Keep positive and negative runs close together to limit loop area, and route them so they cannot be crossed during installation.
- Mark polarity and string numbers on every leg.
- Clip the harness to the structure with UV-stable fixings, and keep it clear of sharp edges and standing water.
- Test every harness for continuity, polarity and insulation before dispatch.
APAR builds harnesses for solar projects, pre-terminated to project drawings for string and combiner applications, and suitable for continuous operation from minus 45°C to 120°C. Sites with high-chloride or acidic soil and possible tidal flooding can be accommodated in the design.