Solar PV Panels: DC String Protection, DC SPD and AC Distribution
Solar PV switchboard: On the DC side of a PV plant the rules change: current does not cross zero, arcs do not self-extinguish and the array is live all day.
ContentsA solar PV switchboard hosts two different worlds in one plant: the direct current side coming from the array and the alternating current side downstream of the inverter. The familiar logic of a low-voltage board applies on the AC side. On the DC side, the fact that current never crosses zero, that an arc does not extinguish by itself and that the array is live whenever there is daylight changes the design from the start.
Why the DC side behaves differently
In alternating current an arc extinguishes naturally because current passes through zero twice per cycle. Direct current offers no such help: once struck, an arc persists in a device that was not designed for it. DC circuits therefore use switches, fuses and connectors designed for DC and selected for the correct voltage class. The second difference is safety: strings generate voltage throughout the day, so an isolation procedure and clear labelling matter even more on the DC side than on the AC side.
String protection and the array combiner box
Fusing each string separately in the combiner box prevents a fault in one string from disturbing the others and makes fault finding straightforward. Fuse selection follows the short-circuit current of the string, temperature correction and the gPV characteristic. As the number of strings grows, so does the need for measurement: per-string current monitoring separates yield loss caused by shading from loss caused by a fault.
DC surge protection and earthing
PV sites cover large areas and usually sit in open terrain, which exposes them to lightning-induced overvoltage. A surge protective device on the DC side must be designed for DC and rated for the maximum system voltage; an AC type device is not safe in a DC circuit. Devices are placed both in the combiner box and close to the inverter, respecting the protection distance. For the selection logic see our article on surge protective device selection.
AC output board and grid protection
| Topic | DC side | AC side |
|---|---|---|
| Arc behaviour | Does not self-extinguish | Extinguishes at current zero |
| Protective device | DC rated switch, gPV fuse | Standard LV breaker and fuse |
| Surge protection | DC type SPD | AC type SPD |
| Isolation procedure | Live in daylight, isolation plan mandatory | Can be isolated by the main device |
| Measurement | String current and insulation resistance | Energy, power quality, export metering |
On the AC side, inverter outputs are combined, selectivity is arranged and the protection functions required by the network operator are provided at the point of connection: islanding detection, over and under voltage and frequency protection, and an export interlock. For the metering infrastructure see energy monitoring in switchboards.
Outdoor conditions: UV, temperature and protection class
Field boards usually operate outdoors in direct sun. Enclosure selection weighs UV resistance, double skin or sun shields, appropriate IP and IK ratings and ventilation that limits internal temperature. Since a high internal temperature derates fuses and switches, thermal calculation cannot be skipped in PV boards. For protection classes see our article on IP protection ratings.
Key takeaways: solar PV switchboard
- Why the DC side behaves differently — In alternating current an arc extinguishes naturally because current passes through zero twice per cycle.
- String protection and the array combiner box — Fusing each string separately in the combiner box prevents a fault in one string from disturbing the others and makes fault finding straightforward.
- DC surge protection and earthing — PV sites cover large areas and usually sit in open terrain, which exposes them to lightning-induced overvoltage.
- AC output board and grid protection — On the AC side, inverter outputs are combined, selectivity is arranged and the protection functions required by the network operator are provided at the point of connection: islanding…
- Standard source — IEC publication catalogue: iec.ch.
Frequently asked questions
Does every string need its own fuse?
In practice, yes once there are more than two strings. A separate fuse isolates the faulty string and limits reverse current into the others. In small systems the inverter manufacturer documentation is decisive.
Can a DC switch be replaced by an AC switch?
No. A device rated only for AC is not designed to extinguish a DC arc and creates a fire risk. In a DC circuit, the DC voltage rating and the number of poles must be verified from the label.
Are residual current devices needed in a PV board?
With transformerless inverters, leakage current can contain a DC component, which calls for a type B device or reliance on the insulation monitoring function of the inverter. The choice follows the conditions stated by the inverter manufacturer.
Related articles
- Load Calculation and Rated Current: Demand and Diversity Factors
- Residual Current Device Types: The Difference Between AC, A, F and B
- Busbar Current Carrying Capacity and Derating Calculation
- Battery Energy Storage (BESS) Panel: DC Protection, PCS Interface and Safety
- All technical articles
Related Devpan solution: low voltage panels.
Devpan designs the board scope of a PV site from combiner boxes to AC output and grid connection boards, matched to outdoor conditions. Send us the single-line diagram of the plant and we will plan the DC and AC sides together.
