In a photovoltaic installation two electrical domains with different risks coexist: the DC side (modules and strings) and the AC side (from the inverter to the internal network). Choosing and coordinating protections correctly prevents faults, extends system life and reduces downtime. If you want a guided assistant to size the board, use the Photovoltaic configurator.
Quick map of differences
| Aspect | DC protections | AC protections |
|---|---|---|
| Area | Between modules, strings and the inverter input | From the inverter to the main distribution board |
| Objective | On-load isolation, limit return currents and surge effects | Protect circuits, people and equipment against overcurrents and surges |
| Typical devices | DC isolator, gPV fuses for each string, Type 2 DC SPD | MCB, RCD, Type 2 AC SPD, or combi devices |
| Selection | Generator Voc and Isc, number of parallel strings, cable runs | Inverter power, rated current, available prospective fault current (Icc), selectivity |
| Key risk | Sustained DC arc and reverse currents | Selective disconnection and protection of persons |
Understand the system before selecting
Define the architecture first. Parallel strings or MLPE (micro-inverters or optimisers)? Single-phase or three-phase inverter? Short or long DC runs? These answers guide the type and location of each protection. To validate the design and generate a bill of materials in minutes, rely on the Photovoltaic configurator.
DC protections: essential criteria
Direct current sustains the arc once initiated. Use devices certified for DC with breaking capacity suited to the string’s voltage and energy.
- On-load DC isolator: allows safe isolation for maintenance or emergency. It must break the array maximum open-circuit voltage (Voc) and string current.
- gPV fuses for each string: prevent return currents with parallel strings. Select by module Isc and temperature/grouping factors. Fit on each pole if required by the manufacturer.
- Type 2 DC surge protective device (SPD): limits transients that damage the inverter and modules. Install near the inverter; on long rooftop runs, add SPD in the PV field.
- Cabling and connectivity: use compatible PV connectors and suitable cable glands. Apply the correct torque and check sealing.
- Enclosures: choose IP and IK ratings fit for weather, UV and dust. Reserve space for heat dissipation and tidy wiring.
Practical rules for DC
- Calculate Voc at low ambient temperature to verify isolator and SPD withstand.
- Size gPV fuses for the worst case of parallel strings and temperature.
- Coordinate the DC SPD with the field’s maximum operating voltage. Ensure the protection level does not overstress the inverter semiconductors.
- If there is an external lightning protection system or high exposure, study higher-category SPDs at the building intake.
AC protections: safety and continuity
On AC, standard low-voltage protections apply. Match them to inverter power, earthing system and available fault current.
- Miniature circuit-breaker (MCB): protects the inverter output feeder against overloads and short-circuits. Select rating and breaking capacity according to the installation point’s Icc.
- Residual current device (RCD): protection of persons against indirect contact.
- Type A: common choice for AC and pulsating DC residual currents.
- Type F: appropriate where residual current may include AC up to 1 kHz and pulsating components.
- Type B: may be required depending on inverter topology where residual current can include AC up to 1 kHz, pulsating and smooth DC.
- Type 2 AC SPD: protects against transient overvoltages caused by:
- Switching operations in the public network
- Load switching
- Indirect lightning
- Type 2 DC SPD: protects circuits and electronics against transients produced by:
- Switching operations
- Inductive load commutations
- Indirect lightning
- Selectivity: a PV fault should not trip upstream protections. Adjust curves and ratings, and check impedances.
DC–AC coordination around the inverter
The inverter is the focal point. Coordinate its limits for voltage, current and surge energy with the board devices.
- On DC, the SPD must clamp at a level tolerated by the inverter’s power electronics.
- On AC, the MCB should not trip on brief inrush or transient currents. Choose curve and rating accordingly.
- The selected RCD must be compatible with the conversion topology. Avoid nuisance tripping due to harmonics.
Use cases and good practice
Residential self-consumption 1–5 kW
Short strings, single-phase inverter and compact board. DC isolator with gPV fuses and DC SPD near the inverter. On AC, dedicated MCB, Type A RCD (or as per the inverter manufacturer) and AC SPD in the dwelling board.
Commercial 10–50 kW
Several parallel strings, long runs and often a three-phase inverter. Coordinated gPV fuses, DC isolator with suitable breaking capacity and DC SPD both at inverter and in field if required. On AC, step selectivity, higher breaking capacity and coordinated AC SPD.
Micro-inverters or optimisers
With micro-inverters the DC section is short and module-level. Most protections move to AC at each branch. With optimisers a DC bus may remain; keep isolation, fusing and DC SPD according to the design.
Selection checklist
- Define number of strings, Isc and Voc of the array at extreme temperature.
- Choose gPV fuses per string and confirm curve and breaking capacity.
- Select DC isolator with utilisation category and voltage rating suitable for the array.
- Add Type 2 DC SPD. Consider an additional device in the field for long cables.
- On AC, set the MCB by inverter current and available Icc.
- Determine the compatible RCD (Type A, F or B per the inverter datasheet).
- Include a Type 2 AC SPD coordinated with any upstream SPD.
- Verify enclosure IP/IK and space for heat and wiring.
- Document schematics, labelling and earthing.
Key parameters and quick rules
| Element | What to check | Rule of thumb |
|---|---|---|
| gPV fuse | Module Isc and number of parallel strings | Size from Isc·k (temperature and back-feed). One per string. |
| DC isolator | Voc at low temperature and string current | On-load breaking with margin over max Voc. |
| DC SPD | Field voltage and inverter protection level | Type 2 DC near the inverter; add in field for long runs. |
| AC MCB | Inverter rated current and Icc | Choose curve and breaking capacity for the installation point. |
| RCD | Inverter topology and harmonics | Type A as baseline; Type F or B as per manufacturer guidance. |
| AC SPD | Coordination with upstream SPD | Type 2 AC in the inverter injection board. |
Common mistakes and how to avoid them
- Using AC-rated devices on DC. The arc will not extinguish the same way, with risk of overheating or explosion. Use DC-specific isolators and SPDs.
- Undersized fuses. Recheck Isc, temperature and number of parallel strings.
- Poor SPD coordination. Ensure protection levels are compatible with the inverter electronics.
- No visible isolation point. Essential for safe work and fast diagnostics.
- Incompatible connectors. Mixing brands can overheat. Use compatible, certified PV connectors.
Regulations and documentation
Requirements vary with location, earthing system, lightning exposure and roof type. Consult and record. Keep drawings, protection calculations and commissioning protocol updated. For a summary of applicable frameworks, see Directives and regulations. For environmental obligations and end-of-life, see Environmental legislation. When designing and installing in the UK, align with the IET Wiring Regulations (BS 7671) and relevant product standards.
Integration in the board: order, safety and maintenance
A well-organised board simplifies diagnostics and reduces intervention time. Leave spare capacity for future expansion. Separate DC and AC routes inside the enclosure. Maintain bending radii and routing so cabling does not strain connectors.
- Label poles, strings, voltages and energy flow direction.
- Physically separate DC and AC with barriers or internal partitions if the enclosure allows.
- Use cable glands with sealing suitable for outdoor exposure.
- Include an accessible isolation point for emergencies.
Impact of storage and EV
If the system includes batteries, the DC section gains extra protections: isolation and fusing suited to chemistry and voltage, plus SPD if runs are long. For EV charging, review AC selectivity and RCD coordination between the charge point and inverter to avoid nuisance trips.
Total cost of ownership and diagnostics
Correct protection prevents early failures and cascade replacements. A well-coordinated SPD costs far less than power electronics. Clear isolation reduces labour hours. Standardise references and generate consistent documentation for installation and after-sales.
Recommended workflow
- Collect array data: Voc, Isc, number of modules and strings.
- Define location and length of DC and AC runs.
- Select gPV fuses, DC isolator and DC SPD.
- Set MCB, RCD and AC SPD by inverter power and Icc.
- Choose an enclosure with suitable IP/IK and spare space.
- Document and label.
This workflow is integrated in the Photovoltaic configurator. If you already know the codes, speed up ordering with the Reference configurator.
Frequently asked questions
When do I need a Type B RCD?
When required by the inverter manufacturer due to installation characteristics and the possible presence of residual currents including AC up to 1 kHz, pulsating and smooth DC. Always follow the equipment datasheet.
Type 1 or Type 2 SPD?
In most self-consumption systems without an external lightning protection system, Type 2 is the standard on both DC and AC. If the building has an LPS or there is high exposure, assess higher-category SPDs at the intake and coordinate with Type 2 devices downstream.
What changes with micro-inverters?
Protection shifts to AC. The DC section is very short and per-module. Keep AC SPD and selectivity. Follow the micro-inverter manufacturer’s instructions.
How do I reduce nuisance tripping?
Coordinate MCB curves, choose an RCD compatible with the inverter topology and review harmonics. Separate DC and AC bonding and improve earthing if necessary.
Conclusion and next step
A reliable PV system is built with DC and AC protections that are properly sized, coordinated and documented. Use certified devices for each side, place them where they add most value and design with maintenance in mind. To simplify selection and generate a client-ready bill of materials, use the Photovoltaic configurator. When your references are defined, complete the process with the Reference configurator.