PV system protection: SPDs, breakers, fuses and combiner boxes

· 6 min read ·

The devices that protect people and equipment in a solar installation, from string fuses to the RCD in the board, and where each one goes in the diagram.

Two sides, different risks

From the panel to the inverter flows direct current (DC), at hundreds of volts. From the inverter to the distribution board, alternating current (AC). Each side has its own protective devices, and those on the DC side must be rated for direct current.

Alternating current crosses zero several times per cycle; direct current never does Alternating current (AC) Crosses zero 100–120 times per second: the arc goes out by itself Direct current (DC) Never crosses zero: the arc is sustained: use DC-rated devices
A standard AC-rated circuit breaker installed on the DC side may be unable to interrupt the current.

Each protection in its place

Interactive

Where does each protection go?

Tap a device to see what it does. Surge protective devices (SPDs) are connected in shunt, between the conductor and earth.

DC side (direct current)AC side (alternating current)Earthing
  1. inverter output

SPDs are connected in shunt: between the conductors and earth, outside the energy path. The SPD branches, the mounting structures and the enclosures all end at the earthing system.

String fuse

DC

Protects each string against reverse current coming from the other strings in parallel. Usually needed from three strings on, depending on the module data.

Where it goes: in the combiner box, one per string

DC isolator (disconnect switch)

DC

Safely disconnects the panels from the inverter for maintenance. Many inverters already include one.

Where it goes: between the combiner box and the inverter

DC SPD

DC

Diverts voltage surges from nearby lightning strikes to earth, protecting the inverter.

Where it goes: in the combiner box, connected to earth

AC circuit breaker

AC

Protects the cable between the inverter and the board against overload and short circuit, and isolates the inverter from the installation.

Where it goes: at the inverter output

AC SPD

AC

Protects against surges arriving from the utility grid.

Where it goes: in the board, next to the inverter breaker

RCD (residual current device)

AC

Detects current leaking to earth, such as when someone touches a live part, and trips quickly. It is additional protection against shock, chosen for the application; it does not protect equipment.

Where it goes: in the distribution board

Earthing (grounding)

Earth

Earth rods and the earth busbar bond structures, enclosures and SPDs, give surges a path and reduce potential differences between the parts.

Where it goes: bonded to the building’s main earthing system

Earthing done right

  • Panel structures, inverter enclosure and SPDs connected to the earthing system
  • Panel earthing bonded to the main earthing of the building, rather than an isolated rod
  • Protective conductors short and without loops: long, coiled cables increase the impedance for surges

How an SPD works

  1. 1
    Normal operation

    Connected between the conductors and earth, the SPD barely conducts.

  2. 2
    A surge arrives

    Nearby lightning or switching on the grid raises the voltage: the SPD conducts for an instant and diverts the energy to earth.

  3. 3
    Wear

    Each surge wears out the device. Many models have a window that changes color when it is time to replace it.

Class Iwhere there is a risk of a direct strike (buildings with a lightning protection system)
Class IIthe most common in boards and combiner boxes
Ucmaximum continuous operating voltage of the SPD: compatible with the string Voc in the cold

Common installation mistakes

Do

  • Devices rated for DC and for the string voltage
  • Respect the polarity of polarized DC breakers
  • A low-impedance path from the SPD to earth
  • Warning labels for solar generation and dual supply

Avoid

  • An AC-rated breaker or switch on the DC side
  • An SPD without proper earthing: it cannot divert the surge
  • Connectors from different brands mated together on the panel cables

Standards, inspection and responsibility

  1. 1
    Visual check

    Combiner box and board: signs of heating, darkened plastic or a burning smell call for immediate attention.

  2. 2
    Re-tightening the connections

    Loose connections heat up with the current. Only with the system de-energized and following the manufacturer’s procedure and torque.

  3. 3
    SPD windows

    Replace the flagged modules.

  4. 4
    RCD test

    Press the test button: it must trip.

  5. 5
    Inverter logs

    Read the fault history in the app or on the display.

Where each protection appears in the diagram

In the diagram below, the combiner box groups the DC protection; after the inverter come the breaker, the SPD and the RCD, and the earthing connects the SPDs.

Draw it in Editor Solar

This is Editor Solar itself, already loaded with the diagram from this article. Move the equipment, edit labels, connect new parts and export to PNG, SVG or PDF. It is free and runs right in your browser.

DC protection grouped in the combiner box and AC protection between the inverter and the distribution board.

Try it: Add a second string: place another panel array with its own DC fuse and connect both fuses to the isolator.

Ready-made templates: see all diagram templates

Open this diagram in the full editor ↗

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