DC arc-fault detection in PV strings — supplementary fire protection

The afdd-arc-fault-detection guide covers arc-fault detection on the AC side of a final circuit (IEC 60364-4-42/§421.7). The DC side of a PV installation (the wiring between solar panels and inverter, see the 712-pv-solar guide) has its own, separate standards framework: IEC 63027 for so-called Arc Fault Protection Equipment (AFPE).

Why the DC side carries a different risk

A series arc fault on the DC side of a PV string (for example a loosened MC4 connector, a rodent-damaged cable, or a degraded crimp connection) cannot be detected by a regular PV string fuse: the current through the arc often stays below the fuse's melting value, while the arc itself, fed by the continuous DC energy from the solar panels as long as light falls on them, can keep burning for hours. A regular RCD (see the RCD-types guide) is moreover not designed to detect a series arc fault either — it does not produce a residual current, only localized overheating at the site of the arc itself.

What AFPE/IEC 63027 does

An AFPE device (often integrated into the inverter, sometimes as a separate device per string) monitors the current and voltage pattern in the DC wiring for the characteristic high-frequency noise pattern an arc produces. Upon detecting an arc pattern, the device interrupts the affected DC circuit and gives a visual indication that an arc has been detected; restart typically requires a manual reset — the inverter does not automatically switch back on without someone confirming the cause has been resolved.

Standards framework: international vs. Dutch practice

• IEC 63027 is the international product standard for AFPE equipment (performance requirements, test regime, detection sensitivity).
• In the United States, DC arc-fault detection for building-mounted PV installations above a certain system voltage is legally mandatory (NEC 690.11); in the Netherlands, NEN 1010 has no comparable general requirement for DC AFPE — similar to the situation with AC AFDD, where the Netherlands has a recommendation rather than a general requirement.

Note: this article gives the standards framework and the underlying protection principle. Whether a specific PV installation needs AFPE (or whether an inverter with an integrated AFPE function is preferable) is a project-specific decision based on system voltage, roof construction/fire risk and insurer requirements — this article does not prescribe a universal obligation.

Practical relevance

When choosing an inverter for a new or replacement PV installation, it is worth checking whether an integrated DC arc-fault detection function is present — especially for installations on a fire-sensitive roof (for example a thatched roof) or with a long DC cable run with multiple connector joints, where the risk of a loosened or degraded contact point is higher than with a short, compact DC wiring layout.

Common mistakes

1. Assuming a regular PV string fuse or an RCD detects a series DC arc fault — neither is designed for this; only a dedicated AFPE device (IEC 63027) does.
2. Confusing AC arc-fault detection (AFDD, §421.7) with DC arc-fault detection (AFPE, IEC 63027) — two entirely separate standards frameworks for a physically different part of the installation.
3. Simply restarting the inverter after an AFPE trip without actually locating and fixing the underlying connector or cable damage — the arc can recur on restart without a repair.

Related

• §712 — PV installations (solar panels)
• Arc-fault detection (AFDD) — AC side
• PV fire-fighter's switch