Protection and Safety in DC Electrical Systems

A safe solar system does not rely on luck. It relies on multiple stacked, independent layers of protection: earthing, surge protection, switching and fusing devices, residual current protection, and safe operating procedures. This document fills the gaps left by the original infographics and consolidates all protection layers into a complete picture for safe design and installation.

For: design and installation technicians, newcomers who need to understand protection devices, and sales staff who need to explain why a properly certified system includes so many safety components.

Quick Summary

Protection in a solar electrical system follows multiple independent layers so that if one layer fails, others remain. Earthing brings the module frames and equipment enclosures to a safe ground potential. Surge protection devices guard equipment against voltage transients. Switching devices and fuses isolate circuits and provide overcurrent protection. Residual current devices protect people from electric shock.

The DC side and AC side each require their own dedicated protection devices, because AC protection devices cannot protect the DC side. This is a principle that is easily overlooked.

The final layer is human. Safe operating procedures, personal protective equipment, and the habit of measuring before touching are things no device can replace.

Earthing and Equipotential Bonding

Earthing is the foundation of every electrical system. Module frames, aluminium rails, and the metal enclosures of all equipment must be earthed so that when a fault current flows, it travels safely to ground and the protective device can trip in time, rather than leaving the equipment enclosure energised and causing electric shock. For residential systems, the earthing resistance should be approximately 10 ohms or less.

In addition to protective earthing, the system also requires equipotential bonding: interconnecting all metallic parts so they share the same potential, eliminating dangerous voltage differences between components. Detailed earthing and protection requirements for solar PV systems are specified in IEC 62548 and IEC 60364-7-712.

Surge Protection

A nearby lightning strike can induce large voltage surges that travel along cables and destroy the inverter and other equipment. A surge protection device (SPD) diverts these voltage transients to ground to protect downstream equipment. The effectiveness of an SPD depends entirely on the quality of the earthing connection: an SPD with poor earthing is almost useless.

The key point is that the DC side and the AC side each need their own SPD. An SPD for the AC side does not protect the DC side, and vice versa. Standard practice is to install a Type 2 SPD on the DC side at the string array and a Type 1 or Type 2 SPD on the AC side at the inverter output. When the cable run between the array and the inverter is longer than approximately 10 m, SPDs should be installed at both ends.

Switching, Isolation, and Overcurrent Protection

The system requires devices to safely isolate circuits during maintenance and to interrupt overcurrent conditions. On the DC side, a DC-rated isolating switch must be used, one specifically designed to interrupt DC current, which is inherently difficult to extinguish. Under no circumstances should an AC-rated switch be used as a substitute. When three or more strings are connected in parallel, each string requires a dedicated PV string fuse to protect it from reverse current flowing from the other strings in the event of a fault on that string.

On the AC side, standard circuit breakers are used for overcurrent and short-circuit protection. The rated current of fuses and circuit breakers is selected based on the short-circuit current and operating current of the PV modules and the load, following the V-I-R-P principles described in document 01-01.

Two additional protection layers specifically for DC arc faults (the arc fault circuit interrupter (AFCI) and rapid shutdown) were covered in document 01-03 and are part of this overall protection system.

Residual Current Protection and Personal Safety

Leakage current is a small current that flows outside the normal conduction path (for example, through the body of a person who touches equipment with degraded insulation) and it can cause a fatal electric shock even at very low levels. A residual current device (RCD) disconnects the circuit when it detects that the difference between inflow and outflow current exceeds the safe threshold.

For transformerless grid-tied inverters, the inverter itself must include an internal leakage current monitoring function complying with IEC 62109-2, capable of detecting continuous leakage current from approximately 300 mA and responding accordingly. When an additional external RCD is required on the AC side, the correct type must be selected according to the inverter manufacturer’s recommendation, because inverters can produce a DC leakage current component that only a Type B RCD can handle: a Type A RCD only handles AC and pulsating leakage current.

People Are the Final Layer of Protection

No device can substitute for safe operating practice. For DC systems, the most important rule is never to switch under load. The system must be isolated and placed in a safe state before any work begins, because switching under load is the direct cause of arc formation. Always measure the voltage to confirm de-energisation before touching any conductors, use appropriate insulating gloves and boots, and for high-voltage systems work in pairs so that assistance is available in an emergency.