PV systems use a variety of protective measures to reduce the risk of electrical hazards and equipment damage. Grounding is one basic measure, and surge protection is another.
Grounding and surge protective devices (SPDs) have different functions. Grounding is mainly used to connect exposed conductive parts to the protective earthing system and to achieve equipotential bonding. SPDs are used to limit transient overvoltages in PV circuits.
Whether a DC combiner box needs an SPD must be judged in combination with the PV system design, wiring layout, lightning strike risk, and applicable installation requirements. Understanding the difference between grounding and surge protection helps determine where protective measures should be installed in a PV system.
What Does Solar Panel Grounding Mean?
“Solar panel grounding” is a rather general term. In PV systems, grounding-related measures include equipment grounding, equipotential bonding, and system grounding. These three concepts differ in what they connect and in their main purpose.
Equipment Grounding
Equipment grounding mainly involves metal parts that need protection, such as solar panel metal frames, mounting brackets, DC combiner box enclosures, and inverter enclosures.
These parts normally do not carry the generating current. However, if the insulation inside the equipment is damaged, live conductors may contact metal parts, causing dangerous voltage on them and increasing the risk of electric shock to personnel.
NEC 690.43 mandates equipment grounding of exposed non-current-carrying metal parts such as PV module metal frames and equipment enclosures.
Equipment grounding carried out in accordance with the electrical design and applicable codes can provide a proper path for fault current and, together with protective devices, reduce the risk of electric shock. However, grounding cannot guarantee that metal parts are at zero potential under all conditions.
Equipotential Bonding
Equipotential bonding is the electrical connection of metal parts in a PV system that need to be connected, in order to reduce dangerous potential differences that may arise between these parts.
If a lightning strike or electrical fault causes a potential difference between different metal parts, proper equipotential bonding helps reduce the associated risk. However, it cannot eliminate all potential differences, nor can it replace a CC SPD to limit transient overvoltages in the DC circuit.
Equipment grounding and equipotential bonding are closely related, but they are not exactly the same: equipment grounding focuses on the connection between the metal parts of equipment that need protection and the protective earthing system; equipotential bonding focuses on the electrical connection between the relevant conductive parts.
System Grounding
System grounding is the intentional electrical connection of specified circuit conductors to the grounding system, in accordance with the electrical design of the PV system. The purpose of this operation is not personnel touch safety (because personnel should not touch current-carrying conductors), but rather:
Stabilizing the voltage to ground: establishing a clear voltage reference point for the entire electrical system and limiting excessive voltages imposed by lightning strikes or line surges.
Facilitating fault detection: in a grounded system, if another circuit conductor accidentally contacts ground, a clear fault current path is created, enabling ground fault detection devices to quickly identify and interrupt the circuit.
Why Is an SPD Still Needed After the Solar Panels Are Grounded?
The main reason is that grounding and surge protection address different electrical problems.
Grounding is mainly used to reduce the risk of electric shock, and to provide a path for fault current that meets design requirements. Surge protection is used to limit transient overvoltages in the circuit, and reduce the risk of surge damage to the inverter and other electrical equipment.
Lightning can produce transient overvoltages on the DC cables between the solar panels and the inverter through electromagnetic induction. Even if the metal frames of the solar panels are already grounded, such overvoltages can still appear in the DC circuit. This is one of the bases for also installing surge protective devices in PV systems.
Must Every DC Combiner Box Be Equipped With a Solar Surge Protectou?
Not necessarily.
Whether a solar surge protectou needs to be installed, and where the surge protector should be installed, must be judged in combination with the following factors:
- The electrical design of the PV system and applicable codes.
- The length and routing path of the DC cables.
- The lightning strike risk at the project location.
- Whether the building has an external lightning protection system.
- Whether the inverter and other equipment are already equipped with surge protection.
If the system requires a DC SPD, the designer should also determine the protection location, the DC rated parameters of the SPD, and the grounding connection method.
Where Should the DC Surge Protection Device Be Installed?
The installation location of the surge protection device, should be determined according to the electrical structure and protection needs of the PV system, and cannot be judged solely by the presence or absence of a caixa combinadora.
In some PV systems, the solar panel surge protection is installed inside the DC combiner box, to protect the relevant DC equipment downstream of the combiner box. In other designs, the SPD may also be installed near the inverter or at another appropriate distribution location.
Solar panel surge protection are usually connected in parallel to the circuit to be protected, rather than simply connected in series in the normal current path like an ordinary circuit breaker.
If the system is equipped with multiple solar SPDs at different locations, the coordination between these protective devices needs to be evaluated, to avoid merely increasing the number of devices without achieving the expected protection effect.
Conclusão
Grounding of the solar panel metal frames does not mean that the PV DC circuit has obtained surge protection. Solar panel grounding is mainly used for electric shock protection and voltage stabilization, while solar SPDs are mainly used to limit transient overvoltages.
Whether a DC surge protector needs to be installed in a DC combiner box depends on the system structure, lightning strike risk, and electrical codes, among other factors. Therefore, when designing a PV system, whether to install a surge protection device should not be decided solely on whether the solar panels are grounded; rather, the surge protection needs should be evaluated based on the complete system design.
