Standards for Grounding and Lightning Protection
This is an overview on standards for grounding and lightning protection of solar home systems (SHS).
Grounding (or earthing) refers to the provision of a low-resistance conduction path from points in the PV power system to the earth ground, and can be divided into two aspects:
- Grounding of equipment casings (e.g. the PV array frames and support structure)
- Grounding of the electrical circuits
Grounding can reduce the risks of damage from lightning-induced surges, may reduce the hazards to people, fix the maximum system voltages relative to the earth ground, and improve radio /TV reception.
Complete protection from lightning cannot be guaranteed, and the extent (and the cost) of technical lightning-protection measures should depend on the nature of the installation. In all cases, the primary concern should be people’s safety.
The installation of a complete lightning conductor system is far from being acceptable in SHSs for economic reasons. For example, under the Bolivian High Plateau conditions, where lightning storms are frequent, the annual losses of PV modules and regulators due to lightning damage are about 0.2%, while the cost of a lightning conductor system would represent an increase of at least 35% in the costs of a SHS.
Moreover, other much cheaper protection options exist:
Exposed metal surfaces and frames of equipment (normally not live) may be grounded for safety. In low-voltage DC power systems (less than 50 V) this is optional, but may be of particular importance in lightning-prone areas, where grounding the PV array frame and structure can provide a safer conduction path. For this purpose, the array frame and structure should be connected by the shortest practical route to an adequate earth contact, using an uninterrupted conductor of at least 16 mm² cross-section.
Optional: In regions with frequent thunderstorms, manual disconnection (switch) of both the positive and negative poles should be installed between the PV generator and the charge regulator, so that the PV generator can be isolated when there is a risk of lightning strikes.
Optional: For improved protection of equipment, an earth electrode system should be installed as close as possible to the PV array, to provide a short conducting path to earth. This earth electrode must then be bonded to any other earth electrode installed. Optional: If an array source circuit runs for more than 20 m in lightning-prone areas, it is recommended to guide the conductors in a well-grounded metal conduit or use shielded cables.
Ground electrodes may be provided by an earth spike (galvanised T-shaped 30 x 15 mm or round Ø 15 mm iron-bars), driven at least 1.5 m deep into the ground. In rocky locations a ring or stripe earth-conductor of galvanised flat or round iron can be laid out and buried in a trench. Metal water pipes providing sufficient ground contact for a low resistance path to earth or reinforcement bars of the concrete foundations may also be suitable.
If other equipment is grounded, it shall either be grounded to the same earth contact ("earth electrode") used for the array, or otherwise to another earth electrode which is bonded to the earth electrode used for the array.
Optional: In a two-conductor DC wiring system, one of the current-carrying conductors shall be grounded by direct connection to a good earth contact. If so, the negative conductor shall normally be the grounded conductor, unless otherwise specified. The connection to earth shall not contain any fuses or switches. There should be a direct connection between its negative input and output terminals of the charge regulator. Conversely, if a regulator is used which has switching in the negative rail, the negative conductor should not be grounded.
Reliable, durable surge arresters are recommended on the input and output of the charge regulator, to reduce the risk of damage from lightning-induced surges. A cheaper, but not as save, alternative is the use of Metal Oxide Varistors (MOVs).
- This overview on standards for grounding and lightning protection of Solar Home Systems (SHS) is an extract of the publication quality standards for solar home systems and rural health power supply.pdf GTZ, Division 44, Environmental Management, Water, Energy, Transport.