By Vvon Engineering Team | Published | Updated
Coordinate lightning protection, earthing, bonding and surge protection around the actual roof and electrical design.
Solar EPC & street lighting RFQ checklist (TXT)
A nearby lightning event can damage inverters or monitoring equipment without leaving a direct-strike mark on the array. The investigation needs to consider bonding, cable routes and coordinated surge protection as well as the failed component.
That is the shape of the problem, and it is not the shape most people expect.
Adding PV changes the rooftop layout and the electrical installation. Reassess the lightning protection arrangement, including protection zones, separation distances and potential surge paths. Do not assume that an existing air terminal protects the new array automatically.
What changes is the consequence of a strike that lands nearby. You have just installed several kilometres of conductor across a roof, arranged in loops that enclose real area, terminated in power electronics that are sensitive to microsecond transients. A strike to a tree, a chimney or a pole a few hundred metres away induces voltage in those loops by simple magnetic coupling. It does not have to touch you. This is one possible route for photovoltaic lightning damage, and the giveaway is that everything looks perfect on the roof while the inverter input stage is destroyed.
Use the site exposure, building use and applicable lightning-risk assessment when specifying protection. A national rule of thumb cannot establish the protection required for an individual roof.
Conversations about this get confused because three separate systems are all described as lightning protection.
The framework that ties them together is the IEC 62305 series. Part two is a risk assessment: you put in the structure's dimensions, its location and ground flash density, what it is built of, what is inside it and what the consequences of a failure are, and the calculation tells you whether protection is required and at which protection level. This matters commercially, because a risk assessment is a document a client can ask for at tender stage. Without one, the decision about how much protection to fit is being made by whoever is quoting, and the incentive there is obvious.
On a building with an LPS there is a genuine design fork, and it is the fork most often fudged.
The isolated approach keeps the array physically clear of the air terminations and down conductors by at least the calculated separation distance, so that lightning current flowing in the LPS cannot flash across to the array. The separation distance is a calculation, not a guess, and it depends on the protection level, the number of parallel down conductor paths and the length of conductor involved. If you can achieve it, the DC side then only needs to deal with induced surges.
The bonded approach accepts that the array structure is connected to the LPS, deliberately or unavoidably. That is a legitimate design, but it has a direct consequence: part of the lightning current can now flow in the array's earthing and, by coupling, in its conductors. Once that is true, the DC surge protection has to be rated for lightning impulse current with the long 10/350 microsecond waveform, which means Type 1 devices, not Type 2 devices rated for the shorter 8/20 microsecond induced surge.
Where lightning current may enter array circuits, the designer must specify suitably rated and coordinated protection. Check the complete arrangement and the SPD manufacturer's application guidance rather than choosing a device from its type label alone.
Module frames and mounting rails are anodised aluminium. Anodising is an oxide layer, and an oxide layer is an insulator. A module clamped to a rail is mechanically secure and electrically unreliable, and the resistance across that joint changes with weather, torque and age.
So the earthing of the array has to be deliberate:
The voltage induced in a circuit by a nearby strike is proportional to the area the circuit encloses. This gives you a genuinely free way to cut the damage, and it is a wiring practice rather than a purchase.
Route the positive and negative of each string together, tied along the same path, from the first module to the inverter input. A string wired with the positive down one edge of the array and the negative back along the other edge encloses the whole array and behaves like a pickup coil. Keeping the pair together, close to the earthed mounting structure, can reduce the induced voltage by a large factor for no material cost at all.
Coordinate DC cable routes with the fire strategy, mechanical protection, required separation distances and earthing design. Keep string loop areas small where practicable, without compromising the approved containment or access arrangement.
Four placements matter, and the fourth is the one that gets left out of every budget quotation.
DC side. An SPD at the inverter DC input, and a second set at the array end if the cable run between them is long. The rule of thumb used in practice is that beyond roughly ten metres of DC cable you need protection at both ends. The device must be specifically rated for photovoltaic DC use, not simply for DC. A PV array is a current source that will feed a fault indefinitely with no zero crossing, so a PV SPD carries a thermal disconnector and an internal arrangement designed for that duty. Its maximum continuous operating voltage has to exceed the array's open circuit voltage at the coldest expected cell temperature, which brings you straight back to the string design calculation.
AC side at the inverter. Select the protection with reference to the inverter, earthing arrangement, upstream protection and installation risk.
AC side at the installation origin. Coordinate the selected SPDs with the assessed surge exposure and any external lightning protection. Follow the manufacturer's coordination and connection requirements.
Data and signal. The RS485 run between inverters, the Ethernet cable to the office, the CT wiring for the export limiter, the weather station cable. These are long, often unscreened, cross zone boundaries, and are usually the first things to die. A signal SPD on each of them costs very little and prevents the classic outcome where the inverters survive and the gateway, the switch and the office PC do not.
One detail decides whether any of this works. The protection level an SPD achieves is its own clamping voltage plus the inductive voltage developed along its connecting leads during the surge, and that inductive term is large. The total lead length, line side plus earth side, should be kept as short as physically possible, which in practice means well under a metre. A properly specified SPD on a long earth tail neatly routed around the panel is not protecting anything. If the panel layout will not allow short leads, use a V-connection or move the device.
Bangladeshi sites accumulate earth pits. There is the original building earth, a separate pit somebody drove for a generator, one for the transformer neutral, one for the lightning protection, and now a new one for the solar. If those electrodes are not bonded together, the site is more dangerous than it would be with a single one, because a surge raises the potential of the electrode nearest the event and the full difference appears across whatever equipment bridges the two systems. That is often the inverter, sitting with its earth on one system and its DC cables running to a structure earthed on the other.
The competent electrical and lightning-protection designers must coordinate electrodes, bonding, neutral arrangements and any required separation. Do not add an isolated solar electrode or bond different systems together without reviewing the complete earthing design.
On resistance targets: the number depends on what the electrode is for, and quoting a single figure without saying which duty it serves is meaningless. In the soft delta soils around much of Bangladesh a low value is straightforward to reach with an electrode driven down into the water table. On higher ground, in laterite, and in the dry season before the monsoon recharges the ground, it is harder, and an electrode measured in August is not the same electrode in April. Measure by a proper fall of potential test at the driest time of year, record it, and install a test link and an inspection pit at every electrode so it can be measured again without excavation. Then put that measurement in the annual maintenance schedule, because earth connections are the part of an installation that is made once and never looked at again.
If you are inspecting a quotation or an existing plant, this is the list, for discussion with the inspection team:
Consult the relevant edition and the documents for the exact equipment and site: IEC 62548-1: PV array design requirements; IEA PVPS: operational and economic impacts of extreme weather.
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