Which Surge Protector Does a Solar System Need? DC vs AC, Type 1 vs Type 2

A solar system needs a DC surge protector at the inverter's PV input, a second one at the panels if the DC cable run is longer than 10 metres, and an AC surge protector in the distribution board. Choose Type 2 unless the building has a lightning rod, in which case choose Type 1+2. On the DC side, pick the lowest voltage rating that still sits above your string's highest open circuit voltage: for the 500 V class off grid inverters most homes run, that is a 550 V or 600 V unit, not a 1000 V one, because a lower rated unit clamps a surge at a lower voltage. A full set for a single phase home costs about $40.

In this article

• Does a solar system actually need a surge protector?
• What voltage rating should the DC surge protector have?
• Type 1, Type 2 or Type 1+2: which do you need?
• Where does the SPD go, and why do short leads matter?
• Do you also need an AC surge protector?
• When is a surge protector not the answer?
• FAQ

Does a solar system actually need a surge protector?

Yes, and the reason is where the panels sit. A solar array is a large metal frame on the highest point of the building, connected by long cables to the most expensive electronic device in the house. Lightning does not have to hit the roof to do damage. A strike a few hundred metres away induces a voltage spike in those cables, and that spike arrives at the inverter's MPPT input in microseconds.

A surge protective device, or SPD, sits in parallel with the circuit. In normal service it does nothing. When the voltage jumps past its threshold it becomes a short path to earth for a fraction of a millisecond, diverting the surge and holding the voltage the inverter sees down to a published ceiling called the protection level, Up. Then it returns to standby. Each surge wears it a little, which is why every unit has a window that turns from green to red when the module is spent.

Three places need protection, and they are three different products:

• The DC side at the inverter, on the PV input. This is the minimum for every system.
• The DC side at the array, in the rooftop combiner box, whenever the cable from panels to inverter is longer than about 10 metres. Suntree's own installation note says their PV SPDs are "recommended at both ends of the DC power supply line (solar panel side and inverter/converter side)".
• The AC side, in the main distribution board, where the utility supply and the generator come in and where the inverter output feeds the house.

What voltage rating should the DC surge protector have?

This is the decision most buyers get backwards. The big number on the label, Ucpv, is the maximum continuous voltage the SPD can sit across all day without conducting. It must be higher than the highest voltage your string can ever produce, which is the open circuit voltage on the coldest, brightest morning, not the voltage you read at noon in summer. Suntree put the rule plainly on their datasheets: the voltage permanently applied across the SPD must not be greater than Uc.

Here is the part the label does not tell you. The protection level rises with the voltage rating. In the same Suntree Type 1+2 family, the 600 V unit is published at Up of 2.2 kV or less, and the 1000 V unit at 4.0 kV or less. Fit a 1000 V SPD to a 450 V string and you have bought a device that lets nearly twice the voltage through to the inverter before it does anything. Bigger is not safer here. The right choice is the lowest Ucpv that still clears your string with margin.

For most homes that margin is easy to find, because the inverter sets a hard ceiling on the string. Voltronic publish a maximum PV open circuit voltage of 500 VDC for the Axpert VM IV Twin and the Axpert MAX Twin, and 400 VDC for the small Axpert VM II 1.2 kW. A string designed to stay under those limits on a cold morning is covered by a 550 V or 600 V SPD. The 1000 V and 1200 V units belong on three phase on grid inverters such as the INVT XG 10 kW, whose MPPT window runs to 1000 V.

Suntree DC SPD	Ucpv	Type	Published Up	Fits	Price
SUP2H-PV 2P	550 V	Type 2	2.8 kV (500 V and 600 V members of the family)	Off grid and hybrid inverters with a 400 to 500 V PV limit	$15.40
SUP2 DC 2P 600 V	600 V	Type 1+2	2.2 kV or less	The same inverters, on a building with a lightning rod	$37.27
SUP2H1-PV 3P 1000 V	1000 V	Type 2	3.6 kV	On grid inverters with strings up to 1000 V	$28.32
SUP2H1-PV 3P 1200 V	1200 V	Type 2	4.0 kV	Commercial strings above 1000 V	$33.91
SUP2 DC 3P 1000 V	1000 V	Type 1+2	4.0 kV or less	On grid systems on a building with a lightning rod	$53.00

Every DC unit in the table is published at 40 kA maximum discharge current on the 8/20 µs wave, so the voltage rating and the type are the real choices. All of them mount on a 35 mm DIN rail and have the green to red status window.

Type 1, Type 2 or Type 1+2: which do you need?

The type tells you what kind of current the device was tested to carry, not how good it is.

Type 2 is tested with the 8/20 µs wave, the shape of an induced surge from a nearby strike or a switching event. Its headline figures are the nominal discharge current In and the maximum Imax, for example 20 kA and 40 kA on the Suntree PV units. It is the standard device for a rooftop system on a building with no external lightning protection.

Type 1 is tested with the 10/350 µs wave, which carries far more energy and represents a share of a direct lightning current. Its headline figure is Iimp, 12.5 kA on the Suntree Type 1+2 DC units. You need it when a direct strike can reach the wiring: the building has a lightning rod, or the array is bonded to a lightning protection system. A combined Type 1+2 unit does both jobs in one module and is what we stock.

Schneider Electric's installation guide puts it in one line: a lightning rod present means a Type 1 SPD at the main board. If the building has none, Type 2 at both the DC and AC side is the normal design, and it is also why Type 2 is the right default for the majority of homes.

Where does the SPD go, and why do short leads matter?

An SPD only protects what is close to it. Schneider's rule for PV is the 10 metre rule: if the DC cable between the array and the inverter is under 10 m, one SPD at the inverter covers both; if it is longer, fit one at each end, because the cable itself picks up the induced surge along its length. Most rooftop to ground floor installations are over 10 m, which is why the rooftop combiner box exists. The Suntree SISO-40/S combiner bundles the fuses, the isolator and a 1000 V SPD in one IP66 box for exactly this spot.

The second rule is about the wires to the SPD, and it is the one installers break most. The surge rises so fast that every centimetre of lead adds inductive voltage on top of Up. Schneider specify that the two connection leads together, from the live conductor to the SPD and from the SPD to the earth bar, must total less than 50 cm. Mount the SPD next to the terminals it protects, keep the leads short and straight, and never coil them.

And it needs a real earth. The SPD does not absorb a surge, it diverts it, and the only place it can send it is the earth conductor. With no earth electrode, or an earth bar connected through a long thin wire, the SPD is decoration.

Do you also need an AC surge protector?

Yes. The DC unit protects the PV input. The inverter's AC input and output, the changeover between the utility supply and the generator, and every appliance in the house sit on the AC side, and surges arrive there too: through overhead supply lines during storms, and from the generator and changeover switching every day.

The AC product is a different device with different ratings. For a single phase 220 V home, the Suntree SUP1H 2P 275 V at $9.21 is a Type 2 unit published at In 20 kA, Imax 40 kA and Up under 1.8 kV. A three phase board takes the 4P 385 V version at $18.04. Where the supply arrives on overhead lines in an exposed area, or the building has a lightning rod, step up to the Type 1+2 2P 275 V at $18.63. Suntree state the AC units suit TT, TN-S, TN-C, TN-C-S and IT supplies, which covers every domestic board you will meet.

System	At the inverter DC input	At the array, if over 10 m	In the AC board	Set price
Off grid or hybrid home, single phase, no lightning rod	DC 2P 550 V, $15.40	DC 2P 550 V, $15.40	AC 2P 275 V, $9.21	$40.01
The same home with a lightning rod	DC 2P 600 V T1+T2, $37.27	DC 2P 600 V T1+T2, $37.27	AC 2P 275 V T1+T2, $18.63	$93.17
Three phase on grid, strings up to 1000 V	Built in on the INVT XG, check your model	DC 3P 1000 V, $28.32	AC 4P 385 V, $18.04	$46.36

When is a surge protector not the answer?

An SPD is cheap insurance, but it is insurance against one specific thing: a transient lasting microseconds. Here is when it will not help, or when buying one is the wrong move.

• There is no earth. Fit the earth electrode first. An SPD with nowhere to send the surge protects nothing.
• A direct strike on the panels. No SPD survives that or stops it. Only an external lightning protection system, a rod and down conductor designed for the building, deals with a direct hit. The SPD then handles what leaks into the wiring.
• The inverter already has one and the cable is short. INVT list DC and AC Type 2 SPDs built into the XG series. If your run is under 10 m, a second unit at the inverter adds nothing. Spend the money on the array end instead.
• The problem is a sustained overvoltage, not a spike. A generator running at 260 V for an hour, or a lost neutral putting 380 V on a 220 V circuit, is a fault the SPD was never designed for. It will try to clamp it, overheat and disconnect itself, and you will have a red window and a cooked appliance. That job belongs to an over and under voltage protection relay.
• Using an AC unit on the DC side. A DC arc has no zero crossing, so PV SPDs carry a DC rated disconnector and are tested to IEC 61643-31. An AC unit on a PV string is a fire risk, not a saving.
• The window is already red. A spent SPD is an open circuit. It is a consumable. Check the window after every big storm and keep a spare module.

Frequently asked questions

Can I use an AC surge protector on the DC side of a solar system?

No. PV surge protectors are built and tested to a separate standard, IEC 61643-31, because a DC fault current has no zero crossing to extinguish an arc. They carry a DC rated thermal disconnector and a fuse or shunt that an AC unit does not have. Always buy a unit marked for PV or DC with a Ucpv rating.

Do I need a surge protector if my inverter has one built in?

You still need one at the array end if the DC cable is over 10 m, and you still need the AC unit in the distribution board. A built in SPD covers only the terminals it sits on. Check the inverter's own datasheet, since many off grid inverters have none at all.

What is the difference between 2P and 3P on a DC surge protector?

A 2P unit protects plus to earth and minus to earth. A 3P unit, often called the Y configuration, adds a third element between plus and minus, so all three paths are covered and a fault in one element does not leave the string tied to earth. For a single string on a small inverter 2P is standard; for larger arrays and on grid systems 3P is the usual specification.

How do I know the surge protector has failed?

The status window on the module turns from green to red, and on units with a remote contact the alarm terminal changes state. The system keeps running, because the SPD sits in parallel, so nothing warns you except the window. Look at it after every big storm.

What do In and Imax mean?

Both are tested with the 8/20 µs wave. In is the nominal discharge current the SPD can carry many times over its life; Imax is the single largest surge it can survive once. Suntree publish 20 kA and 40 kA for the PV units. A Type 1 rating, Iimp, is a different test on the 10/350 µs wave and is not comparable to either.

Does a surge protector make a lightning rod unnecessary?

No. They do different jobs. A lightning rod gives a direct strike a safe path down the outside of the building. A surge protector limits the voltage that reaches equipment through the wiring. A building with a rod needs Type 1+2 SPDs; a building without one needs Type 2. Neither replaces the other.

Sources: SPD placement, the 10 m rule and the 50 cm lead rule from Schneider Electric's Electrical Installation Guide; SUP2H-PV ratings and the both ends installation note from Suntree Electric; inverter PV voltage limits from Voltronic Power's published datasheets. All SPD figures are the manufacturer's own published ratings as shown on each product page. Prices correct at the time of writing. Related reading: which battery voltage your inverter needs.