PV DC Side Protection: Isolation, Fuses, Earthing

The DC side of a PV system is unusual. It cannot be switched off while the sun shines, its fault current is barely higher than its operating current, and its voltage rises on cold mornings. Protection copied from AC practice, with a single breaker sized for the load, does not work there. These are the basics a building services engineer needs to check a PV design.

Quick answer: Size everything on the DC side for the maximum open-circuit voltage at the lowest site temperature (typically 10–15 % above the STC value) and for 1.25 × Isc. Fit gPV string fuses in both poles when three or more strings are in parallel and the reverse current could exceed the module’s maximum series fuse rating. Add a load-break DC isolator at the inverter, a DC SPD with Ucpv ≥ Uoc max, and bond module frames and structures to earth. On the AC side, use a type B RCD unless the inverter maker states that a type A is enough.

Five things that make PV DC different

Always live: modules produce voltage in daylight. Opening the DC isolator does not make the cables between the array and the isolator dead.
Low fault current: a short-circuited string delivers only about its Isc, slightly more than its operating current, so ordinary overcurrent devices never see a “fault”.
Cold means higher voltage: Uoc rises about 0.29–0.30 % per K below 25 °C. Systems are limited to 1000 V or 1500 V DC.
DC arcs do not self-extinguish: isolators, fuses and connectors must be rated for DC at the full voltage. Never unplug connectors under load.
Reverse current: with strings in parallel, a faulted string can be fed by all the others.

Worked example: 4 strings of 18 modules

Module data: Uoc 45.0 V, Isc 13.0 A, maximum series fuse 25 A, Uoc temperature coefficient −0.29 %/K. Lowest site temperature −10 °C.
String voltage: 18 × 45.0 = 810 V at STC → × (1 + 0.0029 × 35) = 810 × 1.10 ≈ 892 V < 1000 V system limit ✓.
Design current per string: 1.25 × 13.0 = 16.3 A → 4 or 6 mm² PV cable (check derating and voltage drop).
Reverse current into a faulted string: up to (4 − 1) × 13 = 39 A > 25 A → string fuses needed.
Fuse: ≥ 1.5 × Isc = 19.5 A and ≤ 25 A → 20 A gPV, rated ≥ 1000 V DC, in both + and − (the array is not earthed).
SPD: Ucpv ≥ 892 V (or ≥ 1.2 × 810 = 972 V if no temperature data) → 1000 V PV SPD.
DC isolator: rated ≥ 1000 V DC, load-break, current ≥ 4 × 16.3 ≈ 65 A if one isolator serves all four strings.

Fuses protect each string from reverse current fed by the other three. The isolator lets you work on the inverter, but the cables back to the array stay live in daylight.

IEC 60364-7-712 vs NEC 690 in short

Item	IEC practice	NEC 690 practice
Design current	≥ 1.25 × Isc STC	Maximum current 1.25 × Isc; conductors and overcurrent devices ≥ 125 % of that (≈ 1.56 × Isc)
Maximum voltage	Uoc at the lowest site temperature	Uoc corrected for the lowest expected temperature
String protection	gPV fuses when reverse current can exceed the module rating	Overcurrent protection per string unless the source cannot exceed the module rating
Isolation	DC isolator at the inverter; firefighter switch per national rules	PV disconnecting means; rapid shutdown on buildings
Earthing	Frames bonded; DC side usually not earthed, insulation monitored by the inverter	Equipment grounding of frames; ground-fault detection

Checklist for the DC side

Use PV cable rated 1.5 kV DC, single-core and double-insulated, with + and − run together to keep the loop area small.
Use connectors of the same type and maker on both halves. Mixed brands are a known fire cause.
Add a DC SPD at the inverter, and also at the array end when the DC cable is longer than about 10 m. See how to connect an SPD.
Bond the frames and structure, and keep the separation distance from the lightning protection system, or bond to it and use Type 1+2 SPDs. See lightning protection basics.
On the AC side, use a type B RCD unless the inverter cannot feed DC fault current (simple separation or a maker declaration). See RCD types.
Label everything: “PV DC – live in daylight”, plus isolation points and dual supply warnings at the board.

Typical values — verify with the edition adopted locally and with the module and inverter data sheets.

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FAQ

When are string fuses needed in a PV system?

When three or more strings are connected in parallel and the reverse current that the other strings can feed into a faulted string exceeds the module’s maximum series fuse rating. With one or two strings it normally cannot, so fuses are usually not needed.

How do I find the maximum DC voltage of a PV string?

Multiply the module Uoc by the number of modules, then correct it to the lowest site temperature using the module’s temperature coefficient (about +0.29–0.30 % per K below 25 °C). This is typically 10–15 % above the STC value (use 1.2 × Uoc STC if no temperature data is available) and must stay below the 1000 V or 1500 V system limit.

Which RCD type does a PV inverter need?

Type B, unless the inverter maker states that the inverter cannot feed DC fault current into the AC side, for example because of simple separation. In that case type A is acceptable.

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PV Systems: DC Isolation, Fuses and Earthing Basics