10.3 DC Circuit-Breakers and gPV Fuses

Quick Answer: The device must be rated to break the maximum DC voltage. Circuit-breakers shall comply with BS EN 60947-2 or BS EN 60898-2. Fuses for module overcurrent protection shall be gPV type in accordance with BS EN 60269-6. An AC MCB is not a DC breaker: DC has no zero-crossing, so breaking capacity and voltage rating must be real DC figures, in both polarities if needed. The gPV fuse curve matches PV reverse-current duty. Place the OCPD in the combiner per the manufacturer.

Rated to break the maximum DC voltage

Learning outcome 4.2.3 starts with voltage, not with the ampere number printed on the handle. A PV array at maximum power sits near Vmp, which is lower than open-circuit voltage. When a fault opens, when a string is isolated, or when the inverter shuts down, the array voltage rises toward Voc. The device that has to interrupt that circuit must extinguish an arc at maximum DC voltage — for the worked array in section 10.1, M × UDC-MAX = 492 V, not the ~330 V you might see at Vmp on a mild afternoon.

If a breaker or fuse is marked 300 V DC, or 400 V AC, it is not a lawful interrupting device for a 492 V DC string. The contacts may close and carry current. The failure mode is on opening: the arc does not go out, the device destroys itself, and the fault continues. Rate voltage first, then current, then utilisation category.

Carry the arithmetic forward so the voltage is not a slogan. UDC-MAX was 41.0 × 1.2 = 49.2 V; ten in series gave 10 × 49.2 = 492 V. Any interrupting device on that string needs a DC voltage rating at least 492 V (in practice you select the next published DC voltage, often 600 V DC or 1000 V DC). A “400 V” marking that is an AC rms figure is not 400 V DC, and even if it were, 400 is less than 492.

Circuit-breakers: BS EN 60947-2 or BS EN 60898-2

Handbook v1.3 requires circuit-breakers used for this DC duty to comply with:

• BS EN 60947-2 — low-voltage switchgear and controlgear, circuit-breakers (the industrial / installer MCCB and many dedicated PV DC breakers), or
• BS EN 60898-2 — circuit-breakers for overcurrent protection for household and similar installations, Part 2: circuit-breakers for a.c. and d.c. operation.

BS EN 60898-1 is the everyday AC MCB standard (the Type B / Type C devices in a domestic consumer unit). It is not the DC standard. A 32 A Type B device to 60898-1 has been type-tested to interrupt alternating current at its AC voltage rating. It has not been shown to interrupt 400 V DC on a PV string.

BS EN 60947-3 covers switches, disconnectors and fuse-combination units — the DC isolator you use to make the array dead. Isolation is not overcurrent protection. An isolator rated to break load at maximum DC voltage still does not provide the string fuse function of section 10.2. Keep the two jobs separate: 60947-2 / 60898-2 (or a gPV fuse) for overcurrent; 60947-3 for isolation where a dedicated isolator is used.

Why an AC MCB is not a DC breaker

At 50 Hz, alternating current passes through zero twice every cycle, every 10 ms. An AC MCB is designed to let the arc go out at that zero-crossing, then hold off the recovery voltage. Direct current has no zero-crossing. The current sits at whatever the PV I–V curve supplies until something forces it to zero. The arc in the contact gap is a continuous plasma column. Unless the breaker has a DC-rated arc chute, magnetic blow-out, and a DC voltage rating high enough for that gap, the arc persists, carbonises the device, and can start a fire in the combiner.

Breaking capacity (Icu / Ics, or the DC interrupting rating on the device) must therefore be a DC figure at the DC voltage. A 6 kA AC breaking capacity printed for 230 V a.c. tells you nothing about 400 V d.c.

Both polarities if needed

PV DC is often an unearthed (isolated) two-pole system: both positive and negative are live relative to earth. A fault can hang on either pole. Some DC breakers and fuse holders are polarised — they interrupt reliably in one current direction only, because the arc chute is magnetically biased. Those devices must be wired with the marked + and − the right way round. Reverse them and they may close, carry, and then fail to break.

Where the manufacturer or the earthing arrangement requires interruption in both poles, you fit two-pole DC breakers or fuses in both positive and negative. An earthed DC conductor (rare on small UK rooftop arrays, more often seen where one pole is intentionally earthed) is the case where you protect the unearthed pole; you still follow the manufacturer, because a polarised single-pole device in the wrong pole is as useless as an AC MCB.

gPV fuses: BS EN 60269-6

Fuses for module overcurrent protection shall be of the gPV type in accordance with BS EN 60269-6. The “g” means full-range breaking (overload and short-circuit). The “PV” utilisation category means the fuse was designed for photovoltaic reverse-current duty, not for a 230 V a.c. ring final.

Why the gPV curve matches PV reverse-current duty

A PV string’s prospective fault current is only a little above Isc. In the worked example, ISC_MAX is 17.25 A. There is no 5 kA transformer behind the modules. A Type B AC MCB is magnetically set to trip around 3 to 5 × In. For a 32 A Type B that magnetic band is about 96 A to 160 A. The array cannot produce 96 A. The magnetic trip may never operate. Thermal trip might eventually, on a timescale and in an arc environment the AC device was not proven for.

A gPV fuse is shaped for that low multiple of current: it can carry the legitimate string current (including the 1.25-corrected ISC_MAX) without nuisance melting, and it still opens on reverse current from parallel strings before the module’s reverse-current rating is exceeded. Typical published DC voltages are 1000 V DC or 1500 V DC, which sit above the 492 V string in section 10.1.

gG fuses (BS EN 60269-1 general purpose) and aM motor fuses are the wrong utilisation category. BS 88 AC industrial cartridges and BS 1361 house fuses are not gPV. Putting any of those in a PV combiner because “32 A is 32 A” ignores voltage, arc behaviour and the time-current curve.

The fuse rating still has to respect IMOD_MAX_OCPR from section 10.2: the fuse cannot be larger than the module’s maximum overcurrent protection rating, or it will not protect the module. It also cannot be so small that ISC_MAX melts it on a bright day. That window is why 15 A or 20 A gPV fuses appear on strings whose ISC_MAX is 17.25 A and whose IMOD_MAX_OCPR is 20 A — you check both ends of the window, you do not copy the AC MCB rating from a domestic board.

Place in the combiner per the manufacturer

String OCPD belongs at the paralleling point, which on a UK rooftop array is usually the DC combiner (or the inverter’s integrated string combiner). Fit one device per string so that a faulted string is taken off the bus while the others remain. Follow the manufacturer on:

• fuse holder voltage and current, including DC rating of the holder, not only the cartridge;
• polarity marks on polarised holders and breakers;
• whether both poles are fused;
• enclosure temperature derating — a 20 A gPV fuse in a hot roof-space combiner may need a larger holder rating even though the cartridge remains 20 A;
• the maximum number of strings the busbar is certified for.

Do not bury a string fuse at the module and then parallel unprotected tails in a plastic box. Do not use the inverter’s AC output breaker as a substitute for DC string protection. The AC breaker never sees the reverse current on the DC bus.

Device versus standard versus trap

Scenario — 32 A Type B AC MCB on a 400 V DC string

A candidate proposes a 32 A Type B MCB from a domestic consumer unit, to BS EN 60898-1, as string protection on a 400 V DC PV string.

Voltage. The device is an AC voltage rating. Even reading “400 V” generously, a 400 V mark does not demonstrate 400 V DC interrupting duty, and the section 10.1 string at 492 V would already exceed 400 V. Maximum DC voltage is the open-circuit design figure, not a guessed operating voltage.

Zero-crossing. Type B AC contacts expect the arc to clear at a current zero every 10 ms. The 400 V DC string never gives that zero. The breaker may not interrupt.

Magnetic trip. Type B operates magnetically around 3 to 5 × 32 A = 96 A to 160 A. PV prospective current is of the order of ISC_MAX (17.25 A in the worked array). The magnetic element is spectator equipment on a PV short.

Utilisation category. This is not a gPV fuse to BS EN 60269-6, and it is not a DC breaker to 60947-2 or 60898-2. It cannot be counted as module overcurrent protection even if someone matches the ampere number to a cable table.

Polarity and placement. A single-pole AC MCB in one AC-shaped DIN slot does not give two-pole DC breaking in a combiner, and it is not installed “per the manufacturer” of any PV fuse holder.

Reject the proposal. Select a gPV fuse to BS EN 60269-6 sized inside the IMOD_MAX_OCPR / ISC_MAX window, or a DC circuit-breaker to BS EN 60947-2 or BS EN 60898-2, both with a DC voltage rating at or above maximum DC voltage, fitted in the combiner as the manufacturer specifies, with both polarities considered.