Protection: fuses, DC SPD, isolators, AC MCB/RCCB, AC SPD

A rooftop system has two distribution boxes. The DCDB sits between the array and the inverter, and the ACDB sits between the inverter and the building's main board. Each device inside has one job. If you understand the job, you can rate it correctly and you will not be fooled by a "solar kit" box that is full of the wrong parts.

DC side

String fuses: often not needed on homes

A fuse protects a string from reverse current flowing back into it from other strings in parallel, for example when one string has a short circuit or a reversed module. With one or two strings per MPPT input, the other string cannot push more current back than a module can take, so string fuses are usually not needed.

The IEC 62548 test, in the conservative form used here: string overcurrent protection is required when the reverse current that can flow into a faulty string, taken as (number of parallel strings − 1) × 1.25 × Isc, can exceed the module's maximum reverse current (on the datasheet as "maximum series fuse rating"). In practice that means three or more strings in parallel on the same input.

Sizing a string fuse. IEC 62548 asks for a fuse rating In with:

1.5 × Isc ≤ In ≤ 2.4 × Isc, and In ≤ the module's maximum series fuse rating.

Example: Isc 13.85 A, maximum series fuse 25 A. - 1.5 × 13.85 = 20.8 A - 2.4 × 13.85 = 33.2 A, but capped at 25 A - Choose a 25 A gPV fuse (IEC 60269-6), rated at least 1,000 V DC.

Use gPV fuses, made for PV, in both the positive and negative of each string unless the inverter maker says otherwise. Ordinary gG fuses or AC fuses are not designed to clear the low fault currents of a PV string.

DC isolator

The DC isolator lets you safely disconnect the array from the inverter for maintenance. It must: - be a DC-rated load-break switch (look for utilisation category DC-PV2 or DC-21B), - break both poles, - have a voltage rating at or above the cold-corrected array Voc, - have a current rating at or above 1.25 × Isc of the strings it carries.

Many inverters have a built-in DC switch. A separate isolator in the DCDB is still common in Indian specs, and it is useful when the inverter is far from the array.

DC SPD

A surge protective device diverts induced lightning surges to earth before they reach the inverter. For rooftop PV: - Type 2 is normal. Use Type 1+2 if the building has an external lightning protection system and you cannot keep the separation distance (see the lightning lesson). - Choose a PV-specific SPD (IEC 61643-31) with a maximum continuous voltage Ucpv at or above the cold-corrected Voc. - Typical home ratings: In 20 kA, Imax 40 kA, with a protection level Up below what the inverter input can withstand. - Wire it in the usual "Y" arrangement: + to PE, − to PE.

If the DC cable between the array and inverter is long (more than about 10 m), fit SPDs at both ends: one near the array and one at the inverter.

AC side

AC MCB

The MCB protects the AC cable from overload and short circuit, and lets you switch the inverter off.

Rule of thumb: MCB rating ≥ 1.25 × inverter maximum AC output current, and ≤ the current rating of the AC cable.

Example, 5 kW single-phase inverter: - Rated current = 5,000 ÷ 230 = 21.7 A; the datasheet maximum may be about 23–25 A. - 1.25 × 23 = 28.8 A, so choose a 32 A, 2-pole MCB. - The AC cable must then be rated for at least 32 A as installed (for example 6 mm² copper).

For a 10 kW three-phase inverter: 10,000 ÷ (1.732 × 415) = 13.9 A per phase; datasheet maximum about 16 A; 1.25 × 16 = 20 A, so a 20 A or 25 A, 4-pole MCB. Follow the inverter manual where it specifies the breaker.

Use C-curve MCBs unless the manual says otherwise, and use 2-pole (1-phase) or 4-pole (3-phase) devices so the neutral is switched too.

RCCB: follow the inverter manual

An RCCB trips on earth leakage current. The type matters: - Type AC detects only sinusoidal AC leakage. Not suitable for inverters. - Type A detects AC and pulsating DC leakage. Many transformerless string inverters state that a Type A RCD (30 mA, 100 mA or 300 mA) is fine because the inverter has its own internal residual-current monitor. - Type B detects smooth DC leakage too, and is needed when the inverter could feed smooth DC fault current and does not have internal monitoring.

PV arrays have natural leakage through their capacitance to earth, which rises in wet monsoon mornings. A 30 mA RCCB on a large array can trip on these mornings. Read the inverter manual: it usually names the type and minimum sensitivity.

AC SPD

Fit a Type 2 AC SPD in the ACDB: for 230 V phase-to-neutral, a maximum continuous voltage Uc of about 275 V; In 20 kA. It protects the inverter's AC side from surges arriving through the DISCOM line. Fit the SPD downstream of a protective fuse or MCB as its maker recommends.

AC isolator

Some DISCOMs ask for a lockable AC isolator near the meter that their staff can open. Check the local DISCOM requirement.

A complete small-system schedule

For a 5.4 kWp single-phase home with one string:

Key points

• With one or two strings per input, string fuses are usually unnecessary; with three or more in parallel, fit gPV fuses between 1.5 and 2.4 × Isc, within the module's maximum fuse rating.
• DC isolators and SPDs must be DC-rated for the cold-corrected array voltage.
• AC MCB ≥ 1.25 × inverter maximum output current, and never larger than the cable can carry.
• Choose the RCCB type and sensitivity from the inverter manual; Type AC is not suitable.
• Keep SPD earth leads short and straight, and fit SPDs at both ends of long DC runs.

Check yourself

1. A system has 4 strings in parallel on one input, Isc 11 A, module maximum series fuse 20 A. What fuse do you choose?
2. Why can an AC MCB not be used to switch a PV string?
3. A 3 kW single-phase inverter has a maximum output current of 14.5 A. What MCB rating would you pick?