Solar DC Protection Selection Tree Pakistan 2026 — Fuse, SPD, MCB Decision Guide
When do I use a DC fuse vs DC breaker vs DC SPD in a Pakistani solar system?
Three Pakistani solar protection devices, three jobs:
- DC fuse (YCF8 PV class) — string-level over-current. Mandatory once you have 3+ strings in parallel per IEC 62548. One fuse per polarity per string.
- DC breaker (YCB8s) — main DC isolator, switchable + resettable. Required at combiner output + inverter input. Sized to combined string current × 1.25.
- DC SPD (YCS6-C / Type 2) — surge protection from lightning + induced spikes. Required at DC combiner + inverter DC input per IEC 61643-31. PV array on rooftop = lightning target.
Rule of thumb: ≤2 strings in parallel → DC breaker + DC SPD (no fuses needed); 3+ strings → add fuse per string AND keep breaker + SPD; any 1000V/1500V industrial → upgrade SPD to Type 1+2.
Selection decision tree (Pakistani residential to industrial)
| System | Fuse needed? | Breaker needed? | SPD needed? | Typical voltage |
|---|---|---|---|---|
| 3 kW home, 1-2 strings | NO | YES (1-2P 600V YCB8s-63) | YES Type 2 (1P+1P 600V YCS6-C) | 350-450V DC |
| 5-10 kW home, 2-3 strings | 3 strings = YES (YCF8 15A PV) | YES (2P 1000V YCB8s-63) | YES Type 2 (2P 1000V YCS6-C) | 500-700V DC |
| 15-25 kW commercial, 4-8 strings | YES (8× YCF8 15-20A PV) | YES (2P 1000V YCB8s-125) | YES Type 1+2 (combined) | 700-900V DC |
| 50+ kW industrial, 1500V | YES (gPV class 14×85) | YES (4P 1500V YCB8s-125) | YES Type 1+2 1500V | 1200-1500V DC |
Why 3+ strings forces you to add fuses
One PV string can't fault into itself with enough current to need a fuse — module Isc is typically 10-13A, and the module is rated for at least 1.25×Isc backfeed. With 2 strings in parallel, a fault in one string sees only the other string's Isc backfeeding through it — still <15A, below module Imax.
With 3 strings in parallel, a fault in string-1 sees strings 2+3 backfeeding (~26A). That exceeds the module's reverse-current rating. Module backsheet burns. Fuse blows the faulted string out of the parallel group, protecting the other modules — this is the IEC 62548 logic.
CNC YCF8 PV-class fuse (gPV) is purpose-designed for this:
- Rated to interrupt low-power-factor DC arcs (PV strings have minimal inductance)
- UV-stable polymer body (rooftop install)
- 10×85mm or 14×85mm standard PV holder format
- 15A / 20A / 25A / 30A common ratings (size to ~1.5× string Isc)
Why DC SPD is mandatory in Pakistan
Pakistan's lightning ground-flash density is high in monsoon zones (Lahore, Islamabad, Faisalabad, Multan): 5-15 strikes per km² per year. A rooftop PV array is an exposed metal grid — perfect lightning antenna. Direct strikes are rare, but induced surges from nearby strikes propagate via DC cabling.
Without an SPD: inverter MPPT input destroyed by 2-6 kV surge. Inverter replacement cost: PKR 80,000-300,000. SPD cost: PKR 2,500-6,000. Payback is one prevented strike.
CNC YCS6-C DC SPD selection:
- 600V version (string up to ~500V) — PKR 2,500
- 1000V version (string 700-900V) — PKR 3,500
- 1500V version (industrial PV) — PKR 6,000
- All Type 2 (8/20 µs waveform, 20-40 kA Imax)
- Replaceable cartridge (after a surge event, change the cartridge, keep base wired)
Common Pakistani solar-installer mistakes
- Using AC SPD on DC side — AC SPD MOVs degrade rapidly under DC stress; fail short, cause string fire. Always YCS6-C (DC-rated).
- Skipping DC breaker because "fuse already there" — you need switchable isolation for safe servicing. Fuse is single-use protection; breaker is operational isolator. Both required.
- Single DC breaker for combined output — if one string faults, the whole array goes down. Use per-string fuses + main combiner breaker.
- 1500V-rated fuse on 600V system — overkill, costs 3-5×. Match fuse Uc to actual string Voc × 1.25.
- SPD at inverter input but not at combiner — surge propagates the full cable length, by the time it hits inverter SPD, damage already done. Install at the lightning-entry end (combiner / array).
- No earthing for SPD ground terminal — SPD must drain surge to ground. Bond SPD earth to PV array frame + main earthing.
Buy + related
YCF8 DC fuse PV class · YCB8s DC breaker · YCS6-C DC SPD · DC Fuse vs DC Breaker depth-guide · Solar Combiner Box Wiring