Pv System Protection Dc Mcb Spd Isolator Korlen

A rooftop or ground-mounted solar PV array is only as reliable as the protection behind it. DC circuits from PV strings behave differently from AC mains — they do not cross zero naturally, so arc faults and surge events are harder to clear. Getting the protection right is what keeps the inverter alive and keeps service crews safe.

What a PV system must be protected against

Solar installations face three recurring risks that the protection scheme must address:

• Overcurrent & short circuit on the DC side — caused by insulation failure, faulty MC4 connectors, or string/inverter rating mismatch.
• Surge / induced lightning — long DC cabling acts as an antenna; a single nearby strike can destroy the inverter and feed overvoltage into connected equipment.
• Maintenance hazard — PV modules keep generating voltage whenever light is present, so a visible disconnect point is required before anyone works on the array.

The three device types below map directly onto these three risks.

DC MCB for the PV string

A DC miniature circuit breaker sits between each string (or pair of strings) and the inverter to clear faults that an AC-side breaker cannot handle. Korlen’s KNB2-63DC 6kA DC MCB is built for PV strings, and the KNB2-63S6Z variant extends breaking capacity to 15kA for larger commercial arrays.

When sizing the DC MCB:

• Rate the MCB at or above the string’s short-circuit current (typically module Isc × 1.25).
• Confirm the DC voltage rating covers the open-circuit voltage (Voc) of the coldest-condition string — do not assume an AC MCB rating applies.
• Use a handle lock or a dedicated PV isolator so the circuit can be made dead before servicing.

Type 2 SPD for surge protection

A Type 2 surge protective device is installed at the inverter AC feed (and often at the distribution board) to clamp overvoltage from lightning and grid events. Korlen’s KNY-40 Type 2 SPD is a T2 AC surge protector with the following verified ratings:

Parameter	KNY-40 (per voltage variant)
Max continuous operating voltage Uc	275V / 320V / 385V
Nominal discharge current In (T2)	20kA
Maximum discharge current Imax	40kA
Protection level Up (L-N)	1.5kV / 1.5kV / 1.8kV
Protection level Up (N-PE)	1.2kV
Response time tA	25ns
Recommended disconnector	LT-SSD40

Select Uc above the system’s highest continuous voltage, and pair the SPD with its dedicated disconnector so a failed module fails safe instead of taking the circuit down.

Isolator switch for safe maintenance

Because PV modules generate whenever exposed to light, a clearly marked disconnect is mandatory before anyone touches the array. A DIN-rail KNH2-100 isolator switch (and the silver-contact KNH1-100) gives a visible break point on the DC side. Place one at the array combiner and another at the inverter so either end can be isolated.

Sample 10 kW residential configuration

For a typical single-phase residential array, a working bill of materials looks like this:

• DC side: one KNB2-63DC per string + a KNH2-100 isolator at the combiner.
• AC side: a standard KNB1-32 MCB on the inverter output, plus a KNBL1-32 RCCB for earth-leakage protection.
• Surge: a KNY-40 Type 2 SPD at the incoming AC distribution board.

Quick selection checklist

1. Compute string Isc and coldest-condition Voc; size the DC MCB above both.
2. Pick a Type 2 SPD with Uc above system voltage and Imax sized to the local lightning level.
3. Add a lockable isolator at both the combiner and the inverter.
4. Keep an AC-side MCB + RCCB on the inverter output.
5. Verify every device carries the relevant IEC/EN mark for your market.

FAQ

Can I use a normal AC MCB on the DC side?

Not recommended. DC arcs do not self-extinguish at the AC zero-crossing, so a DC-rated MCB such as the KNB2-63DC is required for reliable breaking.

Where does the SPD go?

At the AC distribution board feeding the inverter (Type 2), and optionally at the DC combiner for exposed sites. The KNY-40 covers the Type 2 AC position.

Do I still need an isolator if I have a DC MCB?

Yes — an MCB is a protective trip, not a maintenance disconnect. A KNH2-100 isolator provides the visible break point required for safe work.