French
A solar combiner box asks two completely different questions of its protection. Will this circuit survive a fault? And will this circuit survive a surge? A Disjoncteur à courant continu answers the first question. A DC SPD answers the second. The two devices are often discussed together — and just as often confused — because both sit on the same DC busbar, both are DIN-rail mounted, and both are loosely described on site as “the protection”. They are not interchangeable. Neither can be replaced by the other, however carefully it is sized.
This article is about protection coordination — the DC MCB and DC SPD in combiner box pairing. It covers the division of labour between overcurrent protection and transient overvoltage protection, what each device protects and what it cannot, why substitution fails, how the two are coordinated on the same bus including the SPD’s backup protection requirement, what is left exposed if either one is omitted, how the pairing changes between 1000 V DC and 1500 V DC systems, and the selection principles that keep the pair working as a pair.
It deliberately does not re-derive device selection. The DC MCB guide, the DC SPD selection guide and the PV fuse sizing guide each own their own method, and this article links to them at the point where you need them.
Everything below is a design discussion about equipment you will eventually open. A combiner box sits on a live DC system, and DC behaves differently from AC.
The reason a combiner box needs both devices is that the two threats it faces have almost nothing in common.
Overcurrent is a sustained or near-instantaneous flow of current above the circuit’s design value. It happens because a fault path exists: an insulation failure in a string, a short circuit in a connector, a module failure, or a reverse/back-feed current from healthy strings into a faulted one. It rises from the system’s own voltage source, it persists until something opens the circuit, and its energy is delivered over milliseconds to seconds. It must be interrupted.
Transient overvoltage is a brief, very high-voltage impulse — typically from nearby lightning, switching, or a utility-side event — superimposed on a circuit that is otherwise working perfectly normally. It lasts microseconds, it does not necessarily involve any extra current flow in the load, and it does not trip anything. It must be diverted, and it attacks insulation, semiconductors and the devices themselves rather than the load.
A device that interrupts current is useless against an impulse; a device that diverts an impulse is useless against a sustained fault. That single sentence is why both devices exist. The rest of this article is the practical consequence of it.
The DC MCB is the overcurrent and switching device on the combined DC circuit. Inside a combiner box its usual jobs are:
What a DC MCB cannot do is just as important. It has a rated impulse withstand voltage, and a lightning-induced surge can exceed that by an order of magnitude. The breaker does not see a surge as an overcurrent: it does not open, it does not clamp the voltage, and it does not absorb the energy. The impulse simply passes through it, arrives at whatever is downstream, and is dissipated wherever the weakest insulation happens to be — often inside the inverter’s DC input stage, which is far more expensive than the breaker.
This is also why an AC-rated breaker is not a substitute on the DC bus in the first place. DC rating, construction, arc-extinction behaviour and the pole arrangement used to reach a given DC voltage are all device and manufacturer specifics that must be read from the datasheet. If you are selecting the breaker itself, work through this DC MCB guide for solar PV and EV applications, which covers construction and selection in full.
The DC SPD is a voltage-limiting, energy-diverting device. Mounted on the DC bus with a short, direct connection to the protective earth bar, it presents a very high impedance at normal system voltage and a very low impedance once the voltage rises above its threshold. When an impulse arrives, the SPD conducts the surge energy to earth and clamps the residual voltage seen by the equipment to a level defined by its voltage protection level, Up.
That is the whole of its protective function, and it is a function nothing else in the box performs:
There is a second limitation that catches people out, and it is the direct link between the two devices. An SPD’s end of life is not always an open circuit. A degraded metal-oxide varistor, or a gas discharge tube following a sustained temporary overvoltage, can fail to a low-impedance state. At that point the SPD looks like a fault on the DC bus. It has become an overcurrent event, and something must interrupt it before the SPD’s own housing is destroyed. That “something” is the SPD’s backup protection — and it is why the pairing in a combiner box is a genuine coordination problem rather than two independent purchases.
Selection of the SPD itself is covered in the dedicated DC SPD selection guide: Type 2 DC surge suppressor selection for solar, BESS and DC panels. This article assumes that selection step is done and concentrates on how the chosen SPD and the chosen breaker sit together.
Attempts to use a single device for both jobs usually rest on one of three misunderstandings.
“The breaker will trip and protect everything.” It will trip on overcurrent. A surge is not an overcurrent in the sense the breaker detects; it is a microsecond-scale voltage event. The breaker’s contacts stay closed and the impulse passes straight through. The equipment downstream still sees the full impulse.
“The SPD will divert anything dangerous.” The SPD diverts impulse energy. Apply a sustained overcurrent to it — a faulted string, a shorted output, an SPD that has failed to low impedance — and it will conduct until it destroys itself, because it has no interrupting element. An SPD without backup protection is a component that converts a surge problem into a fire risk.
“One device covers both because it is rated for both.” A device can carry a DC voltage rating and a rated impulse withstand voltage without being a surge protective device. Impulse withstand is the level the device will survive, not the level it will limit for the rest of the installation. Only an SPD has a declared voltage protection level and a declared surge current capability.
The cleanest way to see the split is to look at the two devices side by side.
| Question | Disjoncteur à courant continu | DC SPD |
|---|---|---|
| What is it for? | Overcurrent: overload and short circuit | Transient overvoltage: impulse and switching surges |
| What does it do? | Opens the circuit and interrupts current | Conducts the impulse to earth and clamps the residual voltage |
| Typical event duration it responds to | Milliseconds to seconds (instantaneous element for short circuits) | Microseconds |
| Does it protect the inverter DC input from lightning-induced impulses? | No — it does not limit voltage | Yes — this is its purpose |
| Does it clear a faulted string or a shorted output? | Yes, within its rating and coordination | No — it has no interrupting function |
| Does it provide a maintenance isolation point? | Yes, where it is rated as a disconnector for the duty | Non |
| Does it need backup protection of its own? | No — it is the protection | Yes — a maximum overcurrent protective device declared for the SPD |
| What marks end of life? | It operates; it can be re-closed after the fault is found and cleared | A status indicator and cumulative surge exposure; replacement, not reset |
Having both devices fitted is not the same as having them coordinated. Three things must be true.
1. They sit at the right points on the DC bus. The SPD belongs on the DC bus it is protecting, close to the conductors and the equipment it shields, with its connecting leads kept as short and as straight as practicable — long, looped leads add inductance and raise the voltage that actually reaches the equipment. The mechanical layout of the box, the busbar, the earth bar and the sequence of devices is what makes this possible or impossible, and it is set out in the combiner box wiring diagram. Get the layout right on paper before you order hardware; a correctly selected SPD installed with long leads is a compromised SPD.
2. The SPD has its own backup protection, correctly sized. This is the coordination requirement most often missed. Every SPD is declared with a maximum rating of overcurrent protective device that may be used ahead of it — a value the manufacturer publishes precisely because the SPD may fail to a low-impedance state and must then be cleared. The backup device may be a dedicated fuse, or it may be the DC MCB already in the box if that breaker’s rating and characteristic fall within what the SPD permits. It must not be larger than the declared maximum, and it must not be so small that normal inrush or surge currents operate it. The logic of choosing between the two options is covered in SPD backup: fuse or circuit breaker.
3. The type and configuration match the DC system. A DC combiner bus is not an AC distribution board, and the SPD arrangement — including how the connections are made and how many poles are used — must be the type intended for that DC application. The practical question of which configuration is used on a solar DC distribution board is answered in which type of SPD is used in a solar DCDB.
Two further points belong here because they are the difference between a compliant pair and a dangerous one. First, do not reduce the SPD’s backup protection rating to stop it operating during a surge or inrush event; if the backup device operates in normal service, the selection is wrong and must be re-checked, not downgraded. Second, do not treat the MCB as the SPD’s backup device unless the manufacturer’s declared maximum allows it — the number in the datasheet governs, not the fact that a breaker happens to be upstream.
This is the comparison that decides whether the two devices are optional. In both columns the remaining device is correctly sized and correctly installed; the only change is that one device is gone.
| Configuration | What still works | What is now unprotected | First realistic failure |
|---|---|---|---|
| DC SPD removed, DC MCB retained | Short circuits and overloads are still cleared; isolation is still available | The whole DC bus against transient overvoltage — string conductors, busbar insulation, and the inverter DC input | A nearby lightning event or a switching surge damages the inverter DC input stage, with no evidence in the box that anything happened |
| DC MCB removed, DC SPD retained | Impulse overvoltages are still limited and diverted to earth | Every overcurrent condition on the combined output — shorts, overloads, and any SPD that has degraded to low impedance | A faulted output or a failed SPD draws sustained current with nothing to interrupt it, leading to conductor damage and a fire risk inside the enclosure |
| Both fitted, SPD backup protection missing or oversized | Overcurrent protection and surge diversion both appear to be in place | The SPD’s own failure mode — the case it was designed to have cleared | The SPD degrades to low impedance and is destroyed rather than disconnected, damaging the enclosure around it |
| Both fitted, SPD fitted with long, looped leads | The circuit is nominally protected; the SPD does conduct | The clamping performance the SPD was selected for | Equipment damage in exactly the event the SPD was bought for, with an SPD that still shows a healthy indicator |
Nothing in the table is a device-quality problem. Every row is a coordination problem, and every one of them is present on real installations.
The two devices scale differently with system voltage, and that difference is where most 1500 V design errors start.
The DC MCB’s voltage rating is a hard limit, not a margin. A device rated for 1000 V DC must not be used on a 1500 V DC bus. Higher DC voltage ratings are reached through construction: greater contact separation and arc-control design, and in some product families a defined multiple-pole arrangement in series. That arrangement is specified by the manufacturer for the device in question — it is not a rule you can apply yourself to a 1000 V part. Read the DC voltage rating and the permitted pole configuration from the datasheet, and confirm the device is rated for the array’s maximum system voltage at the lowest expected temperature, which is where open-circuit voltage is highest.
The DC SPD has to clear a higher continuous voltage. The relevant parameter is the maximum continuous operating voltage, Uc, which must be at or above the highest sustained voltage the DC bus can present, with the applicable margin for the standard and the system. On a 1000 V DC array that generally leads to a 1000 V or 1200 V class device; on a 1500 V DC array it leads to a 1500 V class device. Choosing an SPD whose Uc is too low for the bus is not a small error — it puts the device into continuous conduction, which is a classic route to premature failure and to a temporary overvoltage event the SPD cannot survive. The voltage-class decision is set out in 1000 V versus 1500 V DC SPD, including how the class interacts with the earthing arrangement and the location of the SPD in the system.
Raising the system voltage changes the coordination, not just the labels. A higher operating voltage across the same installation means the residual voltage the equipment sees in a surge event is a larger fraction of the insulation’s capability, so the voltage protection level matters more, not less. It also means that if the SPD’s backup protection is selected on the assumption of a 1000 V circuit, both the interrupting capability and the voltage rating of that backup device may be inadequate on a 1500 V bus. When you move a combiner box design from 1000 V to 1500 V, revisit the MCB rating, the SPD voltage class, and the SPD’s backup protection together. Never change one of the three on its own.
These are principles, not a selection method. Each points to the guide that owns the detail.
No. A DC MCB interrupts overcurrent; it does not limit transient overvoltage. A surge is a microsecond-scale voltage impulse during which the breaker stays closed and the impulse passes straight through to the inverter DC input. The two devices protect against different events and cannot substitute for each other.
It needs backup protection whose rating is at or below the maximum overcurrent protective device declared by the SPD manufacturer. That backup device may be a dedicated fuse, or it may be the DC MCB already in the box, provided that breaker’s rating and characteristic fall within what the manufacturer permits. If neither satisfies the declared maximum, add a correctly rated dedicated device.
The SPD must be connected to the DC bus it is protecting, with short, direct leads to the protective earth bar. Whether that point is on one side or the other of the output breaker depends on the enclosure layout and on what the SPD is intended to shield, and it should be decided with the box’s wiring arrangement in front of you rather than by rule of thumb.
Both devices are required whatever the voltage class. The need for overcurrent protection and the need for transient overvoltage protection are independent of the system voltage — a 1000 V array is just as vulnerable to a lightning-induced impulse as a 1500 V one. What changes with the voltage class is the rating of each device, not whether you need it.
They are selected for different duties, so there is no single pairing rule. The breaker is selected against the circuit’s continuous current, prospective fault current, and coordination with the string fuses; the SPD is selected against the system’s voltage class, surge environment and required voltage protection level. The only direct constraint between them is that the breaker must be an acceptable backup device for the SPD, which means falling within the SPD manufacturer’s declared maximum. If it does not, a separate backup device is needed.
In a combiner box, the DC MCB and the DC SPD are not two options for one job. They are two devices for two different jobs on the same busbar, and the installation is only as good as the weaker of the two links — including the backup protection that connects them. Confirm both ratings against the actual maximum system voltage, confirm the SPD’s declared backup protection is satisfied by what is in the box, and design the layout so the SPD’s leads stay short.
If you are specifying a combiner box for a project and want the protection content reviewed against your array’s voltage class and string configuration, review the combiner box range and send us your system parameters — the arrangement of the overcurrent and overvoltage devices is the part of the specification that is worth getting right on paper first.
