1. DC Box
The DC box is one of the most critical safety nodes in a solar power system. It is where the energy coming from the solar panels converges, and before it reaches the inverter input, it receives the necessary protection and disconnection capability at this point. In the case of multiple strings, high voltage, or longer outdoor wiring, the DC box is practically the primary line of defense for the system.
Electrically, this is where:
- the strings are collected,
- parallelized if necessary,
- overcurrent and overvoltage protected,
- and the possibility of disconnection towards the inverter is provided.
The DC box does not modify the voltage. The full string voltage is present within it. The components inside exclusively serve protection and disconnection roles.
DC Fuses
The primary role of the DC fuse is to limit reverse currents. This issue becomes a real problem when multiple strings are paralleled. If a string develops a partial short circuit, the other branches can feed a large current back into the faulty circuit. In such cases, the danger comes not from the inverter but from the other solar panel arrays.
General professional principle:
- a single, standalone string usually does not require a fuse,
- for two or more parallel strings, protection may already be warranted.
Choosing the right fuse is not a routine matter. The maximum reverse current specified in the panel's datasheet and the number of parallel branches are the starting points. Only PV fuses specifically designed for DC arc interruption can be used; conventional AC types are not suitable.
DC Overvoltage Protection
DC side overvoltage protection is not needed for normal operating voltage fluctuations. The goal is to handle short-duration, high-energy voltage peaks caused by lightning strikes or switching transients. In normal operation, the SPD is practically "invisible." However, when an overvoltage occurs, it becomes conductive between the positive and negative branches and ground, directing the voltage peak towards the ground.
The effectiveness of the protection depends not only on the quality of the device but also on the design of the grounding system. With a long, high-impedance grounding conductor, the discharge will not be adequate, and some of the energy may be diverted towards the inverter.
DC Disconnect Switch
The purpose of the DC disconnect is to electrically isolate the inverter from the solar array. This is essential during maintenance or in case of a fault. It's important to clarify a common misconception: the disconnect does not eliminate voltage on the strings. Solar panels generate power as long as they receive light. The switch interrupts the current path to the inverter.
The DC disconnect must be capable of interrupting the full operating voltage and current without arc formation. Therefore, an AC switch cannot be used on this side. DC disconnects are designed with special contact and arc quenching mechanisms.
Pre-assembled or Custom DC Box?
With a pre-assembled DC box, the manufacturer has already coordinated the fuses, SPD, and disconnect. This means quick installation and predictable compliance.
Custom designs offer greater flexibility but also greater responsibility. In such cases, special attention must be paid to:
- the electrical parameters of the components matching the system,
- the nominal voltage and current ratings being appropriate,
- the box's heat dissipation and IP rating matching the environment.
For complex systems, a custom solution may be justified, but it does not replace professional design.
Placement Considerations
The location of the DC box is not an aesthetic decision. A position must be chosen where:
- it is accessible for maintenance,
- it is not exposed to prolonged direct sunlight,
- it is protected from rain and mechanical damage,
- the length of the DC cables remains reasonably short.
For outdoor installations, adequate IP protection is required, and indoors, care must be taken to ensure the box is not placed in the immediate vicinity of flammable materials.
2. AC Box
The AC box is the alternating current side safety node of the solar power system. This is where the inverter and the building's electrical network connect, and at this point, it must be ensured that the system complies with all standard, shock protection, and grid requirements.
While the DC side mainly handles the specifics of the PV array, the AC side is already a classic electrical field. Here, faults can cause not only inverter shutdowns but also complete building disconnections or shock protection problems.
Role of the AC Distribution Board
Electrically, the AC box is a protection and disconnection point. It is not merely a distribution board but a unit that allows for the disconnection of the inverter from the grid, protects the inverter and the connecting cable in case of overload and short circuit, and ensures overvoltage and shock protection requirements are met.
The AC box can be a separate unit, but in smaller systems, it can be integrated into the main distribution board. The essence is not the physical design, but that the inverter's circuit appears as an independent, clearly identifiable, and properly protected circuit.
Miniature Circuit Breakers (MCBs)
The inverter is not a typical consumer. It can generate close to its nominal current for long periods. Therefore, the associated miniature circuit breaker cannot be selected using "household logic."
When sizing, the following must be considered:
- the inverter's maximum AC output current,
- the characteristic (typically C),
- three-pole interruption for three-phase systems.
It is important that the circuit breaker protects the cable connected to the inverter and the inverter itself. If it's too small, it will cause unjustified trips. If it's too large, it won't disconnect in time in case of a fault.
AC Overvoltage Protection
AC side overvoltage protection serves to protect the inverter and the building's network from transients coming from the grid. These can originate from nearby lightning strikes, switching operations, or even grid disturbances.
AC SPD is typically located between the phase conductors and ground. It is not active in normal operation, but in case of overvoltage, it clamps the voltage peak and directs it to the ground. Here, too, the effectiveness of the protection largely depends on the quality of the grounding system. With a long or high-impedance grounding conductor, some of the overvoltage may propagate further into the system.
RCDs (Residual Current Devices)
Selecting an RCD for a solar power system is not a routine matter. Due to the internal construction of the inverter, DC component leakage currents may appear on the network. The decision should not be made based on general rules but according to the inverter manufacturer's specifications. Many modern inverters include built-in DC leakage current monitoring, which affects which type of RCD can be used.
Selectivity
The essence of selectivity is that in the event of a fault, only the affected circuit should be disconnected. If the AC box protections do not properly align with the main distribution board protections, a minor fault could cause the entire building to lose power.
The consequences can be:
- complete building shutdown,
- unjustified inverter shutdown,
- difficulty in locating the fault.
Therefore, the nominal current and characteristic of the circuit breakers must be chosen at a system level. One must think in terms of a complete protection chain, not just individual components.
3. String Fuses
The string fuse is one of the most frequently misunderstood protective elements in solar power systems. Many consider it general overcurrent protection, but its role is much more specific.
- It does not protect the inverter.
- It does not react to normal operating current.
The task of the string fuse is to protect against reverse currents originating from parallel strings.
When is a string fuse necessary?
If a single string is connected to the inverter, a separate string fuse is usually not required. In this case, there is no other power source that could impose a reverse current on the circuit. The maximum current of the string is determined by the panel's own characteristics.
The situation changes when two or more strings are paralleled.
In this case, each string potentially becomes a current source for the others. If a fault occurs in one branch (e.g., a partial short circuit or more severe damage), the other strings may be able to "push" a significant current into the faulty branch.
This reverse current can:
- overload the internal conductors of the module
- damage the bypass diodes
- in extreme cases, cause localized overheating or fire hazard
Therefore, in the case of parallel strings, the string fuse is not an extra safety feature, but a fundamental protection requirement.
What happens in case of a fault?
In normal operation, parallel strings operate at nearly identical voltages. The currents are balanced, and there are no problems. However, in a fault situation, the balance is upset.
Typical scenario:
- the voltage of one string drops due to a fault
- the other strings remain at a higher voltage
- current flows towards the faulty branch
This is where the string fuse comes into play. The fuse interrupts the reverse current before it reaches a damaging level. The faulty string is disconnected, and the others can continue to operate. This is an important distinction compared to the main DC protection, because the string fuse intervenes locally, it does not shut down the entire system.
The logic of sizing – where mistakes are often made
The sizing of string fuses is often approached incorrectly. It should not be matched to the inverter, and not simply to the operating current of the string. The starting point is always the module datasheet.
Especially important are:
- the short-circuit current (Isc)
- the maximum allowed reverse current
The nominal current of the fuse:
- must be greater than the maximum operating current of the string
- at the same time, it must be smaller than the reverse current that would damage the module
This is why the fuse value often only slightly exceeds the Isc value. This is not overly sensitive protection, but precisely matched.
The other basic condition: the fuse must be a PV-rated DC fuse and capable of handling the full string voltage.
Why is an AC fuse not suitable in a DC circuit?
In alternating current, there is a zero crossing in every cycle. This helps to extinguish the arc when interrupted. In direct current, there is no such thing. A DC arc can persist if the fuse is not capable of actively interrupting it.
Therefore, DC fuses for PV systems:
- have a special arc quenching design
- are rated for a specific maximum DC voltage
- are specifically certified for photovoltaic applications
A traditional AC fuse in a DC circuit is not only non-compliant but can be downright dangerous. Such incorrect application has already been the source of documented fires.
4. Fire Safety Disconnects
The fire safety disconnect is not part of daily operation. It does not optimize, regulate, or improve efficiency. Its role truly becomes important only in an emergency.
In case of fire, the solar power system should not pose additional electrical risk to emergency responders. Its purpose is as simple and as serious as that.
Why did fire safety disconnection become necessary in the first place?
When solar power systems first appeared en masse on residential buildings, a fundamental characteristic quickly became apparent:
the DC side is continuously energized during the day.
- It's no use shutting down the inverter.
- It's no use losing grid power.
- If the panels receive light, they generate power.
This presented a problem from a fire safety perspective. In a burning building, DC cables running on the roof are potentially high-voltage conductors. Firefighters cannot assume that these are de-energized. As a result, regulations emerged that require the minimization or emergency disconnection of DC voltage sections within the building. In Hungary, this is not found in a single piece of legislation but is determined by multiple sources collectively:
- the provisions of OTSZ (National Fire Safety Regulations)
- related technical guidelines
- and the practical expectations of fire departments and grid operators
The Fire Department's Perspective
For the fire department, a solar power system is not an energy technology issue but a risk factor. It is particularly problematic if high-voltage DC conductors run inside the building, the method of disconnection is unclear, or the presence of the system is not adequately marked.
The goal is not for the panels to be completely "turned off," as this is physically impossible, but to localize hazardous voltage sections so they do not appear in the interior spaces of the building.
What can be disconnected – and what cannot?
It is important to state a fundamental physical fact: a solar panel module always generates voltage in daylight. Therefore, the purpose of fire safety disconnection is not to eliminate the entire DC system, but to localize the voltage.
The principle is as follows:
- the voltage should remain on the roof, near the panels
- the DC section within the building should be disconnected
Therefore, the disconnect is typically located on the roof or directly near the roof penetration. In this way, in case of fire, the internal DC cables can be de-energized, while the modules on the roof continue to generate power.
This is a compromise, but a reasonable one from a safety perspective.
Manual and Automatic Solutions
Fire safety disconnection can be implemented in several ways.
In a manual solution, the DC side can be disconnected with a clearly visible, marked switch. Its advantage is simplicity and transparency. Its disadvantage is that it requires human intervention, which cannot always be guaranteed in case of fire.
In automatic systems, disconnection occurs in response to an external signal. This signal can come from:
- loss of grid voltage
- a fire alarm system
- a separate emergency stop circuit
In the case of an automatic solution, disconnection occurs immediately without human intervention. This is a significant advantage from a fire safety perspective, but it requires a more complex system and more precise integration.
Where should the disconnect be located?
The placement of the disconnect is not an aesthetic question but a safety decision. The goal is for the path of the DC voltage within the building to be as short as possible.
Therefore, in practice, the disconnect is typically located on the roof, in the attic, or directly after the roof penetration.
If the disconnect is placed next to the inverter, deep inside the building, then in case of fire, the entire internal DC cabling will remain energized. This no longer complies with the fire safety principle. Therefore, the correct placement must always be interpreted relative to the building structure and the DC cable route, not to the inverter.
