Home » Blogs » Industry Blog » Solar Surge Protection Device Installation: PV Combiner Box vs Inverter DC Input
2026-10-19
An SPD (Solar Surge Protection Device) in a PV combiner box will not completely protect inverters that are 25 meters away from the combiner box. Over-voltage surges will still occur on the DC lines because of impedance. Other factors like conductor route and the distance between the SPD and equipment protected will also affect over-voltages.
In the design of a PV power plant, the appropriate location for the installation of an SPD is dependent on many factors including the length of the DC cables, the exposure of the equipment to lightning, and the withstand voltage of the equipment.
Why Does Solar Surge Protection Device Location Affect Protection?
Lightning-induced electromagnetic fields can generate transient overvoltages along PV DC cables. A Solar Surge Protection Device limits these voltages and diverts surge current through the equipotential bonding system.
Equipment terminals are generally higher impedance than SPDs. Hence, effective protection at equipment terminals may be greater than the protection level of the SPDs (Up).
Inductive kickback voltage is given by the equation：
ΔU = L × di/dt
Where L is the inductance of the circuit, and di/dt is the rate of change of the current.
Consequently, a correctly rated SPD with unnecessarily long connection leads may still expose inverter electronics to excessive transient voltage.
Two distances must therefore be distinguished:
•Array-to-inverter distance: Determines the need for additional coordinated protection.
•SPD connection length: Influences the effective voltage protection level at the equipment.
Solar Surge Protection Device: Combiner Box vs Inverter DC Input
1. PV Combiner Box Installation
A Solar Surge Protection Device (SSPD) installed in the PV combiner box offers lower level circuit protection of the array.
The following advantages are realized with this protection scheme:
- More effective protection of the PV string array.
- Shorter circuit pathways.
- Ease of integration with DC distribution systems.
- Ease of module replacement and fault identification.
When installing an SSPD outdoors, factor in protection of the enclosure and how the SSPD will manage a fault in the DC circuit.
The Telebahn Type 2 enclosed SSPD uses a Y-circuit with a thermal limiting disconnector and is rated IP65.
Installation limitation: A SPDD installed in a combiner box will not fully protect inverter DC inputs.
2. Inverter DC Input Installation
The protection of the inverter's DC input electronics can be improved by installing an SSPD nearby.
Limitation considerations include:
- Maintain Ue of the SPD and Uw of the inverter.
- Keep connection lengths to DC and PE as short as possible.
- Determine protection of other DC circuit inputs (i.e. MPPTs).
Verify the inverter's existing internal surge protection, if provided.
However, inverter-side protection cannot automatically replace array-side protection where long DC cable runs create separate exposure.
Installation Comparison
| Engineering Factor | PV Combiner Box | Inverter DC Input |
| Protection focus | Array-side circuits | Inverter electronics |
| Installation position | Near PV strings | Near inverter terminals |
| Main advantage | Localized array protection | Shorter equipment connection |
| Main limitation | Long downstream cable | Distant array exposure |
| Maintenance access | Outdoor enclosure | Inverter cabinet |
When Should Solar Surge Protection Devices Be Installed at Both Ends?
Understanding the IEC 10-Meter Consideration
IEC 61643-32 addresses coordinated SPD installation for photovoltaic systems.
For the PV configurations covered by Clauses 6.2.1 and 6.2.2, Clause 9.2.5 specifies that when the distance between PV modules and inverter is 10 meters or greater, two SPD sets are necessary to sufficiently protect both ends.
For shorter distances, one appropriately selected SPD near the inverter may be sufficient, subject to the installation conditions.
| PV Installation | Protection Consideration |
| Short DC cable (under 10m) | Evaluate single-point protection |
| DC cable ≥10m | Coordinate array-side and inverter-side SPDs |
| External lightning protection system | Assess separation distance and lightning current exposure |
| Multiple MPPT circuits | Verify each required protection path |
Type 1 vs Type 2: Does Installation Location Determine SPD Type?
Not independently. SPD type depends primarily on the expected surge exposure.
•Type 1 SPD: Intended to handle partial lightning currents; tested using a 10/350μs impulse waveform.
•Type 2 SPD: Intended for transient surge protection; tested using an 8/20μs current waveform.
Where an external lightning protection system is installed and the required separation distance cannot be maintained, Type 1 or combined Type 1+2 DC protection may be necessary.
How to Select the Correct Solar Surge Protection Device
SPD positioning and electrical compatibility must be evaluated together.
Five Critical Selection Parameters
| Parameter | Technical Meaning | Engineering Requirement |
| UCPV | Maximum continuous PV operating voltage | Accommodate maximum cold-weather string Voc |
| In | Nominal discharge current | Match required surge duty |
| Imax | Maximum Type 2 discharge current | Verify declared discharge capacity |
| Up | Voltage protection level | Coordinate with equipment Uw |
| ISCPV | PV short-circuit current rating | Cover prospective DC fault current |
For example, Telebahn's referenced Type 2 Solar Surge Protection Device specifies In = 20kA and Imax = 40kA using an 8/20μs waveform, with a declared Up of no more than 2.7kV at In.
These ratings must be evaluated against the actual PV configuration rather than selected by Imax alone.
Grounding and Connection Requirements
Poor wiring can compromise a correctly selected SPD.
During installation:
•SPD conductor connections should not be longer than 0.5m combined.
•Avoid extra bends and loops in wiring.
•Adhere to the wiring diagrams provided by the manufacturer for DC+, DC-, and PE conductors.
•Maintain equipotential bonding, and check that protective conductors are not broken.
•Check for conditions of DC overcurrent protection and short-circuits.
•A low ground resistance does not guarantee that there will be no inductive voltage rise.
Assess surge protection for DC, AC, and data communication circuits.
Installation and Maintenance
A Solar Surge Protection Device should be assessed against two International Electrotechnical Commission (IEC) standards, namely:
•IEC 61643-31, which deals with rating, testing and marking of PV DC Surge Protection Devices (SPDs), and
•IEC 61643-32 which covers the selection, installation and coordination of SPDs.
Prior to use, the coordination of the surge protection devices with respect to the other protective devices should be assessed. Other defects in the installation of SPDs include; selecting surge protection devices based on the DC overcurrent level (Imax) only, and failing to control the length of the protective conductors.
Monitoring and Replacement
Telebahn's PV protection design incorporates:
•Visual operating and fault indication.
•Remote signalling contacts for fault monitoring.
•Pluggable protection modules.
•Integrated DC fault protection and thermal disconnection
These features enhance the ability to monitor and service equipment. Isolation of equipment and sectional coordination of protection must be performed in accordance with the manufacturer's instructions.
Selecting the Appropriate Solar Surge Protection Device for Your PV Project
When the supplier has this information, he or she will be able to provide the best surge protection device for your PV project.
The Telebahn Solar Surge Protection Device offers Type 2 DC Protection and is constructed with an impact-resistant, sealed cover. Faults are indicated remotely. It can also be adapted to various user requirements. Check the device specification sheet to assist you in determining how best to utilize this device on your project.
FAQs
Q1. What is Telebahn's Solar Surge Protection Device used for?
Telebahn's DC SPD is designed to protect compatible photovoltaic generators and inverter DC circuits against transient surge overvoltages.
Q2. Where should a Telebahn Solar Surge Protection Device be installed?
At a suitable PV combiner box or near the inverter DC input, depending on cable distance, equipment protection requirements, and system layout.
Q3. Is one Telebahn SPD sufficient for a long PV cable run?
Not necessarily. For applicable PV configurations with array-to-inverter distances of 10 m or more, coordinated protection at both ends should be considered.
Q4. Is Telebahn's Solar Surge Protection Device Type 1 or Type 2?
The referenced photovoltaic DC protection solution is Class II / Type 2. Type 1 requirements must be evaluated separately according to lightning exposure.
Q5. What is the maximum PV voltage rating?
The referenced Telebahn product lists a maximum PV voltage of 600V. Verify compatibility with the array's maximum calculated cold-weather open-circuit voltage.
