Contributed by: Suraj Kumar HS

280V Vs 320V AC SPD


Surge Protection Devices (SPDs) are devices that are used to protect electrical equipment against transient over voltage (TOV).

TOVs often come from one of three places: the grid side (like in a load-shedding situation or switching at the local substation), or the load side, which could be caused by heavy motor loads (such as elevators, compressors, etc.) or a lightning strike. These spikes usually last about 1 – 30 μs and could reach 105 Volts depending on the source.

If exposed to such high voltages, electronic and electrical equipment may burn through insulating protections and exceed voltage ratings, leading to permanent damage or compromise in the affected electrical components.

In a Solar Photovoltaic System, SPDs are installed both on the AC and DC side. However, this article will primarily focus on AC SPDs.

Main SPD Parameters

Types of SPD

Types of SPD depend on the location at which it is installed,

  • Type 1 SPDs – Main Distribution Panel
    • Type 2 SPDs – Sub-Distribution Panel
      • Type 3 SPDs – Load Protection Panel

By technology, there are two types of SPDs, Namely,

  • Gas Discharge Tube (GDT) – Uses a spark gap across electrodes placed in a sealed glass filled with noble gases which are designed to break down at a voltage higher than the operating voltage causing it to conduct. This is a voltage-switching system (acts slower than MOV).
  • Metal Oxide Varistor (MOV) – An MOV is a circular/rectangular-shaped body of sintered Zinc Oxide & other oxides of a single/multilayer sandwich of the same. when a surge occurs, the resistance across the MOV rapidly decreases causing a large current flow. MOVs are Voltage limiting system (acts quicker than GDT)
  • Avalanche Breakdown Diode (ABD) – A P-N Junction diode designed to experience an avalanche breakdown (an electrical phenomenon in a semiconductor that allows large amounts of current to flow) when a voltage is applied that is greater than the rated voltage.

For this discussion, a Type-2 & MOV-based surge protector is being used.

Construction and Working Principle – Citel DS240-280V AC SPD

The heart of the SPD is the MOV (A) and connected to that on top is a weak mechanical solder joint (C), that acts as a fuse when the SPD is pushed beyond its operational voltages for more than the rated length of time. To indicate the failure of the SPD, when the solder joint has melted off, a spring (D) pushes the red shaft (B) forward indicating the failure on the front display. The live/neutral and ground connectors (E) safely carry the current through the housing (F) to ground.  Components A-E are housed in an easily detachable cartridge form that can be replaced after a failure.

Additionally, the SPD also has a feature for remotely signaling a failure. This is done by a contact inside the housing that is activated/deactivated by the red mechanical shaft.

Please refer to the below video for more details:

This could be connected to a contactor that can switch the loads in case of a failure.

Why SPD now?

SPDs have existed for a long time but had niche applications as appliances previously were not susceptible to surges as they didn’t consist of any sensitive electronic/electrical circuit.


A 320V SPD (where Uc=320V) responds to spikes that only occur after 320V, and these might affect some sensitive electronics. To contrast it with the 280V SPD, in a transient over-voltage situation, the SPD starts acting on the surges far earlier than 320V SPD would, further protecting the sensitive loads.

However, as the 280V SPDs are more sensitive, they also tend to fail quicker than their 320V counterpart.


Choosing an SPD should be done based on the sensitivity of the load and/or the inverter of the solar system. Most of the time, the inverter manufacturers specify the required type and ratings of the SPDs that should be used. Generally, microinverters require an SPD rated at 280V and string inverters work with both 320V and 280V.

Case Study: Continuous overvoltage of ~300VAC

Even for a string inverter, it is a good practice to use a 280V SPD as let’s say in a case where the inverter is unable to handle ~300V (more than absolute max cont. v/g and no protections are in place), the SPD will blow & with a contactor connected using the remote signalling feature (if present) of the SPD, the inverter/load could be shut off safely.

In conclusion, the 280V SPD acts quicker and is better for most cases. But the tradeoff is the additional cost of the SPD and its longevity. So, while designing a Solar PV System, it is best to first consider a 280V SPD and then based on if there are already other protections in place to attend to other overvoltage situations, a 320V SPD could also be considered.

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