A surge (a transient overvoltage lasting microseconds) comes from two places: lightning (a direct strike on or near the building, or a strike anywhere on the grid within a few kilometres that induces a surge onto the incoming line) and switching (large inductive loads on the network being switched on and off). Either way, the voltage rises well above the nominal 230 V for a few microseconds, brief, but enough to matter to sensitive electronics. A Surge Protection Device (SPD) sits in parallel across the supply and clamps that transient, diverting the surge current safely to earth so the downstream load keeps seeing a clean voltage.
SPDs are classified by Type under IS/IEC 61643-11. The Type is not a quality grade, a Type 2 is not "better" than a Type 1. The Type describes the kind of surge the device is tested against and, by extension, where in the installation it is meant to sit. This note covers the three classifications you specify most often, Type 1, Type 2 and the combined Type 1+2.
Type 1, the lightning-current SPD
A Type 1 SPD is tested with an impulse current (Iimp) on a 10/350 µs waveform, a long, high-energy pulse that simulates a portion of a real direct lightning current. It is built to carry that energy safely to earth and stay in service.
Type 1 is required where the building can take a direct lightning strike, most clearly, where there is an external Lightning Protection System (a lightning rod / air terminal and down-conductors). On a strike, part of that current flows back into the electrical installation via the earth, and only a Type 1 device is rated to handle it. It is installed at the origin of the installation, the service entrance / main incoming panel.
- Tested against, partial direct lightning current, Iimp (10/350 µs)
- Installs at, the service entrance / origin of the installation
- Specify when. There is an external lightning protection system, or genuine direct-strike exposure (tall or isolated structures, hilltop sites, rural feeders)
- Note, a Type 1 is almost always paired with a downstream Type 2, which brings the let-through voltage down to a level that suits sensitive electronics
Type 2, the main-DB SPD
A Type 2 SPD is tested with a nominal discharge current (In) and maximum discharge current (Imax) on an 8/20 µs waveform, a shorter pulse representing an induced or residual surge rather than a direct strike. It is the everyday SPD, built around metal-oxide varistor (MOV) technology, and it is the workhorse of surge protection.
Type 2 handles two jobs: the residual surge that passes through an upstream Type 1, and, on buildings with no direct-strike exposure and no external LPS, the induced and switching surges that ride the incoming line on their own. For the majority of urban commercial and residential installations, a Type 2 at the main distribution board is the protection. It is installed at the main DB, and repeated at sub-DBs feeding sensitive sections.
- Tested against, residual / induced surge, In and Imax (8/20 µs)
- Installs at, the main distribution board (and sub-DBs)
- Specify when, almost always, on any installation with electronics worth protecting; it is the default specification for offices, IT rooms, hospitals and retail
- Note, for the most sensitive equipment, a Type 3 is added at the point of use, immediately upstream of the device, to clean up the small residual a Type 2 lets through
Type 1+2, the combined device
A Type 1+2 SPD is a single device tested to both Class I and Class II. It withstands the 10/350 µs partial lightning current of a Type 1 and delivers the low protection level of a Type 2 in one DIN-rail body (commonly four modules). It does the work of both stages in a single position.
There are two practical reasons to specify it:
- The service entrance and the main DB are the same enclosure. On many small and mid-size buildings there is no separate origin panel, the incoming supply lands directly in the main DB. A combined Type 1+2 gives lightning-current capability and a low let-through voltage at that single point, where fitting two separate devices would be redundant.
- Panel-room space (or budget) is tight. Where an external LPS makes a Type 1 mandatory but there is no room, or no appetite, for a separate Type 1 and Type 2 with the coordination between them, the combined device covers both duties in the footprint of one.
The trade-off to be aware of: a combined Type 1+2 is rated for the high-energy duty, so it is specified by its Iimp exactly like a Type 1. It is the right call for a single-entry installation, but on a large installation with a separate origin panel and several distribution boards, a dedicated Type 1 at the origin and Type 2s at each board is still the cleaner design.
How they coordinate
SPDs are designed to work as a chain, each stage taking the surge down a level before passing it on: Type 1 absorbs the bulk of a direct-strike current at the entrance, Type 2 clamps the residual at the main board, and a Type 3, where fitted, cleans up the last of it at the equipment.
For the chain to coordinate, a separate Type 1 and Type 2 need a minimum conductor length of about 10 m between them (or a decoupling inductor where that length isn't available), the conductor length lets each stage operate in sequence. The combined Type 1+2 simplifies this: with both functions in one body at one position, the coordination is built in.
The classifications, at a glance
| Aspect | Type 1 | Type 2 | Type 1+2 |
|---|---|---|---|
| Test class | Class I | Class II | Class I + II |
| Test waveform | 10/350 µs impulse | 8/20 µs | Both |
| Key rating | Iimp (kA) | In / Imax (kA) | Iimp + low Up |
| Surge handled | Partial direct lightning current | Residual & switching transients | Both, in one device |
| Installs at | Service entrance / origin | Main DB (and sub-DBs) | Single combined entry point |
| Specify when | External LPS / direct-strike exposure | Almost always, default protection | LPS present but one position preferred |
Reading the ratings on the device
Once the Type is settled, a handful of marked parameters decide whether a particular SPD suits the installation. These are the ones worth checking on the datasheet:
- Uc, maximum continuous operating voltage. The voltage the SPD can sit across indefinitely without conducting. Must clear the system voltage with margin (typically 275 V or 320 V for a 230 V system) so it rides the normal rise and fall of grid voltage comfortably.
- Up, voltage protection level. The let-through voltage the SPD actually clamps to during a surge. Lower is better. It must sit below the withstand rating of the equipment being protected.
- In / Imax, nominal and maximum discharge current (Type 2). How much 8/20 µs surge current the device can divert, repeatedly (In) and once (Imax).
- Iimp, impulse current (Type 1 / Type 1+2). The 10/350 µs lightning-current rating, sized to the lightning risk at the site.
- Status indication. A window or flag that shows when an MOV has reached end of life, plus, on plug-in designs, a replaceable cartridge so the SPD can be renewed after it has done its job without rewiring the panel.
Choosing in practice
- Standard urban building, no external LPS, Type 2 at the main DB. Add a Type 3 at genuinely sensitive equipment (server racks, imaging, instrumentation).
- Building with an external lightning protection system, Type 1 at the origin, Type 2 downstream at the main DB, coordinated over the cable run between them.
- LPS present, single incoming panel, or tight panel space, a combined Type 1+2 at the entry point, with Type 3 at the equipment as needed.
The thread through all three: a Type 2 is the baseline almost everywhere; the lightning risk and the panel layout decide whether that becomes a Type 1 + Type 2 chain or a single Type 1+2.
What we stock
Shreetuvis is an Authorised Stockist for Legrand SPDs in Nagpur, Type 1, Type 2, Type 3 and combined Type 1+2 DIN-rail devices for the coordinated chain, sourced direct from Legrand India, genuine product. See the Legrand SPD distribution page for the range and where each tier fits, or send your panel positions and protection chain via the contact form for a quote.