How Does a 3-Phase Surge Protection Device (SPD) Work?
A three-phase surge protection device (SPD) limits short-duration overvoltage caused by lightning activity or switching events. When voltage rises above its operating threshold, the SPD conducts surge current through its protection paths and limits the voltage reaching downstream equipment.
Quick Answer: A three-phase SPD normally remains in a high-impedance state. During a transient, its internal components conduct, divert surge current and clamp the overvoltage. It then returns to its normal state, although repeated or severe surges progressively use its protective capacity.
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What Happens Inside a Three-Phase SPD?
Under normal supply conditions, the SPD presents a high impedance. When a transient exceeds its threshold, voltage-dependent components such as metal oxide varistors conduct and create a lower-impedance route for surge current.
- The overvoltage is limited to the stated voltage protection level (Up).
- Surge current is handled across the relevant line-to-neutral and neutral-to-earth protection paths.
- After the transient, the device returns to its normal state.
An SPD is sacrificial. Large or repeated events can degrade it, so status indicators and replaceable cartridges should be checked during inspection and maintenance.
How Does It Protect All Three Phases?
A three-phase SPD uses coordinated protection paths rather than one component for the whole supply. The exact arrangement depends on the earthing system and product design.
- 3+1 arrangements commonly protect L1-N, L2-N and L3-N, with a separate N-PE element.
- 4+0 arrangements use protection paths from each live conductor, including neutral, towards PE.
The configuration, maximum continuous operating voltage (Uc), Up, nominal discharge current (In) and maximum discharge current (Imax) must suit the installation.
Where Are 3-Phase SPDs Installed?
Placement matters as much as the device itself. Here’s where 3-phase SPDs are typically fitted to give proper system-wide protection.
- At the main incomer of commercial or industrial boards to protect the entire installation.
- In EV charging hubs, solar arrays, and heat pump systems, where inverter-driven equipment is highly sensitive.
- As supplementary protection inside sub-distribution boards feeding critical loads (IT, HVAC, lifts).
👉 See our full Three-Phase Boards & Accessories for compatible enclosures.
Types of Surge Protection Devices (SPD)
Not all SPDs do the same job. Knowing the difference between Type 1, Type 2 and Type 3 is key to choosing the right protection for your board.
- Type 1 SPD – Handles direct lightning currents (used where buildings have external lightning protection).
- Type 2 SPD – Protects against induced lightning and switching transients and is commonly used at distribution boards.
- Type 3 SPD – Point-of-use protection for specific equipment.
Most distribution-board products are Type 2 SPDs. Whether surge protection is required, and whether coordinated Type 1 protection is also needed, depends on the installation, lightning protection system and consequences assessed under BS 7671.
SPD vs RCD – What’s the Difference?
- SPD protects against voltage surges (transients).
- RCD protects against earth leakage (shock/fire risk).
They’re complementary, not interchangeable — that’s why many of our boards combine Type A RCDs or RCBOs alongside SPDs.
FAQs
Installers ask the same questions time and again when it comes to 3-phase surge protection. Here are clear answers to the ones that matter most on site.
Do I need a 3-phase SPD in every distribution board?
If the board is supplying sensitive or safety-critical equipment, yes. Otherwise, protect at least the origin of the supply.
What kA rating should I choose?
Follow BS 7671 guidance. In most commercial installs, 40 kA Type 2 SPDs are sufficient unless you have an external LPS (then use Type 1).
Can I retrofit surge protection into an existing board?
Yes — with dedicated surge protection devices or pre-assembled metal surge arrestor units.
What Should Installers Check When Specifying a 3-Phase SPD?
- Supply and earthing arrangement
- Type 1, Type 2 or coordinated Type 1+2 requirement
- Uc, Up, In and Imax ratings
- Short-circuit withstand and required backup protection
- Connection arrangement and lead-length requirements
- Replaceable cartridge and remote-status requirements