A 3-phase surge protector (SPD) works by diverting dangerous overvoltages - usually from lightning strikes or switching events - safely to earth before they damage sensitive equipment. In practice, it’s installed at the origin of a 3-phase supply (such as a distribution board or consumer unit) and clamps voltage surges across all three live conductors and neutral, keeping downstream circuits within safe limits.
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What Happens During a Surge?
When a transient spike hits the supply, the SPD’s metal oxide varistors (MOVs) or similar components react within nanoseconds.
- Excess voltage is shunted to earth, keeping the line-to-neutral and line-to-earth voltages below critical thresholds.
- Once the surge passes, the SPD resets and normal operation continues.
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 collection 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 – Standard protection for most distribution boards (stops switching transients and indirect strikes).
- Type 3 SPD – Point-of-use protection for specific equipment.
Most 3-phase units, including our 3 Phase SPD Complete with MCB, use Type 2 protection, which is mandatory under 18th Edition Amendment 2 for most new installs.
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, 40kA 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.