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What Is a 3 Phase SPD - And Do You Need One?
Posted by: Michael Hickey -

With the latest regs making surge protection a talking point on nearly every install, 3 phase SPDs are cropping up in more boards – whether you’re wiring up a commercial unit, an EV charger, or a PME supply.

But what exactly is a 3 phase SPD?

And when do you actually need one?

Here’s a straight-talking guide for UK electricians covering what they do, when they’re required, and how to spec the right one for the job.

Jump to:

What does a 3 Phase SPD actually do?

SPD stands for Surge Protection Device – and in a 3 phase board, the device is designed to limit overvoltage spikes across all three lines (L1, L2, L3) and neutral.

In plain English:

It protects equipment from transient overvoltages (a.k.a. power surges), which are usually caused by things like:

  • Nearby lightning strikes
  • Grid switching events
  • Large motors starting/stopping
  • Faulty upstream connections

These spikes can travel through the supply and damage sensitive kit, from consumer units and RCBOs to EV chargers and servers. A 3 phase SPD safely diverts that excess energy to earth, clamping the voltage to a safe level before it reaches downstream circuits.

Single vs 3 Phase SPD - What’s the difference?

Not all surge protectors are made equal. If you’re used to wiring domestic boards, you might assume a standard SPD will do the job – but once you step into the three-phase world, things change fast.

Here’s a quick side-by-side to show how 3 phase SPDs differ from their single-phase cousins – so you don’t get caught out on your next install:

Feature Single Phase SPD 3 Phase SPD
Voltage protection L–N / L–E L1–L2–L3–N–E
Applications Domestic / small installs Commercial, industrial, 3P EV
Typical wiring 2- or 3-pole 4-pole minimum
Enclosure size Compact Requires larger board (e.g. 8-way)
Surge rating Often 10kA 20–40kA standard

👉 If you’re working with three-phase distribution – you need a surge protector designed to handle all 3 lines and neutral at once.

When do you need a 3 Phase SPD in the UK?

Here’s where BS 7671: 18th Edition (Amendment 2) kicks in.

Under the latest regs, SPDs are required in most three-phase installs unless the client has carried out a documented risk assessment showing they’re not needed.

Common scenarios where a 3 phase SPD is required:

  • 🏢 Commercial premises with sensitive equipment (e.g. office IT, lifts, HVAC, alarm systems)
  • 🧰 Workshops or garages with 3P tools or machinery
  • ⚡ EV chargers fed by a 3 phase supply, especially PME
  • 🔁 Any sub-main boards or distribution boards downstream of a vulnerable supply
  • 🏠 Domestic installs with higher-end electronics or home offices

If there’s likely risk to life (e.g. medical equipment) or disruption to commercial activity, then SPD becomes mandatory – no risk assessment required.

👉 And in many PME systems, surge protection is also used in combination with PEN-fault detection units.

How to spec the right 3 Phase SPD

The right choice depends on a few key factors:

  • ✅ 1. Surge rating
    Most standard installs require Type 2 SPDs rated 20kA/40kA, which are fine for indirect surges. Type 1+2 may be needed near lightning zones.
  • ✅ 2. Enclosure type
    Choose a pre-assembled unit with a metal IP40-rated board for clean, regulation-ready installs – especially in commercial environments.
  • ✅ 3. Protection module
    Your SPD unit might include:
    • MCB (C Curve, 40A or 63A)
    • RCD (30mA high immunity)
    • RCBO (4-pole, A-Type for pulsating DC faults)

⚡ Example:
The Power & Data 3 Phase SPD Unit comes pre-wired with a 4P Type 2 SPD and your choice of MCB, RCD or RCBO – ready to mount, label, and power up.

If you’re regularly wiring 3 phase boards with surge protection, you’ll also want to have these close to hand:

Need a unit built to order with custom modules? Just give us a shout.

FAQs About 3 Phase SPDs

Still got questions about when, where, and how to use 3 phase SPDs?

We’ve pulled together answers to the most common ones UK sparkies ask – from regulation triggers to install locations and protection types.

Q: Is a 3 phase SPD mandatory under the 18th Edition?

Yes – in most commercial or multi-occupancy installs, SPDs are required by default unless a documented risk assessment proves they’re not needed. For PME supplies, EV chargers, or sensitive electronics, it’s generally not optional.

Q: What’s the difference between Type 1 and Type 2 SPDs?

Type 1 SPDs protect against direct lightning strikes (e.g. buildings with external lightning protection systems).

Type 2 SPDs protect against indirect surges and are used in most standard 3 phase installs.

Most UK electricians will use Type 2 SPDs rated 20kA/40kA for distribution boards and sub-mains.

Q: Can I install a 3 phase SPD in a single-phase setup?

Not typically – a 3 phase SPD is built for L1/L2/L3 + N protection. For single-phase installs, use a 2- or 3-pole SPD tailored to L-N/E.

(You can technically use a 3P SPD across fewer phases, but it's not the cleanest or most cost-effective route.)

Q: Where should a 3 phase SPD be installed?

Ideally:

  • At the origin (main incomer) of a three-phase supply
  • In sub-distribution boards feeding sensitive or remote loads
  • Within PME EV protection boards where disconnection timing and voltage clamping are critical

Mount vertically on a dry, indoor surface and follow manufacturer wiring diagrams closely.

Q: Can a 3 phase SPD be combined with RCBO/RCD protection?

Yes – and in fact, that’s the most common setup.

Look for pre-wired SPD units with integrated:

Final Thoughts

Three-phase surge protection is no longer a “nice to have” – in many cases, it’s a regs requirement and a vital safeguard for your clients’ equipment.

By understanding how 3 phase SPDs work and where they apply, you’ll be able to:

  • Stay compliant
  • Protect downstream loads
  • Quote confidently with the right board on hand

And if you’re looking for a ready-to-install 3 phase SPD unit, we’ve got you covered.

👉 Shop the 3 Phase SPD unit now or check out our full 3 phase range.