With three-phase installs now demanding more attention under Amendment 2 of the 18th Edition regs, the question of where to actually install surge protection keeps cropping up - especially on PME and commercial jobs.

Should the SPD go before or after the main switch? What about sub-boards? And what if you’ve got a TT earthing arrangement?

Here’s a straight-talking guide for UK electricians.

Jump to:

• What the regs say about SPD placement
• Where to install SPDs in 3 phase systems
• Should the SPD go before or after the main switch?
• The 10m rule explained
• Do you need SPD + RCBO?
• What about PME or TT systems?
• How to keep lead lengths short (and why it matters)
• FAQs

What the Regs Say About SPD Placement

BS 7671:2018 18th Edition (Amendment 2) makes one thing clear:

Surge protection devices (SPDs) must be installed where overvoltages are likely to cause:

• ✔ Injury or loss of life
• ✔ Interruption to public services
• ✔ Damage to cultural heritage
• ✔ Economic loss or inconvenience

In a typical three-phase setup, that means most commercial, PME, and EV installs will require an SPD - without the option of omitting it.

But the regulations don’t just stop at whether you need one. They also imply where it should go to be effective.

Where to Install SPDs in 3 Phase Systems

In most 3-phase installations, the primary SPD should be installed at the origin of the installation, normally within the main distribution board and positioned so it protects the installation downstream of the main switch. This is the standard starting point for protecting the board and limiting the spread of transient overvoltages through the rest of the system.

• In the main distribution board (DB1)
• Or in the first consumer unit on the line

This ensures that incoming surges are suppressed before they can propagate through the installation.

Other valid installation points:

• Sub-distribution boards (DB2, DB3...) - especially if long cable runs are involved
• Downstream boards feeding sensitive loads (e.g. servers, lifts, HVAC, CCTV)
• EV protection boards with PME connections

Should an SPD Go Before or After the Main Switch?

In most board layouts, the SPD is installed after the main switch so the rest of the installation is protected while still allowing safe isolation for maintenance. The exact arrangement depends on the board design and manufacturer layout, but the key goal is to protect the downstream installation with the shortest practical connection lengths.

The 10 Metre Rule (Explained Simply)

If the cable length between your origin SPD and a downstream board exceeds 10 metres, you're into risky territory.

Why? Because voltage spikes can reform and rise again over long runs.

⚡ If your SPD is more than 10m from the protected load or board, you should install additional coordinated SPDs closer to that load.

This is especially important on:

• Long corridors or warehouse trunking
• Outbuildings or remote supplies
• Industrial tools on long radial feeds

Why SPD and RCBO Protection Are Different Jobs

Yes - both may be needed, but they do completely different jobs. An SPD limits transient overvoltages, while RCBOs or other protective devices deal with overload, short-circuit and earth fault protection on individual circuits.

• SPD = clamps surge voltages to prevent equipment damage
• RCBO = protects against overloads and earth faults on final circuits

Many commercial boards now come with both integrated. If you're buying a pre-wired 3P SPD board, check whether it includes:

• 4 Pole MCB
• 4 Pole RCD
• 4 Pole RCBO (A-Type, high immunity)

PME, TT, and SPD Considerations

Installing SPDs isn’t just about wiring - it’s about understanding how different earthing systems affect surge behaviour.

Here’s how PME and TT supplies influence your SPD placement, module choice, and wiring configuration.

• PME (TN-C-S): SPDs are often combined with PEN-fault detection. Voltage clamping here is vital to avoid excessive touch voltages.
• TT Systems: SPD selection and connection method need careful coordination with the earthing arrangement and the RCD strategy. Upstream and downstream protection should be selected so surge protection works effectively without compromising fault protection or selectivity.

How to Keep Lead Lengths Short (And Why It Matters)

Shorter leads mean better SPD performance. The longer the connecting conductors, the greater the added impedance and the less effective the SPD becomes at limiting the transient voltage seen by the installation.

Every extra centimetre of cable adds impedance, delaying the SPD's reaction time.

Tips:

• Mount SPDs directly in or beside the DB
• Keep connections to live, neutral and earth as short and direct as possible
• Avoid looping or slack cables

FAQs About SPD Placement

Still got questions about where to mount surge protection, how the 10m rule applies, or what to do with PME and TT setups?

This section clears up the most common points of confusion UK electricians face when fitting 3 phase SPDs.

Q: Should an SPD go before or after the main switch?
A: Ideally after the main switch and upstream of any final circuit protection. This way, the SPD protects everything downstream while still being isolated during maintenance.

Q: Can a surge protector go anywhere in the panel?
A: Technically yes, but placement matters. The closer to the origin and shorter the lead lengths, the more effective it is.

Q: Where should a Type 3 SPD go?
A: These are installed right next to the protected equipment and only as a supplement to Type 2 SPDs. Think socket-level protection for high-end kit.

Q: What if I have multiple boards in a commercial unit?
A: Each board may need its own SPD if cable lengths from the origin exceed 10m. Coordination is key.

Q: Do I need an SPD on a sub-distribution board?
A: Often, yes. If the downstream board is more than 10 metres from the origin SPD, or it feeds sensitive equipment, an additional coordinated SPD may be needed at that board.

Final Thoughts

Installing SPDs in the right location isn’t just good practice - it’s a regulatory and performance must.

By placing surge protection as close to the origin as possible, understanding cable length risks, and coordinating with RCBOs or RCDs, you’ll keep your clients' kit safe, your installs compliant, and your call-backs to a minimum.

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