Correct SPD placement is critical on a three-phase system. Whether you're protecting a commercial panelboard, an EV charger supply, a warehouse distribution board or a three-phase heat pump, the SPD must be installed at the correct point in the system to ensure overvoltage protection actually works.
Quick Answer: On a three-phase installation, SPDs should be installed as close as possible to the origin of the supply or distribution board they protect. A Type 2 SPD is usually fitted at the main three-phase board, with additional SPDs installed on submains feeding EV chargers, machinery, or sensitive equipment. Cable length must be kept short to maintain let-through voltage and ensure full compliance with BS 7671.
You can browse all three-phase surge units here: 3-Phase Surge Protection Devices.
This guide explains exactly where to install SPDs on a three-phase system, which types you need, and how placement affects protection and compliance.
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Install the SPD at the Origin of the Three-Phase Supply
On a three-phase installation, the primary SPD belongs at the main distribution board where the supply first enters the building.
This device protects:
- The main panelboard
- Downstream DBs
- Machinery and sensitive loads
- Circuitry exposed to switching or transient surges
Most commercial and industrial three-phase systems use a 4-pole Type 2 SPD at the origin unless lightning protection or external overhead supplies require Type 1.
SPDs on Submains Feeding Other Boards
If a three-phase installation feeds secondary distribution boards, workshops, or remote areas of a site, the submain cable can pick up transient voltage spikes.
This is where additional SPDs are recommended:
- Three-phase board → remote DB
- Panelboard → commercial outbuilding
- Warehouse → mezzanine DB
- Main LV panel → machinery control board
Installing an SPD at each distribution level ensures voltage transients are suppressed locally and do not travel between boards.
SPDs for EV Chargers, Heat Pumps & Renewables
Three-phase EV chargers, heat pumps and inverter-based renewables are particularly sensitive to voltage spikes and almost always require SPDs either:
- At the main 3-phase DB feeding the equipment
- At the submain supplying the unit
- Inside a dedicated 3-phase SPD enclosure placed between DB and equipment
This prevents damage to:
- EV chargers with onboard electronics
- Three-phase heat pump inverter boards
- PVs with string inverters (additional DC SPD may be required)
- Commercial machinery with switching power supplies
SPD Placement Rules for Three-Phase Systems
Correct installation is as important as SPD selection. BS 7671 provides clear guidance on placement:
1. Keep cable length as short as possible
The total cable length between SPD → breaker → busbar should ideally be under 0.5 m. Longer cables increase let-through voltage and reduce protection.
2. Install the SPD on the supply side of sensitive equipment
Always place the SPD as close as possible to the equipment or distribution point being protected.
3. Use a dedicated MCB or RCBO
Many SPDs require a breaker for backup protection. Some units - including pre-assembled ones - come with this built in.
4. Use 4-pole SPDs on three-phase + neutral systems
This ensures full protection across all phase conductors and neutral.
Compliance note: Type 2 SPDs are the standard for most three-phase distribution boards under BS 7671 unless lightning protection or overhead supplies require Type 1 devices.
Recommended Three-Phase SPDs
Here are popular three-phase surge devices that meet 18th Edition requirements and suit commercial, industrial and EV installations:
- 3-Phase SPD with 8-Way Metal Enclosure + 40A MCB
- Pre-Assembled 3-Phase SPD Unit in Steel Enclosure (WME3P40SP)
- Three-Phase 4-Pole SPD – WSPD440
- 3-Phase SPD with 40A RCBO
View all three-phase SPDs: 3-Phase Surge Protection Devices
FAQs
Common questions installers ask when fitting SPDs on three-phase systems:
Do I need a Type 1 or Type 2 SPD?
Type 2 is standard for most three-phase boards. Type 1 is required if the building has a lightning protection system or receives an overhead supply.
Should each sub-board have its own SPD?
Yes - SPDs should protect each distribution level to stop surges travelling through submains.
Does cable length really matter for SPD installation?
Yes. Long cables increase let-through voltage, reducing protection performance.
Can I use a single-phase SPD on a three-phase system?
No - three-phase systems require 4-pole SPDs for full protection.
👉 Protect three-phase boards, EV chargers and commercial equipment: Browse all 3-Phase SPDs