How to Identify the Correct Replacement SPD

If an existing Surge Protection Device (SPD) needs replacing, it is important to identify the correct replacement rather than choosing a device based only on its appearance.

SPDs can look very similar while having different ratings, configurations and compatibility requirements.

1. Find the Manufacturer

Start by identifying the manufacturer of the existing SPD or consumer unit.

Common manufacturers include:

  • Wylex
  • Hager
  • Crabtree
  • Schneider Electric
  • FuseBox
  • Lewden
  • Contactum
  • Eaton

Using the same manufacturer is often the simplest starting point when looking for a replacement.

2. Find the Model or Part Number

Next, look for the model or part number printed on the front or side of the SPD.

This is one of the most useful pieces of information when searching for a replacement.

Do not rely solely on the appearance of the SPD. Manufacturers may produce several devices that use very similar housings but have different specifications.

If the part number is visible, search using the complete number rather than just the manufacturer name.

3. Check the SPD Type

Check whether the existing device is:

  • Type 1
  • Type 2
  • Type 3
  • Type 1 + Type 2

This may be displayed as T1, T2, T3 or T1+T2.

A replacement should not be selected simply because it physically fits. The required SPD type needs to be appropriate for the installation.

4. Check the Electrical Ratings

The front of an SPD normally contains several electrical ratings.

Common markings include:

Uc – Maximum Continuous Operating Voltage
The maximum voltage that can continuously be applied to the SPD.

Up – Voltage Protection Level
Indicates the level to which the SPD limits the transient voltage under specified test conditions.

In – Nominal Discharge Current
The discharge current the SPD is designed to withstand repeatedly under its specified test conditions.

Imax – Maximum Discharge Current
The maximum discharge current associated with Type 2 testing.

Iimp – Impulse Discharge Current
A rating commonly found on Type 1 SPDs relating to their ability to handle high-energy impulse currents.

These values should be considered when identifying an appropriate replacement.

5. Check the Number of Poles and Configuration

SPDs are available in different configurations depending on the electrical system they are designed to protect.

You may encounter:

  • Single-module devices
  • Multi-module devices
  • 1P+N configurations
  • Three-phase configurations
  • Three-phase plus neutral configurations

The replacement needs to be suitable for the electrical system and connection arrangement.

6. Check Whether the Cartridge Is Replaceable

Many SPDs use removable protection cartridges.

If the SPD has failed, it may therefore be possible to replace the cartridge rather than the complete SPD assembly.

Look for a cartridge that can be removed from the fixed base and check whether it has its own part number.

The cartridge and base should be treated as manufacturer-specific components unless the manufacturer explicitly confirms compatibility.

7. Check the Status Indicator

Many SPDs have a visual status window.

Typically:

Green = operational

Red = replacement required / protection no longer available

However, always check the manufacturer's information for the particular device.

A red indicator does not automatically mean the complete assembly must be removed. On SPDs with replaceable cartridges, only the affected cartridge may require replacement.

Can I Use a Different Brand of SPD?

A different manufacturer's SPD should not be treated as a direct replacement simply because it fits onto a DIN rail.

The device may have different:

  • Electrical ratings
  • Dimensions
  • Terminal positions
  • Connection requirements
  • Backup protection requirements
  • Cartridge designs
  • Manufacturer instructions

For an existing consumer unit or distribution board, check the manufacturer's compatibility and installation requirements before fitting an alternative device.

What Information Should I Have Before Buying a Replacement?

Before ordering an SPD, try to obtain:

  • Manufacturer
  • Full model or part number
  • SPD type
  • Uc rating
  • Up rating
  • In / Imax / Iimp ratings where applicable
  • Number of poles/modules
  • Photograph of the existing SPD
  • Photograph of the consumer unit or distribution board
  • Part number from the removable cartridge, if fitted

Having this information makes identifying the correct replacement considerably easier.

Can't Find Your Existing SPD?

Older, discontinued or manufacturer-specific SPDs can sometimes be difficult to source.

ReCircuit Resale stocks a range of new, surplus and discontinued SPDs, including components from major consumer unit and distribution board manufacturers.

If you cannot identify the replacement you need, the model number and clear photographs of the existing SPD are the best information to provide when making an enquiry.

Next Guide: When Does an SPD Need Replacing?