SPD kA Rating Explained — What Does 40kA Mean?

The kA rating on an SPD is its discharge current capacity — how much surge current it can handle without damage. 40kA (Imax) is the standard for most UK domestic and commercial Type 2 installations and provides adequate protection against the surges most installations will ever encounter.

When you look at an SPD datasheet or product description, you will see current ratings in kiloamperes (kA) — typically 20kA, 40kA, or higher for Type 1 devices. These ratings describe the device's ability to conduct and survive large surge currents, not its normal operating current. Understanding what these ratings mean helps you specify correctly and avoid over- or under-specifying protection.

The key rating parameters

Parameter	Applies to	What it means
In (nominal discharge current)	Type 2 SPDs	The current the device can safely discharge repeatedly — its normal working surge capacity. Typically 5–20kA for Type 2.
Imax (maximum discharge current)	Type 2 SPDs	The maximum single surge current the device can survive without being destroyed. Typically 20–40kA for Type 2. This is the headline kA figure.
Iimp (impulse current)	Type 1 SPDs	A different test waveform (10/350µs vs 8/20µs) representing direct lightning energy. Not directly comparable with Imax.
Up (protection level voltage)	All SPDs	The residual voltage across the SPD terminals at rated surge current. Lower is better — typically ≤1.5kV for Type 2.

Why 40kA is the standard for most installations

The vast majority of surge events reaching a UK domestic consumer unit via underground supply cables are indirect — induced by nearby lightning or grid switching events. The typical energy in these surges is well within the capacity of a 20kA device. However, 40kA has become the standard specification for several reasons:

The marginal cost difference between 20kA and 40kA devices is small
A 40kA device has double the headroom for unusual or repeated surge events
Specifying 40kA avoids the question of whether 20kA is sufficient in borderline cases
Most quality manufacturers, including WCED, standardise on 40kA for their Type 2 range

All WCED Type 2 SPDs (WSPD240, WSPD440, WSPDTE-2, WSPDTE-4, WSPDT2-CB) are rated at 40kA Imax.

Protection level voltage (Up) — the other critical rating

The kA rating tells you how much surge current the device can survive. The protection level voltage (Up) tells you how well it actually protects equipment — the lower the Up, the lower the residual voltage that reaches the connected equipment during a surge event. BS 7671 and BS EN 61643-11 specify maximum Up values based on the overvoltage category of the protected equipment. For most domestic and commercial equipment (Category II), a Up of ≤1.5kV is required. All WCED SPDs meet this requirement.

Shop WCED 40kA Type 2 SPDs

40kA Imax, Up ≤1.5kV — 2-pole, 4-pole, and Type 1+2 combined. In stock with next working day dispatch.

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Frequently Asked Questions

Is a 40kA SPD better than a 20kA SPD for domestic use?

For most domestic installations with underground supply cables, both provide adequate protection against typical surge events. 40kA gives a larger safety margin for unusual events and is the standard for new installations. The additional cost is minimal and the peace of mind significant — 40kA is the recommended specification.

Can I compare Iimp (Type 1) ratings with Imax (Type 2) ratings?

No — these use different test waveforms. The Iimp test waveform (10/350µs) contains far more energy than the Imax test waveform (8/20µs). A Type 1 device rated at 25kA Iimp handles much more energy than a Type 2 device rated at 40kA Imax. The two values are not directly comparable and should only be compared within their respective categories.

What does the protection level voltage (Up) mean in practice?

During a surge event, the SPD clamps the voltage to its Up value. If Up is 1.5kV, the maximum voltage seen by connected equipment during a rated surge event is 1.5kV. Equipment rated to withstand impulse voltages above 1.5kV (Category II — most domestic appliances and electronics) will survive. Equipment rated lower than the Up value could be damaged even with an SPD installed — this is why Type 3 point-of-use protection is sometimes also specified for very sensitive equipment.

Further reading:
Type 1 vs Type 2 vs Type 1+2 SPD explained
How to install an SPD
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