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SPD kA Rating Explained — What Does 40kA Mean?

Surge Protection

SPD kA Rating Explained: What Does 40kA Mean?

The kA figure on an SPD datasheet is its surge current capacity, not its everyday operating current. Here is how to read the ratings and why 40kA is the usual choice for UK homes and commercial sites.

Quick answer

The kA rating is how much surge current the device can survive. 40kA (Imax) is the standard for Type 2 SPDs in UK homes and commercial sites, and gives ample headroom for the surges most installations will meet.

The key rating parameters

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

Why 40kA is the standard

Most surges reaching a UK consumer unit through underground cables are indirect, induced by nearby lightning or grid switching. The energy in these is well within the capacity of a 20kA device. 40kA has still become the norm because:

  • the price difference between 20kA and 40kA devices is small
  • a 40kA device has twice the headroom for unusual or repeated surges
  • specifying 40kA avoids arguing over whether 20kA is enough in borderline cases
  • most quality manufacturers, WCED included, standardise on 40kA for Type 2

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

Protection level (Up): the other critical rating

The kA rating says how much surge the SPD can survive. Up says how well it protects. The lower the Up, the lower the voltage that reaches your equipment during a surge.

Up needs to sit below the impulse withstand voltage of the equipment being protected. BS 7671 groups equipment by overvoltage category: most appliances and fixed equipment (Category II) withstand 2.5kV, while sensitive electronics (Category I) withstand 1.5kV. A Type 2 SPD with an Up of 1.5kV or lower leaves a comfortable margin for Category II equipment. All WCED SPDs are rated Up ≤1.5kV.

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 a home?

For most homes with underground supply cables, both give adequate protection against typical surges. 40kA gives a bigger safety margin for unusual events and is the standard for new installations. The extra cost is small, so 40kA is the recommended specification.

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

No. They use different test waveforms. The Iimp waveform (10/350µs) carries far more energy than the Imax waveform (8/20µs), so a Type 1 device at 25kA Iimp handles much more energy than a Type 2 at 40kA Imax. Only compare ratings within the same category.

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

During a surge the SPD clamps the voltage to its Up value. With an Up of 1.5kV, equipment that withstands more than 1.5kV will survive a rated surge. Very sensitive equipment rated below the Up may still be at risk, which is why Type 3 point-of-use protection is sometimes added for delicate electronics.

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