Calculating Iimp for DC SPD in PV

Ian Cheek said:

I've contacted one such manufacturer who offer the 6.25KA, and they are looking into it to see why they on paper seem to offer a product that does not comply with BS7671.

The Regulation you specify is particular to SPDs on the DC side. It does not mandate Type 1 SPDs, but says what you need to do if it has been determined that you need one. PD CLC/TS 51643-32 in Annex A provides a much-reduced approach to that in BS EN 62305 series for determining I_imp.

In most cases where Type 1 SPDs are necessary, BS 7671 alone cannot be used and BS EN 62305 applies ... see Note 1 to Regulation 443.1.1. The decision on whether you need SPDs on the DC side is covered by 712.433.101. SPDs on the AC side is 'general rules' only (Sections 443 and 534).

In Regulation 712.534.102.6, it should also be noted that BS 7671 does not specifically mandate I_imp of 12.5 kA, just says that you should use this if you don't calculate an alternative value ... but that only applies if you have determined you need the Type 1, which invokes other standards anyway. BS 7671 cannot replicate the contents of other standards.

Usually, Type 1 SPDs are required where there is a risk of imported direct stroke current/voltage (e.g. overhead line feeding installation, or installation is on a building with LPS ... or perhaps a large ground mount system connected through transformer to HV with overheads, etc.).

Ian Cheek said:

I've contacted one such manufacturer who offer the 6.25KA, and they are looking into it to see why they on paper seem to offer a product that does not comply with BS7671.

BS 7671 doesn't appear to say you have to use  I_imp of 12.5 kA, or even specifically mandate a Type 1 ... see above.

Well you may find a quick look at the pictures in this brochure helpful to decide which situation you are in , in terms of do you have a lightning protection system, is it spread out so you need multiple arrestors etc. Note that the impulse rating of a surge arrester very much depends on which surge waveform is under consideration (peak current is not the only variable, unsurprisingly impulse duration has a big effect as does rate of rise..)
So the same device may be both 20kA, 12kA and perhaps something else as well when characterized with different standard tests.

regards mike.
PS Example snippet from a datasheet showing what I mean

Hi

It will be Type 1, I've yet to come across an installation were type 1 is not required. So apologise for not clarifying this in my question.

I've yet to come across an installation were type 1 is not required.

That's interesting. Just checking my understanding here ... I was under the impression that for a typical (say domestic) roof mounted PV system a type 2 would be sufficient and type 1 would only be necessary where the building had a lightning protection system or the array was large and vulnerable to lightning - e.g. field systems covering large areas.

   - Andy.

AJJewsbury said:

I was under the impression that for a typical (say domestic) roof mounted PV system a type 2 would be sufficient and type 1 would only be necessary where the building had a lightning protection system or the array was large and vulnerable to lightning - e.g. field systems covering large areas.

That's generally true, although there is the anomaly of an installation supplied by LV overhead lines.

With solar farms, many will have LV-HV transformers with direct connection to HV overhead lines, so Type 1 probably commonly-required even if you ignore the array frames. Industrial on metal-clad steel frame structure is also perhaps a candidate for Type 1 ?

I work in the commercial sector rather than domestic. So all the building have LPS fitted. Even if it doesn't I always invoke 534.4.1.4 "does not require protection against the effect of direct lightning", this has to be proved, as 132.16  the installation has to be suitable now not when it was constructed.

Ian Cheek said:

Even if it doesn't I always invoke 534.4.1.4 "does not require protection against the effect of direct lightning",

Agreed ... direct lightning not directly in the scope of BS 7671, although Section 534 has guidance on selecting and implementing Type 1 SPDs in cases where they are specified.

The important part of BS 7671 is 443, Regulation 443.1.1, that states in no uncertain terms, that BS EN 62305 series should be used to assess the requirements for SPDs where LPS is used.

You can determine Iimp:

• Using BS EN 62305 (which is complicated)
• Using Annex A to PD CLC/TS 51643-32; or
• Using the value in 712