The £1300 AFDD consumer unit

Graham the 443 requirement to fit SPDs is based on the lightning chart and a "magic" formula. True it adds "transients due to switching" as an extra justification, but I am afraid I am far from convinced that they are necessary to protect electronic devices. Induced lightning current in an underground cable is very small, as the conductor spacing is tiny, so why is its length in the formula?


There is an enormous quantity of electronics connected directly to the Mains supply at this time and I am not convinced that there is a significant failure rate due to "Mains transients". Take all the VFDs and Solar inverters in the country, do these regularly fail during thunderstorms? You might say that an extra £50-100 for a bit of insurance is good value, as a cost-benefit exercise, but the question is then"does this guarantee against failure"? I think there is a problem with the theory behind VDR protection here, which I expressed above, and to work properly some series impedance is required unless the device dV/dI slope is essentially zero, like a Zener diode. The clipping action is extremely dependant on the transient energy, and I have yet to see a graph of the protection offered by the various devices on sale, which is curious.

Graham the 443 requirement to fit SPDs is based on the lightning chart and a "magic" formula. True it adds "transients due to switching" as an extra justification, but I am afraid I am far from convinced that they are necessary to protect electronic devices. Induced lightning current in an underground cable is very small, as the conductor spacing is tiny, so why is its length in the formula?


There is an enormous quantity of electronics connected directly to the Mains supply at this time and I am not convinced that there is a significant failure rate due to "Mains transients". Take all the VFDs and Solar inverters in the country, do these regularly fail during thunderstorms? You might say that an extra £50-100 for a bit of insurance is good value, as a cost-benefit exercise, but the question is then"does this guarantee against failure"? I think there is a problem with the theory behind VDR protection here, which I expressed above, and to work properly some series impedance is required unless the device dV/dI slope is essentially zero, like a Zener diode. The clipping action is extremely dependant on the transient energy, and I have yet to see a graph of the protection offered by the various devices on sale, which is curious.