That does look  bit cheap - however, as the calculation above shows, it will still pass the test spec of the 8us/20us  at 20kA, probably with a bit in hand. I imagine there has been a design review and a decision to do some 'value engineering' and save a few pence.

In terms of SPD function it would be better if the links were short and fat, and routed together in a way that their magnetic fields cancelled (*), as this would give the lowest overshoot voltage during the surge. (well not quite, the lowest would be if the mains came in as a piar to the SPD, and then out again from the SPD, but a short 'T branch' is unavoidable in most designs.)

Looking at that, is the SPD on the incoming or outgoing side of the switch ? - I'd expect out going and I presume this is in a box on its own, not in a CU where the bus bar is at the bottom.


No prob with the calculation - it highlights something I first realised when looking at a RADAR designs, that the short pulse ratings of components  can be thousands of times the steady state rating - when limited by thermal considerations, and shorter more peaky pulses are permitted, until we become limited by voltage breakdown, or some other 'near instant' effect, then this scaling up can no longer apply.


Regards Mike


* Aside. If you use a clamp meter as a measure of the  magnetic field around a wire,. you can compare the effect of looking at the live only on a wire feeding a load, or the neutral wire only, or clamping around the whole cable , where suddenly you only see the L-N difference current, as the two fields essentially cancel.out  - energy wasted in filling the space with magnetic fields is the inductive part of the voltage drop, which we can reduce quite noticeably by keeping flow and return currents together in this way.

At 50Hz it only really matters on kA mains wired in singles.

When folk use scope probes to measure microsecond events and find that there is high frequency ringing that changes with how close the earthing wire is brought to the place the probe tip connects, this is the same effect at work at smaller scale.

Where it really matters a lot, ultra low inductance lines can be made of  either coaxial cables with very chunky centres , or if we need extra capacitance, as we might on power supplies to heavy RF devices, then interleaved flat bars of alternating polarity, or on a PCB alternate planes of copper.

That does look  bit cheap - however, as the calculation above shows, it will still pass the test spec of the 8us/20us  at 20kA, probably with a bit in hand. I imagine there has been a design review and a decision to do some 'value engineering' and save a few pence.

In terms of SPD function it would be better if the links were short and fat, and routed together in a way that their magnetic fields cancelled (*), as this would give the lowest overshoot voltage during the surge. (well not quite, the lowest would be if the mains came in as a piar to the SPD, and then out again from the SPD, but a short 'T branch' is unavoidable in most designs.)

Looking at that, is the SPD on the incoming or outgoing side of the switch ? - I'd expect out going and I presume this is in a box on its own, not in a CU where the bus bar is at the bottom.


No prob with the calculation - it highlights something I first realised when looking at a RADAR designs, that the short pulse ratings of components  can be thousands of times the steady state rating - when limited by thermal considerations, and shorter more peaky pulses are permitted, until we become limited by voltage breakdown, or some other 'near instant' effect, then this scaling up can no longer apply.


Regards Mike


* Aside. If you use a clamp meter as a measure of the  magnetic field around a wire,. you can compare the effect of looking at the live only on a wire feeding a load, or the neutral wire only, or clamping around the whole cable , where suddenly you only see the L-N difference current, as the two fields essentially cancel.out  - energy wasted in filling the space with magnetic fields is the inductive part of the voltage drop, which we can reduce quite noticeably by keeping flow and return currents together in this way.

At 50Hz it only really matters on kA mains wired in singles.

When folk use scope probes to measure microsecond events and find that there is high frequency ringing that changes with how close the earthing wire is brought to the place the probe tip connects, this is the same effect at work at smaller scale.

Where it really matters a lot, ultra low inductance lines can be made of  either coaxial cables with very chunky centres , or if we need extra capacitance, as we might on power supplies to heavy RF devices, then interleaved flat bars of alternating polarity, or on a PCB alternate planes of copper.