Would a SPD within a CU at the origin of an installation cover other CU at the origin from surges too? (If the CU with the SPD was the nearest to the mains). What if the CU with the SPD wasn't the closest to the mains? Thanks.
Would a SPD within a CU at the origin of an installation cover other CU at the origin from surges too? (If the CU with the SPD was the nearest to the mains). What if the CU with the SPD wasn't the closest to the mains? Thanks.
As per wiring regulation 534.4.1.1
Where SPDs are required:
By having the SPD in furthest CU the first of the two bullet points above has not been meet all though the surge protection is installed, I would say having surge in the furthest CU would no offer and guarantee of protection to the closest CU and certainly not offer any protection against surges originating from electrical circuits supplied from the closest CU.
G
As per wiring regulation 534.4.1.1
Where SPDs are required:
By having the SPD in furthest CU the first of the two bullet points above has not been meet all though the surge protection is installed, I would say having surge in the furthest CU would no offer and guarantee of protection to the closest CU and certainly not offer any protection against surges originating from electrical circuits supplied from the closest CU.
G
It maybe worthwhile to set aside the letter of the regs for a moment and visualise where the currents go, and that will explain what the reg is trying to achieve.
When the voltage across the SPD exceeds its trigger level, in a few tens of nanoseconds the near insulator becomes more like a very low resistance, and draws a lot of current so the spike voltage is then dropped along the wiring between the SPD and the place the spikes are coming from. This is just a classic voltage drop problem - except it is a transient, and all so fast that that the drop in the wiring is predominantly inductive and not resistive.
Another way of thinking about this is that once the SPD has triggered, the fact that it has done so takes time to travel along the wire - at about the speed of light, so a foot per nanosecond, or 1000ft~ 300metres per microsecond imagine that 'oh there is a short circuit at the far end, better send in some more electrons' info rippling along the wire like one of those cascade lines of dominos )
Clearly moving along that sloping voltage gradient, there is a full height unsuppressed full voltage spike where it joins the system, and a fully clipped one at the terminals of the SPD, and a sort of voltage ramp down the live and up the neutral in the wiring in between.
If the two CUs are side by side, then the SPD can be in either, or even in a small box to one side of both of them, and suppression will work just fine. The problems arise as you separate them - by the time they are a few feet apart, kit on the spikey side of the SPD see a more damaging over-voltage than stuff nearer and maybe a short pulse at high amplitude does damage before the current pulse coming back from the SPD starts to short it out.
I appreciate that this is probably a bit of a 'hang upside down with wet cloth around head' way to think about it, especially for those used to circuits where the normal assumption is that the current is the same magnitude and direction all along the wire at any one time, but at these speeds it really isn't, and it really matters.
Mike.
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