Would a SPD within a CU at the origin of an installation cover other CU at the origin from surges too?

Depends how they were connected - the benefit of an SPD will decline with longer connecting leads (i.e. away from the direct path from surge source to equipment to be protected) - usually 0.5m total is recommended, so if the total (of both L & N) from the SPD to the point where the other CU is connected (e.g. > 200mm to the Henley blocks) I'd say probably not adequately.

   - Andy.

Thanks Andy. Is that in the scenario where the SPD is in the CU further away than the CU without an SPD? If the SPD is in a CU nearest the origin and installed correctly less than 0.5m then this SPD will redirect the surge before it gets to the CU further away?

Agreed.

If you had a double-decker CU, a single SPD could be used. So if instead, the CUs are side-by-side, there is no reason in principle why a single SPD should not suffice, but the cables must still be short enough.

Jimi's second posting suggests that the second CU is supplied by a distribution circuit. My understanding is that an SPD at CU1 protects downstream circuits, but if the distribution circuit is too long (> 20 m?) another SPD may be required.

There will be some degree of spike reduction in almost any wiring arrangement with an SPD near when  compared to having no SPD, but its not a black and white thing, If you have the choice the suppression effect is best when the SPD iis nearer the source of the spikes you want suppressing.

As the current spikes have very fast rise times compared to 50Hz sinewave, the inductance of the wiring becomes far more important, hence the advice for short leads, and although the advice does not say so,  bringing the lines that carry the surge current through the SPD into close proximity so the magnetic field is partly cancelled, also helps, so cable or even a twisted bundle is better compared to a wide spaced loop of  singles.

Mike.

As per wiring regulation 534.4.1.1

Where SPDs are required:

• SPDs installed at the origin of the electrical installation shall be Type 1, Type 2 or type 1-2
• SPDs installed close to sensitive equipment to further protect against switching transients originating within the building shall be Type 2 or Type 3

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.