Just to clarify, twisting wires does provide other advantages. Twisting the signal and return wires in a communications cable reduces the crosstalk between adjacent pairs, reduces interference from noise sources and it is also used to control the impedance of the cable.

This is correct. The intent is to reduce the area enclosed between the two conductors as much as possible as this reduces the inductance. Laying the two conductors side-by-side achieves this area reduction as effectively as twisting the conductors without the additional wire length introduced by twisting.

I also think the same, it will be subtracted from what is applied on SPD, hence SPD will not reduce the resistance.

Length between the SPD and the main earth terminal of the installation. That might be the earth bar in a consumer unit or distribution board, or it could be a separate bar/block located somewhere else.

I would say that it's impossible not to class one of these as a "similar switchgear assembly", and therefore yes, 421.1.201 calls for a non-combustible enclosure.

Also interesting (?) is a what if I've been thinking about. What if you have one of these, so you no longer need a main switch in your CU, and let's say it's an all RCBO one so it has no RCCB either. With no incomer, how do you connect L to the busbar?

@@anonnona8099 Switch off the Isolator in the DNO meter cupboard

@@Mike_5 1) There might not be one. 2) It's irrelevant - that's an answer relating to isolation and safe working. My question is there you are, with tails entering your CU. How do you connect the L to the busbar?

@@anonnona8099 Bellend. Do your homework properly ...

@@Mike_5 Thank you for replying to me with the same level of courtesy as I have shown to you. You are a credit to the human race.

In a consumer unit, it's just a link from the earth bar in the consumer unit to one of the earth terminals on the SPD.

@@jwflame Thanks John

Some have a fuse built in, others require a separate MCB. If built in, it's wired as in this video. For the others, L goes to a suitable MCB first, then from the MCB to the surge protection device. N*E are always directly connected. Type and rating of the MCB will be specified by the manufacturer.

Both E terminals are connected together, it makes no difference which one is used. The differences for SPDs are between TT and TN systems, some can be used for both, others TN only. If it's suitable for TN-S, it will also be suitable for TN-C-S.

@@jwflame thanks for the help John.

The volt drop due to inductance occurs when the SPD is conducting, the current through the SPD and the connecting wires is very high. With no SPD, the voltage will be high but the current low, so any volt drop will be insignificant, with most of the voltage across devices within the installation which will cause damage.

Very little, if installed properly the MCB will only add a few centimeters of length to the wiring,. Although MCBs have a magnetic coil inside, for the ratings typically used with SPDs it's only a few turns of very thick wire.

One for the whole house for normal sized houses. Large buildings will require more than one.

No, these only work for overvoltages. They won't do anything for voltage sag/dip. However they do reduce the overvoltage spikes created when heavy loads are switched off.

@@jwflame thanks for the clarification

@@jwflame are there any products available that would address voltage sag/dip caused by internal switching events? I’ve heard of things like “voltage conditioners” and “voltage stabilizers” not sure if that’s what they do.

Doesn't matter what the earth resistance is for the SPD. The SPD connects to the existing circuit, not to a separate ground.

@@jwflame If an equipment is grounded and someone touches will be get electrocated? 2.why we do system grounding ? 3.the earth is not a perfect conductor does current return back to it's source?

Did you find out, because I have a similar need to interchange one phase and neutral in a 4 pole SPD?

That's what I was thinking too.

exactly. The metal consumer unit thing is just rubbish

Because when everyone has largely got metal they can change their minds again and have some fire retardant plastic with glass fibre reinforcement as a rule - just because.

Best me to it! One rule and logic for one then the opposite for another.

Because it doesn't have any load in it during normal operation. It's a voltage driven device installed in parallel. It fails to open circuit and will quench any transient in nano-seconds so no time for any thermal rise. The correct SPD installed in the right place in the right way poses no fire risk. That's why they can be in plastic enclosures.

That Wylex item is designed to replace any existing isolator. Alternatively the SPD could be installed in the consumer unit.

@@jwflame Wow! Thank you for replying :-) I have been intently watching a lot of your videos as someone whom I trust... If I were to split the tails after the isolator (in "Henley Blocks") to feed two consumer units, would this be sufficient to cover both CUs or would you recommend installing one in each CU? Thanks.

Both earth terminals are connected together in the device, it doesn't matter if there is one or two, only that earth is connected to it.

This has been bothering me as well. The HF transient does not originate 50 cm away from the SPD. When the SPD operates, current will flow from the source of the transient to the MOV. This could be a lightning strike 500 m away. Could it be inductance and capacitance in the long cables causing resonance to occur? Have you come across an explanation yet?

It very much depends on the SPD manufacturer and the size of your incoming supply, in almost all domestic applications you do not need any OCPD for an SPD, in larger industrial /commercial sites you probably will need OCPD. It's worth remembering that SPD's are designed to deal with transients, very high frequency short events so the reason for the OCPD is to protect the SPD from potential long term over voltage or high current events.

BS7671, list of suppliers here: electrical.theiet.org/bs-7671/fake-awareness/

The main factor when the SPD deals with a surge (transient) is the inductance of the wires. Length of those wires will change the inductance significantly, wire size has a very small effect.

Thanks. Never thought inductance was such a key player.

Actually, I've just realised that the N-E combined wiring length for the SPD on these split-load boards must be the best part of 1 metre (assuming the 16 mm² main PE coming in to the CU is connected to the main earthing bar at the closest position to PE supplied for the SPD). See 1drv.ms/u/s!AqBvF7glEgubgshh8kcqCXblGSINYg where I've tried to scale the image to the dimensions of the CU and then measured some lines.

No. It's being pushed by manufacturers to sell more over-priced widgets.

Tim Boler It’s being made explicitly allowed because everything now has electronics in. We used to have incandescent bulbs for lighting, AC synchronous motors running clockwork in our appliances (oven clock, central heating timer, etc), and bimetallic strips regulating temperature (thermostats in your heating, oven, fridge, freezer), etc; none of which were that bothered by a momentary power surges. Now we have LED lights with electronic drivers, and electronic powers supplies and controls in our appliances. Electronics don’t like these kinds of voltages, the insulator layers within an integrated circuit are very thin (more than thick enough for the normal conditions), a component might not totally fail the first time it is hit by a surge, but it creates a weak spot, where subsequent surges will cause further damage. It’s usually an accumulation which causes the final failure, not an obvious event like a near by lightening strike. SPDs are designed to reduce those surges to a level where the electronics can tolerate the remaining energy without being damaged.

I'd bet more on insurance companies etc. wanting more protection for sensitive electronics. Always follow the money.

@@Bin216 Maybe a problem if you live in a rural area where the incoming supply is fed via an overhead line susceptible to lightning strikes, but for most houses fed by underground cable, I'm not sure.

@@benbaselet2026 My line is IT, especially data centre management. We put significant amounts of time, effort and money into securing our power supplies because a single incident could potentially cost many millions. However this is just guarding against an expensive but very remote possibility. Most European data centres are on urban (so usually underground) and notably stable supplies. In 25 years I've not seen such an incident, not even one caught on monitoring systems that did not reach critical equipment. So I would be genuinely very surprised if this figured very high in insurance companies' calculations. For the moment I remain sceptical - even cynical - about the merits of domestic surge protection. They seem to be even less value than the arc fault protection devices that are also being heavily promoted by the manufacturers. If you're following the money I'd say it's there that the trail leads.

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