New CU question.

[SUNRAY]

Why do you say that out of interest? I note the cables are less than 50cm

And what makes you imagine 50cm has any relevance to a SPD?

 

[AndyPRK]

Manufactures state and combined length of the L and E cable to spd should not be more than 50cm

 

[JohnW2]

Manufactures state and combined length of the L and E cable to spd should not be more than 50cm

From where? Do they also state the required CSA of those conductors?

Kind Regards, John

 

[SUNRAY]

Manufactures state and combined length of the L and E cable to spd should not be more than 50cm

Yes I know some do but any one with any radio knowledge will instantly know it's a pile of irrelevant s***e

From where? Do they also state the required CSA of those conductors?

Kind Regards, John

The gauge is almost a secondary consideration when when dealing with transients, but obviously the thicker the better to deal with the potentially high voltages and hence currents involved.

 

[AndyPRK]

I thought the idea was to get it as close to the supply as possible, to ensure it can assist with the surge, rather than it go though the equipment you are trying to protect

 

[SUNRAY]

I thought the idea was to get it as close to the supply as possible, to ensure it can assist with the surge, rather than it go though the equipment you are trying to protect

Totally correct, as I mentioned before the professional way of doing the job is to terminate the feed into the SPD and then take the load (CU) from the SPD. The idea of botching the SPD onto the final part of the busbar, (after all other circuits take the hit first) is a ludicrouss schoolboy error. No I'll step back from that it demonstrates an almost total disregard of R&D and existing approved methods/procedures and certainly an ignorance of how current flows in a wire conductor.

 

[JohnW2]

I thought the idea was to get it as close to the supply as possible, to ensure it can assist with the surge, rather than it go though the equipment you are trying to protect

I would think the same, which is why I asked you where the 50cm quoted is "measured from".

I doubt that many SPDs are installed within 50cm of the point closest to the origin of the installation to which the consumer has access (i.e. the output terminals of meter), yet to measure (the 50cm" or whatever) from some arbitrary point (such as a CU's Main Switch) would seem pretty meaningless.

Kind Regards, John

 

[JohnW2]

The gauge is almost a secondary consideration when when dealing with transients, but obviously the thicker the better to deal with the potentially high voltages and hence currents involved.

Since it's Friday evening, that comment moves me to think a little laterally.

If the concern were about high frequency (rapid rise-time) transients, then, rather than try to address them in a non-ideal way with a 'shunt surge protector", might it not be better to have a requirement for a minimum (possibly quite long!) cable length between origin and any final circuits - or, even better, a requirement for a series inductor in the supply cable?

Kind Regards, John

 

[SUNRAY]

that comment moves me to think a little laterally.


Kind Regards, John

It seems my job is done...

 

[AndyPRK]

I believe it’s measured from the Mcb to the earth bar. Where the reduction in csa occurs. Looks like 6mm cable.

 

[JohnW2]

I believe it’s measured from the Mcb to the earth bar. Where the reduction in csa occurs. Looks like 6mm cable.

If that's the case, it would sound pretty meaningless and non-helpful. In addition to everything else, as Sunray has said, CSA (hence a reduction therein) is not a major issue in relation to transients.

Kind Regards, John

 

[scousespark]

Just read through this thread and have some observations

1. There are 3 types of SPD. (1 is used for lightning protection, 2 is used to protect sensitive electrical and electronic equipment, 3 is used to protect safety critical equipment). They all provide a route to earth when a transient overvoltage (a spike in the supply which can damage the installation when it is not detected). The one in the photo is a type 2, which is there to protect your electrical equipment if a spike occurs. This may be caused by a lightning strike hitting ground near a supply conductor. This voltage would be high enough to damage equipment (such as smart tvs, computers). Note that a direct lightning strike will take out pretty much in the installation. When you add up the cost of equipment (TVS, computers, all you IOT stuff, Sky/BT, routers and wif-fi stuff) it gets high quickly. The cost of an SPD is around the £100 mark. You then assess the risk of being hit by a spike, taking into account your environment and the potential impact of predicted climate change leading to extreme weather events. It's a yes/no decision to adopt SP or omit it.

2. The presence of the MCB is dictated by the manufacturer. Some require an MCB and others don't. The length of cable comes from BS7671, regulation 534.4.8 which states the total length of the cables (live + PE) should preferably be no longer than 0.5 metres, but should never be longer than 1 metre. I assume this is based on research to ensure the cables are not damaged during the spike.

3. The position of the MCB/SPD is less than ideal, but is not that bad. There are 2 probabilities here. The first is that a transient overvoltage occurs - the SPD clears the fault and no equipment is damaged. The second is that the second RCD has tripped when the transient overvoltage occurs - the SPD won't clear the fault, but the damage will be limited to circuits protected by the first RCD.

FYI, a number of manufacturers are now making consumer units with an SPD built in linked to the main switch. This addresses the cable length requirement and also ensures the whole board is covered by the SPD.

These are just my thoughts and feel free to criticise me. I've typed this in on my phone, so can't really tell how clear my post is.

I think the draft for the next amendment made SPD a mandatory requirement, may be wrong here. I deleted the docs a few weeks ago and somebody may still have a copy.

 

[AndyPRK]

Thank you for the sensible post

 

[JohnW2]

1. There are 3 types of SPD. (1 is used for lightning protection, 2 is used to protect sensitive electrical and electronic equipment, 3 is used to protect safety critical equipment). They all provide a route to earth when a transient overvoltage (a spike in the supply which can damage the installation when it is not detected).

Good to see you, and I hope all is well with you and yours.

Indeed - and I assume (some contributors have actually said this) that we are taking primarily about Type 2 ones.

2. The presence of the MCB is dictated by the manufacturer. Some require an MCB and others don't.

Fair enough. Do you know whether that means that some have built-in over-current protection and others don't, rather than that some manufacturers are happy to rely on protection by the cutout fuse, but others aren't?

The length of cable comes from BS7671, regulation 534.4.8 which states the total length of the cables (live + PE) should preferably be no longer than 0.5 metres, but should never be longer than 1 metre. I assume this is based on research to ensure the cables are not damaged during the spike.

As I've asked in this thread more than once, where is this 0.5m (or 1m) measured from? To measure it from the point of incoming supply to a CU/DB would be completely arbitrary, given that that place could be quite distant from the origin of the installation, is very commonly more than 0.5m, and not infrequently >1m. On the other hand, if one wanted it to be within 0.5m of the most upstream consumer-accessible part of the installation (the output terminals of the meter) that would in many/most cases mean that the SPD would have to be a standalone item, external to the CU/DB

I assume this is based on research to ensure the cables are not damaged during the spike.

I somewhat struggle to understand that. The length of a cable does not significantly influence its ability to survive high currents without damage - and, in fact, if anything the longer the conductors (hence higher their impedance) the (slightly) lower will be any current. I cannot think of any situation in which a shorter conductor would be more able to survive (without damage) a particular current than could a longer one - can you?

3. The position of the MCB/SPD is less than ideal, but is not that bad. There are 2 probabilities here. The first is that a transient overvoltage occurs - the SPD clears the fault and no equipment is damaged. The second is that the second RCD has tripped when the transient overvoltage occurs - the SPD won't clear the fault, but the damage will be limited to circuits protected by the first RCD.

Agreed.

FYI, a number of manufacturers are now making consumer units with an SPD built in linked to the main switch. This addresses the cable length requirement and also ensures the whole board is covered by the SPD.

As above, it only "addresses the cable length requirement" if that length is to be measured from the input of the CU - which, as I said is, in relation to the installation as a whole, merely an arbitrary location, hence seemingly leading to a meaningless requirement.

I think the draft for the next amendment made SPD a mandatory requirement, may be wrong here. I deleted the docs a few weeks ago and somebody may still have a copy.

I still have my copy of the DPC somewhere - I'll try to find it this evening.

Kind Regards, John

 

[AndyPRK]

The length is not to protect the cable, but to ensure the spd is able to do its job.