SPD's - are they required or just nice to have?

[ericmark]

I have sat considering where we are likely to get a surge from which the surge protection built into the appliance will not protect from, but in the consumer unit it will, the size of the unit in the consumer unit is unlikely to protect from a lighting strike, neither is it likely to absorb enough energy to rupture the DNO fuse, and loss of PEN and resulting high voltages is not really a surge, so about the only thing I can see has changed is micro generation.

So can micro generation produce a surge? I think unlikely as there is so much included in the grid tie inverters to insure should the supply be lost, it will fail safe, however there is clearly a time period between a DNO supply failure and the micro generator inverters closing down, however the consideration seems to be over head lines, not micro generation when considering inclusion of SPD's.

The same problem as earth rods with TN-C-S also exists with SPD's, if every one from one transformer has either personal earth rods or SPD's then if required the energy dissipated is shared between all the homes on that supply. So with a new build estate great, but unless shared then the energy dissipated by one device is simply not enough.

I saw the result of a radio hams shack supply using a 4 mm² earth cable from the CU to shack when the PEN was lost, little balls of copper, OK with a 65Ω earth rod no problem, but the 4 mm² was bonded to the earth bar in the shack which was connected to 4 earth rods with bare copper tape between them for the radio transmitter so was better than my meter could measure, I would say less than 1Ω. He had realised there was some thing wrong, turned off main isolator to house, and called me on 2 meter as this had not stopped the earth wire melting.

It was caused by some road works, and the DNO said it was impossible, but responded very quick, and the radio ham was only person in street not to loose equipment, since he had turned off the main isolator before his wire burnt out. Why the person wiring his shack had used twin core SWA and not used the SWA as earth I don't know.

But had the supply been PME and actually had multiple earth rods it could not have happened, but TN-C-S is not always PME.

So are we being asked to install earth rods and SPD because those who should be fitting them the DNO and micro generators are not fitting them?

 

[JohnW2]

But had the supply been PME and actually had multiple earth rods it could not have happened, but TN-C-S is not always PME.

My understanding is that, in the UK (and quite probably most/all other places that allow TN-C-S), TN-C-S is only allowed if it has "PME".

However, that appears to mean very little. I had originally presumed that the "M" of "PME" meant what it says, and that there would be 'multiple' (which I took to mean 'several', distributed along the length of the supply cable) earth connections to the CNE/PEN conductor.

However, the late-lamented westie disillusioned me about that, by telling us that "PMEing" a supply often meant no more than adding one extra earth, at the far end of the CNE/PEN - so that there would only be two earths in total - one at the transformer and the other at the far end of the cable. I would not personally call that 'multiple', although I suppose the word is not (literally) incorrect!

As you imply, it would be different (better) if every supplied premises had a TT rod - and that really would be what I would could 'multiple earths'! Given that that nearly got into 18th ed., I suspect it might not be all that long because it becomes a 'requirement' - although, as has been discussed, implementing it for existing installations would be problematical, since it would really be desirable that all earth rods connected to one CNE/PEN were installed 'simultaneously.

Kind Regards, John

 

[ericmark]

If the DNO fit the earth rods, then they can be fitted simultaneously, so that would seem the way forward.

 

[SimonH2]

I have sat considering where we are likely to get a surge from which the surge protection built into the appliance will not protect from, but in the consumer unit it will, the size of the unit in the consumer unit is unlikely to protect from a lighting strike, neither is it likely to absorb enough energy to rupture the DNO fuse, and loss of PEN and resulting high voltages is not really a surge

Indeed. It would have to be mostly differential to kill equipment, it would need to be within the capability of the SPD to absorb it, but more than the ability of the end equipment to absorb it after adding the extra impedance of a length of cable - which is very significant for the fast rise time pulses induced by lightning.

So can micro generation produce a surge? I think unlikely as there is so much included in the grid tie inverters to insure should the supply be lost, it will fail safe, however there is clearly a time period between a DNO supply failure and the micro generator inverters closing down

On over-voltage, I'd expect the inverters to shut down "very quickly" - that's so easy to detect, and it's not like there's a bit rotating machine with a big inductor of a field winding to make it slow to shut down. In any case, I don't think we're looking at something with masses of droop that's going to spring up it's output voltage several fold on loss of load.

As an aside, at a previous job, one of our clients put in an 89kW water turbine. The people putting it in were <coughs> "a bit lacking in networking basics" resulting in their system killed the phones if connected to the network - IP clash even though I'd given them an IP to use - they had 3 bits of kit and only asked for one IP, so picked 2 more out of their backsides. So I got a visit to sort that out, and see the system running - not the first I've seen BTW. One of the safety features these have is a diverter flap which normally does nothing - but in the event of an overspeed which would happen if the load was lost, the flap very quickly diverts the jet off the turbine wheel to stop the machine self-destructing before the control valve can close.

I saw the result of a radio hams shack supply using a 4 mm² earth cable from the CU to shack when the PEN was lost, little balls of copper, OK with a 65Ω earth rod no problem, but the 4 mm² was bonded to the earth bar in the shack which was connected to 4 earth rods with bare copper tape between them for the radio transmitter so was better than my meter could measure, I would say less than 1Ω.

There must have been a heck of an unbalanced load. Checking the tables, a single core thermosetting plastic insulated cable is rated for 40+A (depending on installation method) and MICC at over 50A - so to actually melt the copper there must have been (sticks finger in air) 100+A of load imbalance. Perhaps the digger managed to cut the PEN and one or two of the phases, but not the phase your mate was connected to - quite a trick to achieve

So are we being asked to install earth rods and SPD because those who should be fitting them the DNO and micro generators are not fitting them?

Earth rods - well any system with combined N&E is going to have a problem - even if the DBO earthed your PEN where you are tee'd off the service cable, you could still have a fault between there and your house.
SPDs, well I really don't know. As already discussed, they are of marginal benefit for most installations - but I don't think we can blame anyone for their need, I don't think embedded generation is at fault.

 

[SimonH2]

If the DNO fit the earth rods, then they can be fitted simultaneously, so that would seem the way forward.

Except that AFAICS there's no plan for the DNO to be fitting loads of earth rods - it's going to be individual properties fitting them as/when the regs come into play for them. New builds should be trivial since it would be easy to just drop some rebar in the foundations at negligible cost for a good earth. Retrofit is another matter.

 

[JohnW2]

If the DNO fit the earth rods, then they can be fitted simultaneously, so that would seem the way forward.

Indeed - but, as Simon has said, I don't think that there has ever been any suggestion that DNOs would fit them, except perhaps for new builds with new supplies.

We don't know what caused them to remove this requirement from the draft of BS7671:2018 'at the eleventh hour' - maybe it was representations from DNOs?

Kind Regards, John

 

[JohnW2]

... any system with combined N&E is going to have a problem - even if the DBO earthed your PEN where you are tee'd off the service cable, you could still have a fault between there and your house.

If the fault were literally in the cable solely supplying one's house, there wouldn't be any real problem, would there? Even if the installation's MET were bonded to an extremely-low-impedance extraneous-c-p, the bonding cable (assuming CSA as required for PME) would presumably not 'melt' on the basis of a single installation's load, would it?

As I understand it, the main potential problem with CNE faults is when they occur in a location which leaves the neutrals of several installations connected together, but not connected to the supply CNE. In that situation, if one of the installations has a very low impedance path to true earth [unusually low-impedance extraneous-CP of earth rod(s) ], then much of the total load of all the installations can go through the bonding conductor in the installation with the low impedance path to earth.

Kind Regards, John

 

[SimonH2]

If the fault were literally in the cable solely supplying one's house, there wouldn't be any real problem, would there?

Hmm, I see I didn't put down what was in my mind at the time. Lose the CNE to your house, and in the absence of any local earthing your installation "earth" will be pulled up to 240V via your own connected loads. Thus no matter how many earth points the DNO added to the system, you would not remove the risk of dangerous voltages in your home.
Yes, in theory it should be "safe" because all your extraneous conductive parts are bonded, but can you be sure you've not missed anything at all ?

As I understand it, the main potential problem with CNE faults is when they occur in a location which leaves the neutrals of several installations connected together, but not connected to the supply CNE. In that situation, if one of the installations has a very low impedance path to true earth [unusually low-impedance extraneous-CP of earth rod(s) ], then much of the total load of all the installations can go through the bonding conductor in the installation with the low impedance path to earth.

Indeed, that is the case.
But, I wonder how much current that would be in most cases - and hence why I was curious about the radio shack example above. In the general case of losing only the CNE, some of the loads will be balanced out - so even if you were earthing a whole housing estate (which you couldn't be), you might not actually be carrying a huge current. In the case of the radio shack with a very good earth, I can't help thinking that there must have been one or two broken phase conductors as well - otherwise how would you get 100+A of unbalanced 3P loads in what would presumably be a smallish group of houses affected by the broken CNE ?

Or do you get a situation where the houses on the lighter loaded phases all see an over-voltage, the equipment "pops" which reduces their loads, further increasing the over-voltage so more stuff pops - until you end up with only the houses on one phase which are powered vi your low impedance earth ? Hmm, no. If the earth is such that it'll pull that sort of current, it's going to hold the neutral "not too far" from the real neutral and you're not going to see that sort of effect.

 

[JohnW2]

Hmm, I see I didn't put down what was in my mind at the time. Lose the CNE to your house, and in the absence of any local earthing your installation "earth" will be pulled up to 240V via your own connected loads. Thus no matter how many earth points the DNO added to the system, you would not remove the risk of dangerous voltages in your home.

Well, it would not remove the risk of high voltages relative to true earth being present within the home but, as you go on to acknowledge, those voltages (relative to true earth) are not, in themselves, 'dangerous' if the house has been properly constituted as an equipotential zone.

Indeed, that is the case. ... But, I wonder how much current that would be in most cases - and hence why I was curious about the radio shack example above.

I suspect not an awful lot of current in the great majority of cases. It obviously depends upon the number of installations whose 'floating' neutrals are connected but, because even if the path to earth had zero impedance, the current would be restricted to the total instantaneous loads of the 'connected' installations - which, unless it happened at a time when everyone was taking (electric) showers, would probably be far less than many people would guess.

Also, as you elude to, in practice (for domestic installations), the loads of several ('neighbouring') installations which share a (broken) CNE are likely to be reasonably balanced across phases, so the net neutral current might be very low, even when the CNE conductor is intact. And then, of course, these postulated extremely low impedance incidental/extraneous paths to earth (or 'super earth rods'!) are probably pretty rare!

Kind Regards, John

 

[ericmark]

The radio ham example was extreme, most earth rods are more like 50Ω so likely no more that 5 amp, the problem is when gas pipe used to earth, 20 amp through a gas pipe may cause the flexible pipe to rupture, with sparks, and so ignite the gas, very important bonding is after and electrical isolation on the incoming gas pipe.

However we are looking as SPD's and the one I have is two modular wide, so not able to sink much energy without damage and connected between lives, line to neutral, so the question is what could that unit absorb which the equipments network could not handle? I can see the worry about the knock on effect, a spike if allowed to reach the equipment could kill a diode which in turn could mean there is a DC component being caused which in turn can freeze the RCD, however we have 6mA DC protection included in the [url=https://s3-eu-west-1.amazonaws.com/podpoint-website/PP-D-130012-14-Instalation-Guide-Solo.pdf] PodPoint [/URL]for charging electric cars, it would be better to include one of them to disconnect if over the 6 mA DC is present rather than using a SPD.

 

[SimonH2]

For full protection, SPD should have 3 elements connected L-E, N-E, and L-N; add additional 4 (2off, L-N & L-E) elements for 3P supplies. If you don't have those earthed elements then the protection is incomplete.
As I've mentioned previously, my personal experience has been of damage caused by a common mode surge where the equipment has another connection to a different system. So you need to try and clamp those common mode transients.

One factor that complicates things is PME. With the neutral being earthed, there's scope for common mode surges to be converted to differential - i.e. the surge on the neutral gets earthed leaving the surge on the line(s).

 

[JohnW2]

How many domestic installations have suffered damage from surges, excluding lightning. Are spd's a solution to an imaginary problem.

As you will be aware if you've been following this discussion, my thinking corresponds with those questions.

I very much doubt that any significant number of domestic installations, per se, will have been damaged by surges (as opposed to actual/nearby 'lightning strikes'), so nearly all of the arguments one sees for SPDs relate to the theoretical risk of damage to 'sensitive' electronic equipment - but, as I've said, even then one is hard-pressed to find many instances of such damage which was definitely caused by a 'surge' in the electricity supply.

I said early in this discussion, I think that, certainly as far as domestic installations are concerned, SPDs are "a solution looking for a problem". I would not go quite as far as you in describing the problem (qualitatively) as 'imaginary', since it is a theoretical possibility, but, in quantitative terms I am certainly inclined to think that it is such a rare issue as to probably not warrant any action to 'guard against it'!

It's interesting that you 'exclude lightning' in your rhetorical question, and maybe you were only talking about actual 'lightning strikes' (to the property of nearby (against the effects of which an SOD won't help)? In a domestic environment large 'switching surges' are probably rare - so, in practice, the concept of domestic SPDs is presumably primarily aimed at 'lightning-related' surges. Indeed, the 'risk assessment' calculation described in 443.5 of BS7671 to determine whether SPD protection is 'required' all relates to such risks - since the only parameters of that calculation are (a) whether the location is rural or urban, (b) the 'lightning flash density' in the area of the UK in which one lives and (c) the length of the local LV and HV supply network cables (majoring on the overhead parts of the cable).

As I recently wrote, that calculation puts my house in an allegedly 'very needy' (of an SPD) category, given that the calculated "Calculated risk level" ('CRL') is around 43, and anything under 1,000 is regarded as indicating the need for an SPD - but, for what it's worth, and obviously anecdotally, in nearly 35 years of living here I have never been aware of any damage to anything which was definitely related to 'power surges'.

Kind Regards, John

 

[JohnW2]

The radio ham example was extreme, most earth rods are more like 50Ω so likely no more that 5 amp .... However we are looking as SPD's and the one I have is two modular wide, so not able to sink much energy without damage and connected between lives, line to neutral, so the question is what could that unit absorb which the equipments network could not handle?

Are two totally unrelated issues getting muddled here? The excessive currents in paths to earth arise, under some circumstances, if there is a fault in the CNE of a TN-C-S supply - and there is nothing which an SPD can do (or would be expected to do) about that.

Kind Regards, John

 

[SimonH2]

Not that they are getting muddled - just two separate sub threads going on : one on lightning and SPDs, the other on loss of CNE and the potential for thermal damage from currents flowing to other sources of an earth connection. Yes, it's not hard to get muddled as to which is which

 

[JohnW2]

Not that they are getting muddled - just two separate sub threads going on : one on lightning and SPDs, the other on loss of CNE and the potential for thermal damage from currents flowing to other sources of an earth connection. Yes, it's not hard to get muddled as to which is which

All true, but (although I may have misunderstood) in the message I commented on, eric appeared to be trying to link the two different issues!

Kind Regards, John