I suppose it would with 90%, but what would you do with 70, 60 or 50%?
What's inside an SPD?
- Thread starter aptsys
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As I said, it's when one has those 'intermediate' probabilities, that that then raises the $64,000 question.
I suppose that some information is better than no information. If it's 50, 60 or 70%, then one at least knows that it is not 10% or 90% (figures which would probably strongly influence one's decision) - that it essentially becomes 'pot luck' (i.e. not much helped by the predictions).
However, it's very much a personal decision and, as we often discuss in relation to bernard (and others) is a question of an individual's degree of "risk-averseness" (i.e. how optimistic/pessimistic, or 'how much of a gambler', they are). Some people might, for example, gamble on sending out invitations for their 'garden party' if the predicted risk of rain was less than, say 50%. Others might feel that even a 20% risk of rain was 'too much of a gamble', and therefore would postpone the party. Statisticians, or relevant experts like meteorologists, can provide the figures, but it's only an individual who can decide how to act upon them.
Kind Regards, John
And in practical terms, had they been fitted beforehand I would have expected the outcome to be unchanged - the SPD would not have helped in the least. As suggested, there was a (common mode) surge induced into the cables to the lights which the controller couldn't cope with. As you were able to discern a time lag between flash and bang, it wasn't a direct strike - given the rise time of the current in a lightning strike, protection that far away from the device would be worthless due to the inductance in the cables.
On the subject of extension leads, as John suggests, many people will be buying this to get a false sense of safety. As most are used, they will provide little if any benefit and may even make things worse.
A few decades ago I recall a distinctly violent thunderstorm - talk about skies darkening, it went quite dark and the flashing and banging was such that it was impossible to try and time the lag. Not long after it subdued, we had a call from a customer about 4 miles away to say that there's been a very large flash and bang and there was now a burning smell from where the main server lived. I reckon they'd had a ground strike quite close to the site - and the effect on ground potentials was what did the damage.
Back then it was all serial terminals, and it was clearly a case of ground differentials across the site (multiple buildings, TN-S supplies from their on-site 11kV substation) given the rough correlation between distance from server (and expecially if in a different building) and damage done.
Separately, I've seen the insides of a laptop subject to a lightnig induced surge up it's modem port - it wasn't pretty
The bosses lived in a house in the sticks, and they lost equipment fairly regularly due to thnderstorms - and it was always only equipment connected to both mains power and phone line that "got it". SPDs in the CU would not have helped UNLESS the phone lines were routed via the CU (or at least, diectly (within a few inches) adjacent) and provided with their own SPDs sharing the same "earth" bar. That adjacent bit is key - just as we use main and supplementary bonding to maintain an equipotential zone - we want to create a "safe" zone that may not be at "earth" potential if there's a lightning strike nearby, but at least will have everything inside it at the same potential. That means having a single entry point as a few feet of 10mm² bonding cable will have enough inductance to stop the protection working.
The same can be done to a small group of items (eg a PC with modem etc) using a surge protected mains lead IF (and only if) everything (power, phone line if there's a modem, network cable, whatever) goes through that extension lead. That way, while a common mode surge will still get to the equipment, it probably won't cause any damage as it will be common mode to all of it's connections to the outside world.
TL;DR - yes I also think that SPDs in the CU are unlikely to be of much use in most installations - something expensive but a waste of money.
Why will they?
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I would hope that they won't ("become commonplace", unnecessarily), but I fear that aptsys may well be right, and everyone knows the reason as well as I do. Not installing them requires thought and a sensible/intelligent judgement, but fitting them can be done mindlessly. Worse, it will probably snowball - as the number installed creeps up very slowly, those of limited confidence will become increasing nervous about a decision not to install them, so the rate of installation could start rising rapidly. Then, of course, if the number being installed gets past some threshold, 'pre-populated' CUs which include them will probably start appearing on the wholesaler's shelves
And sadly...
Is it like the police?
Does society get the tradesmen it deserves?
Nobody unwilling or unable to do that should be working as an electrician.
In an ideal world, anybody found to have done that would have his ability to work as an electrician taken away from him.
Nobody lacking the confidence to make proper judgements should be working as an electrician.
Is it like the police?
Does society get the tradesmen it deserves?
Yes, you are right - people who can't or don't want to think things like that through shouldn't be doing the job. But as we all know, there are a heck of a lot of people who really don't seem to even know that these questions are there to be considered.
As an example of the level of knowledge amongst some people ...
Several years ago I had a job to network up a new office for a customer. The electrician was doing the containment (dado trunking) and backboxes, I had to pull in and terminate the cables. The trunking was nice stuff from Rehau, and where the bottom bevel cover was integral to the trunking back - so cables would stay in rather than trying to fall out while you are trying to get the cover on.
But ... This trunking was 2 section with an option to add a divider to make it 3 section. Needless to say, the sparky had taken the big section for an RFC and left just the small top section for data cables. On one run there were 24 cables to go in, plus a 10 pair phone cable from the building next door - not a chance of fitting the latter in as the 24 data cables only just fitted. I observed that the T&E cables would fit in the top section at which point the sparky shows his ignorance. He was 110% adamant that he could not put the power cables in the smaller top section - they must be in the middle section as the insulating barrier only works one way. WTF He was also adamant that there wasn't a divider available to section off the bottom bit - I looked it up later, and yes there is
It was "quite hard" not saying anything when he said (something like) "it must be nice when someone else does all the hard work of putting the trunking in and all you have to do is pull a few cables in"
As an example of the level of knowledge amongst some people ...
Several years ago I had a job to network up a new office for a customer. The electrician was doing the containment (dado trunking) and backboxes, I had to pull in and terminate the cables. The trunking was nice stuff from Rehau, and where the bottom bevel cover was integral to the trunking back - so cables would stay in rather than trying to fall out while you are trying to get the cover on.
But ... This trunking was 2 section with an option to add a divider to make it 3 section. Needless to say, the sparky had taken the big section for an RFC and left just the small top section for data cables. On one run there were 24 cables to go in, plus a 10 pair phone cable from the building next door - not a chance of fitting the latter in as the 24 data cables only just fitted. I observed that the T&E cables would fit in the top section at which point the sparky shows his ignorance. He was 110% adamant that he could not put the power cables in the smaller top section - they must be in the middle section as the insulating barrier only works one way. WTF
He was also adamant that there wasn't a divider available to section off the bottom bit - I looked it up later, and yes there is It was "quite hard" not saying anything when he said (something like) "it must be nice when someone else does all the hard work of putting the trunking in and all you have to do is pull a few cables in"
Indeed so!
Hmmm. It must be a few years since I last told this story here - so, if only for the sake of newcomers ...As an example of the level of knowledge amongst some people ... He was 110% adamant that he could not put the power cables in the smaller top section - they must be in the middle section as the insulating barrier only works one way. WTF
... many years ago (but obviously in the days of RCDs), I had a very heated argument with an 'electrician' as regards an electric shower which had a pretty long cable run back to the CU, where it was RCD protected. He was adamant that I had to have a second RCD, much closer to the shower, since otherwise "any earth leakage current will have reduced to a level which wouldn't trip an RCD by the time it had got all the way back to the CU" !
Kind Regards, John
Edit: Typo corrected
Last edited:
Who won?
Well, he remained 'adamant' - but I sacked him (there were other reasons as well!) - so, in one sense, I suppose I won!
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It's already happening. People getting hammered on other Electricians Groups for not installing SPDs
Does anybody know if the DNO can provide the distances which apply to any given property?
Re my house IHNI what LPAH or LPCH are. I do know that LPAL is 0, and I could have a stab at estimating LPCL.
But this guidance seems bonkers:
Of all the lengths in the formula, the only one I do know for sure is LPAL, and I know that it is zero - there are absolutely no overhead cables between the substation and the houses around here.
The whole business of calculating a CRL for "rural and suburban" environments seems utterly pointless. fenv is 85, so to get to ≥ 1000 needs LP x NG to be ≤ 0.085.
Best case NG is 0.1, so the longest that LP can be is 850m
Vast swathes of the country have NG between 0.3 and 1.0, meaning that LP can be no more than 283m down to 85m.
And what do rural, suburban and urban mean? Presumably there must be some kind of population density thresholds, but in my outer London borough I would (and I expect most people would) say that we were suburban. So with an NG of 0.8 I'd need my LP to be no more than 106m.
2LPAL + LPCL + 0.4LPAH + 0.2LPCH ≤ 0.106
Hmmm.
But this guidance seems bonkers:
Of all the lengths in the formula, the only one I do know for sure is LPAL, and I know that it is zero - there are absolutely no overhead cables between the substation and the houses around here.
The whole business of calculating a CRL for "rural and suburban" environments seems utterly pointless. fenv is 85, so to get to ≥ 1000 needs LP x NG to be ≤ 0.085.
Best case NG is 0.1, so the longest that LP can be is 850m
Vast swathes of the country have NG between 0.3 and 1.0, meaning that LP can be no more than 283m down to 85m.
And what do rural, suburban and urban mean? Presumably there must be some kind of population density thresholds, but in my outer London borough I would (and I expect most people would) say that we were suburban. So with an NG of 0.8 I'd need my LP to be no more than 106m.
2LPAL + LPCL + 0.4LPAH + 0.2LPCH ≤ 0.106
Hmmm.
A more common set up in rural areas
I don't really get ....
At least in my part of the world (and I would imagine countless other 'rural' locations), I feel that LPAH alone is often going to be far more than 1 km. I've never bothered following the overhead 11 kV feed to my village to see where it originates, but it's certainly 'quite a long way away'
Kind Regards, John
At least in my part of the world (and I would imagine countless other 'rural' locations), I feel that LPAH alone is often going to be far more than 1 km. I've never bothered following the overhead 11 kV feed to my village to see where it originates, but it's certainly 'quite a long way away'
Kind Regards, John
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