Lightning - From the mouths of babes!

[OnOff]

LIGHTNING NOT lighting! An odd title given, but bear with me as I think it's a subject worthy of airing here. So, a serious electrical storm over the house today, a proper window rattling, pull the aerial lead out job. My five year old said "Dad, where does the lightning go?". So I said "Into the ground". So pretty much she now knows as much as I do on the subject! I got to then thinking............is one earthing system any more susceptible to lightning damage than another? We're on TT. IF we had a strike close to the rod (how close is "close"?) then could it travel up into the house and make live everything that's connected to the MET - sort of an earth fault "in reverse". Is it something you can guard against?

Would say a PME system with the traditionally lower Ze offer the best "protection"? As in multiple earthing points for the strike to dissipate.

What would happen if you were in contact with say a metal fitting? Or say you switch off at the incomer and are changing a fitting at the precise moment there is a strike. Your L & N are disconnected as you've switched off the main switch at the cu and maybe even removed the mcb. All padlocked up with the key round your neck. I'm now thinking "So what!". As your cpc is still connected to the met etc.

Apologies if a total numpty question, I almost didn't post for fear of ridicule but hey ho, bring it on, hopefully I come out of it having learnt something! Reading elsewhere of 3' blue sparks coming out of chimney breasts etc and insurance companies terming such strikes as acts of God!

(....and lastly why, can someone tell me do I only write these sorts of posts after a drink? )

 

[Andy™]

with the power lightning has, i dont think TT or TNCS will make much difference

 

[PC Electrics]

About 2 years ago we had a stonking storm (we get quite a few here). I stood in my lads bedroom window watching it approach across the fields from Rugby. Next thing I knew a bolt of lightning about as thick as a telegraph pole had landed just across the road, about 40m away. What little hair I have left stood promptly to attention and the thunder clap pushed me backwards. Later i learned a house about 2 miles to the north of us had been struck and destroyed. Close call!! No electrical equipment was affected on a permanent basis.

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And I'd say 40m was quite close enough, thank you very much!!

 

[OnOff]

[quote name='Andy

 

[Manator]

I think this is a very good question and one that very few would have considered. I doubt any earthing arrangements would offer any protection other than basic. We have to remember that lightening protection is only required when the area of the installation is within a target zone. The protection is provided by solid conductors direct to earth by multiple rods. An average bolt of negative lightning flash carries an electric current of about 30,000 to 100,000 amperes (30-100 kA) at a voltage of over a billion volts.

Amazingly we are often protected by simply being not directly connected to earth, and if we are, protected by the laws of average.

 

[steptoe]

OK, here is my take on it,

if the lightning strikes the ground, as opposed to you, then you will not get killed by it[pacemaker wearers etc may want to be worried though!]

you are on the general mass of earth, the lightning has risen the potential of the general mass of earth, no worries,

the problem arises when you become a conductor between earth and something else,

now, dont get too worried, if the lightning HAS entered your property via an earth rod or similar, chances are it will have fried the insulation on most cables anyway thereby bringing everything to the same potential,

and we all know what potential makes!?

only, and ONLY if you become the direct conductor between the source [lightning bolt] and earth [the general mass of] will it become a major problem[ read that as MAJOR],

so, lets hope you dont have a direct strike on your bonce, :|

now, this raises a few more issues with the exported earth theme though,

do we have time for that tonight,? :|

 

[Sidewinder]

BS7671 even with its "surge protection" section, is not and has never been designed to protect against direct atmospheric lightning strikes.

 

[gavindrummond]

I thought lightning will only strike such buildings like churches and castles higher buildings, that's why they have special earthing arrangements attached to them and local occupied buildings near by have to have special earthing protection installed just in case it enters their premises through growing mass.

Sorry ground mass. The protection stops all the domestic appliances getting damaged.

 

[sellers]

I've been 150meters away from a strike, its one of the most impressive things I've ever seen and experienced. Never heard anything so loud or intense.

 

[OnOff]

So, should there be a rider in all the test and inspection books to consider whether it is safe to "undertake" (what an apt word) electrical works IF there's an electrical storm about...............Brings a whole new meaning to the phrase DEAD TESTS!

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With the TT I'm thinking more on the lines of NOT the house getting a "direct hit" as such but the strike hitting the ground local to the install, then dissipating through the general mass of earth and "finding" the rod which must present a nice easy path for it into the house. Seem to remember years back messing around with home RF projects that you could buy surge arresters for the coax in case your aerial got hit.............

150m.............40m...........any improvements on 40m?

 

[binky]

Had call out a few years ago - lightning had struck overhead pole on other side of road, and burnt-out old board main switch. Neighbours were ok, as were all electrical items in the house.

 

[jopo]

perhaps another way of looking at this is too ensure that in the event of a lightning strike that all extraneous conductive items are at the same potential as mass of earth. i.e what earthing arrangements do we partake in to ensure that touch potential is kept to a minimum. excluding the use of an rcd off course. we bond everything together.in close proximity to each other. we refer to them back in oz as equipotential conductors.

an example was a submersible bore and control equipment that quite often had to be replaced every year during our lightning season. a lot of money was thrown at the pump station with regard to surge protection. that did nothing to solve the ongoing problem. it was just another expense to replace the now stuffed surge protection.

We ended up bonding everthing together. the metallic water pipe (submersible side ) to the motor frame then to the supply side water line. then to the overhead metal tripods for the submersible pump. then to the perimeter fencing. we still left in the surge protection. Have never had a problem since. all cable bonding was done with lengths as short as possible.

Again i would guess this would be the equivalent of equipotential conductors

definiton of equi-potential earthing in our electrical bible

Electrical connections intended to bring exposed conductive parts or

extraneous conductive parts to the same or approximately the same

potential, but not intended to carry current in normal service.

 

[steptoe]

jopo

we did have a very similar system until a couple of years ago,

now, normally in most circumstances [domestic at least], it 'can' be omitted, and most people dont do it for a number of reasons,

1 they think that if they have RCD protection it automatically means they dont have to install equipotential bonding

2 they are too lazy to do it

3 they are just doing monkey see monkey do

the truth is, a lot, if not most, people dont understand earthing,

 

[binky]

It's brown stuff you plant tomatoes in :coat

earthing arrangements are truly an in-depth subject and much mis-understood. Been so up to neck in PV, that a good dose of revision wouldn't go amiss...

 

[westom]

Sort of what I was getting at, is it a case of "there ain't nothing stopping it and it ain't your day"?

Nothing stops lightning. Potection from direct lightning strikes is routine. For example, BT suffers about 100 surges with each storm. How often is your town without phone service for four days? Never? Because routine is to never stop lightning. Routine is to do what Franklin demonstrated in 1752. Divert it safely to earth on a path that is not destructive.

Lighning is a connection from the cloud (maybe 3 kilometers up) to earthborne charges maybe 4 kilometers away. Lightning uses the electrically shortest path. Not five kilometers across the sky. It goes down 3 kilometers to earth. Then uses an electrical conductor - earth - to connect to those charges.

If a shortest path is via your building or appliances, then damage occurs. A lightning rod connects it harmlessly to earth for building protection. A 'whole house' protector connects it harmlessly to earth for appliance protection. But neither a lightning rod nor protector does protection. Protection is defined by the quality of and connection to an earthing electrode. Because that is where hundreds of thousands of joules dissipate. That is where lightning goes to become irrelevant.

Where does lightning go? Into what absorbs all that energy.

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With the TT I'm thinking more on the lines of NOT the house getting a "direct hit" as such but the strike hitting the ground local to the install, then dissipating through the general mass of earth and "finding" the rod which must present a nice easy path for it into the house.

Lightning strikes a nearby tree. Therefore a cow some 10 meters away is killed. A direct strike to the cow. That current is 3 kilometers to a tree. And 4 kilometers to earthed charges. A shortest path was up the cow's hind legs and down its for legs. Since an incoming and outgoing current path exists, then the cow is 'surge damaged'.Same applies to what protects all appliances in the house. Single point earth ground. jopo describes part of the concept. Essential is both equipotential and conductivity. If any wire enters and is not earthed, then these concepts are violated.

TT makes no difference. Every AC electric wire must connect low impedance (ie 'less than 3 meters') to the single point ground. Otherwise equpotential and conductivity are compromised. Then appliances may be damaged just like how the cow was killed. Or a surge simply enters on one unearthed AC wire to find earth destructively via appliances.

Its about how that current connects a cloud to those distant charges. Most important is the current path. And how that current gets conducted into and by earth. Single point earth ground (as described by jopo) is critically important so that a current need not flow inside any building. A human stands with feet together so that a current need not flow up one leg and down another. Same concept.

 

[binky]

so if cow stands with feet together it's OK ray

Nice explanation Westcom

 

[OnOff]

Some eloquent thoughts here................. say then if lightning were to strike in the vicinity of your TT rod then could that rod (on which you've worked hard to get the Ra down) not present a shall we say "preferred" path of least resistance WITHIN the mass of earth (surrounding the rod) - depends on the soil resistance etc etc. So it then comes INTO the installation and you happen to be in contact with either an exposed or extraneous conductive part. Just then supposing all plastic water pipes and no gas - so no parallel paths. I can't see why that couldn't happen........someone enlighten me please!

 

[westom]

Some eloquent thoughts here................. say then if lightning were to strike in the vicinity of your TT rod then could that rod (on which you've worked hard to get the Ra down) not present a shall we say "preferred" path of least resistance WITHIN the mass of earth (surrounding the rod) - depends on the soil resistance etc etc.

If that ground is the best ground, then a surge need not blow through existing appliance protection to only get to the same earth ground. Also important is a concept often not taught to electricians.- impedance. Not resistance; Impedance.

For example, wire 'thickness' reduces resistance. A shorter wire 'length' mostly reduces impedance. Critically important is the lowest impedance possible to earth (ie 'less than 3 meters', no sharp wire bends, ground wire not inside metallic conduit, no splices, etc). Protection further increases when impedance between protector and appliance increases. For example, to increase protection, BT wants up to 50 meters separation between protector and electronics.

Examples of why the single point (service entrance) ground is so important to protection. And what defines impedance. A lowest impedance connection to the building's best (most conductive) earth ground means a surge need not hunt for earth destrutively inside the building. Then best protection already inside every appliance is not overwhelmed.

Geology is a major factor. Whereas one or a few earth rods may be sufficient. In poor conductive soil (ie sand), then more electrodes may be necessary. Protectors are simple science. The art is earthing.