To Bond or not too Bond??

[Sidewinder]

Yes

In that case, each flat (MPAN) is it's own electrical installation and totally separate from all others, and thus must be bonded as required individually, to whatever exposed and extraneous conductive parts are accessible in the installation, or able to bring a potential into the installation in any way means shape or form, such as is required to ensure that an equipotential zone is maintained within the premises at all times, including under fault conditions, and that includes faults introduced into the zone.

 

[Sharpend]

Thanks Sidey, that was my initial thought but the IET technical guy was more concerned as to the lightning protection system having a bearing on the bonding, he thought that the bonding would make matters worse, he was saying it would be out of the scope of bs7671 and would be subject to bsen62305?, which immediately made me doubt myself.

would the fact that all walls being metallic and each flat being bonded to the metsec cause any issues? 

 

[Sidney]

Thanks Sidey, that was my initial thought but the IET technical guy was more concerned as to the lightning protection system having a bearing on the bonding, he thought that the bonding would make matters worse, he was saying it would be out of the scope of bs7671 and would be subject to bsen62305?, which immediately made me doubt myself.

would the fact that all walls being metallic and each flat being bonded to the metsec cause any issues? 

The only point of connection with the LP and the installation is at the MET. TBH it just makes a better earthing system and will lower the EFLI readings when tested.

If the building ever gets zapped the duration will be short but very high and will more than likely really screw things up.

TBH I would be more concerned about transient voltages from strike further away than direct building strikes, this is where the SP really comes into play and breaking the installation down into zones. People tend to forget that all service points of entry into the building need protection, from the BT lines in the ground to the TV antennas on the roof.

Likelihood of a strike, direct or indirect, is extremely slim, I've been to thousands of telecoms masts across the UK and only seen one damaged from a direct which fried all the earthing and killed the fire alarm. All other atmospheric events have been transient voltages from strikes further away, one site in particular has been hit three times but the customer refuses to fit surge protection as its cheaper just to replace rectifiers.

Anywho, beer to drink...

 

[apprentice87]

Hi all,

Slightly off at a tangent, but why would anyone think it is a good idea to connect a lightning conductor to the MET??

Having seen the effect a lightning strike had on a house not 75 yards from where i am sitting, and on a tree about four feet in diameter that USED to be about 200 yards from here it is obvious that the DNO service cable and the wiring in your house would simply cease to exist.. What is the benefit supposed to be??

Another thing.. I have heard about "surge protectors" that are supposed to protect you electrical stuff from lightning.. Given that the lightning ionises the air and manages to arc several thousand feet to the ground, a surge protector will achieve precisely nothing..

The tree i saw that was blown to bits was quite something to see.. It was, like i said, about 4 feet in diameter, i "think" it was a cedar tree, [ i know nothing about trees] but all that remained was basically a splintered stump and all the rest was torn into long strips that dangled from nearby trees..

john..

 

[Rob.]

The SPDs are there to protect against increased transient voltages caused by lightning strikes to the grid. Not to protect against a direct strike. Well, domestic ones aren't.

Edit: As for bonding to the MET, I've seen it done, but this isn't something I've ever had to deal with so my knowledge isn't really there on the subject. 

 

[Blue Duck]

Some info on LPS here and here http://new.abb.com/low-voltage/products/earthing-lightning-protection/furse/literature

 

[Sidewinder]

Thanks Sidey, that was my initial thought but the IET technical guy was more concerned as to the lightning protection system having a bearing on the bonding, he thought that the bonding would make matters worse, he was saying it would be out of the scope of bs7671 and would be subject to bsen62305?, which immediately made me doubt myself.

would the fact that all walls being metallic and each flat being bonded to the metsec cause any issues? 

As I said each supply with its own MPAN needs to be completed in accordance with BS7671.

The LPS must be bonded to the earthing and bonding of the LV installation in the building.

Personally I would do this at the "master" supply, you mention shop I think, at ground level?  I'd do it there.

This all needs to comply with 62305, you'll need earth rods down < 20 Ohms I think from memory, but you need to check.

There are 4 parts to 62305, £306, £318, £254, £254, in no particular order!

So that's over £1100!!!

Unless you get BSI member discount.

On top of this, if you have an LPS, you really should fit surge to all incoming lines, that's electric, phones, broadband, basically, anything conductive entering the building.

Nothing will protect against a direct strike, it's indirect strikes, and conducted emissions you are trying to mitigate against.

Look to 7671 regs below & GN8.

411.3.1.2

443.1

534.2

541.3

542.4.1(iv)

Appendix 16

GN8, Ch3, 5, & 6

EN 62305 parts 1 -4 & PD 62305 which is just the flash density map.

 

[Sharpend]

Ok clear and understood. 

The lightning protection is being designed/installed by specialists and covers the whole building but I believe it to be connected to incoming supply for shop. 

It was the IET that raised the questions over it. 

I was personally treating each incomer (for flats and shop) as separate entities of their own,, and thus each flat as a separate dwelling. 

Just wasn’t sure after speaking to the IET. 

 

[Sidney]

Its <10 ohms combined for LP rods.

 

[Sidewinder]

Its <10 ohms combined for LP rods.

Thank you Sidney, I couldn't recall, and didn't have chance to check the standards before I left the house this morning.

 

[Sidney]

Thank you Sidney, I couldn't recall, and didn't have chance to check the standards before I left the house this morning.

I have a head full of useless things like that, I can never remember my wedding anniversary though...

 

[Sharpend]

Ok an update:

spoke to lightning protection guys today, it appears that they were originally informed that the building was going to have a metallic roof, so tagged cooper tape to bottom of the steel structure to be staked into ground and thus using the steel structure as the down conductor from roof. It now turns out that the roof is not metallic, so they are going to run a tape from top of steel structure around the roof, thus still using structure as down conductor. 

Surely by doing this, in the event of a strike the whole structure will be livened up for the duration, thus sending voltage down the earth bonds to dwellings and main incomes to both shop and dwellings, it will bypass the surge arrestors as not coming into the building by the transient voltage from the ground but by direct strike?

Am I over thinking this and is this the norm? I thought the purpose of the LPS was to  take the voltage away from the structure of the building? 

 

[Tony S]

A metal building depending on construction will form a faraday cage. I was on the top floor of our kilns when it got a direct strike, I didn’t notice anything and everything carried on working. In the light of day one of the exhaust chimneys had a few lumps missing.

A second occasion I was in a local brick built pub, tiles blown off the roof and all electrical equipment destroyed. It was OK as I was on hand pulled beer.

PS, if I invite you for a drink, wear asbestos underwear.

 

[apprentice87]

I would not be worrying about surge arrestors and other silly little bits of kit, it the building gets struck by lightning that will be the last of the owners worries. Where i live a tree was blown to bits not 200 yards from here, and the house across the road had the corner blown off together with half the roof, not 50 yards from here.

Personally, i cannot see that a lightning conductor does anything anyway, it will merely be vaporised..

 

[Sidney]

Ok an update:

spoke to lightning protection guys today, it appears that they were originally informed that the building was going to have a metallic roof, so tagged cooper tape to bottom of the steel structure to be staked into ground and thus using the steel structure as the down conductor from roof. It now turns out that the roof is not metallic, so they are going to run a tape from top of steel structure around the roof, thus still using structure as down conductor. 

Surely by doing this, in the event of a strike the whole structure will be livened up for the duration, thus sending voltage down the earth bonds to dwellings and main incomes to both shop and dwellings, it will bypass the surge arrestors as not coming into the building by the transient voltage from the ground but by direct strike?

Am I over thinking this and is this the norm? I thought the purpose of the LPS was to  take the voltage away from the structure of the building? 

No amount of SPDs will protect from a direct strike. The LP will reduce the time of the strike therefore reduce the impact and potential damaged caused by it.

The SPDs will protect against the induced voltage created by said strike on any incoming services. I'm not fully conversant with the effects of strikes as I did the course a while ago and haven't really touched the subject fully since then, but basically its an attempt to direct the flow away from critical areas of the structure. The building is also broken down into zones if required with more sensitive equipment placed in a central zone with less critical kit on the outside of that but this only really applies to commercial buildings that use a lot of IT and have servers etc.

LP can actually increase the risk to people of the ground near an electrode as step voltages can be quite severe in the area so if you're walking past one you could get a bit of a tingle...

TBH try not to over think it. The actual likelihood of a strike is slim, there's more chance of induced transient voltages from strikes in the area causing damage. Off the top of my head a ground strike can travel up to three miles and a strike on overhead lines up to ten miles.

Metal framed buildings seem to fair better than most. I've seen pictures of brick built structures in very bad shape, the damp in the plaster had vapourised and took a very large chunk out of supporting wall.

I should really start reading up on lightning again as I do find it fascinating on how much energy is produced.

 

[Sharpend]

Thanks all, I was concerned that I may have been overthinking the whole thing, I tend to do that to try and gain understanding.  I've got it  

 

[binky]

A house near me suffered a direct strike on the chimney. It tracked down through things like plastering corner metal thingies, arced across any gaps in that, jumped to floor, vapouried a piece of carpet and continued down thourgh the house. Did very little damge to loft conversion electrics - i had to replace 1 length of lighing cable. Bonding prevents the arcing which is what does the most damage and can cause fires ( at least thats the way I understand it).

SPDs - I've read the spec on these several times and they all talk about lighting strikes and big surges on mains, but do they give any protect against ' normal surges' like switching power stations On and Off and much smaller transient voltages that the grid is prone to? 

 

[Sidney]

SPDs - I've read the spec on these several times and they all talk about lighting strikes and big surges on mains, but do they give any protect against ' normal surges' like switching power stations On and Off and much smaller transient voltages that the grid is prone to? 

Switching doesn't have the energy. The average strike in the UK is around 70,000 volts and 20,000 amps, I doubt even one of Tonys kilns turning on will create that sort of energy...  :slap

I did see an interesting figure of 1,000 Giga Watts measured by one research team, that'd make your halogen GU10s glow...

 

[apprentice87]

There is a large disused chimney not far from me.. It was struck by lightning and fared pretty well, all apart from a section from ground level up to about 20 or 30 feet high.. I will get a photo in the next few days.. Wonder the entire thing did not collapse..

john..

 

[Sidney]

Wouldn't surprise me if the bottom 20-30ft didn't have more moisture in, the higher parts would be more exposed to the wind so would have a lower moisture content.