Impedance value

[Lectrician]

I should have started a new paragraph, sorry for confusion. I meant that that if you altered the earthing arrangement, that the armour and third core must be kept isolated at the garage end, but connected at the house end.

To carry the eq zone over, you need the third core/armour, and this must be sized taking into account the requirements for main eq bonding aswell.

I have corrected my post.

 

[Lectrician]

The armour need earthing - a must. This must be from the house side, as this is where the protection for that cable is.

Yes it must be connected at the house, but no reason, surely, for it not to be connected at the garage as well? [If it is a seperate earthing arangement, it must be isolated]

If you are not taking an earth to the garage, there is no need for a third core, so it will be un terminated each end, non existant, or earthed at the house end, and isolated at the garage end.

I don't think that not taking an earth to the garage is a Good Idea.

 

[allsorts]

Thanks guys, however everything you have said is the opposite to what we are being taught by the lecturer at college????
we are advised to connect the earth at the garage end but not at the house end and therefore configuring to have independant earth arrangement at garage???

So to conclude?

 

[Damocles]

Well it has to be connected one end or the other. If you are deliberatley not connecting one end, then the question becomes which way is safest?

I guess the cable armour may not be at the same voltage as you are, standing in the garden and holding it? But that may be the same either end is connected.

Or there is a fork through it, and 1000A fault current is flowing to earth. Is it better for this to run to the house or to the garage?

 

[allsorts]

Damocles... My argument exactly!

What about.... pick axe slices through cable leaving neutral intact, if the cable is earthed at the garage then the rcd would trip out however the phase would still be live from the house side surely? Danger to someone who has no knowledge of electric??

 

[Lectrician]

Depends on the location of the RCD.

If the RCD is at the shed end, the cable in the ground will have no rcd protection, and therefor the armouring must be connected to the TN earth to ensure the disconnection time is met.

If the RCD is at the house end, the armour could indeed be earthed to the TT rod, but this rod should then be fitted at the house end of the cable.

 

[Damocles]

Don't see why there should not be an earth rod at the garage, at least in principle. It just comes back to the fact that the earth return loop down from the garage through earth to the supply has to be good enough to guarantee disconnection of the devices in the house, as well as anything in the garage.

The garage RCD has no power to stop a cable fault in the feed from the house, but also it would not trip from any current leak in the cable, whichever end is earthed.

Someone saying it should be earthed garage end only is introducing the problem of creating a brand new earth. Remind me, someone, why relying on importing an earth through the cable is considered dangerous.

 

[securespark]

According to NIC, the cable's armour should be conected at both ends. Further, their book "earthing snags & solns", recommends all unused cores be grounded.

 

[Lectrician]

It is recommended that an outbuilding fed from a PME service doesn't make use of the PME earth, this earth should not be taken out of the equipotential zone. This a recommendation of all DNO's It is not a regulation.

If you do decide on a PME service to use a TT earthing arrangement at the garage, the earths MUST be kept seperate. If the RCD is at the garage end, the submain to it must be earthed from the house end, as this is where the protection for this cable is.

 

[ban-all-sheds]

Um.

If you do that, presumably you use 2-core SWA, as you're not taking an earth to the outbuilding. What do you do with the armour at the shed-end?

Also, PME is AKA TN-C-S, so the neutral of your supply is at the earth potential of your house. And then in the shed you have a local earth which is not at the same potential as the house earth, nor therefore the neutral. In the event of a close lightning strike, might not the difference between the neutral and earth rise to a high value?

 

[plugwash]

in britan we don't install lightning earthing in most cases the probability of strikes is just considered too low in most cases

lightning earthign often involves using things like copper tape as bonding conductors to take the insane instantanious current (the problem with wires is skin effect which limits the rise time on any current in them)

anyway neutral is treated as live by the wiring regs so a potential between neutral and earth should be no more dangerous than one between live and earth

 

[Lectrician]

Yup, always treat N as a live conductor (hence why most people call 'live' phase.)

The armour of the 2 core should be terminated into a plastic enclosure. I have a NIC drawing of the recommended set up somewhere, I will dig it out if i have a moment.

 

[ban-all-sheds]

anyway neutral is treated as live by the wiring regs so a potential between neutral and earth should be no more dangerous than one between live and earth

Any idea what that potential difference could rise to given a lightning strike and a separation of 10's of meters?

 

[ban-all-sheds]

The armour of the 2 core should be terminated into a plastic enclosure.

That would make sense..

 

[plugwash]

anyway neutral is treated as live by the wiring regs so a potential between neutral and earth should be no more dangerous than one between live and earth

Any idea what that potential difference could rise to given a lightning strike and a separation of 10's of meters?

about the same as the live-earth potential could rise too under similar conditions

yes close lightning strikes have a tendency of blowing up electrical gear this can be mitigated but the procedures are extremely complex and expensive

the risk level in most of the uk is generally considered to be sufficiantly low that lightning protection is not worthwhile