Scariest Post/ Thread?

[Risteard]

I don't think I'd be too happy with a smaller conductor to the electrode than one for a main protective conductor tho.

There are generally rules as to the minimum cross-sectional area of the conductor to the earth electrode (usually called the Earthing Conductor but BS7671 defines that as a particular conductor regardless of earthing arrangement, so I will avoid using the term to avoid confusion) depending on whether it is or isn't mechanically protected, so it wouldn't be a tiny conductor.

 

[Risteard]

As here in the U.S. where TN-C-S is the norm and the NEC requires an earth electrode connected to the incoming neutral at every house.

Likewise in the south of Ireland an Earthing Conductor is required to connect the earth electrode to the Main Earthing Terminal (MET), and a Main Protective Conductor (aka the Neutralising Conductor) connects the MET to the ESB's PEN conductor.

 

[Risteard]

Very true, of course, but nothing could get him to see that the pipe was still going to behave as an earth electrode anyway.

Indeed. It shows a lack of understanding at the very least.

 

[EFLImpudence]

The point is, is it not, that a water pipe is not allowed to be the sole means of earthing - in case it disappears - therefore it is an extraneous-conductive-part because it cannot be removed and may be at a different potential and so must be bonded.

An internal and accessible electrode would be the same but could be removed.
An external electrode, because of its position outside, is not really an extraneous-c-p, is it, so would not have to be bonded and could be removed. It's not going to be satisfactorily effective as an earth, if the rest of the installation is not treated as TT, should the neutral be lost.

Do the authorities which demand an electrode on TNC-S supplies insist that the installation be treated as TT?

 

[PBC_1966]

The removal of a water pipe as an acceptable means of earthing from the 14th edition was due to the increasing use of plastic and the idea that a pipe providing an acceptable earth electrode today could cease to be such tomorrow. The later amended editions of the 13th edition, while permitting it, contained warning notes about the increasing use of plastic pipe/couplings.

It's also worth noting that these earlier editions while not requiring a supplementary electrode with PME supplies did actually encourage the addition of one as an additional safeguard.

Do the authorities which demand an electrode on TNC-S supplies insist that the installation be treated as TT?

The TT and TN terms aren't used as such here, but TT isn't permitted at all as an arrangement. By the way, metallic water pipes (subject to certain qualifications) are still permitted to act as the required earth electrode.

 

[ban-all-sheds]

Although BS7671 does not require an earth electrode for a TN-C-S system, where one was installed I would not class it as anything other than an earth electrode.

What if it wasn't something sold. or marked, as an earth rod, but was just a random bit of metal coming out of the ground and entering the house?

 

[Risteard]

Although BS7671 does not require an earth electrode for a TN-C-S system, where one was installed I would not class it as anything other than an earth electrode.

What if it wasn't something sold. or marked, as an earth rod, but was just a random bit of metal coming out of the ground and entering the house?

If you are talking about extraneous conductive parts which are bonded then that is a different issue. Anything installed as an earth electrode (which is not limited to earth rods - it could be earth tape etc.) is exactly that.

 

[ban-all-sheds]

So if you can't tell why it was installed, what is it?

 

[bernardgreen]

A few years ( decades ) ago I found a house with a "good" earth in that faults of Live to CPC did trip new fangled RCD. The earth was via the screen of the Co-ax lead to the TV aerial amp and splitter on the next door house. ( the two houses shared the TV aerial and amp. )

 

[bhm1712]

That's how I know about them, I collect valve TVs and Radios!

You don't know of any Mullard EL84's knocking about in working order do you?

 

[ericmark]

I disagree. You most certainly can have an earth electrode with a TN-C-S system (and indeed most countries require it).

Although BS7671 does not require an earth electrode for a TN-C-S system, where one was installed I would not class it as anything other than an earth electrode.

BAS did work out what I was saying when he stated unless fitted by the DNO, you can have a copper coated steel rod connected to the house earthing system with a TN-C-S supply, but it is not called an earth electrode it is called "extraneous conductive part" same as the water pipe and gas pipe.

The exception is where a resistor is placed between the DNO earth and the installation earth. I have only seen this on a gas plant clearly they were very worried about sparks and there were earth electrodes all around the plant.

Measuring the resistance of the earth electrode with a loop impedance tester is rather erroneous, as we have a 0 ~ 15Ω variation to the true earth reading as the DNO earth is permitted to be 15Ω above true earth, so with a TT installation the reading can be out by 15Ω. Although one can fit earth rods with a TN system one has to be careful they are not too good. A 60Ω earth rod is not a problem but as we get to 10Ω or less we can be talking about a lot of current flowing and not fuse protected.

A gas pipe 15mm dia is likely got a wall thickness of 0.7mm so is around 35mm² CSA, so the earth cable should be selected so should there be a loss of the DNO earth the cable burns out before the gas pipe keeping to 6mm² does mean cable before gas pipe as long as no reduction with for example some flexible connection. We hear of gas house fires and explosions due to copper theft causing the DNO earth to be lost, so either 6mm² is too big or it had more earthing connections possibly the cooker and boiler also earthed so we have multiple 6mm² earth wires?

Fitting earth electrodes of say 8Ω would allow up to 50 amp to flow to earth and therefore reduce the current in the gas pipe, but the M in PME stands multiple and the DNO should have multiple earths without there being a need for the house holder to install them. Also the gas supplier should use a insulator between their earthed supply and your gas pipes so in real terms the explosion and fire is down to the utilities not doing their job correctly rather than simply some one stealing copper.

 

[bernardgreen]

the explosion and fire is down to the utilities not doing their job correctly rather than simply some one stealing copper.

or go back one step to the days when the supply from the street had a real "earth" in the cable.

 

[ban-all-sheds]

Trouble is they corrode and degrade.

 

[securespark]

Although BS7671 does not require an earth electrode for a TN-C-S system, where one was installed I would not class it as anything other than an earth electrode.

What if it wasn't something sold. or marked, as an earth rod, but was just a random bit of metal coming out of the ground and entering the house?

What, like a girder?

 

[bernardgreen]

Trouble is they corrode and degrade.

So would the Live and Neutral then, the cable in our street, ( 1950 or before ) was 5 core

The cable in the street here has 4 cores and the "wave formed" earth "screen" that is inside the outer sheath so protected. Like wire armour but not wound around the conductors to allow it to be moved out of the way for joints to be made.