Whats wrong with ' overlapping resistance areas ' ?
John Rumm wrote: "Multiple rods are ok (and are commonly used in the supply side of PME installations for example), but ideally don't want to be too close together such that they are in overlapping resistance areas. "
The existing earth rods are about a yard outside the backdoor, the proposed indoors one would be just inside the back door under the consumer units.
The existing TT earth inductance was measured a new weeks ago and was OK, but I dont have the figures here.
I dont know why Iggy is talking about Lightening: My On Site Guide says the earthing conductor need only be 2.5 mm-squared for TT if protected, I think that'd melt with lightening.
But 2.5 mm-squared does seem very thin compared to the new meter tails to the consumer units and earth wires i was sold with them. I dont understand why OSG Table 4.4.ii says only 2.5 mm-squared
I thought the earthing conductor is for use in case of an electrical fault, a loose live wire, earthing most of the current through copper not through me until a fuse blows or RCB trips.
There's an 80 or 100 amp main fuse isnt there, which would be of no use for lightening. Or would it?
Woa there horse, you are romping off at at a tangent...
You said "you bring unregulated and unwanted electricity into the building"
I was pointing out this was nonsense. If you give it a moments thought you would realise that all the metalwork of any class 1 appliances in the building, plus the other main metallic services, and any extraneous conductive paths around the inside of the property are *already" connected to the earth spike outside. Having an additional one inside does not in any material way change this.
You also said "and you may SEVERELY worsen your earthing" which I what I was highlighted was unlikely.
Again, that really makes no sense.
How is having an earth rod connection inside the property any different from having a main equipotential bond to metallic gas and water mains, which are also long lengths of copper "rod" driven into the soil within the perimeter of the property?
So the OP has two external electrodes, plus let's say two de-facto internal ones[1] already (assuming he has gas/water mains supplies). You are suggesting that one additional electrode is going to fundamentally change either the likelihood or the deleterious effects on the property of a lightening strike?
[1] Note that while one may not rely on the fortuitous earthing effect provided by main equipotential bonds to achieve adequate earthing, the effects can't be ignored for the purposes of this discussion.
I don't believe you did.
BS7671 17th edition section 542.2 does not preclude multiple earth electrodes.
If you refer to BS 7430:2011 (Code of practice for earthing electrical installation), section 9.5.3 "Rod electrode" it notes:
"A number of rods may be connected in parallel and the resistance is approximately reciprocal to the number of rods employed, so long as the separation is outside of the resistance area of the each rod."
I did not say that.
Perhaps to achieve a usable impedance all year round.
What exactly are claiming is "wrong"?
You are no doubt aware that bonding connections between a lightening protection system (when installed) are also required[2] to be made to the buildings protective earthing system?
[2] See BS EN 62305-1:2006 Protection against lightening general principles
Perhaps I am not the only one.
There is a requirement with PME to extend the equipotential zone to all parts of the installation where you also export the PME earth. That can make its use impractical, but that is not a "ban" as such. Do you have a citation?
Within a single installation, it just means that the rods don't behave as fully independent connections to earth - so its better to test them as a single system rather than in isolation.
Its more problematic when different dwellings have supposedly independent connections to earth but they are via electrodes with an overlapping resistance area. Then a fault current flowing to earth from one property can induce a voltage to be present on the other properties earthing system.
Because with a TT system its assumed that the external earth impedance will exceed 1 ohm, and in most practical situations will be at best several times that.
(if your TT earth impedance is less than 1 ohm, then the same rules as as for TN systems should be used instead)
So in practice you maximum prospective fault current would be limited to
230A.
Well within the fault withstand capability of a 2.5mm^2 CSA conductor with the maximum allowed disconnection time of 1 second for a TT system.
You can do an adiabatic check to be sure:
if you take the minimum required conductor size S as:
S = sqrt( I^2 x t ) / k
(and k = 115 for PVC insulated wire)
So S = ( 230^2 x 1 ) / 115 = 2mm
(and in reality even a time delayed RCD will operate more quickly than 1 sec)
Generally TT systems will rely on RCDs to clear faults to earth since in may cases there may be inadequate prospective fault current to operate a fuse or MCB.
No. If its just forced a conductive path through 100's or 1000's of metres of air, an extra few mm of fuse is not going to help!
It's all to do with the fault current the earthing conductor will be carrying under fault conditions, i.e. a Line to earth fault. As the Earth Fault Loop Impedance will be pretty high (20 ohms+?), it will only be required to carry a low fault current, and can withstand that current long enough to allow the RCD to trip.
Yes, that's right.
No, it'd be no good, as it would blow fast enough to stop any lightening surge current damaging any equipment in your house. However, the odds of that happening are so remote, that we mostly dont even consider it a risk in a typical house. A large house on top of a hill, then maybe.
The 80 or 100 amp fuse will also be pretty useless for you if you touch live and earth at the same time, as it'd never blow. In a typical TT installation, a 32Amp circuit breaker is also unlikely to trip when you touch L + E simultaneously. That's because your Earth Fault Loop Impedance will be so high that there will not be enough fault current to trip the circuit breaker. A 32A CB will trip instantaneously (~0.1 second) with ~160A flowing through it. Your total fault current with a 20 ohm EFLI (your earth rod resistance) will be about 80 Amps, so you'll be holding that L+E for quite a while before the circuit breaker trips. You'll be holding it forever for the main fuse to blow. This is why RCD protection is needed for TT installations. The RCD will trip in less then 300mS, probably faster for a 30mA RCD. Your current rod EFLI will be less than 200ohms, possibly as low as 10 ohms, but, IME, usually in the range of 30-50 ohms. Adding more rods, and deeper rods will bring it down, but there is a limit, especially in rocky areas, where it just is not possible to get a lower EFLI.
replying to George Miles, Iggy wrote: I'm talking about lightning, as one threat, because it's the most threatening and most common. Lightning energizes the air and the earth. If you only provide an outlet with no inlet, then you're dissipating lightning's residual effect that is very weak. The facts about electricity are that it always returns to the source of the electricity in seek of balance.
And therefore, if you add another rod and connection you're inviting more than a residual of the energy, because you create both an inlet and an outlet, this is very bad since neither you nor your devices can handle lightning's power. Meaning, your current setup hasn't failed you and it has been confirmed to be working properly. Don't fix what ain't broken and spray some herbicide around the earth rod so you stop bumping into it.
1 - I didn't go off on any tangent. You seem to think Earthing is only ever a
1-way road and here you are telling people that I'm talking gibberish by saying some appliances are Earthed, just to then confirm and tell me they're Earthed...I know, I said it. You saying it to tell me I'm wrong is absurd. That's how and why they get fried in a flawed Earthing setup.
2 - Like I said, if you aren't adding a 2nd rod and connection you aren't supplying an inlet and are only supplying an outlet. However, by breaching the building's natural insulation from what's going on outside, you now draw and bring the outside in to create instead of avoid the hazard.
3 - Yes, adding another connection to the wiring system will effect everything attached to it, because you're providing an in and out loop for the energy to travel. Plumbing isn't providing any electrical source and is Earthed by itself being in the Earth. Again, no inlet provided and only an outlet.
4 - Yes, this is what he has, in parallel or ONLY connected to each other with just 1-connection. This, is extremely different than having multiple connections to multiple rods...a proven disaster.
5 - Sorry, I misunderstood.
6 - Not where I'm from. Never ever should an electrical wiring system or structure protection system be connected to a lightning protection system. They are to be separate and even distanced from each other to avoid sharing lightning's charge. I saw nothing to the contrary in your document, they make it very clear that the lightning system shall have its own separated and dedicated Earthing rods.
7 - See - 5.6.1 - What is protective multiple earthing? at
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this very clearly restricts the use to the Utility Supplier and the last 2 sentences lock it out of public use.
8 - I've said my piece, you may have the last word.
(that daft web site you use, does let you quote - just select the text of the message you are replying to before you start entering your response and it will quote the source material)
I was pointing out to you that what you claimed the additional earth electrode could do (i.e. that it could bring a potential from outside the building into it) was already being done.
Hence the additional electrode makes no difference.
ISTM that you don't understand how main equipotential bonding is used and configured in UK electrical systems.
There is a summary here:
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Something connected to earth *is* a source of electrical potential. Also, due to the presence of main equipotential bonds like those which connects the water supply pipework to the main earthing terminal and hence to the earth electrode in a TT system, there is already an internal earth connection.
Connections to multiple earth electrodes may be made by a single unbroken conductor which loops to each in turn, or via individual dedicated conductors. Both are acceptable.
You misunderstood what?
Where are you from exactly? It does have a significant relevance if its not the UK, since wiring practices and codes vary significantly around the world. Advice based on what is say common practice in the US would be of little relevance to the UK for example.
That explicitly contradicts the requirements specified in BS 7430 "Code of practice for earthing of electrical installations".
I quote verbatim:
"6.4 Protective equipotential bonding
NOTE 1 The protective measure automatic disconnection of supply, requires:
a) automatic disconnection in the event of a fault to earth; b) protective earthing (to facilitate this); and c) protective equipotential bonding to reduce touch voltages.
Where the automatic disconnection of supply is adopted, protective equipotential bonding is recommended (see Figure 6), whatever the type of supply system. Bonding, by means of main equipotential bonding conductors, of extraneous-conductive-parts to the main earthing terminal of the installation is recommended; these extraneous-conductive-parts include:
a) water installation pipes; b) gas installation pipes; c) other installation pipework and ducting; d) central heating and air conditioning systems; e) exposed metallic structural parts of the building structure; and f) other metallic parts such as cable trays and cable ladder racks.
Connection of a lightning protection system to the protective equipotential bonding should be made in accordance with BS EN 62305-4.
#### my emphasis:
NOTE 2 BS EN 62305-4 requires that a bond is made between the lightning protecting system and the installation equipotential bonding system. The responsibility for specifying and installing this bond is that of the organization responsible for the lightning protection system.
####"
(not that for completeness, lighting protection systems are very rarely installed for domestic properties in the UK)
They have separate electrodes, however the are also bonded together. This helps mitigate shock risk in the event of a strike by ensuring that all accessible conductive paths will be elevated to the same (or similar) electrical potential.
Most new UK supply installations are PME (TN-C-S) (Older properties tended to more commonly be TN-S).
Significantly fewer properties use TT Earthing (i.e have to supply their own earthing system - typically via earth rods etc[2])
So PME is widely installed for public use. The limitations spelt out at the end of the page you cited (i.e. "The difficulty of ensuring that bonding requirements are met on construction sites means that PME supplies must not be used. Electricity Supply Regulations forbid the use of PME supplies to feed caravans and caravan sites.") does not really have any bearing on the current discussion, however there is a fuller explanation here if you are interested:
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Explanation of the general earthing terms here:
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[2]
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For the avoidance of doubt, I am not picking on you are arguing for the sake of it, it is just that the advice you are offering does seem to be at odds with standard wiring practices here in the UK. I accept that things are done differently elsewhere.
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