PME and main bonding
- Thread starter Podooser
- Start date
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It is important for all types of EEBADS systems, however with PME (TN-CS) there is a possibility where there is an open circuit neutral, for all the exposed/extraneous metalwork in the installation to become live at mains voltage. Equipotential bonding keeps all metalwork which can introduce a potential at or about the same potential hence in theory reduces the risk. I wouldn't like to be on the receiving end of it though!
It should also be noted that in normal use, the voltage of the MET may be slightly different to that of mother earth owing to the impedance of the supply neutral conductor and the current flowing in it.
It should also be noted that in normal use, the voltage of the MET may be slightly different to that of mother earth owing to the impedance of the supply neutral conductor and the current flowing in it.
And that last line......
Due to this, in certain types of an installation, PME is not permitted by many DNO's.
A wet person can recieve a 'percieved' shock due to potential differences between the extraneous or exposed conductive parts and mother earth (soil).
Swimming pools and communal shower rooms are typical examples. If PME is the only option here (ie, TT cannot be adopted in that area due to parallel earth paths to the remaining PME installation....pipe, structural steel), then earthed metalic grids should be installed in the floor screed.
Another good example of percieved shock is an outside tap fed in copper. Tap is at the potential of the PME earth, and you are standing on mother earth outside. There have been many cases of surfers recieving or complaining of shocks due to 'percieved shock' when they wash their surfboards and themself down using the tap (when they turn it on/off).
Due to this, in certain types of an installation, PME is not permitted by many DNO's.
A wet person can recieve a 'percieved' shock due to potential differences between the extraneous or exposed conductive parts and mother earth (soil).
Swimming pools and communal shower rooms are typical examples. If PME is the only option here (ie, TT cannot be adopted in that area due to parallel earth paths to the remaining PME installation....pipe, structural steel), then earthed metalic grids should be installed in the floor screed.
Another good example of percieved shock is an outside tap fed in copper. Tap is at the potential of the PME earth, and you are standing on mother earth outside. There have been many cases of surfers recieving or complaining of shocks due to 'percieved shock' when they wash their surfboards and themself down using the tap (when they turn it on/off).
Can you give me an example of that?
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Neutral on the pole outside corrodes, and goes open circuit or high resistance.
Loose connection within service head, arcs and sparks, goes high resistance.
Tree falls, and severs the bottom neutral on a pole (missing the top 3 lives).
list goes on.....
Loose connection within service head, arcs and sparks, goes high resistance.
Tree falls, and severs the bottom neutral on a pole (missing the top 3 lives).
list goes on.....
Neutral on the pole outside corrodes, and goes open circuit or high resistance.
Loose connection within service head, arcs and sparks, goes high resistance.
Tree falls, and severs the bottom neutral on a pole (missing the top 3 lives).
list goes on.....
So you have lost neutral and therefore also earth and all there is indoors is a potential of 230V and no path to earth. How does connecting the incoming gas and water to the MET make things safer. (Bare with me, penny has not dropped on this one yet)
In theory yes, but it aint a reg.......so.......?
I don't follow
Equipotential zone... think of birds on a HV transmission line...
Plus, if they haven't been replaced with plastic yet, then they contribute to the M in PME and drag down the potential towards that of the body of the earth... whic is why there they have to be sized to carry a reasonable amount of current... see table 54H of BS7671
If you don't bond the incoming water and gas then the neutral and earth conductors will rise to full live potential, the pipework will remain at ground potential. Touch both at the same time and you get a nasty (potentially fatal) shock.
if you do bond them then the neutral and earth will rise to somewhere between true earth potential and live potential, exactly where in between will depend on how good an earth the pipework provides. but the important thing is that the pipework and the electrical earth will be at the same potential so touching both of them at once will not be a shock risk.
and yes adding more earth electrodes to a combined neutral and earth core is generally a good thing because it helps reduce the affect of neutral faults and local ground potential differences. In the uk the supplier is required to do this on the main cable at regular intervals but it is not required for individual houses to have thier own electrodes.
btw rods aren't the only type of earth electrode that can be used where an electrode is required, a bare wire in a trench is also very effective (though you have to make sure it is thick enough to keep corrosion problems down, i belive 25mm is required for bare underground earth wires by the regs).
50mm hard drawn copper is often used as the norm.
Not sure that's 100% correct, if you have PVC incoming water main and gas main, how does this earth all the pipework ???
What you have if you don't bond is a resistance of all pipework if not bonded, you then have Volts/Resistance = Amps (current will flow), should you touch both.
Also if not bonded and the pipework becomes live then touching anything earthed will allow current to flow.
What you have if you don't bond is a resistance of all pipework if not bonded, you then have Volts/Resistance = Amps (current will flow), should you touch both.
Also if not bonded and the pipework becomes live then touching anything earthed will allow current to flow.
May be this is obvious (so excuse me for this) but just in case; The difference between NC-S and TN-C-S (PME) is that in the NC-S the main earth conductor is connected to the incoming supply cable shield (in general), so if you have a neutral fault you will still have connection to earth. In the PME arrangement the earth and neutral connected to the same point meaning that if you loose the neutral you will loose your earth. (Please do not jump I tried to make it simple as I am sure that there are many other people that are a bit confused with this staff)
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