OK, can someone please explain...

[flyingsparks]

Just to put you in the picture, I am recently qualified, I know that I should understand all this stuff by now, but there are still a few things I dont quite get! Different earthing systems... I understand how the earths are generated for each system but its more the technical stuff, and why the rules are there..

I know that main bonds on a TT system can be 6mm, and that this is because the fault current on a TT system is very unlikely to be large. Here are a couple of other questions I have:

Why is it for a TNS system the max size main bond you need to install is 35mm depending on tail size, yet PME you could need 70mm bonds??

Why is PME not allowed on garage forecourts?

Any other info you can give me will be much appreciated..

 

[Lectrician]

The requirements for PME and TN-C-S are the same - Look at th tables in the regs. On larger size supplies, PME and TN-C-S can be a PITA.

TN-S requires a conductor atleast half the size of the main earthing conductor, but not more than 25mm.

PME is not allowed on forcourts due to possible network circulating currents (current in the earthing conductors and any bonded services such as pipework, tanks etc, and of course......mother earth). As the earth is connected the neutral in a PME install, your neutral current and that of neighbours and any other building can use the neutral AND your earth bonding etc as a return route......including possible the petrol tanks! Arcs and Sparks anyone?

 

[flyingsparks]

Why is it for a TNCS system the max size main bond you need to install is 35mm depending on tail size, yet PME you could need 70mm bonds??

Woops... I ment TNS!

Thanks for your reply Lectrician..

 

[Spark123]

Not allowed to use TN-CS (PME) for caravans or boat supplies either.

 

[Lectrician]

Not allowed to use TN-CS (PME) for caravans or boat supplies either.

Ten points for the reason why........

 

[Spark123]

Increased risk from a broken neutral for the both the caravan and boat.
Electrolysis for the boat too?

 

[RF Lighting]

Or farms....

 

[Spark123]

Don't think it is banned on farms, not recommended unless extra precautions are taken.
BS7671:2008 doesn't allow TN-C to be used, the ESQCR doesn't allow us to use TN-C anywhere without permission from the secretary of state anyhow.

 

[Lectrician]

WPD will not provide PME to farms, they always opt for TN-S which good.......

With farms being rural, and usually supplied by their own tranny on a pole, TN-S is easy to deliver.

If for any reason TN-S is not a possibility (because local tranny already supplies services as PME), then WPD will not provide any earthing terminal.

As a note, WPD will only provide one earthing type per substation or tranny. Thy will not mix systems.

PME on a caracan site can be nasty......

Lost neutral gives a rise in volts (upto phase volts) on the earth. The earth is connected to the caravan chassis. Caravan chassis and metal cladding sits at phase volts, you step out of said caravan onto 0v mother earth. Lovelly shock.

 

[flyingsparks]

Lectrcian, I have been to many farms where WPD have put in PME?! Infact, only 3 days ago I was at a farm where split phase has just been put in, and thats PME... We always put farm outbuildings on RCD..

 

[Lectrician]

All th farms I have ever done (loads of milking parlours and dairys), WPD have always installed TN-S. We where refused a TN-S connection not so long ago as th only systm available to us was PME, and they would not present this to us.

They did however fit a split con cable incase they did eventually upgrade the network.

We recently did an upgrade on a farm which had a bottling dairy. It nded a 250amp TP supply. The original was an 80 amp TT.

The tranny on the pole was PME feeding the farm, a bungalow and two mobile phone masts. All PME except for the farm.

With a 250amp supply TN-S is all you want! nothing else!

They added an extra pole, and sat a tranny just for the farm on it, giving us TN-S. The rest of the network stayed on the other tranny as PME. The WPD desinger had said he would have prefered one tranny and all supplies to be TN-S.

I have heard that mobile providers do not like PME, but have never worked on their sites to know what happens.

I know many remote masts rely on LPG gas generators for their supply.

 

[Spark123]

An RCD won't help in the problems associated with PME - your earthing system must not be connected through an RCD. The problems in some installations i.e. farms are more associated with the small voltages which can be present between the PME and true earth, animals are more susceptable to shocks for instance when they are drinking from a metal trough whilst in contact with mother earth. Ever put your tongue across a PP3 battery
One method of reducing the effect of this is to install an equipotentially bonded metal grid in the ground (practicallity aside!).
Another thing to be carefull of on farms is when installing something like an earth electrode for the likes of a TT system is that animals cannot gain access to the immediate area - a fault can cause a potential pool radiating out from the electrode, any of our 4 legged friends can become part of the path through their front legs and down their back!

 

[Lectrician]

Even without PME it is recommended to fit a grid in the floor of milking parlours. Cows legs are far apart, and with any PME network currents running in the ground, or any other fault currents, they can recieve a rather 'perky' jolt.

This makes them ****......not good for the milker!

 

[flyingsparks]

Could you please explain what you mean by the PME network currents!? Not quite grasping it!!!

 

[Spark123]

Do you understand TN-CS (PME)? I'll try to explain!
In a TN-CS the network from the supply transformer to the cutout in a premises consists of up to 3 phases and a combined neutral earth (CNE).
The CNE is connected to electrodes along the network in order to keep it close as practicable to the potential of earth, also reduces some of the risk of open circuit CNE.
At the suppliers cutout in the premises the CNE is connected to both the neutral and an earth terminal i.e. they are separate from here-on. (This is where the S comes from in TN-CS, it is TN-C up to the premises, TN-CS after.)
As the CNE as it is being used for a neutral and it has an impedance, current flowing down it causes small voltages (ohms law) which are above that of true earth and is apparent on any metalwork connected to it.