Things that make you go Hmmm.....

[plugwash]

It's a problem with TN-C-S in general, it can easilly cause significant currents in things that were never really meant to carry signficant currents be they water and gas pipes or the lead sheaths of former TN-S cables.

It's not usually too much of a problem because things with low CSA also have relatively high resistance so in most cases will only get a small share of the current. Larger CSA items will get more current but will also be better able to deal with it. The exception to this is when you have a long section of large CSA metal and a short section of small CSA metal.

That is why we have sizing requirements for main bonding conductors in TN-C-S systems.

You say that cable is carrying about 1/3 to 1/2 of the instalaltions neutral current. Even without harmonic loads in a worst case loading scenario the neutral current could be as high as 400A before the phases are overloaded making the earth current betwen 133A and 200A. Harmonic loads could make things worse still.

IMO you really need to call them back and tell them that you belive the conductor sizes they have used in the PME conversion are too small.

 

[JohnW2]

It's a problem with TN-C-S in general, it can easilly cause significant currents in things that were never really meant to carry signficant currents be they water and gas pipes or the lead sheaths of former TN-S cables.

That's clearly true, but aren't former TN-S cable sheaths, if intact, in a different ballpark from everything else? The resistance (which also includes the resistance of the DNO's electrode at the transformer) of the return path via mother earth and service supply pipes, structural metalwork etc. is going to be relatively high, such that not all that high currents are going to flow. On the other hand, the return path via an intact metallic sheath which goes all the way back to the transformer is presumably going to have a much lower resistance, hence will take a much higher proportion of the neutral current.

Kind Regards, John

 

[bernardgreen]

A system described as TN-S has separate conductors in the mains and services cables for the Neutral and the Earth.

But exactly how and where is the Earth conductor grounded ? At the sub-station is it connected to the same ground rod as the star point ( neutral ) ? Or are there two seperate ground rod to ensure the -S applies to the whole of the distribution network.

 

[JohnW2]

A system described as TN-S has separate conductors in the mains and services cables for the Neutral and the Earth. But exactly how and where is the Earth conductor grounded ? At the sub-station is it connected to the same ground rod as the star point ( neutral ) ?

That's always been my understanding (well, probably connected to, or very close to, the star point, not to its rod), but I may be wrong. Why would one 'unecessarily' introduce two earth rods into the fault current path, thereby largely destroying the concept of TN-S? In fact, if the two rods were far enough apart not to interact, you would effectively have created a TT supply - with a very long earthing conductor!

Or are there two seperate ground rod to ensure the -S applies to the whole of the distribution network.

I think that, quibbling apart, the "-S" probably still applies even with the first option above, if the CPC is connected to, or close to, the star point.

Kind Regards, John

 

[westie101]

That's always been my understanding (well, probably connected to, or very close to, the star point, not to its rod),

I may of mentioned this on another thread.

In a substation we are connecting two different things to earth.
1/ The star point of a 3 phase transformer, or for single phase one end of the secondary winding. (in our terminology the LV earth)

2/ What is classed as the metal work, which is the steel tanks, the cable sheaths, the metal within a substation. (in our terminology the HV earth)

Now we then have two options.
If we can reduce the earthing system (system as it will include multiple rods & bare wire, we also try to "surround a substation with rods (on each corner) and wire between, to control voltage gradients within the substation) to an impedance to below 1ohm we can connect all the earthing together as a common system.
This is measured with the earthing system not connected to any cable or equipment 1.e. stand alone value.

If we cannot do that we would install one earth system to below 20ohm for the star point or single end, and a seperate system for the rest of the equipment with a value of below 40ohm. There has to be a minimum 10m seperation between these seperate systems
This is more common on overhead networks or in areas with poor ground conditions like rock.

Obviously if the associated sytems are underground and interconnected the earth impedance will reduce as we then interconnect everything, if using older lead sheathed type hessian covered cables these laid in the ground will also become part of the earthing system.

The only difference for PME is the addition of the extra rods, usually at the ends of main cable runs

Looking at the "lash ups" on here we are prohibited from doing this sort of thing by our CoPs, in fact I'm surprised that other DNOs allow it!

 

[JohnW2]

That's always been my understanding (well, probably connected to, or very close to, the star point, not to its rod),

I may of mentioned this on another thread....

Thanks. All interesting stuff, but it doesn't really address the question which Bernard raised. If there is a TN-S/SNE main, where is the end of that connected?

Kind Regards, John

 

[westie101]

If there is a TN-S/SNE main, where is the end of that connected?

If you mean this one?

But exactly how and where is the Earth conductor grounded ? At the sub-station is it connected to the same ground rod as the star point ( neutral ) ? Or are there two seperate ground rod to ensure the -S applies to the whole of the distribution network.

It will be connected at the substation on our LV distrubution equipment which will connect to the earthing systems I have described

 

[JohnW2]

If you mean this one?

But exactly how and where is the Earth conductor grounded ? At the sub-station is it connected to the same ground rod as the star point ( neutral ) ? Or are there two seperate ground rod to ensure the -S applies to the whole of the distribution network.

Yep, that's the one.

It will be connected at the substation on our LV distrubution equipment which will connect to the earthing systems I have described

Thanks. As I wrote to Bernard, that had always been my understanding. As I also said, if the 'TN-S' earth conductor/sheath were (as Bernard was suggesting/asking) connected to a separate earth rod, the services provided to consumers would effectively be 'TT with a very long earthing conductor', wouldn't it?

However, I suspect that there could still be some 'nit picking'. Exactly where would the sheath/whatever of the main usually be connected to your LV distribution equipment - at the star point, close to the star point, at or close to the earth electrodes, somewhere else or 'any of the above'?

Kind Regards, John

 

[westie101]

'any of the above'

Pretty much that as it can vary physically from site and equipment to site and equipment. Electrically it can be close to the star point on combined earthing systems or 10m away for seperated earthing systems.

Don't also forget, as I've said, that for underground SNE cable, of which the vast majority is lead sheathed & hessian covered, that also is one huge earth network.

Unlike the concept of the single earth rod as often seen in TT systems we, again as I've said, use networks of multiple rods, bare copper wire and often interconnect the whole system to adjacent substations.
In effect this would mean that for a city like Manchester virtually the whole of the network is interconnected and, with the exception of PVC covered cables, forms one huge earth system!
In these terms the actual point at which an earth sheth connects becomes a bit insignificant!

 

[JohnW2]

'any of the above'

Pretty much that as it can vary physically from site and equipment to site and equipment. Electrically it can be close to the star point on combined earthing systems or 10m away for seperated earthing systems.

Thanks - that's what I rather suspected - which is why I offered you that option

Don't also forget, as I've said, that for underground SNE cable, of which the vast majority is lead sheathed & hessian covered, that also is one huge earth network.

Yes, I realise that, but I would imagine that the metal path directly to the substation will almost always provide a lower impedance path to the transformer than any route invoving 'mother earth', won't it?

Kind Regards, John

 

[westie101]

but I would imagine that the metal path directly to the substation will almost always provide a lower impedance path to the transformer than any route invoving 'mother earth', won't it?

Probably