Idle question about TNCS systems

[Cigaro]

Recently I have been reading up on various earthing systems, not for any real reason other than I'm interested in electrics

My understanding is that in a TNCS system (like in my house) you have a neutral and a live from the supplier, and your house's earth is connected to the neutral at the cutout.

Now I am aware that in the event of, for instance, a broken neutral connection back at the substation, your house and a house on a different phase will see a total of 480 volts (between phases), which is dangerous in itself and causes things to blow up (apparently there have been a few instances of this).

What I'm confused about is this: Doesn't that also mean that, because the earth is linked to the now floating neutral conductor, any bonded metalwork in your house would then rise to 240 volts with respect to true earth?

Isn't this dangerous?

I am just puzzled about this.

Cheers

 

[JohnW2]

What I'm confused about is this: Doesn't that also mean that, because the earth is linked to the now floating neutral conductor, any bonded metalwork in your house would then rise to 240 volts with respect to true earth?
Isn't this dangerous? I am just puzzled about this.

That's the whole point of the main bonding - to attempt to ensure that no metalwork in the house can be at a different potential from the DNO's 'earth''connection, even if the potential of that 'earth' connection rises to considerably above true earth. In other words, main bonding seeks to prevent any supply pipes etc. bringing 'true earth' into the house, by connecting it via a fat cable to the DNO's 'earth' terminal.

Kind Regards, John.

 

[jaysean1]

With any luck if the neutal has broken from the cable the live would have broke too and the REC fuse would have blown

 

[Dazz0i]

Surely due to PME the neutral would still be earthed?

 

[JohnW2]

Surely due to PME the neutral would still be earthed?

That surely would depend entirely upon where the neutral was broken relative to the multiple earths of the PME system. If there was no such earth between the break and the installation of the consumer in question, then the incoming neutral would presumably not be earthed by the PME.

In practice, of course,main bonding will usually/often have the effect of earthing the incoming neutral/'earth' via incoming supply pipes under these circumstances, although there's no telling how good a path to earth those pipes etc.will represent.

Kind Regards, John.

 

[Dazz0i]

As the OP mentions at the substation. I'd assume there'd be some "along the way".

As more and more gas/water becomes plastic, how long before they're totally useless to us for these instances.

 

[JohnW2]

As the OP mentions at the substation. I'd assume there'd be some "along the way".

Whoops, I hadn't noticed that - so, yes, you're probably right. A neutral fault in the cabling itself could obvioulsy be a different matter.

As more and more gas/water becomes plastic, how long before they're totally useless to us for these instances.

We're obvioulsy moving in that direction. Mind you, I don't think the utility companies (particularly gas) usually run plastic right into the property, do they? If not, there would usually still be at least some underground metalwork, albeit probably not much.

As I understand it, some countries require properties with a TN-C-S supply to also have its own earth electrode, and one can see some merits in that.

Kind Regards, John

 

[Dazz0i]

Gas supply is heat welded plastic pipe right up to the meter box now, onto a gate valve union, followed by the flexi and regulator, meter, then consumers copper supply pipework.

So not much continuity there.

Can also see merit in an electrode, would remove this possibilty for the sake of £100ish.

 

[JohnW2]

Gas supply is heat welded plastic pipe right up to the meter box now, onto a gate valve union, followed by the flexi and regulator, meter, then consumers copper supply pipework.

So much for my theory, then

Can also see merit in an electrode, would remove this possibilty for the sake of £100ish.

Indeed. As you may have seen, I've been known to express my surprise that TN-C-S supplies ever came to be allowed, and suggested that if I had such a supply (which I don't) I'd be tempted to ignore the DNO's 'earth' terminal and run the installation as TT.

Kind Regards, John.

 

[Spark123]

Now I am aware that in the event of, for instance, a broken neutral connection back at the substation, your house and a house on a different phase will see a total of 480 volts (between phases), which is dangerous in itself and causes things to blow up (apparently there have been a few instances of this).

In bog standard 3 phase systems the phases are 120 degrees apart so the max you'll see between any two phases is 415v (or as they like to call it now 400v).
I have seen the damage that happens when the neutral is lost before, it is effectively a battle of the phases - the one with the lowest impedance (i.e. highest load) wins and the other two phases get more volts across them with respect to neutral.
It is not just common to a TN-CS supply, this can happen on any supply type. The major difference as you say is the neutral is used as the earthing for the supply hence you will get a varying voltage on the ecps with respect to mother earth however as your house is still connected to say next doors via the neutral then the risk is reduced, you shouldn't see 415v between the two.
TN-CS also uses PME or protective multiple earthing, this ties the combined neutral and earth down to mother earth at points along the system to reduce the effect should the CNE go open circuit.

 

[bernardgreen]

As the OP mentions at the substation. I'd assume there'd be some "along the way".

One can assume that but how effective would they be at returning the current of a broken neutral via the ground to the star point at the sub-station transformer. They are after all only ground rods along the route of the cable. Not much current if the supply is phase balanced but few are that well balanced.


So the PME "earth" derived from the neutral can go to a potential significantly different to that of the ground.

Inside the equipotential area of the house all is OK even in the neutral goes to some significant voltage above ground.

But tingles from water taps on the outside of a house and noticable shocks from gas meters on oustide walls are often mentioned. And that is when the neutral is intact but has a voltage drop along it..

 

[Paul_C]

As I understand it, some countries require properties with a TN-C-S supply to also have its own earth electrode, and one can see some merits in that.

That's the case in the U.S., where all regular residential supplies are arranged as TN-C-S and the NEC (National Electrical Code) requires a local earth electrode to be connected to the incoming neutral at the service entrance.

 

[JohnW2]

As I understand it, some countries require properties with a TN-C-S supply to also have its own earth electrode, and one can see some merits in that.

That's the case in the U.S., where all regular residential supplies are arranged as TN-C-S and the NEC (National Electrical Code) requires a local earth electrode to be connected to the incoming neutral at the service entrance.

That certainly makes sense to me. Indeed, if the supplier installed an earth electrode for each service entrance, that could presumably be the 'PME', couldn't it?

Of course (and even though some people use the terms as if they were synonymous), TN-C-S, as defined, does not have to have 'PME' - but I suspect that every supplier, all over the world, probably does implement PME with it, to make the TN-C-S a bit less potentially dangerous!

Kind Regards, John.

 

[ericmark]

In theory the M in PME means multiple and TN-C-S should have many points where the supply is earthed and so no problem.

However in the real world that is not the case. I have only come across it once in the 40 odd years as an electrician, but it did happen, and once the load melted the 4mm earth wire a radio ham had going to his radio earth, the voltage went crazy.

But although most people in street lost equipment, no one other than the worker who hit the cable was injured in any way.

Having said that, if it is safe? then why for Petrol stations, Caravan sites and Marinas is it banned?

I have never heard of a single person being injured as a result of a PME supply.

The only supply type where I know of injuries due to supply type, is with an IT supply. The TBM in Hong Kong had an IT supply, and that was shocking.

Maybe some one has heard of an injury? As to equipment lost losing a neutral will likely always cause that what ever type of supply. So since the multiple earthing will likely limit the amount the supply will vary as far as equipment damage the TN-C-S supply is less likely to damage equipment due to neutral break.

 

[JohnW2]

In theory the M in PME means multiple and TN-C-S should have many points where the supply is earthed and so no problem .... However in the real world that is not the case. .....But although most people in street lost equipment, no one other than the worker who hit the cable was injured in any way..... I have never heard of a single person being injured as a result of a PME supply. ... Maybe some one has heard of an injury?

I suppose the main hazard (risk of injury/death) which people think about is the occurrence of a supply-side neutral fault in the supply to an installation which does not have (any or adequate) main bonding. Whether that theoretical scenario has ever resulted in harm, I don't know.

Kind Regards, John.