TN-S Earthing

[Gardner]

Are earth rods or earth electrodes required for TN-S supply?

 

[Gardner]

Combined Neutral and Earth

OK, that would make it a MEN system.

In a TT system do DNOs usually earth the neutral repeatedly or only at the 11-400Y substation? Of course this would not matter at the customer but just trying to get a sense of whats going on.

 

[spark 68]

OK, that would make it a MEN system.

In a TT system do DNOs usually earth the neutral repeatedly or only at the 11-400Y substation? Of course this would not matter at the customer but just trying to get a sense of whats going on.

Have a look at this attached pdf,

 

[davesparks]

OK, that would make it a MEN system.

In a TT system do DNOs usually earth the neutral repeatedly or only at the 11-400Y substation? Of course this would not matter at the customer but just trying to get a sense of whats going on.

Never heard of MEN as an earthing arrangement, it is a TNCS system and may be PME.

What the DNOs get up to is entirely up to them, if the supply was designed as a TT system then you'd expect it to only be earthed at the substation transformer yes. But if it has been made TT because the DNO have declined to make an earth connection available to the customer it could be anything.
It could also be that the DNO have made their TNCS earth connection available but the regulations have prohibited its use in the installation.

 

[Gardner]

Have a look at this attached pdf,

That clears up a lot! Thanks

One question that stands out. One page 3 where a TN-C-S system is shown it says "Additional source electrode" What do they mean by that? Is this electrode part of the DNO or electrician supplied connected to the MET?

 

[davesparks]

That clears up a lot! Thanks

One question that stands out. One page 3 where a TN-C-S system is shown it says "Additional source electrode" What do they mean by that? Is this electrode part of the DNO or electrician supplied connected to the MET?

That is an example of one of the many Earth electrodes which would be connected along the length of a PME distribution cable (on the DNO side)

But an electrode can be installed by the electrician and connected to the MET if they deem it to be necessary. This is common practice in some countries and a requirement in others (Ireland springs to mind for this)

 

[Gardner]

That is an example of one of the many Earth electrodes which would be connected along the length of a PME distribution cable (on the DNO side)

But an electrode can be installed by the electrician and connected to the MET if they deem it to be necessary. This is common practice in some countries and a requirement in others (Ireland springs to mind for this)

But optional in the regs. I think its beginning to fall into place now.

As for my final point, caravans must employ a TT system when the DNO is TN-C-S? At least this is what the document makes not of, unless I am misinterpreting the term "Pitch".

Is this a practical safe guard in the event the PEN breaks in the DN which would energize the caravan frames?

 

[UKMeterman]

Correct, this is to prevent a caravan park mass slaughter due to an open combined neutral and earth conductor.

 

[Gardner]

Correct, this is to prevent a caravan park mass slaughter due to an open combined neutral and earth conductor.

If the DNO is TN-S this can be skippted or must TT also been employed?

 

[davesparks]

But optional in the regs. I think its beginning to fall into place now.

As for my final point, caravans must employ a TT system when the DNO is TN-C-S? At least this is what the document makes not of, unless I am misinterpreting the term "Pitch".

Is this a practical safe guard in the event the PEN breaks in the DN which would energize the caravan frames?

It's not just caravans, there are many installations where PME is not permitted, construction sites, outdoor events, parts of farms etc.

 

[UKMeterman]

if it is declared as TN-S then the supply company are declaring that N and E are electrical seperate except at the star point of the local transformer.

 

[Gardner]

It's not just caravans, there are many installations where PME is not permitted, construction sites, outdoor events, parts of farms etc.

if it is declared as TN-S then the supply company are declaring that N and E are electrical seperate except at the star point of the local transformer.

All this places would then be fine with either TT or a true TN-S, since a broken PEN will not electrify anything. I have to say, a bit different than the NEC. In the NEC TN-C-S supplies feed caravans all the time after the main disconnect. The supply to the caravan is always 4 wire, but 3 wire up to the pedestal.

 

[UKMeterman]

I will rephrase my comment, in a TN-S the neutral and earth conductors are physicaly different and are only connected together at the local substation.

With TNCS by law they are seperate in a consumers installation but may be the same conductor in the supply network.

For interest sketch out what hapens if you say have a 25 ohm load on the consumer side and and the PEN conductor to one customer breaks and note the voltage on the consumer installation "earth" to "true earth"

 

[Gardner]

The MET goes to 230 volts relative to remote earth. So while people inside the caravan might not feel it because the frame of any appliance to pluming will be close to zero volts (both bonded to MET), the skin to earth soil will be 230 volts. So anyone standing on grass will feel it.

 

[davesparks]

All this places would then be fine with either TT or a true TN-S, since a broken PEN will not electrify anything. I have to say, a bit different than the NEC. In the NEC TN-C-S supplies feed caravans all the time after the main disconnect. The supply to the caravan is always 4 wire, but 3 wire up to the pedestal.

And American electrical safety is widely regarded as being decades behind us. Simple things like testing are rumoured to be an alien concept to American electricians.

 

[UKMeterman]

You have now understood one of the major problems, persons stood on the grass and in contact with the frame of the caravan are at risk of a fatal electric shock.

 

[Gardner]

And American electrical safety is widely regarded as being decades behind us. Simple things like testing are rumoured to be an alien concept to American electricians.

Not a rumor, its true. Testing is an alien concept, being none existent. Not to mention many are lacking in solid electrical theory.

 

[UKMeterman]

Also I have seen in the states 4 wire HV systems with the N combined with the local LV N!

 

[Gardner]

Also I have seen in the states 4 wire HV systems with the N combined with the local LV N!

Yup. POCOs will combine the function of the MV neutral and the LV neutral into one conductor, with nearly all MV-LV transformers being wye-wye (Yyn0). NEC requires ground rod(s) at every service and all TELCO/phone shields are bonded to it as well. Combined with metal water mains the POCO has tons of free neutral paths. The earth in the US is basically a giant neutral conductor. I have designed and worked on such system being so common, but its one of the dumbest ideas ever conceived.

 

[UKMeterman]

Along with SWER HV supplies....

 

[Knobhead]

Deleted

 

[Gardner]

Along with SWER HV supplies....

Those are rare in most of the US, but when a MV PEN breaker here it is practically the same thing.