TN-S vs TNC-S (PNB) Earthing System

[Deleted member 26818]

A TN-S will have a single earth conductor from the star point of the DNO transformer.

As in ‘S’ - seperate

A TN-C-S is combined on the DNO side ‘C’ and seperate on the installation side ‘S’.

A PME will be earthed at multiple points along the PEN conductor and then linked out at the termination within the installation.

A PNB will only be linked out at the installation with no other earthing points. As soon as another earthing point is put in along the PEN conductor it ceases to be a PNB.

If it has a PEN conductor, then it’s TN-C-S.

 

[buzzlightyear]

PNB is TNS. It cannot be TNC-S as there is no conductor that shares the "earth" and "neutral" functions. It also cannot be "PME" as there is only a single connection to earth. PNB is used where a transformer supplies a single large customer. The earth reference for the transformer star-point is made at the customer's end of the supply cable, as there will not usually be a fuse rack in the DNO's substation.

 

[Deleted member 26818]

Yep.

 

[DPG]

Has Buzz got an alter ego on the IET forum then?

 

[David Prosser]

I may be missing the point, looks to me like the original question was for a HV DNO supply to customer owned switch gear and transformers. But that's just my take on it.