Neutral Earthing at source

[ProMbrooke]

Why is earthing of the neutral forbidden at the supply transformers in a 3 phase 3 wire system? This has me beyond words confused. 

 

[poni]

I don't think it is, what you have shown is two different source's, one could be from a Network and one genny or two genny supply's ,I would think that its in case one source is removed. 

 

[ProMbrooke]

Correct- two sources- but the foot notes say neither can be earthed. Only in the main gear and beyond.

 

[Phoenix]

You only want one N->E bond and it needs to be in place all the time, you don't want the earthing of one source dependant on a link fitted elsewhere. There will be even though its not shown in the diagram, provision for switching the feeds from the source, you shouldn't want for instance, a situation of planned maintenance on one transformer, so all the esstenial load gets switched onto the other one, with it isolated from the bus, and the transformer isolated and earthed at the HV end, you wouldn't want to be a possition where you could be loosing the N->E link with the installation live, that would be very bad.

Why are you against the link being located in the main switchboard?

 

[ProMbrooke]

I want the neutral earthed in all 3 places, at source 1, source 2 and at the main gear.  Nothing gained by isolating it.

 

[kerching]

Post deleted as I was talking  

 

[Tony S]

Industrial installations are by nature TN-S unless dispensation is granted. Public supplies can be any mix of TN-S, TN-C-S, PME or TT.

Read this.

Have a look at this.

 

[ProMbrooke]

Ok, but why require that the neutral be isolated from earth when it does not carry any load?

 

[Tony S]

Neutral does carry current and that current will take the easiest rout.

 

[ProMbrooke]

It won't, not with everything connected line-line. Perhaps PE would be a better term.

 

[Phoenix]

Do you really want the conductors between the transformers and the switchboard to be PEN/CNE? You'd have to class the installation as TNC-S then, and that is likely to cause you some headaches in terms of bonding conductor sizing and exporting earths to cabins, car chargers, etc

Plus once you have created your separate neutral and earth by putting the link in and connecting onto the LV electrode, if you then connect neutral to earth later on, that would run into difficultly with ESQCR as you can't connect neutral to earth legally within an installation, if the only link is placed in the main switchboard then the conductors prior to this point are simply transformer unloading tails, the concept of neutral and earth starts from where you tie down to the LV earth and branch out into separate neutral and earth conductors

 

[Tony S]

One of the reasons for locating the N→E link at the switchboard is so Restricted Earth Fault protection can be utilized.

 

[ProMbrooke]

Do you really want the conductors between the transformers and the switchboard to be PEN/CNE? You'd have to class the installation as TNC-S then, and that is likely to cause you some headaches in terms of bonding conductor sizing and exporting earths to cabins, car chargers, etc

Plus once you have created your separate neutral and earth by putting the link in and connecting onto the LV electrode, if you then connect neutral to earth later on, that would run into difficultly with ESQCR as you can't connect neutral to earth legally within an installation, if the only link is placed in the main switchboard then the conductors prior to this point are simply transformer unloading tails, the concept of neutral and earth starts from where you tie down to the LV earth and branch out into separate neutral and earth conductors

Ok, I don't thank folks are understanding. There are no line to neutral loads, so there is no PEN or TN-C. It is TN-S without the neutral distributed.  But for some reason the regs want a none current carry conductor to be treated as it would be carrying current.

I'd like to earth at both sources and the gear.

One of the reasons for locating the N→E link at the switchboard is so Restricted Earth Fault protection can be utilized.

Valid point- but why not CTs around each phase?