Earthing arrangements to a generator and change over switch

[Ian1981]

Need someone either confirming or someone to put me right!
Regulation 551.4.3.2.1
Protection by ADS shall not rely upon the connection to the earthed point of the system for distribution to the public when the generator is operating as a switched alternative.
A suitable means of earthing shall be provided.

To me this means when the generator becomes the main source of power to the installation, then the generator TNS earthing arrangement shall not be connected to the DNO supply authority earthing arrangement,say it’s pme arrangement.
Am I correct?

 

[buzzlightyear]

recommending that all earths should remain connected?.
Just for clarification
The earth bar has the following permanently connected:
equipotential bonding,
Generator earth spike
incoming earth from the DNO

 

[buzzlightyear]

recommending that all earths should remain connected?.
Just for clarification
The earth bar has the following permanently connected:
equipotential bonding,
Generator earth spike
incoming earth from the DNO

 

[jamie-spark]

It’s just an argument I’m having with an ‘electrical designer/engineer ‘ on a job I’m on.
I say a link needs to disconnect the generator earthing from the dno arrangement when the changeover switch is utilised.
He says I am wrong.

I’m thinking it could be near impossible. consider 4 industrial units, all with a seperate DNO supply, common steel frame , all bonded ect ect , you have a backup generator how can you disconnect all there earthing connections

 

[davesparks]

I would assume this would then change the characteristics of the installation to a TT system while the DNO earth link was disengaged, and all TT regulations would be applicable at this stage

No, generators with a neutral-earth link are normally a TNS system.

 

[David Prosser]

WPD are perfectly happy for us to leave the DNO earth connected when running on generators. HSE guidelines suggest connecting to true earth through an earthing point of an adjacent fixed installation. This I would think would it self be connected to the DNO earthing.

 

[buzzlightyear]

Correct Dp the generators will have a spike in ,the three core will be feed to the building to the earth term .

 

[Ben Hutchinson]

Morning all - this is something im currently looking into but don't have much experience in myself.

To clarify; Does the mains earthing arrangement need to be switched in the changeover switch to a TT system when the generator is used as the mains supply?

Everyone I have spoken to has only ever seen 3pole + switched neutral change over switches so I'm unsure of what is the best way to complete the works.

 

[Lucien Nunes]

I think post 5 sums it up. You must not rely on the DNO supply for an earth when using the genny, but probably don't need to disconnect it either. Ask the DNO.

to a TT system when the generator is used as the mains supply?

Having an earth rod at the genny doesn't make it TT. The earth terminal of the genny is referenced to real earth by the rod but the installation MET connects to that terminal by a cable, which makes it TN-S. Fault current from the installation flows through the cable, not the rod. It would only be TT if the genny had a rod and the installation had a rod, and the only connection between the two was the mass of earth, which is daft.

 

[pc1966]

Just to add that often generators have output RCD in case they are used with sockets (stand alone supply) but also often to make sure they disconnect quickly as the PFC on a real fault from the generator is going to be far short of the grid's capacity. It that context they look a little like a TT supply, but as @Lucien Nunes points out, really are TN-S.

 

[Rockingit]

BS7430 is the reference for this - a genny servicing a building should be connected to that building's MET.

 

[Sean keyser]

Yes, you must isolate the supply earth from the generator earth. Engineers working on the DNO network don't like back-feed from gennys, it makes their hair curl...

If I have a stand alone ATS panel. Do I run the earth from the generator to the main Distribution board and bond the ATS panel.