Standby Generator Earthing (TT or TN-S)

[Locus]

Hi Everyone

The utility supply for an installation is of TT system.

The Generator start point is connected to the its frame and earthed to an electrode. Thus, the installation has its own earthing electrode.

I would like to know whether the installation is a TT or TN-S system when the standby generator is in operation.

 

[Ian1981]

Generally generators 10kva and above will be TN-S with the rod connected to the neutral of the generator via the chassis giving the neutral 0 volts potential to earth or there abouts

 

[davesparks]

With the star point connected to the general mass of earth, and an earthing conductor from the greater to the installation then it is TNS

With the star point connected to the general mass of earth, and the installation earth connected to the general mass of earth but no earthing conductor from the generator it is TT (and a pretty much ridiculous design)

 

[Locus]

Why should the earthing of the generator connected to the installation earth since the utility is a TT else as stated it will be a TNS.
What about the electrical protection shifting from TT to TNS during generator operation.

Thanks

 

[Locus]

I understand that if the power cable shield is connected through cable gland at the Generator and the other end is connected to the LV panel through cable gland, then the generator neutral/earth is connected to the installation. Since the installation has its own earthing is being connected to the generator earthing through the cable shield, this create a TNS system.
However, if the cable shield is not connected to the generator frame, then it is a TT system.

Since, all the electrical protection was calculated for the TT system, is it not better to allow a TT earthing system for both the generator and utility operation i.e. both earthing system should be the same either TNS/TNS or TT/TT.

 

[bill01803]

If the generator is under 10kva you will have to make it tns after the cable leaves the generator and label it thus.
FYI under 10kva generators are not supposed to be used with more than one class 1 appliance

 

[davesparks]

If the generator is under 10kva you will have to make it tns after the cable leaves the generator and label it thus.
FYI under 10kva generators are not supposed to be used with more than one class 1 appliance

Where have you got that from?

 

[davesparks]

Since, all the electrical protection was calculated for the TT system, is it not better to allow a TT earthing system for both the generator and utility operation i.e. both earthing system should be the same either TNS/TNS or TT/TT.

Not at all, connecting the generator up as a TT is pointless when you can actually run an earthing conductor and have it as a TNS which gives you an a good conducting path all the way back to the star point.
Whatever protection has been calculated for the TT from the DNO will almost certainly not be right for the generator however it is connected. A generator cannot deliver the fault current that the distribution network can, the engine will stall if it attempts to deliver that much current. This is why generator outputs are protected with variable RCDs.

 

[Davisonp]

FYI under 10kva generators are not supposed to be used with more than one class 1 appliance

It is floating earth generators (earth of generator outlet not connected to neutral at all) that are not suitable for use with more than 1 class 1 piece of equipment.

 

[Davisonp]

Can I ask what size generator it is? And if under 10K, have you confirmed the earthing arrangements (Eg measured voltage between Live / Neutral at the Generator outlet and live to earth at the generator outlet)? I would not trust what someone says about how the generator is set up as suppliers and even manufacturers can give out incorrect information.
In an installation I did, the manufactures told me they did not know the earthing arrangements in the generator. The suppliers told me all I needed was to bang in an earth rod and it would be fine. They had installed dozens of this model exactly this way and not had a problem, but when I tested the generator, it was a floating earth system, so I informed the suppliers that they had installed several potentially lethal generators and they were not happy.

I would agree with Davesparks on earthing. For domestic use, you MUST have a generator that is configured as TN-C-S, so a very low resistance earth is supplied from the generator. To throw that away and instead use two earth spikes creating a much higher resistance earth path is silly.


I have attached some documents that better explain the earthing within generators and requirements if you are interested. The IET document is not for backup generators directly but it does explain what is required.

 

[Locus]

Hi Everyone

I wish to point out that I'm referring to industrial installation of Utility and Standby Generator operation and changeover for three phase system.

My query to the TT and TNS earthing system during standby generator was in regard to an Engineer comment. The latter stated that when the generator is in operation, the system become TN.

My question was that is it really TN or TT in regard to the following argument.

Firstly, the Main switchboard is a TT system for Utility supply as a separate earth electrode is installed for the metallic of panel and equipment. Hence, the earth conductor is connector to the main earth terminal in the switchboard.

Now, the Generator star point is connected to its Frame and connected to its own earth electrode. Thus, if the power cable shield is connector to the generator frame and the other end is connected to the main switchboard panel, the generator earthing is now connected to the Main Switchboard earth electrode. In this respect, it form a TNS.

On the other hand is a non armoured 4C cable is connected from the Generator to the Main LV Board, then it form a TT system.

Now, my concern is what is the requirement and why should it be TNS or TT.