Confused at Standby Generator earthing.

[Locus]

Dear All

I have read several posts but still confused. I hope someone can shed some light on the Regs.

1. The Incoming Power Supply from the Utility to a Plant is 3 Phase (4 wires) TT Earthing System.

If a Standby Generator says 100 kVA (3 phase +N) is installed at the Plant with an Automatic Transfer Switch install (4 Poles), then should we need to install independent earth electrodes for the Generator Set i.e. 1 No. for the Generator Set Body Frame AND 1 No. for the Star Point (Neutral) OR Link the star point to the frame & connected to a single earth electrode.

Thanks in advance.

 

[R-fur]

I would expect to see one LV earth system for the neutral and the frame, with a maximum resistance of 20 ohms.

 

[pc1966]

Usually the generator will have the neutral earthed internally (or via a configurable link) so the ATS should be 4-pole switching so you never connect the incoming supply's neutral to your local earth arrangement. However you can share the local earth with the generator's earth terminal.

What does the generator instructions say?

 

[davesparks]

Having a TT system makes this quite simple, the whole installation and generator can be connected to the one earth electrode network/system permanently.

 

[Locus]

@R-fur, PC166 & Davesparks. Thanks.

The following points are noted:
1.@R-fur: 1 No. Earth Electrode connected to the Generator Body & Neutral (i.e. Neutral Link to Generator Body & connection to its dedicated earth electrode).
The Earthing system is TT since the whole installation earth electrode and the Generator have separate earth electrode.

2. @PC166 & Davesparks: Having the Generator Neutral/Body connected to the Whole installation earth electrode is that the earthing system is TNS.

Is both systems ok and is there any additional protection for the TNS setup.

Usually, what I have observed is that the Generator body & Neutral is connected together via the neutral link and earthed to its own earth electrode. Then when connecting the power cable to the transfer switch, the armour is connected at both end i.e. Main LV Panel and Generator Panel. Thus, in this case, two separate earth electrodes are installed and are always bonded together. Is this right a right set up or use a single earth electrode as @ Davesparks.

If the armour is connected, then the set up would be as per 1. above.

Any comments is appreciated.

 

[davesparks]

If the armour is connected at both ends then you have a metallic return path back to the star point so you have created a TNS setup, I don't see any reason why you would create a TT system for an on-site generator.

 

[pc1966]

2. @PC166 & Davesparks: Having the Generator Neutral/Body connected to the Whole installation earth electrode is that the earthing system is TNS.

Is both systems ok and is there any additional protection for the TNS setup.

No, in reality you have less protection needs for TN-S as your R2 (earth return resistance) is normally low enough so the over current protection works rapidly on a phase-to-earth fault without needing the RCD.

However the standard TT protection of an incomer delay RCD is perfectly good for TN-S as well (in the UK that is often seen in agricultural setups, etc, for additional fire protection if cables get chewed by rats, etc).

Usually, what I have observed is that the Generator body & Neutral is connected together via the neutral link and earthed to its own earth electrode. Then when connecting the power cable to the transfer switch, the armour is connected at both end i.e. Main LV Panel and Generator Panel. Thus, in this case, two separate earth electrodes are installed and are always bonded together. Is this right a right set up or use a single earth electrode

Having more earth rods are usually a good thing! So you can happily retain a local generator earth rod and have the SWA armour linking it to the main TT earthing system.

As Davesparks has already said, there is no real advantage in making the generator TT here as you will get more reliable fault clearing on TN-S

 

[Locus]

Thanks, everybody for their contributions and useful comments. I really appreciate the good discussions.

 

[R-fur]

The IET do a very good book 'temporary power systems' it is aimed mainly at events installations but has a good chapter in generator installations, might be worth a look for you.

 

[Locus]

The IET do a very good book 'temporary power systems' it is aimed mainly at events installations but has a good chapter in generator installations, might be worth a look for you.

Thanks. I will check it.
Regards to all.

 

[davesparks]

The IET do a very good book 'temporary power systems' it is aimed mainly at events installations but has a good chapter in generator installations, might be worth a look for you.

As you say it is for temporary installations, the permanent installation of a generator with an automatic changeover system can be somewhat different.

 

[R-fur]

Absolutely I wasn't trying to suggest otherwise, but it does cover generator earthing in detail.