UK Where are the TN-S generators?

[Dazzler]

Hi all,


I've recently had 2 customers enquire about installing a backup generator for their domestic premises.

After a lot of research, contacting generator manufacturers, and generally going around in circles, it's become apparent that TN-S, or "centre tap earth" generators are practically non-existent in this country, honestly I cannot find one anywhere, and when you do contact manufacturers, all you get is a either very vague answer, or a straight "no, none of our generators are centre tapped to earth", together with the warning that any modifications will void the warranty.

I just can't understand the lack of this type of generator, when it's a requirement for supplying multiple class 1 equipment.

I'm finally at the end of my tether now, and about to reject both jobs due to the hassle involved in trying to design an installation that complies with BS7671, just because the unavailability of a product that makes this possible, or am I missing something here?

Anyway, enough ranting, now here's my question: Does anybody out there know of anywhere I can purchase a TN-S centre tapped to earth generator?


Thanks,


Daz.

 

[Rockingit]

My guess is that you're looking at smaller sized sets? Generally speaking anything over around 20kVA will be TN-S.

You can always create a PEN link, too, although there's been lots of discussion on that on here - have a search.

 

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[davesparks]

Anyway, enough ranting, now here's my question: Does anybody out there know of anywhere I can purchase a TN-S centre tapped to earth generator?

The generator itself isn't TN-S, it is how you install it that makes it a TN-S supply. So if you connect one pole of a two pole generator to earth you create a TN-S supply.

I think you may be looking/asking for the wrong thing, unless these properties have a two phase 460V (230-0-230) supply?

A centre tapped 230V generator would be one which two 115V outputs either side of a neutral, 115V-0V-115V style.

What you need for the average single phase domestic installation is a single phase 230V generator, you can reference one pole of this to earth to make it a neutral conductor and give you a TN-S system.

So to answer your question of whether you are missing something then yes, you are missing the correct terminology.

 

[Dazzler]

The generator itself isn't TN-S, it is how you install it that makes it a TN-S supply. So if you connect one pole of a two pole generator to earth you create a TN-S supply.

I think you may be looking/asking for the wrong thing, unless these properties have a two phase 460V (230-0-230) supply?

A centre tapped 230V generator would be one which two 115V outputs either side of a neutral, 115V-0V-115V style.

What you need for the average single phase domestic installation is a single phase 230V generator, you can reference one pole of this to earth to make it a neutral conductor and give you a TN-S system.

So to answer your question of whether you are missing something then yes, you are missing the correct terminology.

Thanks for the info guys. Yes generator installation is new to me, I've never installed one before so I'm erring on the side of caution with this.

The one customer has a dual voltage 115V/230V generator that he wanted me to install but after doing continuity tests it became apparent that the 115V was the centre tap (as you state, 115V-0V-115V), therefore I'd risk shorting the windings if I installed a link in the wrong place.

So I guess you're right in what your saying, by having a 230V single voltage generator, linking the neutral to earth would provide the TN-S, without the risk of shorting any windings.

My only worry is that by modifying it would void the warranty. Problem is, I don't see any other way.

I'll just have to very careful on the type of generator I get, and if course with any modifications.

Anyway, that's pretty much answered my question, so thanks again for the feedback.

 

[davesparks]

My only worry is that by modifying it would void the warranty. Problem is, I don't see any other way.

I'll just have to very careful on the type of generator I get, and if course with any modifications.

You don't need to modify the generator.

 

[pc1966]

One thing to be aware of is you do need an independent means of earthing, for example an earth rod permanently connected to the MET (my choice), or connected to the generator's earth/chassis, as under loss of supply conditions you cannot assume the DNO's earth would still be present.

However, as above, the TN-S aspect is down to a N-E link at the generator so you have L, N and E from the generator. The presence of an earth rod does not make it TT, unless it was independent of another earth rod for the installation. Which would be a little bizarre!

 

[timhoward]

There's a good John Ward video on generator earthing. Worth a watch.

 

[Dazzler]

There's a good John Ward video on generator earthing. Worth a watch.

Thanks Tim, I have watched that one. Some useful info on there.

 

[Dazzler]

You don't need to modify the generator.

Sorry for the delayed response here. Now I'm intrigued, is there a way of creating a PEN conductor on the generator without modification? I was under the assumption it would have to be done by linking the neutral terminal of the windings to the earth terminals of the socket outlets on the generator, and the frame?

 

[James]

I would expect the generator frame is already connected to the earth terminal of its socket.

N to E link could be made in the plug that is going to be inserted into the generator.

 

[Dazzler]

I would expect the generator frame is already connected to the earth terminal of its socket.

N to E link could be made in the plug that is going to be inserted into the generator.

Thanks for the reply. I was wondering that myself but thought this would not be considered as being centre tapped "at source".
Although with regards to the installation it would be supplying then it would still be considered as being an external PEN conductor, so I'm happy with that.
Things are becoming much clearer now. Thanks everyone for the advice.

 

[davesparks]

Thanks for the reply. I was wondering that myself but thought this would not be considered as being centre tapped "at source".
Although with regards to the installation it would be supplying then it would still be considered as being an external PEN conductor, so I'm happy with that.
Things are becoming much clearer now. Thanks everyone for the advice.

I thought we'd established you don't want it centre tapped?

There won't be a PEN conductor, you still run seperate L N and E conductors.

 

[pc1966]

It should definitely NOT be centre-tapped (115-0-115V style as they do in the USA) for UK use as the house breakers/fuses are only on the L line and not in N which would be needed if it were well above E!

 

[Dazzler]

I thought we'd established you don't want it centre tapped?

There won't be a PEN conductor, you still run seperate L N and E conductors.

Yes, understood. Just getting my terminology mixed up again! So my plans are to link the neutral & earth within the plug at the generator, to create the TN-S earthing arrangement, then run separate live, neutral & earth from the plug to the changeover switch.

 

[James]

I thought we'd established you don't want it centre tapped?

There won't be a PEN conductor, you still run seperate L N and E conductors.

If it was a 3 phase generator, it would be considered center tapped.
but a single phase, single voltage generator doesn’t have a center to tap!

it is a single winding where both ends are floating unless you tie one end down to earth.

 

[davesparks]

If it was a 3 phase generator, it would be considered center tapped.
but a single phase, single voltage generator doesn’t have a center to tap!

it is a single winding where both ends are floating unless you tie one end down to earth.

A 3 phase generator would be star point tapped rather than centre point tapped as it is not the centre point of a winding but the point where three windings connect together.

 

[James]

You are correct of course.
well spotted