portable generators and earthing?

[HappyHippyDad]

A customer has asked me to install a generator to supply her house for when they have a power cut. I have already said this is beyond my capability and I will definitely not be taking the job. However, I'm interested in understanding it a little better.

So you have the correct size live and neutral going into a transfer switch from the generator. You would also have the mains supply going into this transfer switch (or change over switch). From here you have a live, neutral and earth going to the CU. This would mean you can choose which source you use, also it would stop any back flow of power.

My main question is what happens to the earthing in each of the 3 main domestic earthing types. PME, TNS and TT.

The transfer switch would need to disconnect both lives and the earth from the mains supply. However, there could well be extraneous conductive parts in the property which need to be considered. Would you somehow utilise an earth rod for use with the generator? If you did this, would the earth rod need to be connected to the neutral of the generator at some point?

If the house was TNS or TT, would this alter your earthing needs for the generator compared to PME?

 

[davesparks]

If we assume it is a sensible sized generator designed for the application which has a neutral-earth link inside it (big assumptions!) then it's relatively straight forward. You would install an earth electrode, ideally with a nice low Ra, and connect it to the generator thus earthing the neutral and creating your own TNS system when the generator is in use.
As far as I know there is no need to disconnect the DNO earth, even with TNCS, in this setup. But I do normally deal with stand-alone generators rather than backup sets so I may be wrong on this point.

Your changeover switch will disconnect L+N of the DNO supply before the generator is connected so there is no chance of a back feed that way.

 

[timhoward]

I'm actually sure you could do this. The OSG has some diagrams. This isn't anywhere near as complex as people seem to want to believe.

The generator's I've had experience with have a connection for an earth electrode which is internally joined the the side of the coil used for Neutral, and there's a 2nd separate internal link from the the same side of the coil to the earth terminal for the load, which basically means it's a TN-S supply.

The grid supply earth doesn't need disconnecting. There's a reg somewhere that says when the generator is switched in you can't rely on the grid earth for ADS which basically means the genny needs it's own independent earthing setup.

Beyond that it's a glorified changeover switch really. Whether the grid supply is TN-S or PME doesn't really change anything.
A TT supply could potentially share an electrode for the generator I suppose but I'd keep things simple.

 

[davesparks]

If it's not a nice perfect generator you may have a little more to think about.

If the generator doesn't have a neutral earth link then you'll need to fit one, ideally at the generator.
But first you'll want to be checking that it isn't a centre tapped output.

 

[davesparks]

Another thing to consider with generators is fault protection, you're pretty much guaranteed to need RCDs to achieve any earth fault protection.

Small generators can't supply the levels of fault current necessary to operate most common MCB's. The engine will bog down or stall when a short circuit occurs.

I've seen a 60KVA diesel generator nearly stall from the inrush to a strobe light,

 

[HappyHippyDad]

If it's not a nice perfect generator you may have a little more to think about.

If the generator doesn't have a neutral earth link then you'll need to fit one, ideally at the generator.
But first you'll want to be checking that it isn't a centre tapped output.

Thanks for all the posts on this. It's opening up a few more questions.

If you need to make a neutral earth link on a generator that does not already have one, why do they not always have one already installed? I'm guessing it's a) cheaper and b) you don't always want an earth?

oo.. just having a look at the regs. Looks like a generator without neutral earth link is limited by reg 413.418.3. meaning you would never use one for the scenario I have mentioned, unless you made the N-E link as you suggest.

 

[davesparks]

Thanks for all the posts on this. It's opening up a few more questions.

If you need to make a neutral earth link on a generator that does not already have one, why do they not always have one already installed? I'm guessing it's a) cheaper and b) you don't always want an earth?

If the generator hasn't been designed as a standby generator to feed an installation, or has been designed to supply a power tool or a couple of lights then it may well have been intentionally designed to rely on electrical seperation (IT system style) then it won't have a N-E link.

Also it could just be due to being cheap and nasty.

Center tapped outputs are usually the generators which have both 230V and 115V outputs, the centre tap being used as the second leg of the 115V output.

 

[HappyHippyDad]

I'm actually sure you could do this. The OSG has some diagrams. This isn't anywhere near as complex as people seem to want to believe.

The generator's I've had experience with have a connection for an earth electrode which is internally joined the the side of the coil used for Neutral, and there's a 2nd separate internal link from the the same side of the coil to the earth terminal for the load, which basically means it's a TN-S supply.

The grid supply earth doesn't need disconnecting. There's a reg somewhere that says when the generator is switched in you can't rely on the grid earth for ADS which basically means the genny needs it's own independent earthing setup.

Beyond that it's a glorified changeover switch really. Whether the grid supply is TN-S or PME doesn't really change anything.
A TT supply could potentially share an electrode for the generator I suppose but I'd keep things simple.

I'm not sure Tim.
I think the earthing gets complicated, although I do tend to over think things a bit and perhaps make them complicated.
I've just tried to get my head around this article by the IET, the earthing aspects do seem to get a bit tricky!

https://electrical.------.org/wiring-matters/years/2020/82-september-2020/mythbusters-6/

• Wiring Matters
• Years
• 2020
• 82 - September 2020
• MythBusters #6

 

[HappyHippyDad]

https://electrical.the iet.org/wiring-matters/years/2020/82-september-2020/mythbusters-6/

 

[HappyHippyDad]

Sorry, the link doesn't work. It's actually the link above but without the space between the words 'the' and 'iet'. For some reason it wont let me enter 'the iet' as all one word.
I'll try ------,
nope

 

[brianmoooore]

Bac in the day, when all the dairy farmers on my books were installing tractor driven generators, SWEB issued guidance as to how they wanted them connected. As well as provision of a secondary earth, in case the reason for the power loss included loss of the DNO earth, they required a double pole isolator switch between the meter and the change over switch, to make doubly sure that backfeed could not occur. The DP isolator was always to be turned off when the change over switch was operated, as was the MCB on the generator.

 

[loz2754]

Sorry, the link doesn't work. It's actually the link above but without the space between the words 'the' and 'iet'. For some reason it wont let me enter 'the iet' as all one word.
I'll try ------,
nope

There are some sites that don't seem to be allowed to have direct links posted here, the IET being one of them.
I usually use a link shortening site like tinyurl.

 

[timhoward]

I've read the article now and it's basically elaborating on what @davesparks already said above, but in a not terribly clear way. If someone was confused before reading that lot they would be even more confused afterwards!

In this order they describe:
-the most complicated centre-tapped setup (why did they START with this!)
-'common and normal', their words, Page 10 (this bit is actually quite good)
-non-compliant floating earth setup - really not sure WHY they included this
-small suitcase generators (IT system) - irrelevant unless for a single appliance

I'd suggest you
1) watch this video which is a million times clearer:

2) Read page 10 and ignore everything else, for now at least!

 

[HappyHippyDad]

I've read the article now and it's basically elaborating on what @davesparks already said above, but in a not terribly clear way. If someone was confused before reading that lot they would be even more confused afterwards!

In this order they describe:
-the most complicated centre-tapped setup (why did they START with this!)
-'common and normal', their words, Page 10 (this bit is actually quite good)
-non-compliant floating earth setup - really not sure WHY they included this
-small suitcase generators (IT system) - irrelevant unless for a single appliance

I'd suggest you
1) watch this video which is a million times clearer:

2) Read page 10 and ignore everything else, for now at least!

That was a fantastic video, thanks so much Tim (and JW). It's great when it all clicks in to place. It all made perfect sense..

Still not doing the job though. I'd have to work alongside @davesparks for a few weeks at one of his hippy festivals before I even considered it