Standby Generator Earthing Question

[Dave-sparks]

i suggest you go on voltimum and ask the question and you will get an answer of a top electrical engineer or from the niceic informing you and quoting the regulation number

But that's the thing, I have had this conversation with an NICEIC area engineer on an assessment visit and he said no it's not in the regulations.

I was first told this by the tutor on a regulations course, a guy by the name of Chris Kitcher, who is the author of some well respected and popular text books on electrucal installation

 

[Dave-sparks]

Do you hire yourself out to insomniacs? 5 minutes and they would be fast asleep

Probably make more money than this sparkying nonsense

Thanks for the insults but I am just asking for some simple information.

 

[dobbslc]

I'm not going to have a pointless, willy waving, I know more than you debate over the regs, but it is in the books that are currently in my shed, the books you can only buy with a valid gas safe card

So as an electrician I can't buy the book that tells me what I need to do to comply with the gas regs as I'm not a plumber/gas safe bod?
How am I meant to know what to do then?

Anyway I don't have gas here...

 

[awemawson]

OK Generator frame (and it's earth stud) is bonded to the earth pin of it's 32 amp commando socket but is not connected to neutral - does this make any difference?

 

[Dave-sparks]

OK Generator frame (and it's earth stud) is bonded to the earth pin of it's 32 amp commando socket but is not connected to neutral - does this make any difference?

Does the manual for the generator have anything to say on the subject of earthing, or does it have a diagram of the internal wiring?

Did you check for a connection to neutral by testing continuity or visually or both?

 

[sparkysy]

if the generator is going to be solely used for the supply of a fixed installation I would tie the neutral to earth in the generator.
If the generator is going to be used elsewhere this is not good as without a fixed earth electrode connected the chassis of the generator can become live so only do this if that is its sole purpose, if you sell it on or move it temporarily remember to remove the link. It can be seen in the picture below just to the left of the earth labled as 4.

 

[GRW]

As a general rule - a generator (as a supply/source) should have an independent Earth from its chassis to ground.
Many years ago we asked the NICEIC for what the Earth measurement should be (they had never considered it, so went away and had a think, and came back with a minimum of 20 Ohms, but as close to zero as feasible).
Generator should be neutral-earth linked (either in the alternator terminal box or in the output breaker box).

You should not rely on any utility Earthing when powering the site from a backup generator (e.g. the loss of utility could be because someone's put z digger bucket through the supply and severed).

 

[Ubique]

Generators are generators, standby or prime. They all need an earth of some type to provide a 'reference' or to tie the 'neutral' to. I agree in principle that using a PME terminal 'could' endanger life, however, remember what the 'M' stands for. If you wanted to be absolutely stand-alone, then you would need an Earth bus-bar similar to this:-

The left side would have the PME bonded to it, the right side would have your local earth plus all your CPC's bonded to it, you would need to open the link (commonly used for testing, not functional) before switching over your supply to generator.
All the standby generators I've ever installed have had something similar but I doubt very much if the link was ever opened apart from testing. There is also the potential (!) for both sides to be at different voltages once opened, so be careful or use a voltmeter.

 

[Dave-sparks]

As a general rule - a generator (as a supply/source) should have an independent Earth from its chassis to ground.
Many years ago we asked the NICEIC for what the Earth measurement should be (they had never considered it, so went away and had a think, and came back with a minimum of 20 Ohms, but as close to zero as feasible).
Generator should be neutral-earth linked (either in the alternator terminal box or in the output breaker box).

You should not rely on any utility Earthing when powering the site from a backup generator (e.g. the loss of utility could be because someone's put z digger bucket through the supply and severed).

Did they give any reason behind the 20 ohms they quoted?

I'm curious as 20 ohms is a figure which often appears in various documents and comments on the subject of supply earthing, but it doesn't really seem to have any discernable reason behind it.

 

[GRW]

Note on an operational level, that not having a N-E link in your generator can (that's can, not definitely will) cause nuisance tripping of RCD's.
With no N-E bonding, there's potential for a voltage differential (& therefore current between N & E, which trips the Dis Board RCD).
A lot of dual voltage portable gennys (because of the way they are wired and switched between 230 & 110vac) aren't N-E linked, so (as an example) it's possible for the voltage to be 230v between L & N.... but between L & E 240v, and between N & E, 10v.

 

[GRW]

Did they give any reason behind the 20 ohms they quoted?

I'm curious as 20 ohms is a figure which often appears in various documents and comments on the subject of supply earthing, but it doesn't really seem to have any discernable reason behind it.

Yeah... they didn't say! Our old boss and Co Founder was a bit of an anorack/boffin, and he asked the NICEIC as it wasn't (at the time, maybe some 25-years-ago) clarified or mentioned anywhere in any of their bumf.
They seemed caught unawares, went away and just came back with 20 ohms, but without any explanation or reasoning why that was the upper limit.

I think at the time our little generator firm was the only generator Co the NICEIC seemed to have. Consequently they were a bit lost as to how to assess our installations/work annually to qualify as a member. They wanted to see wiring/distribution work - when 99% of the time we were a subbie doing the generator for a main elec cntr... and they didn't give us cabling work they could do themselves! (We just told them what cables we needed, and they'd provide em )

 

[awemawson]

Does the manual for the generator have anything to say on the subject of earthing, or does it have a diagram of the internal wiring?

Did you check for a connection to neutral by testing continuity or visually or both?

Genny is a Bohmer-AG 9000W-E (German design but I'm sure Chinese made) - The manual says utterly nothing regarding earthing, not even referring to the earth stud that is adjacent to the output socket.

I tested neutral (O/C) and earth terminal (connected) connection using my Fluke.

Probably safest then to drive an earth spike in where it will operate and have a flying lead off that stud.

 

[GRW]

Genny is a Bohmer-AG 9000W-E (German design but I'm sure Chinese made) - The manual says utterly nothing regarding earthing, not even referring to the earth stud that is adjacent to the output socket.

I tested neutral (O/C) and earth terminal (connected) connection using my Fluke.

Probably safest then to drive an earth spike in where it will operate and have a flying lead off that stud.

I just looked that genny up online... & it looks to be 230v only.
If so, it should be N-E linked... but in case you can add a link wire between N & E (easiest place would probably be in the back of the output socket).
Then, add your independent Earth Rod.

 

[Onoff]

You're all missing a trick afaic! Portable earth is where it's at for a genny

 

[sparkysy]

@GRW
You are spot on In what you say, always tried to keep the Ra as low as possible! Chances are that unless the generator is going flat chat it would not have enough output current to trip the protective device anyway!
I would agree with your link position too if there is not a suitable panel on the side of it with the connections in.
Sy

 

[GRW]

The weird thing for me.... is that next to no small portable gennys - are fitted with any Earth Leakage Protection Device.
Yet, they are sold with the primary purpose of running hand tools or kit via surface flexible leads.
Any industrial portable generator (e.g. a hire set or towable set), HAS to be fitted with Earth Leakage Protection - BECAUSE of the potential and likelihood that surface cabling and Distributions, and power tools will be connected to it.

I don't know how/why the mfrs of these small sets are able/allowed to get away with building/supplying gennys without any Earth Leakage devices incorporated.

 

[awemawson]

It’s obviously intended for domestic rather than industrial use so no panels with links etc.

I’ll run it stand alone tomorrow and measure live / earth and neutral / earth voltage while running. Assuming that neutral isn’t floating mid way I’ll strap it to earth

 

[Ubique]

The 20 Ohm limit seems at variance with their own guidance on TT earthing arrangements

Earth Electrode Testing

The NICEIC offer their technical support regarding the necessary earthing arrangement to comply with the requirements of BS 7671. Where an earth electrode provides the means of earthing for an install

professional-electrician.com

Also no mention of the <25V PD guidance for farms etc.

 

[sparkysy]

Got to love the NIC
I draw your attention to this contradiction from earlier in the article
First it says to water the area around the rod and then not to as it will invalidate the test!!

A sufficient reduction in the resistance of the temporary electrodes may be achieved by driving longer temporary rods into the ground, or by watering the ground in the immediate vicinity of the temporary rods.

The ground adjacent to the earth electrode under test must not be treated with water or brine, as this would make the resistance measurement invalid for the purpose of verification or reporting.

As to the 20ohm value I did have a technical document with a list of earth rod values for most protective devices but I can't find where I saved it currently, and they were lower values than those listed in that article and BS7671. Also can't remember why they were lower! But obviously the lower you can get them the better for disconnection under fault conditions.

 

[Ubique]

I honestly can't remember the values we worked to - thankfully the NIC were not involved, we had our own Authorising Engineers from WS Atkins to design and sign-off the installations - I do remember that we pushed a lot of rods into a mountain-top site, probably 12 x 6-8 metres deep, it was suggested that the testing was done before the morning mist cleared to get the benefit of the moisture on the surface.....
This was all overseas but the spec was to UK standards.