Fixed generator questions

[John-]

Sorry fir the length of this.

Am adding some lights and sockets to existing circuits in a SP board. And providing bonding to metal oil pipes coming into this workshop from underground.
They have a diesl SP generator supplying ‘esential’ SP Dis boards across the site feeding lights and basic power - in case of supply interruptions. It seems that existing TP boards had SP circuits removed and installed into these boards, with a 3 position change change over switch about 6 years ago ir so. The positions are. Mains on, all off, generator on. The idea is that the three position switch is throwen to generator, isolating street supply and the generator is then manually started.

where can i find more about the regs governing SP generators and their integration into fixed TNCS systems please?

These 3 position switches also isolate and switch over the earths. I have looked inside...

My concern here is that in all areas there is always a TP Dis boards supplying TP and SP non essential circuits in the area. Isolating the earth in this way seems dangerous? We are also in a reasonably high lightning area, so if earth was isolated this could also impact any surge pritection - not that there is any yet ?.
Is earths like this common practice with fixed generators?
Thanks

john

 

[pc1966]

The usual requirement in the regs is not to have more than one N-E link at any point in time, so there is the utility supply link (TN-S or TN-C-S, both before the installation by varying degrees) and then usually larger generators also have a N-E link inside (so are TN-S supplies in their own right). Though some generators are isolated internally (L&N floating, not linked to earth/chassis).

I would be very unhappy about switching the earth!

 

[pc1966]

Did someone use a TP switch to change over all 3 lines?

I would expect only to switch the L & N

 

[R-fur]

I wouldn't switch the earth either. I would expect the generator to have its own earth and N-E link. The change over switch should be double pole.

 

[John-]

Yes, triple poll switch, changes Line, Neutral and Earth. This also means that if it is switched over to generator the Oil and building bonds are then isolated from the SP Dis board not a good thing to do... I have checked regs, and there is just 1 page on alternative supplies, nothing of any worth though.

What about prospective current values, Values of KA, retesting while generator is running to ensure ZS value is equal to or less than that of the mains supply (obviously it should be, but...), bonds continuous to both supply supplies, marshalling earths? Strange how there is little to no guidance.

Or am i missing some other other non 7671 docs?

I also assume the earth stake at the generator (pretty large cable) is just to bring a ground connection, no floating earth - like the sub station? And that when I look into the system more deeply, i should expect to see earth cables, appropriately sized, leading back to the generator for the alternative supply i.e. the gene is nota TT supply?

 

[Simon47]

I too would be really uncomfortable having a switch in the earth - especially if other items are still connected to the DNO earth which is a recipe for differentials between items. Switching some items but not others is just so wrong and dangerous.
I was under the impression that for a generator you need a sufficient earth to safely run the entire system in the absence of the DNO earth as there's no guarantee that during works on the system, the CNE (combined neutral & earth) won't get disconnected.
So you then either tie the local earth grid to the DNO earth, or you have to switch the earthing for the whole system (I don't like the idea of a switch in the earths), or you don't use the DNO earth at all (go TT). AIUI at one time it was prohibited to connect a local earth to a DNO earth, but now they are talking about mandating it - it was one of the things that didn't make it into the final version of the last amendment to the wiring regs.

 

[John-]

Thanks for your thoughts. I have often thought what would the implications be if an earth stake was provided at the head connected to a TNCS system? If it was best case 100ohm, in terms of a parallel earth path it wont make much difference to a ZE of the TNCS side is under an ohm, but if suitably sized might help avoid issues if a Neutral is lost?

i was thinking the same that I would ensure continuity between the Generator MET and the TNCS MET, at all satellite points. The generator serves multiple buildings, where they have removed esentusl circuits from existing TP boards and installed a single phase boards with the three switch position mentioned in the earlier thread.
This does mean though that i will have a TNCS system earthing system that is also connected to an earth stake at the generator...

Thoughts?

 

[pc1966]

The main issue with having the generator and related local earth on the TN-C-S earth is the same as the one of extraneous conductive parts, you then need (typically) 10mm bonding at least, and you have the potential issue of an open PEN fault making external metalwork hazardous.

If your local earth is very good, say a couple of buried mats or deep rods, then you may well be able to pull the open PEN potential down (and pull the surrounding earth up) to keep it OK.

Back to an earlier point about Zs, etc. I don't know much about how you generator would test but I suspect even if it shows a "stiff" well-regulated voltage and so a low Zs / high PFC on a MFT style of test, you simply won't see that on a real fault. Probably the generator would stall, or go under-volt and disconnect.

Quite a lot of generators have RCD on the output for obvious safety reasons, but also it would allow a fast fault disconnection for low PFC. The problem you might find is selectivity with final 30mA RCD unless it is a generator designed for feeding backup power (with something like a 100mA or mode delay-RCD), instead of a building site style with multiple sockets each requiring 30mA "additional protection".

 

[i=p/u]

Make up a plug and play system, power goes out swap the plug. See next doors lights come on , swap it back..

 

[John-]

Make up a plug and play system, power goes out swap the plug. See next doors lights come on , swap it back..

It is an existing system, with multiple dis boards and change over switches - not quite that simple

 

[Simon47]

I stand corrected (surprised no-one else has done it) on switching the earth ...
537.1.5 Where an installation is supplied from more than one source of energy, one of which requires a
means of earthing independent of the means of earthing of other sources and it is necessary to provide that not more than one means of earthing is applied at any time, a switching device may be inserted in the connection between the neutral point and the means of earthing, provided that the device is:
(i) a multipole, linked switching device arranged to disconnect and connect the earthing conductor for the appropriate source at substantially the same time as the related live conductors, or
(ii) a switching device interlocked with a multipole, linked switching device inserted in the related live conductors such that the earthing conductor for the appropriate source shall not be interrupted before the related live conductors and shall be re-established not later than when the live conductors are reconnected.
Switching devices provided in accordance with (i) and (ii) shall meet the requirements of Chapter 46 for a device for isolation.
So it's is permitted to switch the earthing, but going back to the OPs situation, I cannot see how it can be considered safe to do anthing other than switch the entire earthing system at the MET of the supplies. Only switching it for some items is ripe for creating hazardous voltage differences - and indeed it is effectively prohibited :
411.3.1.1 Protective earthing
Simultaneously accessible exposed-conductive-parts shall be connected to the same earthing system individually, in groups or collectively.
Also applicable will be :
444.5.1.1 Within a single building
All protective and functional earthing conductors of an installation within a building shall be connected to the main earthing terminal, as required by Regulation 542.4.1, except where this is precluded by the requirements of legislation or Part 7.
The bit in Part 7 that would apply is 722.411.4.1 (iii) which details a device that will disconnect the protective earth after disconnecting the live conductors in the event of a PEN fault.

Ah, here we are, the condition for switching the earth conductors :
543.3.3.101 No switching device shall be inserted in a protective conductor, except:
(i) as permitted by Regulation 537.1.5
(ii) a multipole, linked switching in which the protective conductor circuit is not interrupted before the live conductors and is re-established not later than when the live conductors are reconnected
(iii) a switching device interlocked with a multipole, linked switching device inserted in the live conductors such that the protective conductor circuit shall not be interrupted before the live conductors and shall be re-established not later than when the live conductors are reconnected, or
(iv) a multipole plug-in device in which the protective conductor circuit shall not be interrupted before the live conductors and shall be re-established not later than when the live conductors are reconnected.

But given that switches can and do fail, I would suggest that having any switch in the earthing system ought to be considered poor design unless it is completely unavoidable.

When I said that switching of the earth was not permitted, I could have been thinking of :
411.4.3 In a fixed installation, a single conductor may serve both as a protective conductor and as a neutral conductor (PEN conductor) provided that the requirements of Regulation 543.4 are satisfied. No switching or isolating device shall be inserted in the PEN conductor.
Although PENs are not generally permitted in consumer installations (by ESQCR)

 

[John-]

I stand corrected (surprised no-one else has done it) on switching the earth ...
537.1.5 Where an installation is supplied from more than one source of energy, one of which requires a
means of earthing independent of the means of earthing of other sources and it is necessary to provide that not more than one means of earthing is applied at any time, a switching device may be inserted in the connection between the neutral point and the means of earthing, provided that the device is:
(i) a multipole, linked switching device arranged to disconnect and connect the earthing conductor for the appropriate source at substantially the same time as the related live conductors, or
(ii) a switching device interlocked with a multipole, linked switching device inserted in the related live conductors such that the earthing conductor for the appropriate source shall not be interrupted before the related live conductors and shall be re-established not later than when the live conductors are reconnected.
Switching devices provided in accordance with (i) and (ii) shall meet the requirements of Chapter 46 for a device for isolation.
So it's is permitted to switch the earthing, but going back to the OPs situation, I cannot see how it can be considered safe to do anthing other than switch the entire earthing system at the MET of the supplies. Only switching it for some items is ripe for creating hazardous voltage differences - and indeed it is effectively prohibited :
411.3.1.1 Protective earthing
Simultaneously accessible exposed-conductive-parts shall be connected to the same earthing system individually, in groups or collectively.
Also applicable will be :
444.5.1.1 Within a single building
All protective and functional earthing conductors of an installation within a building shall be connected to the main earthing terminal, as required by Regulation 542.4.1, except where this is precluded by the requirements of legislation or Part 7.
The bit in Part 7 that would apply is 722.411.4.1 (iii) which details a device that will disconnect the protective earth after disconnecting the live conductors in the event of a PEN fault.

Ah, here we are, the condition for switching the earth conductors :
543.3.3.101 No switching device shall be inserted in a protective conductor, except:
(i) as permitted by Regulation 537.1.5
(ii) a multipole, linked switching in which the protective conductor circuit is not interrupted before the live conductors and is re-established not later than when the live conductors are reconnected
(iii) a switching device interlocked with a multipole, linked switching device inserted in the live conductors such that the protective conductor circuit shall not be interrupted before the live conductors and shall be re-established not later than when the live conductors are reconnected, or
(iv) a multipole plug-in device in which the protective conductor circuit shall not be interrupted before the live conductors and shall be re-established not later than when the live conductors are reconnected.

But given that switches can and do fail, I would suggest that having any switch in the earthing system ought to be considered poor design unless it is completely unavoidable.

When I said that switching of the earth was not permitted, I could have been thinking of :
411.4.3 In a fixed installation, a single conductor may serve both as a protective conductor and as a neutral conductor (PEN conductor) provided that the requirements of Regulation 543.4 are satisfied. No switching or isolating device shall be inserted in the PEN conductor.
Although PENs are not generally permitted in consumer installations (by ESQCR)

Thank you for your efforts. I think I’m still going to link them out and ensure a common earth arrangement through out. ?

 

[i=p/u]

Does it mention anything about the size of generator, upto 10kva and no class 2 items???

 

[John-]

Does it mention anything about the size of generator, upto 10kva and no class 2 items???

 

[John-]

what's the relevance of class 2 items please?

 

[John-]

I stand corrected (surprised no-one else has done it) on switching the earth ...
537.1.5 Where an installation is supplied from more than one source of energy, one of which requires a
means of earthing independent of the means of earthing of other sources and it is necessary to provide that not more than one means of earthing is applied at any time, a switching device may be inserted in the connection between the neutral point and the means of earthing, provided that the device is:
(i) a multipole, linked switching device arranged to disconnect and connect the earthing conductor for the appropriate source at substantially the same time as the related live conductors, or
(ii) a switching device interlocked with a multipole, linked switching device inserted in the related live conductors such that the earthing conductor for the appropriate source shall not be interrupted before the related live conductors and shall be re-established not later than when the live conductors are reconnected.
Switching devices provided in accordance with (i) and (ii) shall meet the requirements of Chapter 46 for a device for isolation.
So it's is permitted to switch the earthing, but going back to the OPs situation, I cannot see how it can be considered safe to do anthing other than switch the entire earthing system at the MET of the supplies. Only switching it for some items is ripe for creating hazardous voltage differences - and indeed it is effectively prohibited :
411.3.1.1 Protective earthing
Simultaneously accessible exposed-conductive-parts shall be connected to the same earthing system individually, in groups or collectively.
Also applicable will be :
444.5.1.1 Within a single building
All protective and functional earthing conductors of an installation within a building shall be connected to the main earthing terminal, as required by Regulation 542.4.1, except where this is precluded by the requirements of legislation or Part 7.
The bit in Part 7 that would apply is 722.411.4.1 (iii) which details a device that will disconnect the protective earth after disconnecting the live conductors in the event of a PEN fault.

Ah, here we are, the condition for switching the earth conductors :
543.3.3.101 No switching device shall be inserted in a protective conductor, except:
(i) as permitted by Regulation 537.1.5
(ii) a multipole, linked switching in which the protective conductor circuit is not interrupted before the live conductors and is re-established not later than when the live conductors are reconnected
(iii) a switching device interlocked with a multipole, linked switching device inserted in the live conductors such that the protective conductor circuit shall not be interrupted before the live conductors and shall be re-established not later than when the live conductors are reconnected, or
(iv) a multipole plug-in device in which the protective conductor circuit shall not be interrupted before the live conductors and shall be re-established not later than when the live conductors are reconnected.

But given that switches can and do fail, I would suggest that having any switch in the earthing system ought to be considered poor design unless it is completely unavoidable.

When I said that switching of the earth was not permitted, I could have been thinking of :
411.4.3 In a fixed installation, a single conductor may serve both as a protective conductor and as a neutral conductor (PEN conductor) provided that the requirements of Regulation 543.4 are satisfied. No switching or isolating device shall be inserted in the PEN conductor.
Although PENs are not generally permitted in consumer installations (by ESQCR)

I still need to check, but as the site supply cables are SWA, and speaking to the customer last night, there are only two change overs. One in the same building at the geneset, and the other near the mains intake that seems to do multiple boards.

My assumption, still need to check, is that the change over in the meter cupboard does all single phase disboards, also changes over the earth, but does not do the dis board in the building with the geneset installed. Maybe this one is fed directly from the gen set.

There will be fortuitous connections to earth via the SWA back at the mains intake, and possibly direct connections of earths, which means the building bonds may still be at similar potentials to that of the changed over earth when the genset is running, but there are two runs of supply cable between the Generator earth when the switch is in the Gen position: the CPC for the TP board that the Bond is connected to that then goes back to the street MET, then through Disboard and back out to the same building on the alternate supply cable via the change over to the SP disboard. If so it could still introduce a potential difference, all be it small. So assuming the above proves correct. I still see that not switching the the earths and instead linking them out is still the best option. Further, i will link together the two supply CPCs in the change over. This will introduce deliberate parallel paths, but so long as each CPC complies on its own, I don't see this as an issue.

I will also contact the manufacturer of the genset and ask for their advise also.

Thoughts welcome please?

 

[i=p/u]

Richard burns could tell ye , telectrix will tell ya if ya pay him