Generator Installations and EIC issue

[JohnW2]

Although I'm even far less of a generator expert than you, a few comments/questions....

OK, Trojan, as best as I can........but like I said, I'm no generator expert. My BLUE

a) Earthing arrangement - single core connected to MET of installation so TN-S? Should the generator have it's own earthing too?

The generator doesn't need 'earthing' - you are providing an earth path back to the generator to be used by the installation. I thought it had to be a seperate earth from the supplied (DNO) - but maybe it doesn't. The earth from the generator is connected to the MET because this is where all of the installation's CPCs / Bonding etc should be connected to......it's not to 'earth' the generator - it's to give the installation a path back to the generator.
I have to say that I'm rather struggling to see what problem would be posed by a generator supply that was truly floating relative to true earth (e.g. if a DNO-supplied 'earth' were to 'fail'), but 551.4.3.2.1 of BS7671 (which I quoted recently) seems to very clearly state that a standby generator must not 'rely' ('rely' for what?) on a DNO-supplied TN earth, but should be provided with its own earth. Although I've not seen it (costs money!), I've been led to believe that BS7430 says much the same. However, as I said, I don't really understand why.

b) PFC and Ze. It's a back up generator but considered the source of supply so how do you get around this? It can't run just to test at the changeover panel.Surely you can isolate the main supply and run the generator - then you do the necessary 'live' tests for the generator 'supply'.
I would have thought so, too. However, isn't PFC, in practice, going to be determined/limited by the maximum current the generator can supply? I would have thought that a PFC calculated from Ze would usually be 'unsupplyable', at least for more than a few cycles.

a) Means of earthing - Distributors facility as the generator is earthed via the MET?
No. The generator isn't earthed. The generator is providing an earth to the installation.
I fear that we at risk of the usual seantic confusion! Unless it has it's own earth electrode, it's not 'providing an earth'. However, as you go on to say, the conductor from MET to genny is providing an 'alternative' neutral return path from CPCs; 'earth' does not come into it

You need to treat it as two supply - one supply has the DNO provided earth.......the other has the earth provided from the generator.
As above, in the absence of a local electrode, it's not really an 'earth' - so I wouldn't think one could respond to the 'means of earthing' question with an answer 'generator', could one? If no local electrode was being used, I would have thought the correct/honest answer (in relation to the generator supply) would be 'none', wouldn't it?

b) Main protective conductors - None seem to apply
These should be in place anyway (for the existing installation/supply.)
Indeed - although, as you say, it's hard to see that they would theoretically be required if the genny supply really were truly floating wrt true earth. Whatever, assuming that it's true (in terms of the main installation), I would assume that, for the purpose of 'the form', the answer would be 'present and correct'.

Kind Regards, John.

 

[JohnW2]

Christ. As you correctly identified this isn't an IET Platinum Membership forum so please don't act like you are the presidents of one. ..... Get with the real word and provide help when asked rather than acting like some jumped up fleshend response from someone who thinks and expects it all to fall in line with British Standards 100% of the time.

Just for the record, as most here know, I'm not an electrician, and I most certainly do not expect "it all to fall in line with British Standards 100% of the time.". You've only got to look around this forum to see that I am a strong critic of many aspects of the 'rules, regulations and laws' surrounding electrical work (including BS7671) and am personally a believer in treating some of it with the contempt that I believe it deserves! Professional electricials don't really have the luxury of being able to do that!

However, even as an 'amateur' (in many fields ), rules and regulations aside, I would never embark upon anything until I had satisfied myself that I fully understand what I was doing and what was required, and I think I'm not being unreasonable in expecting that the same should be at least as true of someone undertaking tasks professionally. That's really all I was saying when I agreed with Electrifying.

Kind Regards, John.

 

[Electrifying]

Hi John,

I have to say that I'm rather struggling to see what problem would be posed by a generator supply that was truly floating relative to true earth (e.g. if a DNO-supplied 'earth' were to 'fail'), but 551.4.3.2.1 of BS7671 (which I quoted recently) seems to very clearly state that a standby generator must not 'rely' ('rely' for what?) on a DNO-supplied TN earth, but should be provided with its own earth. Although I've not seen it (costs money!), I've been led to believe that BS7430 says much the same. However, as I said, I don't really understand why.

It wouldn't be a problem, as such, because the installation would still have an earth via the 'single core' CPC back to the generator.

There are two problems with not 'rodding' the generator, though, which is probably why BS7671 says you should.

Firstly, the neutral would indeed be floating without a reference to 'true earth',

and secondly, there would be no path back to the source (generator), via the general mass of earth, so, if you were to come into contact with a live part of the installation, any RCD protection wouldn't trip and clear the fault.

Ive always thought this isn't such a bad thing - as you wouldn't get a shock, either.
I've also thought how much safer things would be without 'earth'

I would have thought so, too. However, isn't PFC, in practice, going to be determined/limited by the maximum current the generator can supply? I would have thought that a PFC calculated from Ze would usually be 'unsupplyable', at least for more than a few cycles.

To a degree, but that's no different to a substation supply - you would still need to measure it - probably between the generator 'Line' and 'Neutral', because, although your measuring the fault current at 'negligible' impedance, you still have a fault loop via the supply cables.......similar to a DNO supply.

I fear that we at risk of the usual seantic confusion! Unless it has it's own earth electrode, it's not 'providing an earth'. However, as you go on to say, the conductor from MET to genny is providing an 'alternative' neutral return path from CPCs; 'earth' does not come into it

I don't look at it that way - the path provided back to the 'star point' or 'neutral' is the 'earth fault path'.....regardless of whether that's via the general mass of earth or a conductor.

The reason the 'star point' or 'neutral' are 'rodded' is to reference the neutral to the 0 volts of 'true earth'.....not, as you suggest, to provide an earth.
Sub station transformers are still 'rodded' and referenced to earth, even if the supply is TN-S and the installation has no extraneous conductive parts - because it's referencing the neutral, not providing an earth.

The earth provided by the generator is there to ensure that the protective devices will trip under earth fault conditions - regardless of whether the generator is rodded or not.

As above, in the absence of a local electrode, it's not really an 'earth' - so I wouldn't think one could respond to the 'means of earthing' question with an answer 'generator', could one? If no local electrode was being used, I would have thought the correct/honest answer (in relation to the generator supply) would be 'none', wouldn't it?

To this I would reply 'Other', and attach a description of the 'other' arrangement.



The world as I see it - others might disagree.

 

[JohnW2]

It wouldn't be a problem, as such, because the installation would still have an earth via the 'single core' CPC back to the generator.

Agreed, except I would call it 'a return path for fault currents', rather than 'an earth'.

There are two problems with not 'rodding' the generator, though, which is probably why BS7671 says you should. .... Firstly, the neutral would indeed be floating without a reference to 'true earth',

Yes, but that's just words - it only constitutes 'a problem' if there is some undesirable consequence of having a floating neutral.

...and secondly, there would be no path back to the source (generator), via the general mass of earth, so, if you were to come into contact with a live part of the installation, any RCD protection wouldn't trip and clear the fault.

That surely shouldn't be the case. Provided that the premises are correctly constituted as an equipotential zone, RCDs should work just as they would in an 'earthed' installation - all CPCs and exposed metalwork should be at the potential of the generator neutral, and any L-'E' fault (whether through a human being or anything else) would be between L and CPCs/MET/metalwork, in the usual way, via the 'alternative' return path to the generator, hence creating a L/N imbalance to operate RCDs.

Ive always thought this isn't such a bad thing - as you wouldn't get a shock, either. I've also thought how much safer things would be without 'earth'

Yes, there are many learned writings about the pros and cons of earthing electrical systems. The greatest problem is probably that 'unintended' connections to earth are effectively unavoidable - which creates the situation of 'if you can't beat it, join it'!

To a degree, but that's no different to a substation supply - you would still need to measure it - probably between the generator 'Line' and 'Neutral', because, although your measuring the fault current at 'negligible' impedance, you still have a fault loop via the supply cables.......similar to a DNO supply.

I think it's pretty different, unless the installation in question has its own substation. Substations generally have the capability to supply a relatively latge number of premises, and the grid itself effectively has unlimited current-providing capacity. In constrast, an approporiately-specified standby generator is unlikley to be capable of supplying much more current than the installation requires - it's likely to struggle even under modest 'overload' conditions, let alone fault conditions. My main point was that I would have expected the actual PFC for a standby genny to be pretty low - which could, of course, be a problem for satsifactory operation of OPDs. For that reason, I would imagine that it's usually essential for standby generator installations to be RCD protected - but, even then, required disconnection times for L-N faults (where the RCD doesn't help) might be difficult to achieve.

I don't look at it that way - the path provided back to the 'star point' or 'neutral' is the 'earth fault path'.....regardless of whether that's via the general mass of earth or a conductor.

That's the reason I think such terminology confuses so many people.

The reason the 'star point' or 'neutral' are 'rodded' is to reference the neutral to the 0 volts of 'true earth'.....not, as you suggest, to provide an earth. Sub station transformers are still 'rodded' and referenced to earth, even if the supply is TN-S and the installation has no extraneous conductive parts - because it's referencing the neutral, not providing an earth.

Again, I think many people get confused by such language.

To this I would reply 'Other', and attach a description of the 'other' arrangement.

That sounds like a reasonable approach.

Kind Regards, John.

 

[SgtTrojan]

Looking at BS 7430, The code of Practice for Earthing it quotes:

Generator protective earthing is achieved by
connecting the frame of the generator, associated
exposed-conductive-parts and
extraneous-conductive-parts to a main earthing
terminal. The earthing terminal or bar is connected
to an independent earth electrode and, where
appropriate, to other earthing facilities associated
with the installation.
The earthing of the electrical system may require
that the neutral point of the generator is connected
to earth, either directly or via an impedance.
Where an installation is supplied by more than one
source of energy (e.g. a public supply and a
generator), the earthing system of the installation
should be designed so that each source which can
operate independently of other sources remains
earthed if all the other sources are not connected.