TN system for generator

lyledunn said:

This is the resistance (281 ohms) of the earth electrode connecting star point and frame of a stand alone TPN gen set. It was one of 15 used at a recent outdoor festival. I do appreciate the desire to keep the resistance within the norms usually applied, say around 20 ohms, but I don’t think there is anything in BS7671 that puts numbers on a TN system.

Not that I'm suggesting we can apply a draft standard, but out of interest, the latest Draft for Public Comment for BS 7430 (see here, comments period closes on 10 September) steps away from 20 Ω in cases where generating sets are supplying an installation, and all circuits are protected by RCD, and recommends 200 Ω for TN system (Clause 6.1, Table 1). Where OCPDs are used, the recommendation of Clause 6.1 remains 20 Ω.

200 Ω is of course provided as a recommendation as above this value it's generally accepted that the earth electrode might not be stable.

20 Ω is a lot more interesting ... probably the true "source" is legislation and UK supply industry practice, particularly around PME ... interestingly, an earlier value used was 10 Ω.

Thank you GK, at least that gives me some idea of a number. However, it would be a blessing if someone could explain the engineering rationale of the 200 ohms for RCD protected arrangements as distinct from 20 ohms for those without.

the engineering rationale of the 200 ohms for RCD protected arrangements as distinct from 20 ohms for those without.

Maybe I'm being unfair, but my suspicion of the underlying logic would be something along the lines of "20 Ohms can ****** difficult to achieve and isn't really justifiable - while 200 Ohms is a well recognised number (not from any direct electrical calculations, but from a stability across the seasons point of view) and reasonably achievable in the majority of soils ... so what excuse can we come up with to justify a number that's 10x higher than the standard (BS 7430) says...

  - Andy.

Well both 20 and 200 ohms seem to be a peculiarly British thing.

There are a number of real considerations however. One is British soil. If you move to the middle east, or even Malta, the chances of getting anything as low as 20 ohms  with a sensible cluster of electrodes is almost nil. Whereas in the UK in Essex clay it is quite practical to hit 20 ohms with a couple of 4ft rods. Of course where I am in Hampshire, the ground is sand and gravel, and in the recent dry spell a similar 2 rods managed about 300 ohms but ho hum.

So practically to blow all but the smallest fuse or trip a breaker in any sensible time, relying on a loop with 2 electrodes - one intentional at the genset, and the other accidental at the fault, is a pretty long shot, even for a 5A MCB on a lamp post - which is the sort of thing the 20 ohm figure seems to come from, or rather if 5A is diverted into a 20 ohm electrode, the step voltage is about 100V - and much more than that is getting more than a little dangerous..

Before my time, I'm told, the way to verify the earth on a TT house was to put a 60W lamp between live and electrode and see how bright it lit. A dim light or no show indicated a bad earth.  So, what sort of value was that checking for ? well a 60W lamp draws 1/4A and is noticeably dim by 200V(?) so if we said 50V dropped at 1/4 amp, that would be about 200 ohms. Hmm. (there are some nasty approximations here, as the filament resistance falls when it is dimmer, but of course the current is not constant.)

Enter the RCD, now we can have a 'safe' step voltage of say 50V near the electrode at our largest non disconnected fault current  (30mA/100mA or 300mA depending on RCD) so the resistance to terra-firma can be 1.6k ohms /500 ohms or 160 ohms . Well the 160 ohm case needs real electrodes, but the 30mA one will be happy with a screwdriver or a garden fork in the lawn, and now we credibly are protecting the person with wet feet picking up the severed lead to the hedge trimmer or whatever.

But a high resistance electrode is not good, as if unlucky you can have a fault that is much better earthed than the genset - a live core being spiked by a metal fence comes to mind, and also the falling of a 3 phase connector into a ditch full of muddy water. The 230V is shared between the 2 'electrodes' with the bulk of terra firma at the mid point, and it is possible for the genset and all CPCs to be elevated to most of the supply voltage wrt the ground beneath your feet.

its arbitrary but worth thinking of the likely fault cases.

Mike.

mapj1 said:

So practically to blow all but the smallest fuse or trip a breaker in any sensible time, relying on a loop with 2 electrodes - one intentional at the genset, and the other accidental at the fault, is a pretty long shot, even for a 5A MCB on a lamp post - which is the sort of thing the 20 ohm figure seems to come from, or rather if 5A is diverted into a 20 ohm electrode, the step voltage is about 100V - and much more than that is getting more than a little dangerous..

Agreed, although the discussion is about TN-S, in which the electrode (and the ground it is in) does not partake in the fault path itself for ADS.

However, for additional protection, the ground and the source earth electrode might be used as a "return". In this case, provided an RCD with residual current rating not exceeding 30 mA is likely to operate, all is good.

mapj1 said:

Enter the RCD, now we can have a 'safe' step voltage of say 50V near the electrode at our largest non disconnected fault current  (30mA/100mA or 300mA depending on RCD) so the resistance to terra-firma can be 1.6k ohms /500 ohms or 160 ohms . Well the 160 ohm case needs real electrodes, but the 30mA one will be happy with a screwdriver or a garden fork in the lawn, and now we credibly are protecting the person with wet feet picking up the severed lead to the hedge trimmer or whatever.

Agreed

mapj1 said:

But a high resistance electrode is not good, as if unlucky you can have a fault that is much better earthed than the genset - a live core being spiked by a metal fence comes to mind, and also the falling of a 3 phase connector into a ditch full of muddy water. The 230V is shared between the 2 'electrodes' with the bulk of terra firma at the mid point, and it is possible for the genset and all CPCs to be elevated to most of the supply voltage wrt the ground beneath your feet.

Also agreed.

In fact, guidance for some time has been pointing installers away from "earth rod does the job" to the use of earth mat or conductive disc type electrodes for a number of reasons (including reducing the risk of buried services strike, which, in the case of gas, can be very nasty). Having said that, for temporary installations in a field or similar, well, it's going to be some sort of rod isn't it?

Having said that, for temporary installations in a field or similar, well, it's going to be some sort of rod isn't it?

I'd agree for general garden fetes and so on. 
Or rely  the skids of the genset if the ground is wet, but the sort of scouty event I do, involve so many spikes in the ground, that a few more make no odds to the risk, The sort of things you use to secure a marquee or a climbing tower are probably better electrodes than the official electrodes.

For some mil stuff we have plates that you drive on that go under the fronts wheel of the vehicle and get pressed into the earth,  these are more popular if there is a risk of buried munitions... some of the stuff I have been involved with  in the past has had some unusual looking risk assessments.

Mike.

If I read the HSE guidance correctly, they advise that even for large 3p gens used for short term events, there is no significant concern in not connecting frame/neutral to earth. That being the case, then perhaps hammering in electrodes could pose a risk that should be avoided altogether?

lyledunn said:

If I read the HSE guidance correctly, they advise that even for large 3p gens used for short term events, there is no significant concern in not connecting frame/neutral to earth. That being the case, then perhaps hammering in electrodes could pose a risk that should be avoided altogether?

There is a very nasty fault scenario that can occur if you do that ... if a line conductor becomes connected to the ground somehow (e.g. long nail or tent peg or similar going through a flexible cable that only penetrates the insulation of a line conductor), then the frame becomes live with respect to the ground.

I have been led to understand that this has killed a security operative on trying to enter a site portacabin with conductive outer fabric connected to the "mains earth" inside the unit.

The current version of BS 7430 therefore suggests armoured cables are used for distribution with unearthed generating sets - this will operate protection to prevent the nasty scenario. 

However, if flexible cables are supplied from, say, portacabins or other mobile/transportable units with unearthed generating sets, the same nasty issue is back again.

I think the 20 and 200 ohms proposed in the DPC version of BS7430 is a reasonable tack for gen sets at a festival event, particularly to address the "nasty fault scenario" that you describe. 

That was the kernel of my original post. I wanted to establish the maximum electrode resistance that could legitimately describe a generator arrangement as TN. 

My thinking was taken from the TN requirements for a public supply as outlined in the wiring regs in the ROI (IS10101-2020 A1 2024). 

The designer of the electrical installation for the festival event I identified in my original post merely specified that the gen-sets were to be arranged as a TN-S system with adjustable RCD set at 300mA time-delayed to 0.5s (apart from the stage supply where he had permitted the RCD to be bypassed). There was no specification for the electrode resistance establishing the "T". 

Often the electrodes that accompany the gen sets are thumped in with a few clouts of a sledge hammer without any thought of their intended function. In fact, the electrician in charge of this particular festival was a veteran of such events across the UK and Ireland said that it was rare that the electrodes were subject to test as we had done.

Following the exactness required for a TN system for a public supply might be onerous and would obviously require speculation about the value of Re, however, as a way of reference for compliance at such events in the future, I will run with the recommendation in the DPC version of BS7430, as well, of course, by ensuring cables are of adequate mechanical strength and appropriately installed. 

lyledunn said:

That was the kernel of my original post. I wanted to establish the maximum electrode resistance that could legitimately describe a generator arrangement as TN. 

My thinking was taken from the TN requirements for a public supply as outlined in the wiring regs in the ROI (IS10101-2020 A1 2024). 

Don't these requirements revolve around TN-C-S? Which in the UK is where, I believe, the 20 Ω figure came from that ended up as a recommendation for all TN systems.

The question that was posed in developing the guidance previously published in the IET CoP for Electrical Energy Storage Systems, that are now proposed for BS 7430, was 'Why should this apply to TN-S? Provided the connection with earth is low enough to operate RCDs for additional protection, i.e. residual current rating of 30 mA or less, in a TN-S system do we mind?'

There is still not full agreement in the industry that 200 Ω is the threshold of instability ... there are discussions about it being too much in some cases, too little in others. However, I don't think anyone has an alternative reasoned proposal at this stage to reduce or increase the 200 Ω value.

I’ve been doing some reading of the HTM 06-01 regarding this.

With reference to HTM 06-01 (2017) section 13.11 - generator earthing: 

It should be ensured that an adequate fault current can be developed to operate any protective device within the electrical network. 

This supersedes the resistance target (old 13.13) of less than 20 ohms. 

I’ve been doing some reading of the HTM 06-01 regarding this.

With reference to HTM 06-01 (2017) section 13.11 - generator earthing: 

It should be ensured that an adequate fault current can be developed to operate any protective device within the electrical network. 

This supersedes the resistance target (old 13.13) of less than 20 ohms. 

AMK said:

This supersedes the resistance target (old 13.13) of less than 20 ohms. 

I don't agree with the logic that this means you don't need to meet the BS 7430 recommendation (generally 20 Ω for TN systems) if the system is TN-S or TN-C-S ... the earth electrode is not part of the earth fault path for ADS, so meeting the 13.13 requirement of HTM 06-01:2017 has nothing to do with the earth electrode !

And in the latest DPC for BS 7430, the recommendation is still 20 Ω unless all circuits are protected by RCD (which broadly aligns with the IET CoP for EESS).