TN system for generator

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. 

gkenyon said:

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

You might be right but can it not be used as a guide to deal with that nasty fault scenario you referred to in terms of earthing gen sets to establish a TN-S system? Specifying a maximum of 200 ohms might be spot on reasonable for a 300mA RCD at the generator output which is the maximum setting for compliance with Section 740. Even if the grounded phase was without resistance, whilst the voltage drop experienced would be close to phase voltage, it would be relatively short lived. On the other hand, where there is no RCD, keeping the earth electrode to a maximum of 20 ohms helps attenuate touch voltage accepting that the grounded phase will likely have a resistance of more than 20 ohms??  

That is about as good as the logic gets. It rather depends on the type of fault and the RCD/ earth fault relay selected.  Faults paths that include  terra firma are not really guaranteed to be automatically disconnected if the resistance is too high - after all that is why we do have a CPC, and why that 300mA or less RCD is mandatory for all normal use cases.

Having just said that for  reasons of all the downstream NE bonds,  this is not true for genset used by DNOs to energise a neighbourhood while a transformer is being changed or when one genset is to run in tandem with another one, So it is possible to hire gensets that either do not have this feature or can have it locked off, so on receipt of such a machine,  from companies that  hire for these  applications,  it is important to check what state it is actually in.

I have seen a variant of the live CPC problem first hand, with a trailer mounted genset  where a cable was snagged on a metal fence and the installers were a bit 'liberal' about omitting earth rods. The effect was a surprise when someone reached for the door handle of the vehicle, it could have been worse, but even when it  isn't it is quite thought provoking.

Mike.

lyledunn said:

You might be right but can it not be used as a guide to deal with that nasty fault scenario you referred to in terms of earthing gen sets to establish a TN-S system? Specifying a maximum of 200 ohms might be spot on reasonable for a 300mA RCD at the generator output which is the maximum setting for compliance with Section 740. Even if the grounded phase was without resistance, whilst the voltage drop experienced would be close to phase voltage, it would be relatively short lived. On the other hand, where there is no RCD, keeping the earth electrode to a maximum of 20 ohms helps attenuate touch voltage accepting that the grounded phase will likely have a resistance of more than 20 ohms?? 

I'm not sure I follow.

If we determine that BS 7671 (or IEC 60364 series) is conformed to "to the letter", this means that an earth fault is one to exposed-conductive-parts, and not the Earth itself.

Only in additional protection (where there is always an RCD) do we ever see a fault to Earth.

(in the above, 'earth' is an earthed part of the system, and 'Earth' is the general mass of the Earth).

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).

Hi Graham. Are you involved in the latest DP. For BS 7430?The previous edition of the HTM explicitly specified a 20-ohm value for earth electrodes, but this has been replaced with the new statement I mentioned. I understand that an electrode connection is essential to allow fault currents to flow back to the supply in the event of cable insulation failure (same way, the public distribution is earthed to detect the collapse of overhead power lines. ) The updated HTM no longer references the 20-ohm value for generator earth rods. Why do you think this is ?