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.

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 ?  

AMK said:

Hi Graham. Are you involved in the latest DP. For BS 7430?

I am the current Chair of GEL/600, which is responsible for BS 7430.

Also, see Page 5 of HTM 06-01 (2017).

AMK said:

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 agree the statement about an earth electrode was removed in the 2017 version of HTM 06-01, and at that Clause number there is another requirement.

AMK said:

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. )

I would agree this might be the case in a public distribution system for overheads or cables not in the ground., but I think buried cables are different and require an earthed metallic screen ('protective screen') ... see Regulation 13 of ESQCR.

BS 7671 doesn't recognise an earth fault path in a TN system that doesn't flow through exposed-conductive-parts and protective conductors ... except for additional protection where the fault path is not defined, only the maximum residual current rating of the RCD. 

AMK said:

The updated HTM no longer references the 20-ohm value for generator earth rods.

Agreed.

AMK said:

Why do you think this is ? 

Why repeat something that's covered in a British Standard (BS 7430)?

See HTM 06-01 (2017), Clause 13.1, and the shaded note at the beginning of Section 9, which specifically call out the standard.

AMK said:

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. 

I don't think this is a replacement statement for a requirement for an earth electrode resistance of 20 Ω.

In fact, why do we think it is? Surely, a fault to the 'general mass of the Earth' (or at least, to something metallic connected to the Earth with a negligible effective earth electrode resistance) will only provide an  maximum prospective fault current of 11.5 A ... which won't operate an OCPD of rating much above 6 A !!!

Surely the statement in  HTM 06-01 (2017) clause 13.11 is as much about co-ordinating protective devices (RCD or OCPD) with the source of supply ... such as UPS's and inverters, or using a rotary generator or battery-backed static converter as an alternative supply to the grid, and making sure the protective devices are good for all modes of operation (connected mode and island mode)?

Remember also, the N-PE connection (now 'system referencing connection' used to be 'Neutral bond' or similar term) has a resistance of its own that is different to that of the PE-Earth connection (the electrode itself).

Going back to the statement “The connection between a generating set and Earth must have sufficiently low resistance to allow adequate current flow for protective devices to function effectively”. For instance, in the unlikely event that the generator’s tails come into contact with the general mass of Earth, a resistance of 20 ohms back to the generator’s star point would be too high. As you mentioned, this resistance would not allow enough current to trip a 6-amp MCB within 0.4 seconds. However, generators used in critical applications, such as hospitals, are typically not equipped with MCCBs or ACBs (Generator main breaker)  that incorporate RCD functionality. So how do you ensure disconnection in this scenario ? 

AMK said:

“The connection between a generating set and Earth must have sufficiently low resistance to allow adequate current flow for protective devices to function effectively”

A similar consideration seems to be associated with the requirements for isolating the neutral conductor on TN systems. If, the source earth electrode is allowed to be up to 20 ohms I fail to understand how protection could be arranged to operate unless by RCD.

The wording is exactly the same in the ROI regs, IS 10101. 

So how do you ensure disconnection in this scenario ? 
You don't, and you don't need to - it's not  a single fault condition, so long as all class 1 kit has a CPC, and everything without a CPC has suitable sheathing/ cladding/ enclosure or whatever to qualify as double insulated or equivalent protection.

Double fault to danger - e,g, a live to case fault plus a broken CPC, or cable sheath and basic insulation both damaged are not considered likely enough to require action.

'Additional protection' by RCD or earth fault relay is just that, additional.

Mind you that cable plus sheath damage is actually quite a common double fault, and is the thinking behind BS7909 requiring RCDs for temporary wiring at outdoor events etc.

Mike

AMK said:

For instance, in the unlikely event that the generator’s tails come into contact with the general mass of Earth,

What is the means of protection against electric shock?

• If the tails are in armoured cable, or metallic conduit or trunking, it's ADS
• If the tails are insulated and sheathed, then it's double or reinforced insulation.

But, more importantly, what if the tails come into contact with something metallic connected to the general mass of earth.

AMK said:

For instance, in the unlikely event that the generator’s tails come into contact with the general mass of Earth, a resistance of 20 ohms back to the generator’s star point would be too high. As you mentioned, this resistance would not allow enough current to trip a 6-amp MCB within 0.4 seconds.

Correct, I made this point. We could go to 1 Ohm (the old value from earlier Editions of BS 7430, the old "combined earthing resistance" that permitted connection of HV and LV systems ... but even that would limit the OCPD nominal current rating to 160 A or so !

gkenyon said:

In fact, why do we think it is? Surely, a fault to the 'general mass of the Earth' (or at least, to something metallic connected to the Earth with a negligible effective earth electrode resistance) will only provide an  maximum prospective fault current of 11.5 A ... which won't operate an OCPD of rating much above 6 A !!!

But that kind of fault is only considered by BS 7671 in the case of additional protection (RCDs), and NOT other means of protection addressed in Chapter 41, because there is, in general, always at least 2 "faults" needed to create a shock hazard.

AMK said:

The connection between a generating set and Earth must have sufficiently low resistance to allow adequate current flow for protective devices to function effectively

But in this case, does 'Earth' include the protective earthing system ("PE")? Otherwise, as above (discussion regarding what current is needed for OCPD to operate) we'd be in a right pickle?

lyledunn said:

I fail to understand how protection could be arranged to operate unless by RCD.

Exactly, which is why it can't mean that. It must mean the combined earthing system (including PE). There is really no other explanation.

I think it's as simple as the PE is connected to Earth, and the generator is Earthed to PE ... then the statement makes sense?

lyledunn said:

If, the source earth electrode is allowed to be up to 20 ohms I fail to understand how protection could be arranged to operate unless by RCD.

I think this says it all, but to support my point:

... and perhaps HTM 06-01 Clause 13.11 is really aimed at Regulations 411.4.1, 411.4.2 and 114.1?

It may be worth pointing out that the whole voltage of the cpc 'earth' and exposed metal  relative to terra-firma ('Earth' with a capital E as Graham denotes it) is also a great headache for designers of substations, and when using generating plant with a transformer that steps up to HV for distribution of some distance and down again where the load  is.

There are several things that make it impossible to know the fault or shock current accurately , plus the added complication that the impedance of a person in the fault loop is a very variable thing and the rise of earth potential affects how much the near fields of the various earth electrodes can be overlapped, or if they should be interconnected.


Then the preferred approach to co-coordinating electrode impedance and disconnection times is that  more or less captured in BS-EN 5022
That is to start  with some assumptions about the human body.

To which multipliers are applied for hand to hand or hand to foot or foot to foot etc 
The situation is complicated by the fact that as the voltage increases, the skin is damaged and becomes more conductive

Leads to

(so implying much above 690V to earth the ADS can never be fast enough - not really true - after all electric fences have many kV but very much shorter duration)

But the voltages can be much higher if one can assume various types of footwear or a gravel or other self draining surface adding resistance in series with the person. 

The lowest of these  - 'not really insulating at all wet leather ' footwear, aligns very approximately with the familiar 0.4 seconds for TN systems at 230V, though that is also the no footwear case  and an assumption that the voltage at fault point is about half that of the unloaded supply i.e. when the CPC and phase line resistances are equal.
all these curves share a kink between 0.2 and 0.5 seconds as that relates to the period of the human heartbeat, and how long the muscles can be shocked for without it losing its rhythm.

This is only a side branch to the discussion, but may explain where the numbers come from, and why various authoritative documents don't quite line up, as assumptions vary.
Mike

It may be worth pointing out that the whole voltage of the cpc 'earth' and exposed metal  relative to terra-firma ('Earth' with a capital E as Graham denotes it) is also a great headache for designers of substations, and when using generating plant with a transformer that steps up to HV for distribution of some distance and down again where the load  is.

There are several things that make it impossible to know the fault or shock current accurately , plus the added complication that the impedance of a person in the fault loop is a very variable thing and the rise of earth potential affects how much the near fields of the various earth electrodes can be overlapped, or if they should be interconnected.


Then the preferred approach to co-coordinating electrode impedance and disconnection times is that  more or less captured in BS-EN 5022
That is to start  with some assumptions about the human body.

To which multipliers are applied for hand to hand or hand to foot or foot to foot etc 
The situation is complicated by the fact that as the voltage increases, the skin is damaged and becomes more conductive

Leads to

(so implying much above 690V to earth the ADS can never be fast enough - not really true - after all electric fences have many kV but very much shorter duration)

But the voltages can be much higher if one can assume various types of footwear or a gravel or other self draining surface adding resistance in series with the person. 

The lowest of these  - 'not really insulating at all wet leather ' footwear, aligns very approximately with the familiar 0.4 seconds for TN systems at 230V, though that is also the no footwear case  and an assumption that the voltage at fault point is about half that of the unloaded supply i.e. when the CPC and phase line resistances are equal.
all these curves share a kink between 0.2 and 0.5 seconds as that relates to the period of the human heartbeat, and how long the muscles can be shocked for without it losing its rhythm.

This is only a side branch to the discussion, but may explain where the numbers come from, and why various authoritative documents don't quite line up, as assumptions vary.
Mike