Ze of PCE (Code of practice for EESS)

I've upgraded the EESS at my off-grid property (new batteries, new inverter/charger/mppt), all up and running as expected and everything working as it should. An EICR is required, and the electrician performing the inspection and testing has raised an interesting point, how to complete the Ze test and what value to put down on the certificate.

The IET Guidance Note 3: Inspection & Testing, when referring to prosumers installations suggest that the Ze test should be taken with the distribution board isolated, and at the output terminals of the PCE (the inverter), but this will just give the impedance of the output stage of the inverter, currently around 5-6ohms. This value being a fail because the supply from the PCE to the distribution board is T-N-S and this requires a lower value.

Again from guidance note 3, from 643.7.3.1 note 1, B, point 2:

“For island mode: if applicable, verification of earth fault loop impedance is determined using measured (r1+r2) values, plus the manufacturer’s information regarding the value of Ze to be assumed for the EESS or the relevant PCE within it.”
All clear there, except I can't get this value from the manufacturer, I've asked and they've gone silent on me.
It has been suggested by another electrician that I've spoken to (who admits to not having extensive EESS experience) that I install another earth rod and make the system TT from the PCE to the distribution board, then anything <200ohms would 'technically' be a pass, but as the PCE and the distribution board are within 1m of each other, this seems a bit of a fudge when T-N-S is preferred according to the latest Code of Practice for EESS (an excellent publication btw, my copy arrived yesterday)...
Something interesting to discuss, how we move forward from here?
Parents
  • I am trying to square this up with a public supply, in which the LV end has either one or two conductors at earth potential, which is ensured by earthing at the transformer.

    So is the output of the inverter any different from the output from a public transformer. Granted the cables are short, but they are also short if you live adjacent to a transformer.

    If the PCE has an internal impedance of 5 Ω to 6 Ω, then ignoring whether this is Ze or not, how can any Zs be satisfactory?

  • Chapter 82, 2.6.24 says:

    Supply characteristics may change between installation operating modes;

    (b) The prospective fault current is likely to be far less and the effective EFLI greater, than in connected mode. 

    I won’t quote it all, but basically ‘use RCDs or the overcurrent protection built into the PCE to provide protection against electric shock’. MI’s should be consulted…

  • If the PCE has an internal impedance of 5 Ω to 6 Ω, then ignoring whether this is Ze or not, how can any Zs be satisfactory?

    You just have to accept a high Zs and design accordingly for shock protection. If using ADS then pick an appropriate device (most likely an RCD - just like for TT), or use double/reinforced insulation (likely upstream of the 1st RCD), or even rely on the characteristics of the inverter to collapse the voltage to acceptable levels in the case of a L-PE fault of negligible impedance (section 419 style).

       - Andy.

  • You just have to accept a high Zs and design accordingly for shock protection. If using ADS then pick an appropriate device (most likely an RCD - just like for TT)

    Thank you, Andy. Yes, I can see the need to protect in a similar way to TT, so is Zs relevant?

    Ze is defined as the loop impedance external to your installation - if your system is entirely self-contained (off grid) then there is no external part - so the test and concept simply don't apply. I'd record it as "N/A".

    Yes, I think that sums up the situation.

    but this will just give the impedance of the output stage of the inverter, currently around 5-6ohms. This value being a fail because the supply from the PCE to the distribution board is T-N-S and this requires a lower value.

    Which is why I mentioned Zs. Is the installation really TN-S?

    Earth rod is connected to the MET, N-E link is made in the PCE (although code of practice states it should be in the distribution board). All circuits are on 30mA rcbo’s.

    Still doesn’t help with the original issue/question though.

    I think that Andy has answered it. In Section I of the EICR, put "N/A" for Ze, but you must also give details of the earthing in Section J including the resistance of the rod.

    With a public TT supply, you can estimate the resistance by undertaking a Ze test, which ignores the resistance of the transformer's earth. You cannot do that in your case so the rod's resistance needs to be measured properly.

  • Thank you, Andy. Yes, I can see the need to protect in a similar way to TT, so is Zs relevant?

    It is relevant in TT systems also ... even if RCDs are used ... see 411.5.3 (i).

    Regardless of "very high" Zs, it's still a requirement to know that the RCD will operate in the stated disconnection time. To achieve 0.1 s disconnection times, we are talking in excess of 1 k ohm for 100 and 300 mA RCDs ... but can be as low as 250 ohms Zs for 500 mA RCDs.

    I don't believe, though, that for island mode there is a common standard for protections built into the inverter, so I'm not sure that you can always guarantee the RCD will operate before the inverter current limits ?

  • I think that Andy has answered it. In Section I of the EICR, put "N/A" for Ze, but you must also give details of the earthing in Section J including the resistance of the rod.

    This is not part of the earth fault path unlike a TT system, so it's not appropriate if you form a TN-S system in island mode (which is the preferred option and regardless of the technicalities will operate protective devices more quickly).

    So, whilst it's correct to record the earth electrode resistance (it must be measured before energization in initial verification in any case), I don't think Ze is "N/A" and you can't rely upon 411.5.3 (ii) ... the value of RA (which is the sum of R2 from exposed-conductive-parts to MET, resistance of protective conductor between MET and  electrode, and the earth electrode resistance itself).

  • So similar to a typical earthed generator configuration 

  • So similar to a typical earthed generator configuration 

    Well ... yes, but I can measure a loop impedance with a rotary generator (although noted the results can be variable with the wrong test instrument). With an inverter, the loop impedance/prospective fault current measurement is meaningless because the rms voltage is kept pretty constant during the test by the inverter.

    It might mean, for an inverter, that Ze ≈ 0 for currents below the current limit, and Ze →∞ [or at least a very large number] for currents above the current limit, after a particular time (which may vary from manufacturer to manufacturer).

  • Thank you, Graham. Could you please confirm if it is appropriate to treat a PV system in the same manner as a standard generator with respect to earthing arrangements and related considerations? The value of the earth electrode should be low enough to allow sufficient fault current to flow in the event of a fault to earth; such as May arise from insulation damage on a cable.

  • Thank you, Graham. Could you please confirm if it is appropriate to treat a PV system in the same manner as a standard generator with respect to earthing arrangements and related considerations?

    Yes, in so far as there is currently no distinction in BS 7671, AND Regulations 551.1.1 and 551.1.2 pull in "static converters" ...

    The value of the earth electrode should be low enough to allow sufficient fault current to flow in the event of a fault to earth

    Well, which 'earth' are you talking about/

    'Earth' (capital E) is the 'general mass of the earth':

    BUT, BS 7671 considers 'single fault conditions' which would be L to PE ... but not L to 'Earth' unless we are talking about additional protection? However, ADS is not 'additional protection' in general ...

    The value of the earth electrode should be low enough to allow sufficient fault current to flow in the event of a fault to earth; such as May arise from insulation damage on a cable.

    According to?

    You see ... as far as BS 7671 is concerned, an insulated and sheathed cable can't have a single fault to Earth.

    But, an armoured cable, or a cable with a cpc, can have a single fault to PE ... which in a TN-S or TN-C-S system, does not include Earth in the fault path ?

  • Hi Graham, yes I was referring to protection against shock by ADS. And the value of the electrode resistance should be low enough to cause the protective device on the output of the Genny to operate within 0.1-5 seconds. In most circumstances an RCD should be used to protect the distribution cable.

Reply
  • Hi Graham, yes I was referring to protection against shock by ADS. And the value of the electrode resistance should be low enough to cause the protective device on the output of the Genny to operate within 0.1-5 seconds. In most circumstances an RCD should be used to protect the distribution cable.

Children
  • TN-S arrangement 

  • But in a TN-S arrangement, the electrode simply references the N to Earth, it doesn't form part of the earth fault loop. Consider that a DNO's electrode may be between 1 and 20 Ohms, yet Ze for a TN supply is usually < 0.3Ω.

    Faults direct to the general mass of the earth aren't quantifiable, as there will inevitably be a very significant resistance at the point of the fault - as unlike metals soil, even wet soil, has a considerable resistance - so the fault loop could easily be many kΩ so even an RCD can't guarantee immediate disconnection.

      - Andy.

  • I am only considering a generator as the primary supply here. 

  • Understood - I was just comparing it with a DNO supply, since they're both the same TN arrangement and work in the same way (electrode not part of the earth fault loop).

      -  Andy.

  • Interesting. So is the value of resistance of the electrode not to be considered in a typical earthed generator configuration ? I was under the impression that the value should be low enough to allow ADS in the event of insulation damage on the distribution cable to true earth (Ground) 

  • Faults direct to the general mass of the earth aren't quantifiable, as there will inevitably be a very significant resistance at the point of the fault - as unlike metals soil, even wet soil, has a considerable resistance - so the fault loop could easily be many kΩ so even an RCD can't guarantee immediate disconnection.

    I was imagining a mobile generator on standard rubber tyres (like a car). If you touched a live conductor when standing beside the generator, you would not get a shock because there is no circuit.

    However, the situation becomes different if the generator (along with the chassis, etc.) is earthed. So why do we earth a generator please?

  • So is the value of resistance of the electrode not to be considered in a typical earthed generator configuration ?

    Yes, it's to be considered - as I recall BS 7430 recommends max 20Ω (although many seem to be on the opinion that significantly higher values can be satisfactory in some circumstances) - what I'm saying that's got little to do with Zs or normal ADS though in a TN system.

    The problem with faults to true Earth is the resistance of the fault - i.e. the resistance of the soil that's in contact with the exposed live conductor - we just don't know what it's likely to be - it's likely to be many kΩ - so even if you could calculate something, the numbers are unlikely to work even for RCDs. That's why buried cables with any chance of damage need to have a concentric c.p.c. (e.g. armour).

       - Andy.

  • I was imagining a mobile generator on standard rubber tyres (like a car). If you touched a live conductor when standing beside the generator, you would not get a shock because there is no circuit.

    Yup - protection by separation. Often used with small generators (e.g. ≤ 3kVA) especially when feeding a small system with a limited number of Class I items.

    However, the situation becomes different if the generator (along with the chassis, etc.) is earthed. So why do we earth a generator please?

    Separation is only good for small systems - larger systems have a couple of disadvantages for a separated system. Firstly long cable runs can mean the system goes get referenced to Earth - via stray capacitance (or even imperfect insulation) to Earth (BS 7671 set a limit of 500m of cable in total, and 100,000 Vm) . Secondly while separated system are only OK on 1st fault - as there's no disconnection on 1st fault, a 2nd fault can then occur which can be fatal - and larger systems will typically have more faults than smaller ones (all else being equal). There's also an EMC issue that some equipment with filters etc, like the PE to be connected to the same system as the live conductors, so they can actually divert the unwanted signals somewhere.

    So all in all, larger systems tend to go down the TN route instead.

       - Andy.

  • Thanks Andy. How about let’s say HO7RNF generator tails on the surface. What happens if they get cut and exposed copper touches the ground?

  • Surly the path  of the rod back to the star point needs to be low enough to operate the protective device.