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?
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  • 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?

  • 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. 

  • Also, in regards to three phase generators, if one of the lines comes into contact with mass of earth ( True earth) how does the value of the electrode influence potential neutral inversion ? 

  • Although slightly of topic , this incident is still somewhat relevant. Recently, I became aware of a 630mm single neutral conductor from a transformer to a store being severed and stolen. Remarkably, the electrical supply remained uninterrupted, and the theft was only discovered upon review of CCTV footage. The Distribution Network Operator subsequently visited the site and isolated the supply. I am astounded by the expertise of these thieves, who specifically targeted the neutral conductor.

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

    All depends on the soil resistance and the contact area. As a for instance, let's imagine that the soil is of a type where a 4' x 3/8" rod (so surface area about 30,000 mm²) gives a resistance of 200Ω - and say the gash in the cable exposed say 20x1 mm of conductor (20mm²) - then as a very simple approximation you might imagine that the fault would gave a resistance about 1500x higher than the rod (30,000/20) - so around 30kΩ.. (All very rough, if the soil eas dryer near the surface it could be a lot higher, of if in a puddle it might be lower) Anyhow 230V and 30kΩ would mean a fault current of less than 8mA - so nothing trips, not even a 30mA RCD,

    Cue comments about choosing a cable that's suitable for the environment. RN sheathed is pretty robust and isn't easily cut into by the usual blunt trauma - but if sharp implements are a credible risk and other precautions aren't sufficient, then consider armoured (or at least braided) cables instead.

      - Andy.

  • or to put it another way, ADS works when you can have a fault of negligible impedance - i.e. a fault directly between metallic parts, so you can then know the overall earth fault loop impedance, and calculate on that basis. That's really the essence of "Indirect" shock protection (as it used to be called) - faults to exposed-conductive-parts. Other problems - e.g. due live conductors becoming accessible - is closer to what used to be called direct contact and there's little ADS can do for that. It's either a case of doing your best to make sure that basic insulation remains intact (e.g. by physically protecting the cable) and/or by additional protection (e.g. 30mA RCD) - but even then you don't expect the fault to disconnect immediately it occurs - just when someone is gets a large enough shock for it to be potentially fatal.

      - Andy.

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  • or to put it another way, ADS works when you can have a fault of negligible impedance - i.e. a fault directly between metallic parts, so you can then know the overall earth fault loop impedance, and calculate on that basis. That's really the essence of "Indirect" shock protection (as it used to be called) - faults to exposed-conductive-parts. Other problems - e.g. due live conductors becoming accessible - is closer to what used to be called direct contact and there's little ADS can do for that. It's either a case of doing your best to make sure that basic insulation remains intact (e.g. by physically protecting the cable) and/or by additional protection (e.g. 30mA RCD) - but even then you don't expect the fault to disconnect immediately it occurs - just when someone is gets a large enough shock for it to be potentially fatal.

      - Andy.

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