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Ze of PCE (Code of practice for EESS)

Hugh Frater

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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  • 0

    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?

  • 0 in reply to Chris Pearson

    It’s different in many ways, the main one being its very limited fault current. The inverter manufacturer calls for over current protection between the inverter and the loads, and I’ve fitted the required B curve mcb to satisfy this requirement.

    There’s no way the inverter can clear this 32a breaker though, it has such a limited overload/surge capacity.

    Guidance notes for testing do cover this this, but I don’t have it in front of me to remember exactly what they say.

  • 0 in reply to Chris Pearson

    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…

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  • 0 in reply to Chris Pearson

    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…

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