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Simultaenous Contact

Dom

Good afternoon all, 

I am seeking the collective views of the group regarding the issues we are currently encountering within the EV installation environment in relation to simultaneous contact.

In simple terms, we are seeing a significant number of installations where customers have lamp posts located within their driveways. In many of these cases, it is not possible to achieve the recommended 2.5‑metre separation distance.

My position is that, where physical separation cannot be achieved, the application of layered protection at the EV charge point and associated switchgear would leave the customer in no worse position than prior to the installation. It is accepted that the proposal only outlines protection on the vehicle, but there are no other solutions for industry, are we with the protection detailed taking enough measures to carry out an install and in affect leave the install without a subsantial increase in risk?

My proposed approach to managing simultaneous contact risk would follow a structured hierarchy as set out below:

  • Can the maximum separation distance be achieved?
  • Can barriers or enclosures be installed to prevent simultaneous contact?
  • If neither of the above options are achievable, can layered protection provide an acceptable level of risk mitigation?

With layered protection, the proposed measures would include:

  • Installation of a compliant open‑PEN (O‑PEN) protective device
  • Provision of a double‑pole 30 mA Type A RCBO
  • Use of a charge point - M3 21 mA protection

Based on the above, the assumption is that, if all protective measures are correctly installed and verified, the installation would incorporate:

  • Automatic disconnection within the required times
  • Residual current protection
  • Open‑PEN fault detection

This combination of protections would significantly reduce the likelihood of a fault condition persisting for any meaningful duration.

On this basis, the key question for consideration is:

Where physical separation and barriers are not achievable, would it be considered acceptable to proceed with installation relying on this layered protection approach?

  • 0 in reply to gkenyon

    My point was not to suggest that the two pole RCB was not required but to point our that if there is sufficient coupling (capacitive or conductive fault) between the core, body or any interwinding screen of the transformer and its secondary to present a hazard then the arrangement shown does not provide protection in the context under discussion and that an additional 3 pole RCD would be required to achieve that.

    Whilst I accept that the PC is necessary for various reasons the most significant of which is allowing the two pole RCD on the secondary to be effective, there is a risk if this becomes connected to anything, including any PEs that might be present (the context of this discussion).  The PC is an unbonded conductive part so shouldn't be exposed anywhere other than at the vehicle itself and should be treated as a live conductor - e.g. the use of T+E or armoured cable for wiring on the transformer secondary side shouldn't be permitted - I don't know if this is clear?

  • 0 in reply to DShutt

    I think we’re largely in agreement on the principle. My point is that the transformer secondary is intended to remain electrically separated from earth, with the PC acting as the reference conductor for the separated system rather than as a protective conductor in the conventional sense.

    If the PC were inadvertently connected to PE or any earthed conductive part outside the vehicle, then yes, the electrical separation would be compromised and the assumptions behind the arrangement would no longer apply. That’s why the integrity of the separated system is critical.

    Where I differ is on the need for an additional 3-pole RCD. If the transformer complies with the relevant product standards (including limits on leakage and insulation between primary, core/screens and secondary), and the secondary is installed as a separated system in accordance with BS 7671 and BS 7430, then the 2-pole RCD on the secondary should provide protection against a second fault. A 3-pole RCD would only become necessary if we assume a fault path that effectively defeats the electrical separation, in which case the installation is already outside its intended operating conditions.

    I do agree that the PC should never be treated as a conventional PE and should not be exported or bonded to external earths. Maintaining that separation is fundamental to the safety of the arrangement.

  • 0 in reply to DShutt

    The PC is an unbonded conductive part so shouldn't be exposed anywhere other than at the vehicle itself and should be treated as a live conductor - e.g. the use of T+E or armoured cable for wiring on the transformer secondary side shouldn't be permitted - I don't know if this is clear?

    That does seem to be going beyond normal requirements for separated circuits - section 418.3 for instance only requires the bonding conductor to be insulated (rather than say insulated & sheathed as would usually be the case for live conductors) and clearly accepts multiple exposed-conductive-parts on the same system. Conventionally the sheath of SWA is considered adequate for separating different earthing systems (consider the typical arrangement for a TT'd outbuilding where the supply SWA is TN and then gapped at the intake position of the outbuilding - so the final length of the SWA is within the TT equipotential zone).

    That said the A722 approach isn't really a separated system in a conventional sense (normally the connection of exposed-conductive-parts or a protective conductor to a live conductor is prohibited) - so some differences will be inevitable. It might be clearer if BS 7671 recognised it as a different system again (IN-S).

    Also BS 7671 generally only provides safety under single-fault conditions - risks from 2nd faults (e.g. earth fault and broken c.p.c. in ADS) can often remain (although BS 7671 does require protection from some 2nd faults in some situations - e.g. for additional protection). The PC conductor becoming connected to another earthing system (or true Earth) probably wouldn't of itself be an immediate hazard - you'd normally also need another fault elsewhere (e.g. broken PEN or uncleared earth fault) - so into 2nd fault territory.

       - Andy.

  • 0 in reply to AJJewsbury
    AJJewsbury said:
    It might be clearer if BS 7671 recognised it as a different system again (IN-S).

    IN-S is now recognised in BS 7430:2016, so that might be possible in future.

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