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TNC installation - Continental Europe

vantech
  1. Hi,

I am looking at re-wiring a small apartment on the continent. The supply to the building is a TNC.

I was quite surprised to find a 4 core 6mm cable coming into the building from an underground cable, with the 3 phases landing into 80A fuses and the neutrals twisted together with an aged bit of tape around them. There are then multiple 4mm cables that connect to the secondary side of these fuses, which go off to the various properties.

The properties themselves have a consumer DB, with the phase conductor landing in a 32A MCB. The neutral then lands into the neutral bar, with a link to the earth bar.

I just wanted to clarify the following. 

Q1: 

When upgrading this apartment, I obviously want to use RCDs. I believe in order to do this, I would need to land the incoming neutral into a suitable connector, which then links to the neural terminal of the RCD and also the earth bar - this would then give me a TNC-S earth and neutral. The neutral output of the RCD is then in essence measured and compared to the phase conductor for subsequent outgoing circuit protection. 

Q2: 

I am assuming that the supply authority rely solely on the consumers “main switch” which is in essence an MCB to ensure the cable between the suppliers main feed and the property isn’t overloaded, because the suppliers fuses are not rated to protect the 4mm cable that comes into the property. I believe they have rated the fuses high enough to cope with the maximum demand for the connected properties. 

This would then mean, that the phase conductor needs to land into this MCB before the RCD. Is a single pole MCB really the best choice, or should I look to use a double pole MCB that disconnects the phase and the neutral conductor, and if so, what then happens to the earth?  

Obviously all of this falls outside of BS7671, because this standard doesn’t apply on the continent. I am just looking to clarify my understanding. I have actually looked at various installations and all of them are the same. The standard of wiring and associated safety devices/methods doesn’t even compare to what we do in the UK.  

I have also looked at using a local electrician - really not an option considering the state of some of the installations I’ve seen! 

Thanks. 

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    Interestingly the (presumably metallic) box's own earth is connected to the unprotected PE (which is probably correct in one way, given the possibility of a fault from the incoming conductors) - but that would mean that the box itself would likely be live under a broken PEN condition. That's probably fine for the intended use where the matt:e is indoors within the equipotential zone and it's just the EV/EVSE outside that needs disconnecting, but probably not what you want if you're trying to protect an entire installation.

       - Andy.

  • 0 in reply to AJJewsbury

    Hi Andy, 

    There is an option for a plastic box, as well as a metal box with an RCD before the contactor, so in this instance the box’s earth could be connected to the output of the contactor so that the enclosure becomes disconnected during a PEN failure. 

    There would be a small and insignificant risk of the incoming supply touching the metalwork, but I see this as unlikely, compared to someone climbing a ladder trying to reset an RCD which hasn’t tripped (not knowing the PEN fault is active).

    I see a greater benefit in having PEN fault detection, because there is absolutely no way I would be able to ensure neutral connectivity downstream. It’s sort of a belt and braces approach. 

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  • 0 in reply to AJJewsbury

    Hi Andy, 

    There is an option for a plastic box, as well as a metal box with an RCD before the contactor, so in this instance the box’s earth could be connected to the output of the contactor so that the enclosure becomes disconnected during a PEN failure. 

    There would be a small and insignificant risk of the incoming supply touching the metalwork, but I see this as unlikely, compared to someone climbing a ladder trying to reset an RCD which hasn’t tripped (not knowing the PEN fault is active).

    I see a greater benefit in having PEN fault detection, because there is absolutely no way I would be able to ensure neutral connectivity downstream. It’s sort of a belt and braces approach. 

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