Hi,

I was looking for a clear explanation of TN-C-S earthing.

I found what was a very clear video on youtube:

Earthing Arrangements TN-C-S in a Single Phase Installation and in Picture form to aid with Exams

Short version: "Terra (earth) and Neutral carried on a single conductor on the distribution side, but separately in the customer wiring.

There's an associated term "PME" - Protective multiple earth - which simply means that the distribution network earths the neutral at multiple points along the way.  Which tries to ensure that the earth is not lost.

In my house that appears to include actually at the point where the supply cable terminates.

Be happy to hear further info about TN-C-S design.

21 minutes ago, Elbow said:

In my house that appears to include actually at the point where the supply cable terminates.

Correct. Two wires come from the street to the connection point at your house. The neutral is usually already earthed at the transformer, so the neutral serves as both neutral and earth (it is combined in other words, hence the C).

An earth spike is installed nearby and a cable goes from the earth spike to this same connection box on the back/side of your house. Neutral is then bonded to earth at this point, and then from that point onwards earth is a separate conductor (hence the S in the moniker).

So TN (terra neutral), C (neutral and earth combined initially), S (but separate after the bonding point).

9 hours ago, plonkster said:

An earth spike is installed nearby and a cable goes from the earth spike to this same connection box on the back/side of your house. Neutral is then bonded to earth at this point, and then from that point onwards earth is a separate conductor (hence the S in the moniker).

So TN (terra neutral), C (neutral and earth combined initially), S (but separate after the bonding point).

So I looked again at the old board where my supply connects.  I can see that the power arrives in a sort of coaxial cable.  the centre (live) goes through a breaker to the old meter.

The screen heads off into an old metal box ("R10 fine for opening") and out of that box comes two earth wires - one goes to an earth spike and the other used to go to a nearby water pipe.  Now its loose - not sure if that is a fault or whether the rules changed?

Anyway - from the same black box emerges a neutral that goes into the old meter.

Out of the meter a live and neutral heads down a conduit and presumably to the main board inside.

BUT - there is no earth wire going to the main board.

Now my competent electrician friend did check my house when I moved in and the earths at the plugs do have a low resistance back to earth.  So where does it connect?  Does the main board actually depend on the neutral wire from the bonding point as earth too? 

 

 

1 hour ago, Elbow said:

I can see that the power arrives in a sort of coaxial cable

Probably armouring around the cable. Sometimes the armouring on the outside of the cable is used as a ground connection (though nowadays it is frowned  upon). It's possible that the outside armouring serves as a combined earth/neutral. Maybe that was allowed in the 70s. I'm not enough of an electrician to know.

1 hour ago, Elbow said:

old metal box

That's where they bonded earth and neutral. At least in British homes it is common to also earth your copper pipes to the same potential. Not sure about this side, but that's likely why one wire went to a pipe, it's to earth the piping system of the house and ensure it is at ground potential.

1 hour ago, Elbow said:

Out of the meter a live and neutral heads down a conduit and presumably to the main board inside.

It sounds like earth and neutral remains combined  to the main DB and is only separated there. Again... I don't know if this is a 70s thing and if it is still allowed. How these things usually work, is it remains fine until you start working on it... then it must be brought up to date.

Not to worry. It is safe and bonded as it should be. The only danger (as is the case with all TN-C-S systems) is a broken neutral between your earth spike and the main DB: Cause then you also lose your earth.

Have a look in the NRS-097 document (you can freely download it - search via Google).

It has a nice graphic.

TN-C-S has a link between Earth and Neutral at the consumer supply point (the meter) and an earth spike at that location.

The relevance of this is that there is no need to switch neutrals if you have a change over switch between inverter or any generator and grid provided the generator (whatever it is) does not itself have a link between neutral and earth. If it has not it usually has is own earth connection to a earthing spike.

Theoretically this also means there is no need to split neutrals in a split DB board as all neutrals are always at the same potential and must be. 

For any other type of connection you MUST split the DB board neutrals and switch neutrals. The reason is that the connection point between neutral and earth is not on your property and thus not under your control. 

I found what was a very clear article:

TN-C-S Earthing System: Detailed Explanation, Diagrams

Quote

TN-C-S earthing system (or TN-C-S system) is a distribution system in which one live part of a power source is earthed, exposed-conductive-parts of an electrical installation are connected to the earthed live part of the power source in a head part of the electrical installation (from the power source) by PEN, PEM or PEL conductors, and in other part of the electrical installation by protective earthing conductors (PE)

This definition is based on IEC 60364-1 and describes the meaning of the term well.

Example: TN-C-S system 3-phase 4-wire. The PEN conductor is separated at the origin of the electrical installation.

 

Edited February 14, 20233 yr by Vernonstanley