TNC-S Loss of Neutral

[FarmerJohn]

Hi DIYnot,

I have a bungalow that is out in the sticks which is connected using TNC-S. The cable feed is a combination of overhead and underground and must be about 300-400m from the tranformer in a garden nearby.

I considered the case of a loss of the incoming combined TN which could result in the entire earthing system of the bungalow becoming live. I know it is quite rare but have read of cases where this has happened which quite nasty results.

I have been mulling over this problem for quite some time now.

I am soon having the consumer unit changed to a Hager Design 50 where all circuits will be RCBO protected. At the same time I wondered if it would be possible to add one or more earth rods which I could connect to the incomming TN connection to provide an earth connection if the incomming TN connection was lost.

Would there be a problem in doing this?

As an alternative I did consider converting the TNC-S system to TT in which case the incomming TN would only be used as the neutral and I would then provide my own earth again using a number of earth rods only this time there would be no connection to the incomming TN.

Would there be a problem in doing this?

Is either method better than the other?

Any help/comments would be greatfully appreciated.

Regards
FarmerJo

 

[bernardgreen]

A few decades ago an overhead supply had to be TT due to the significant risk of the overhead Neutral becoming open circuit.

I would go for TT

 

[securespark]

I have seen folk bury old copper cylinders deep in the ground to get a decent Ze. In the most recent case, they brazed an earthing rod to either end of the cyl, linked them together above ground and ran a conductor into the house.

 

[SUNRAY]

TNC-S or TT but not both.

It happens more often than make the news, due to metal theft usually.


https://www.diynot.com/diy/threads/a-real-moan-tonight.552183/

 

[bernardgreen]

TNC-S or TT but not both.

When a TNC-S "Earth" is accidently or purposely connected to a very low impedance TT Earth and the supply Neutral has gone open circuit the result can be severe damage.

A TT Earth achieved by connection to a metallic water supply pipe could be taking the entire Neutral current going back to the sub-station when the Neutral fails..

The bonding wiring has to take account of this current and there have been fires in properties which were TT using the water mains as the Earth and then "upgraded" to TNC-S ( PME ). The fires occurred when the Neutral went open circuit ( metal theft being one reason ) and the small gauge bonding wires were overloaded and melted

 

[EFLImpudence]

If such a TT earth (no longer allowed) were 'upgraded' to TNC-S and then the neutral was lost, what would be the difference from what it was before?

If that resulted in damage then the earthing conductor must have been inadequate when simply (so-called) TT.

If the water pipe is now technically an extraneous-c-p then the bonding conductor must be 10mm² for that reason.

 

[bernardgreen]

If that resulted in damage then the earthing conductor must have been inadequate when simply (so-called) TT.

Provided the TT Earth could sink enough current from a fault in the house to blow the fuse(s) in the house the earthing conductor could be considered to be adequate. Just the consumption of a single home.

The difference is when the "redundant" connection to the TT earth rod(s) and/or metallic supply pipe is the only connection between Earth and Neutral in the area and has to carry the current from many homes.

 

[JohnW2]

TNC-S or TT but not both.

Except when there are no extraneous-c-ps, all TN-C-S installations are 'both' (TN-C-S and TT).

... and don't forget that it was only at the 11th hour that a 'requirement' for all installations (including TN-C-S ones) to also have an earth rod was removed from BS7671:2018 (a requirement which exists in other countries) - and that may well creep back in when a new edition (or, maybe even in an Amendment) comes along!

Kind Regards, John

 

[JohnW2]

I have a bungalow that is out in the sticks which is connected using TNC-S. The cable feed is a combination of overhead and underground and must be about 300-400m from the tranformer in a garden nearby. I considered the case of a loss of the incoming combined TN which could result in the entire earthing system of the bungalow becoming live. I know it is quite rare but have read of cases where this has happened which quite nasty results.

It sounds as if, in your situation, it would be extremely unlikely that you would experience the sort of 'nasty results' (usually due to very high currents flowing in bonding conductors) that one (very rarely) hears about. For that to happen, the break in the CNE conductor has to be downstream of where it connects to multiple other installations - in which case the combined neutral currents of all the ('still connected together') installation tries to flow to earth through bonding conductors and extraneous-conductive-parts (water pipes etc.).

In your case, it sounds as if by far the most likely fault would be downstream of the CNEs connection to any other installations (i.e. affecting only yours), in which case zero current would flow through your incoming CNE (from other installations), and the only current trying to find a route to earth would be that from your own installation.

Provided that any/all required bonding was present in your installation, "the entire earthing system of the bungalow becoming live" would not, in itself, be much of a problem - the only theoretical risk being to someone 'with one foot inside and the other foot outside' of the bungalow or, say, for someone touching an outdoor tap whilst standing on the ground (at a time since something was obviously wrong, since little, if anything, in the electrical installation would be 'working').

At the same time I wondered if it would be possible to add one or more earth rods which I could connect to the incomming TN connection to provide an earth connection if the incomming TN connection was lost. ... Would there be a problem in doing this?

No problem in doing that - in fact, if you have bonded extraneous-conductive-parts (e.g. water pipes), then you effectively already have such a situation. The difficulty would be in getting earth rod(s) with low enough impedance to earth to have a major impact on the situation.

As an alternative I did consider converting the TNC-S system to TT in which case the incomming TN would only be used as the neutral and I would then provide my own earth again using a number of earth rods only this time there would be no connection to the incomming TN. Would there be a problem in doing this?

Again, no problem, provided that the installation took into account the fact that it had become TT (which would mean more reliance on RCDs/RCBOs for fault protection than is the case with TN). In fact, that's essentially the situation I have here. Also relatively 'in the sticks', my (overhead) supply was traditionally TT. When they PMEd the local network, they offered me TN-C-S, but I declined that offer and stuck with my TT - so no different from a situation in which I had had TN-C-S but decided to disconnect the N-E connection.

Kinbd Regards, John

 

[plugwash]

The downside of TT is it leaves you totally reliant on RCDs for protection against faults to earth and also (particularly with a metal CU) leaves you with no protection at all if a fault to earth happens prior to the RCDs.

 

[flameport]

and that may well creep back in when a new edition

It's already in Amendment 2 as a recommendation for all properties with TN supplies.

 

[JohnW2]

The downside of TT is it leaves you totally reliant on RCDs for protection against faults to earth ...

True - but that, in itself, is not much of an issue, since almost all circuits are now required to have RCD protection. However ...

.... and also (particularly with a metal CU) leaves you with no protection at all if a fault to earth happens prior to the RCDs.

Yep, another (in my opinion) 'danger' of the requirement for metal CUs! I could understand metal CUs (etc.) being forbidden, at least in TT installations, but to require them seems to be (albeit very improbably) 'a death waiting to happen'!!

Kind Regards, John

 

[JohnW2]

It's already in Amendment 2 as a recommendation for all properties with TN supplies.

I wasn't aware of that, but am not surprised.

Of course, at least prior to the increasing deployment of plastic utility pipes, almost all properties had (electrically) such a situation, anyway, so it's perhaps nothing like as 'revolutionary' as many seem to think!

Kind Regards, John

 

[bernardgreen]

The downside of TT is it leaves you totally reliant on RCDs for protection against faults to earth

With the Earth being a ground rod that is true, but decades ago the "ground rod" could be several miles of metallic water main, often bonded to the Neutral the sub station.

 

[FarmerJohn]

Great help so far. I have some additional information which I should have included initially.

There is no gas pipe to the bugalow, just water and oil. The water pipe is plastic and the oil pipe is copper and just goes to my own oil tank about 10m away for heating purposes. The oil pipe is earthed as normal where it enters the bungalow and is run in a protective ducting as well as being plastic coated. I don't expect that the copper of this oil feed to ever touch ground and therefore should not accidently make an intended TT system to be TNC-S.

Does that make sense so far?

I am more convinced now that I need to use one or more earth rods to provide an earth if the incomming TN is broken. This is my thinking so far.

At the moment I am thinking about using a number of earth rods to connect to the incomming TN connection. That way I get a fairly low earth impedance (to be measured and I would be surprised if it was anywhere near 0.35-ohm), which in normal operation will provide a good fault path under fault conditions to ensure that the RCBO's trip etc.

Now the problem becomes down to the earth rod impedance and what I need to get it to and also how stable is it throughout the seasons. I live in an area where the water table is quite high (but varies on the time of year) so I should be able to get a fairly low impedance. But I don't want it to be too low since the incomming TN connection may vary relative to true ground (depending on my loading and other users loading) which could mean a continuous earth current would flow through my earth rods. If for example TN was say 10V above true ground and the earth rod impedance was 100-ohm, this would be a current of 100mA continuously. Of coarse, if TN was losted, TN could be 240V (presumably) with an earth rod current of 2.4A. Please correct me if this logic is incorrect in anyway.

Any ideas on what a good target earth rod impedance should be would be appreciated.

Regards
FarmerJo