Relative risks of TN-C-S and TT

[JohnW2]

I dunno... the earth they supply has a hazard cause by them, but TT also has a hazard, that the RCD may not operate.

Because of a faulty RCD, you mean? As I've said before, those who are concerned about that should ignore the 'usual advice' and 'double up' on the RCDs. Indeed, even if all final circuits are otherwise RCD-protected, one can do as I do (albeit for an additional reason) and have an up-front time-delayed RCD - which, in practice, has no real 'nuisance value', since it is never going to operate unless there is a fault and a non-functioning primary RCD.

... and what, I wonder, is the 'failure rate' of MCBs if one is relying on them for ADS - given that they are essentially untestable in a domestic environment.

They are saying PME is not too risky in general, but if there is a GF shower, etc., the risk outweighs the benefit therefore they won't supply PME. Everything comes down to shades of grey ....

It may not be "too" (whose opinion?) risky in general but, I think we can probably agree that it comes with (very small) risks that do not exist with any other type of earthing arrangement. As I hinted before, I would imagine that it has to be a largely cost-driven decision because they could avoid all the specifically TN-C-S-related hazards (albeit extremely small) by providing everyone with TN-S earths, and maintaining those earths satisfactorily.

Kind Regards, John

 

[securespark]

RCD failures do occur, and this is why regular operation of the test button is prescribed.

 

[bernardgreen]

I wrote this years ago ( some typing errors may exist )

In the beginning there was no Earth,
Then there came the earth. For some it was the water pipe that went to ground, for others the earth came to the house by wire from the electric company.
Any electricity that leaked from the wires went to earth if it could. When it could leak to earth in large amounts then the fuse would melt and all would be safe.
But sometimes the wire would melt before the fuse and things were not safe but this didn't happen often if the fuse wire was the right size.
Along came plastic water pipes
There was now no way the leaking electricty could get to earth in large amounts. Fuses now longer melted and leaking electricity made things dangerous.
So they invented the VOELB ( voltage operated earth leakage breaker ) .
All the bits that leaking electricity could get to were connected together with a third wire called the Earth wire. This wire was connect to a coil in the VOELB and the other side of the coil was connected to a rod stuck in the ground. The earth wire would collect leaking electricity and take it to the VOELB. This electricity would flow through the coil and operate the VOELB to switch off all the electicity in the house and all was safe.
Modern plumbing put an immersion heater in a tank with metal pipes connecting it to the the metal pipe in the street. When electricity leaks to the metal tank some goes along the earth wire to the VOELB and some goes along the pipes to the street. The VOELB has a resistance to electricity passing through it to ground but the pipe work offers no resistance. The leaking electricity chooses the easy path along the pipes and avoids the difficult route through the VOELB. The VOELB can no longer operate. When a lot of electricty is leaking along the pipes the fuse will melt, ir it may not and wires get hot and may melt or cause a fire.
Electricity comes into the house on the live wire, does its work and returns to the power station on the neutral wire. If no electricity is leaking from the wires the electricity flowing along these two wires will be equal but in opposite directions.
The RCD ( residual current device ) checks that the amount of electriciity leaving the house is the same as the amount of electricity going into the house. If it is not the same there has to be a leak in the house. The RCD will turn off the electicity if it sees a difference caused by a leak.

 

[EFLImpudence]

The RCD ( residual current device ) checks that the amount of electriciity leaving the house is the same as the amount of electricity going into the house.

Why do we have to pay for it, then?

 

[JohnW2]

RCD failures do occur, and this is why regular operation of the test button is prescribed.

Such failures do indeed occur, and I have indicated what can be done (and, indeed, what I do) to mitigate that risk, by having redundancy.

MCB "failures" undoubtedly also occur, but there are no test buttons and no information I've seen about the frequency of such failures. Fortunately, today, an increasing proportion of installations have 'additional protection' (RCDs) on some/most/all of the circuits, so ADS does not rely on satisfactory MCD operation.

In the final analysis, if all prootective devices in the installation fail, there is at least the cutout fuse as a 'back stop'.

Anyway, returning to the title of this thread, I would be interested to hear what you feel are the "downsides" (higher risks) associated with TT?

Kind Regards, John

 

[securespark]

Anyway, returning to the title of this thread

I wasn't aware I had deviated. But grovelling apologies for doing so. I shall make sure it never happens again.

 

[JohnW2]

I wasn't aware I had deviated. But grovelling apologies for doing so. I shall make sure it never happens again.

I wasn't accusing you of having deviated. I probably should have written "..turning to the title of this thread" (which I started and named), rather that "...returning to the title of this thread".

In the thread which spawned this one (part of which is quoted in my OP here), it was seemingly being suggested that although TN-C-S comes with some additional hazards peculiar to just TN-C-S, in most situations (other than ones like livestock and showers with ground floor concrete floors, petrol stations etc. etc.) is "not too risky" (and therefore deemed to be acceptable) - but, to my mind, "not too risky" means that it needs to have some adequate counter-balancing advantages (say, over TT - but also TN-S) before it becomes reasonable to accept the additional 'risks' which come only with TN-C-S.

I would therefore be interested to hear what you, and others, feel are the 'balancing advantages' ('risk'-wise) of TN-C-S? I interpreted you comment about faulty RCDs as meaning that you regarded TN-C-S's non-reliance on RCDs for ADS as being an 'advantage'. Was that your meaning, and do you consider that there are other 'upsides' of TN-C-S (or downsides of TT, or TN-S)?

Kind Regards, John

 

[ban-all-sheds]

I dunno... the earth they supply has a hazard cause by them, but TT also has a hazard, that the RCD may not operate.

Because of a faulty RCD, you mean?
.
.
.

Did you start this thread because by virtue of your own actions you were left with no other option if you wished to 'contribute' further to the discussion?

 

[JohnW2]

Seemingly so, but if it was the consequence of my actions (and not someone else's), those actions were only my being part of an extended discussion which, although somewhat tangential and academic, was by no means totally unrelated to the topic of the OP. If that was the reason, I would say that it represents a new development in management policies of this forum.

 

[ban-all-sheds]

... but if there is a GF shower ...

https://www.bing.com/images/search?q=gf+shower

 

[plugwash]

It may not be "too" (whose opinion?) risky in general but, I think we can probably agree that it comes with (very small) risks that do not exist with any other type of earthing arrangement.

I think that applies to all earthing systems, each one has risks that don't apply to the others.

TT has a high earth impedance, that means that you are totally reliant on RCDs for disconnection of phase-earth faults. It also means that during a phase-earth fault the voltage between the earthing system and real earth will become much higher.

TN-C-S has the problem of what happens in the event of a lost neutral and also the fact that some of the current that would normally flow in the neutral may flow through earthing/bonding systems (and ultimately the earth itself) instead.

TN-S has the problem that in the event of a neutral-earth fault significant current can potentially flow through the fault. Earth never has overcurrent protection and neutral rarely does, so unless there is a RCD current will flow through the neutral to earth fault indefinitely.

 

[EFLImpudence]

...

 

[JohnW2]

I think that applies to all earthing systems, each one has risks that don't apply to the others.

Exactly - hence the reason I started this thread about 'relative risks'. I should rerally have included TN-S in the title, but didn't think of that at the time, since the 'parent thread' had not really said much about it.

TT has a high earth impedance, that means that you are totally reliant on RCDs for disconnection of phase-earth faults.

Indeed - but, as I've said, that risk can be mitigated. It could be said that one advantage is that it is not at all dependent upon (essentially untestable) MCBs for ADS.

It also means that during a phase-earth fault the voltage between the earthing system and real earth will become much higher.

True - but if the building is properly constituted as an equipotential zone (i.e. all appropriate bonding is in place), that will not have any consequences (other than for a person 'half in and half out' of the building!). In any event, an RCD should clear the fault pretty smartish.

TN-C-S has the problem of what happens in the event of a lost neutral ....

Indeed, the 'notorious' (albeit very rare) scenario.

... and also the fact that some of the current that would normally flow in the neutral may flow through earthing/bonding systems (and ultimately the earth itself) instead.

Indeed - and the potential of a TN-C-S 'earth' will inevitably be at least a bit above true earth potential even during 'normal operation'

TN-S has the problem that in the event of a neutral-earth fault significant current can potentially flow through the fault. Earth never has overcurrent protection and neutral rarely does, so unless there is a RCD current will flow through the neutral

I'm not sure that I fully understand that one.

Anyway, those are at least some of the 'cons'. Other than the corollaries of what you have said, do you feel that any of the options have
any noteworthy 'pros' - and, in an ideal world, which do you think would, on balance, probably be the best for an average domestic installation?

Kind Regards, John

 

[John D v2.0]

I had no idea this thread was going on! Well, since I'm here, if your earth has a low impedance you have an additional protection against high touch voltages.
If your earth has high impedance you're only relying on the RCD.
Also with a low earth impedance even if the MCB is not tripping, you'll notice other issues like lights going dim.
It's all pretty much unlikely things, but unless you're going to go around in a chain mail suit or something, there's always a trade off between cost and safety.

 

[JohnW2]

Well, since I'm here, if your earth has a low impedance you have an additional protection against high touch voltages.

Comments like that are often made in this type of discussion, but I'm not sure what is meant. What do you mean by a 'touch voltage' - a potential difference between what and what?

On the face of it, given proper CPCs and bonding, the greatest PD which should/could exist as a result of an L-CPC fault would be that between (a) the exposed-c-p of something on a circuit with an L-CPC fault and (b) the exposed-c-p of something on a circuit without a fault, or something (like a pipe) which is earthed and/or bonded (i.e. connected to the MET). In that situation, as far as I can see, that PD (which exists only because of the 'voltage drop' in the CPC due to fault current flowing between the L-E fault and the MET) will surely be higher if the earth resistance is low, since the fault current is then higher?

Kind Regards, John