Understanding the relationship between TNCS, TNS and TT

[HappyHippyDad]

Some very interesting and detailed replies, thanks a lot to you all for replying. Especially @pc1966 and @davesparks .
I'll ponder it all for a while now I think.

 

[LastManOnline]

@Happpyhippydad yes it is safe and needed. What he did was bond the neutral bar to the ground rod. Without it your voltage would fluctuate.

That an earth rod prevents "the voltage from fluctuating" is an explanation I have heard before but have yet to see the the science behind it. The voltage at the rod in a TNC-S system is always dependent on the voltage dropped over the main neutral as the rod is connected in parallel with it.

 

[davesparks]

That an earth rod prevents "the voltage from fluctuating" is an explanation I have heard before but have yet to see the the science behind it. The voltage at the rod in a TNC-S system is always dependent on the voltage dropped over the main neutral as the rod is connected in parallel with it.

Megawatt is referring to the USA system where they make the N-E link at the main DB by installing earth rods there.
I think this is sometimes the only N-E link on the system, making it technically TNS, but I may be misunderstanding this.

 

[Lucien Nunes]

That an earth rod prevents "the voltage from fluctuating" is an explanation I have heard before but have yet to see the the science behind it.

I have heard that too, and asked the same question without adequate answer. The implication is that the PEN is of too high a resistance and the voltage drop in it too severe, but the Ra of the earth rod is somehow low enough to provide an effective parallel path thereby mitigating the voltage drop. Since a typical customer's earth electrode Ra is 100 - 1000 times higher than a typical Ze, the reduction of voltage drop it can provide will be in the order of 0.1 - 1% (of the variation, not the supply voltage). Noting re. voltage drop that in a US domestic installation the PEN is normally a centre conductor between split-phase lines.

If an electrode of vanishingly low Ra were provided, the overall Zpn would be reduced and the voltage drop mitigated, but only because a significant fraction of the load current (and possibly other peoples' load currents) would return via the electrode. We accept that these currents do flow, hence the bonding requirements for TN-C-S, but surely no regs would sanction relying on the electrode as part of the load circuit absent a solid PEN connection, (other than for a SWER supply.)

 

[LastManOnline]

Megawatt is referring to the USA system where they make the N-E link at the main DB by installing earth rods there.
I think this is sometimes the only N-E link on the system, making it technically TNS, but I may be misunderstanding this.

But TNS by definition requires a separate earth from DSO to consumer. Earth rods (while they may be added) are not strictly speaking included in the arrangement.
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I have heard that too, and asked the same question without adequate answer. The implication is that the PEN is of too high a resistance and the voltage drop in it too severe, but the Ra of the earth rod is somehow low enough to provide an effective parallel path thereby mitigating the voltage drop. Since a typical customer's earth electrode Ra is 100 - 1000 times higher than a typical Ze, the reduction of voltage drop it can provide will be in the order of 0.1 - 1% (of the variation, not the supply voltage). Noting re. voltage drop that in a US domestic installation the PEN is normally a centre conductor between split-phase lines.

If an electrode of vanishingly low Ra were provided, the overall Zpn would be reduced and the voltage drop mitigated, but only because a significant fraction of the load current (and possibly other peoples' load currents) would return via the electrode. We accept that these currents do flow, hence the bonding requirements for TN-C-S, but surely no regs would sanction relying on the electrode as part of the load circuit absent a solid PEN connection, (other than for a SWER supply.)

?Agreed

 

[LastManOnline]

Yes, no extraneous in the new shed.
If there was a broken PEN, wouldn't the RCD trip immediately (if there was a rod installed <1667ohms)
as any current would be flowing to earth?

This is a widely held but erroneous belief in the electrical Industry. When the PEN breaks and someone touches bonded metalwork that becomes live, rcd, s will NOT trip. Reason is the fault current flows first through L of rcd and then back out N of rcd and subsequently through the unfortunate individual in contact with bonded metalwork. Quite simply this individual has become the "new neutral" from the rcd, s point of view (though a higher resistance one).
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In most case having additional earthing paths via dedicated rods or by means of metal water pipes, etc, is a good thing. But of course one must never rely on service pipes as an earthing means (unless specifically with the agreement of the pipe owner, but that is not a good path to try and pursue anyway).

If you have RCD additional protection, as pretty much most new circuits will have anyway, then you would become TT in this case and be OK even though faulty and less safe than before. But obviously not if any circuits depend on the OCPD for clearing!

Why less safe? Basically as the RCD is more complex than the MCB so more likely to fail, and you might have a fault that is not RCD protected (e.g. supply line to CU case) that would cause a big BANG! in the TN-S / TN-C-S case but when TT-ing would just roast the earth rod and leave metalwork at a dangerous voltage.

Mayby I misunderstood but how would the earth rod be roasted?. The standard resistance of most rods would not allow that to happen
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I hope that they haven't done this as it is dangerous and illegal to combine the neutral and earth in a consumers installation in the UK.
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Good question, this has the potential to be a very interesting discussion.

The third core is perfectly adequate for earthing (cpc) but not for main bonding if any extraneous conductive parts are present in the outbuilding (based on the assumption that the required size of bond is 10mm)

I assume in this scenario that main bonding is not required at the shed?

Based on these assumptions that earth rod is not necessary.

What is the earth rod actually doing electrically? That depends a little on its Ra, generally its hard to get a low enough Ra that it will make a practical difference to the earthing of a PME supplied installation. You would only start to notice it affecting the Zs of the circuit, and at the origin, if the Ra was a couple of ohms or less.
If the Ra is that low then the fault current that could flow through the rod could be higher than the 4mm CPC could safely handle and it could be non-compliant.

That earth rod, in my opinion, will in itself be an extraneous part because it is introducing an earth potential into the installation. As such it would need to be connected back to the MET by 10mm copper or equivalent.
Based on that logic it is in fact making the installation worse rather than better.

As you probably know I am an advocate of earth electrodes being connected to the MET of all installations with a PME earth, I think 10mm would be the minimum acceptable conductor size for this.

That situation of more current flowing through the rod than the CPC could handle could only occur if the OCPD was incorrectly rated. After all what flows through the CPC must first flow through the L. But in any case 1,2 or even 3 rods will ne er get you you under 1 ohm resistance and its not a situation I could ever envisage occurring. You are correct about the 10mm to the rod. This is the generally accepted size. Most of that 10mm is specified however fir mechanical protection rather that currant carrying capacity. For current carrying capacity a 4 ft rod (the standard in most TNC-S) countries will never require more than a 1.5mm
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True, I should have been precise than answering 'no' to the assumed question about the 10mm minimum.

As you say, in all cases you need to look at the various minimum CPC requirements based on the electrical and physical situation.

Interestingly, your initial comment about "Bonding with phased sized conductors" seemed reasonably to me (still does!). I do not work with TT so I am speculating here. If the extraneous metal part (copper tap) in the shed was turned out to have a reasonably high resistance, would it then be possible to Bond with phase size conductors?
Secondly would it be acceptable practice in UK to insert some plastic pipe in place of the copper and remove this Extraneous part?

 

[pc1966]

Mayby I misunderstood but how would the earth rod be roasted?. The standard resistance of most rods would not allow that to happen

Roasted is a bit of an exaggeration, but if you have 230V on the rod with say 30 ohm resistance you have around 1.8kW being dissipated all of the time (mostly in the soil immediately around the rod.

It was more the point that the OCPD won't disconnect even though a significant amount of power is going astray.

 

[davesparks]

That situation of more current flowing through the rod than the CPC could handle could only occur if the OCPD was incorrectly rated. After all what flows through the CPC must first flow through the L.

You have missed the point of my post, the point of it was to describe the theory of tjso hypothetical situation. I described a number of scenarios to try and give a full explanation of the theory.

The OCPD will not have any bearing on this, you have again misunderstood my post, plus an OCPD does not monitor the current in the CPC.
In the hypothetical example I gave of the earth rod, which is connected to the CPC of the garage circuit at the garage,being in perfect contact with the mass of earth and a broken PEN occurring. The CPC of the garage circuit will be connecting neutral at the cutout, via the MET, to the earth rod. All of the installation's neutral current will flow through that cpc which may be undersized.

Not also that one of the conditions of this hypothetical situation was that a number of purely resistive loads were connected.

 

[LastManOnline]

Roasted is a bit of an exaggeration, but if you have 230V on the rod with say 30 ohm resistance you have around 1.8kW being dissipated all of the time (mostly in the soil immediately around the rod.

It was more the point that the OCPD won't disconnect even though a significant amount of power is going astray.

?

Roasted is a bit of an exaggeration, but if you have 230V on the rod with say 30 ohm resistance you have around 1.8kW being dissipated all of the time (mostly in the soil immediately around the rod.

It was more the point that the OCPD won't disconnect even though a significant amount of power is going astray.

Agreed?. Third time trying to post this. Has to be 3 words minimum apparently.
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You have missed the point of my post, the point of it was to describe the theory of tjso hypothetical situation. I described a number of scenarios to try and give a full explanation of the theory.

The OCPD will not have any bearing on this, you have again misunderstood my post, plus an OCPD does not monitor the current in the CPC.
In the hypothetical example I gave of the earth rod, which is connected to the CPC of the garage circuit at the garage,being in perfect contact with the mass of earth and a broken PEN occurring. The CPC of the garage circuit will be connecting neutral at the cutout, via the MET, to the earth rod. All of the installation's neutral current will flow through that cpc which may be undersized.

Not also that one of the conditions of this hypothetical situation was that a number of purely resistive loads were connected.

The "Theory of tjso." Not a misprint?

 

[davesparks]

The "Theory of tjso." Not a misprint?

It should say this, 'the theory of this hypothetical situation' , its a typo.