Wiring Rules amendment re twin and earth cables

Hoagy

This one came out in August, it slipped by me until today.

ET101 Amendment 2.2

Basically from September 2017 there will be no more twin and earth cable as we know it.

The protective conductor will have to be same size as the phase conductors and insulated.

The smaller sizes will be all right, but 10sq and 16sq are going to be very big.

Stoner

That's correct the manufacturers got extra time to adjust

So phase and earth will be the same size and the earth to be sleeved all the way through.

meath4sam

And cable will be more expensive...

Dardania

What's the thinking behind this? Are they trying to prepare for increased fault currents coming in?

Bruthal

Dardania wrote: »

What's the thinking behind this? Are they trying to prepare for increased fault currents coming in?

That shouldn't come into it really. Increased fault currents are usually due to increased circuit sizes. The circuit protection will be appropriate for circuit size. Loop impedance tests will be better though. Measuring cpc resistance will be slightly easier probably.

Whether it's necessary is debatable though.

I think the earth's in some twin and earth's is less than half the conductor size too. So there is that possible factor.

Risteard81

Bruthal wrote: »

Increased fault currents are usually due to increased circuit sizes.

The prospective fault current will decrease with increased circuit length owing to extra impedance. Larger csa of circuit conductors will increase the prospective fault current.

The current sizes of cpc for T&E are as follows:

1mm[FONT=Times New Roman, serif]^2 (1mm^2) - not permitted by ET101
1.5mm^2 (1mm^2)
2.5mm^2 (1.5mm^2) - some older versions used a 1mm^2 cpc
4mm^2 (1.5mm^2)
6mm^2 (2.5mm^2)
10mm^2 (4mm^2)
16mm^2 (6mm^2)
[/FONT]

2011

The objective should remain i.e. to ensure that under fault conditions the protective device operates within the required disconnection time. Once that objective is achieved I can't see the benefit in this change.

My guess is that it is a crude attempt to dumb things down. Under this new regulation chances of achieving acceptable earth fault loop impedance values are dramatically increased simply by selecting cabling that has sufficient current carrying capacity.

Risteard81

That is true, although a cpc with a greater csa will also have the additional benefit of reducing touch voltages under earth fault conditions.

Bruthal

Risteard81 wrote: »

That is true, although a cpc with a greater csa will also have the additional benefit of reducing touch voltages under earth fault conditions.

Touch voltage is usually brought into it by having a relatively high fault path impedance. The difference between an intact 2.5 and intact 6 square will be very small in that scenario.

Bruthal

Risteard81 wrote: »

The prospective fault current will decrease with increased circuit length owing to extra impedance.

True, although I was posting about circuit size rather than length, in reference to another posters post.

Risteard81

Bruthal wrote: »

Touch voltage is usually brought into it by having a relatively high fault path impedance. The difference between an intact 2.5 and intact 6 square will be very small in that scenario.

If the impedance of the phase conductor and protecive conductors are equal then the touch voltage will be circa half of the supply voltage. If the protective conductor is lower impedance then the touch voltage will decrease. If this is by a factor of around 5:1 (e.g. with some MIMS cables; containment etc.) then essentially you have an inherently safe system from a shock prevention point of view (although disconnection might be important from the point of view of thermal effects) as the touch voltage would be below 50V.

On the other hand, as with say 16mm^2 T&E (which only has a 6mm^2 cpc) then the touch voltage will be much closer to the supply voltage.

2011

Risteard81 wrote: »

If the impedance of the phase conductor and protecive conductors are equal then the touch voltage will be circa half of the supply voltage.

Agreed, I notice that you correctly imply that there are multiple protective conductors. This aligns with the reality that the fault current generally takes multiple paths back to the origin (star point of the transformer).

If the protective conductor is lower impedance then the touch voltage will decrease.

Not always that straight forward.
If one of the parallel paths is of lower impedance (due to the use of a larger CPC) the overall earth fault loop impedance value will be lower, but often only by negligible amount.

For example if you were to disconnect the CPC from a motor in an industrial installations and measure the earth fault loop impedance the reading may remain unchanged. Why? Because the motor is mechanically bolted to the frame, which is bolted to the piping and it is electrically bonded to a local earth bar. So there are multiple parallel low impedance paths even when there is no CPC.


One of the limiting factors of touch voltage should be equipotential bonding. That is because it keeps all exposed conductive parts at the same potential. This has nothing to do with the size of the CPC.

Bruthal

Risteard81 wrote: »

If the impedance of the phase conductor and protecive conductors are equal then the touch voltage will be circa half of the supply voltage.

If the protective conductor is lower impedance then the touch voltage will decrease. If this is by a factor of around 5:1 (e.g. with some MIMS cables; containment etc.) then essentially you have an inherently safe system from a shock prevention point of view (although disconnection might be important from the point of view of thermal effects) as the touch voltage would be below 50V.

On the other hand, as with say 16mm^2 T&E (which only has a 6mm^2 cpc) then the touch voltage will be much closer to the supply voltage.

True in theory, and very simple basics. But it would want to be seriously bad circuit protection that could present such a scenario for long, as in a direct short which divides the supply voltage between the phase and earth conductors, and nothing else. It will be a very low impedance fault. Id be having the fire extinguisher on standby if that event is likely (phase to earth staying energised with supply voltage divided only between them)

In reality, such a short alone wont likely be the cause of touch voltages, but faults such as neutral failure, or earthing problems along with phase to earth faults are far more likely to be the cause. Or a phase to earth in a non neutralised installation, and no main rcd. Again, these scenarios will vary little with the proposed earth conductor size increase, since they involve relatively high impedances as mentioned already.

Dont forget, the cpc only forms a small part of the overall circuit, so this change in size is not such a big change to the characteristics of the circuit.

2011

Bruthal wrote: »

Dont forget, the cpc only forms a small part of the overall circuit, so this change in size is not such a big change to the characteristics of the circuit.

+1
That is the point I was making in my last post.

Risteard81

2011 wrote: »

Agreed, I notice that you correctly imply that there are multiple protective conductors. This aligns with the reality that the fault current generally takes multiple paths back to the origin (star point of the transformer).

Of course. Other than for very simple all-insulated installations the removal of parallel earth paths is both impossible and undesirable.

My point about increased csa of cpc reducing touch voltage is accurate - although as you say given all the parallel paths in an industrial or commercial installation the effect may be negligible. However, the reality is that T&E is a domestic cable and is not particularly appropriate for industrial/commercial installations where singles in steel containment are frequently the norm.

Bruthal

Risteard81 wrote: »

My point about increased csa of cpc reducing touch voltage is accurate

No one says it isnt. Its not some obscure characteristic beyond all but the elite.

2011

Risteard81 wrote: »

Of course. Other than for very simple all-insulated installations the removal of parallel earth paths is both impossible and undesirable.

My point about increased csa of cpc reducing touch voltage is accurate - although as you say given all the parallel paths in an industrial or commercial installation the effect may be negligible. However, the reality is that T&E is a domestic cable and is not particularly appropriate for industrial/commercial installations where singles in steel containment are frequently the norm.

In a domestic installation touch voltage is of particular concern in a bathroom. This risk is primarily mitigated with eqipotential bonding, ELV fittings, double insulated fittings, isolating transformers and RCDs. Increased CPCs will have a negligible impact IMHO.

Bruthal

2011 wrote: »

In a domestic installation touch voltage is of particular concern in a bathroom. This is primarily dealt with eqipotential bonding, ELV circuits, double insulated components isolating transformed and RCDs. Increased CPCs will have a negligible impact IMHO.

It will have no real affect in real life. The scenario outlined by Risteard with supply voltage divided between the phase and earth is describing a direct short. I cant see any circuit in that situation staying on long enough to cause a touch voltage problem. There are protective devices to protect against that.

In other words, the larger cpc would indeed make a difference in very low impedance, high current faults, but those faults are protected by protective devices. But the level of voltage drop difference on the cpc wont be that big between the size changes. It is of little risk difference in a house.

And as you say, bonding protects against potential differences in the installation anyway.

Risteard81

2011 wrote: »

In a domestic installation touch voltage is of particular concern in a bathroom. This risk is primarily mitigated with eqipotential bonding, ELV fittings, double insulated fittings, isolating transformers and RCDs. Increased CPCs will have a negligible impact IMHO.

That is the reason for supplementary bonding, but exposed conductive parts could be at a significant potential in other parts of the installation before disconnection occurs.

Of course main and supplementary equipotential/protective bonding mitigate against this. I was simply pointing out that there are certain benefits to increasing the csa of the cpc - I wasn't saying that I necessarily agree with the change.

You could always wire in singles and apply the adiabatic equation to confirm whether smaller cpcs would be satisfactory and would withstand the fault current without damage until disconnection occurs (although it is unlikely that anyone would bother in a domestic installation).

2011

This is an interesting paper on the topic:

http://dit.ie/media/electricalengineering/documents/martinbarrett/200.pdf

Bruthal

Risteard81 wrote: »

That is the reason for supplementary bonding, but exposed conductive parts could be at a significant potential in other parts of the installation before disconnection occurs.

Well if the supply voltage is being split along the phase and cpc conductors, it would seem the voltage will be a lot lower at the neutralised earth bar than at the cpc point of the fault.

Bruthal

2011 wrote: »

This is an interesting paper on the topic:

http://dit.ie/media/electricalengineering/documents/martinbarrett/200.pdf

I dont think cpc size comes into those situations probably. Once its not neutralized, the earth fault path is a relatively high impedance, so the increased cpc size in this thread would make even less difference than it might on a neutralized one.

Protective devices are required such as a main rcd, or rcds on all circuits due to the high impedance fault path. The difference between the existing and new cpc sizes would be negligible.

Although that`s what we are saying anyway