TN-CS Combined With TT Question

[OnlQQker]

So I've just watched this and the chap explains all the different Earth arrangements used in the UK. He mentions about how in a TN-CS earth system if the neutral was broken just before it came into your house then this would be very dangerous. So I was just wondering if a TT system combined with a CN-CS would be safer.
Or am I missing something he is saying and barking completely up the wrong tree? (which is always quite possible)

 

[jakew009]

What are the chances of the neutral leg being broken between the service head outside your property and your meter without causing some other fault? Pretty much zero.

pme = protective multiple earth. It’s earthed at pretty much every joint; every time the dno do any work on a cable they pme it if not already and put an earth rod in.

you could always bang an earth rod in on a tncs system if it makes you feel better. That’s what other countries do and I think was considered for the 18th?

 

[UNG]

Talking to an ENWL jointer a few weeks ago they don't necessarily PME their cable joints

 

[Julie.]

Talking to an ENWL jointer a few weeks ago they don't necessarily PME their cable joints

Not every one, but the actual number that are is fully detailed in enwl's code of practice 332.

 

[Strima]

It was proposed as part of the 18th edition to install supplementary earth electrodes but it never happened.

On a side note it is a DNO requirement to have an electrode to supplement TNC-S systems on metal street furniture subject to certain load and resistance criteria.

 

[Julie.]

It was proposed as part of the 18th edition to install supplementary earth electrodes but it never happened.

On a side note it is a DNO requirement to have an electrode to supplement TNC-S systems on metal street furniture subject to certain load and resistance criteria.

Part II of the Electricity Safety, Quality and Continuity Regulations 2002, as amended, which covers protection and earthing, applies to supplies to street lighting installations and other street electrical fixtures.

An earth electrode shall be provided at the end of every service supplying more than one street lamp or road sign.

An earth electrode shall be provided at the source pillar or supply point servicing more than one street lamp or road sign.

There are a few more requirements, such as if the furniture is in insulation, then it needs a rod irrespective etc

Forgot to say, the requirements of G12/4 apply as well

 

[pc1966]

The safety trade off of TN-C-S versus TT is not just about the open PEN conductor risk.

The PEN fault situation is what usually sparks the discussion when looking at hot tubs, etc, outside that may be fed from a TN-C-S supply and where the risk of 80+ volts to true Earth is a whole lot more serious (it also applies in agricultural visitations where animals might be sensitive to far lower voltages that may be present even without a PEN fault as such). In those cases your options are either supplementary earth rods/mesh on to the TN-C-S CPC to keep the step potential down in such a situation, or to go TT and have the local CPC from an earth rod, etc, and basically ignore the TN-C-S derived earth.

But the other side of the risk assessment is the case of a fault to earth within the installation where rapid and certain disconnection is desired. In this case the low Zs from TN-C-S allows you to (usually) meet fault disconnection by means of a fuse or MCB alone, and you are not reliant on an RCD to allow clearing. Given the order of magnitude or more difference in complexity between the two, you can rely on a fuse/MCB (or equivalent OCPD side of RCBO) with greater certainty than the electronics of RCD detection.

While the serious nature of the PEN fault should not be discounted, I suspect installation L-E faults are more common!

 

[LastManOnline]

Part II of the Electricity Safety, Quality and Continuity Regulations 2002, as amended, which covers protection and earthing, applies to supplies to street lighting installations and other street electrical fixtures.

An earth electrode shall be provided at the end of every service supplying more than one street lamp or road sign.

An earth electrode shall be provided at the source pillar or supply point servicing more than one street lamp or road sign.

There are a few more requirements, such as if the furniture is in insulation, then it needs a rod irrespective etc

Forgot to say, the requirements of G12/4 apply as well

But exactly what purpose does an earth electrode (size /length resistance to earth not specified) installed at the base of a street lamp serve? If its not of a very low resistance its of little value on a TNC-S system.

 

[Wilko]

So I was just wondering if a TT system combined with a CN-CS would be safer.

Hi - given that it’s unlikely that DNO N will be broken outside your house I don’t think adding a rod would improve safety. Unfortunately if the N did break we would need a very very good Ra to keep the touch voltages in check.

 

[pc1966]

Having one small rod would make much difference, as above, but if every 2nd house had a 2m rod or similar then it would greatly reduce the risk.

Which is what the 'M' of PME really tries to do anyway, but with the onus on the DNO side.

 

[LastManOnline]

The safety trade off of TN-C-S versus TT is not just about the open PEN conductor risk.

The PEN fault situation is what usually sparks the discussion when looking at hot tubs, etc, outside that may be fed from a TN-C-S supply and where the risk of 80+ volts to true Earth is a whole lot more serious (it also applies in agricultural visitations where animals might be sensitive to far lower voltages that may be present even without a PEN fault as such). In those cases your options are either supplementary earth rods/mesh on to the TN-C-S CPC to keep the step potential down in such a situation, or to go TT and have the local CPC from an earth rod, etc, and basically ignore the TN-C-S derived earth.

But the other side of the risk assessment is the case of a fault to earth within the installation where rapid and certain disconnection is desired. In this case the low Zs from TN-C-S allows you to (usually) meet fault disconnection by means of a fuse or MCB alone, and you are not reliant on an RCD to allow clearing. Given the order of magnitude or more difference in complexity between the two, you can rely on a fuse/MCB (or equivalent OCPD side of RCBO) with greater certainty than the electronics of RCD detection.

While the serious nature of the PEN fault should not be discounted, I suspect installation L-E faults are more common!

I notice your concerns about using rcd, s with electronics. Why not turn to rcd, s that don't include electronics? I like the arrangement best where we use the TNC-S, arrangement for ADS but also have an rcd for additional protection. In reality rcd, s are now becoming seen as, required protection, which in reality they are. They are singlehandedly responsible for reducing electrocutions massively in countries that use them as standard

 

[Julie.]

But exactly what purpose does an earth electrode (size /length resistance to earth not specified) installed at the base of a street lamp serve? If its not of a very low resistance its of little value on a TNC-S system.

Primarily it ties the local ground to the same potential as the metal furniture, if this was not the case then the ground - and thus the feet of the drunk leaning on the lamp post could raise to some random potential in the event of a fault elsewhere - such as a line-ground fault at the next lamp post - or that one itself so with say 100....230V actually connecting to the ground/drunk's feet, whilst the lamp post itself would be tied to earth at the substation.

Tying the local ground to the same potential as the metalwork in the area prevents this, its one of the principles of equipotential bonding used throughout the 18th.

Rather than of little value, it's of great value

The resistance value to earth isn't unspecified at all; it depends on the load or the unbalance of a 3 phase load - so 500W would be 100ohm, through to 5kW which would be 9ohm - with corresponding values between.

 

[LastManOnline]

Primarily it ties the local ground to the same potential as the metal furniture, if this was not the case then the ground - and thus the feet of the drunk leaning on the lamp post could raise to some random potential in the event of a fault elsewhere - such as a line-ground fault at the next lamp post - or that one itself so with say 100....230V actually connecting to the ground/drunk's feet, whilst the lamp post itself would be tied to earth at the substation.

Tying the local ground to the same potential as the metalwork in the area prevents this, its one of the principles of equipotential bonding used throughout the 18th.

Rather than of little value, it's of great value

I think you have missed my point. What you state is correct PROVIDING as other posts mention the RA of the rod is very low. If the RA is above 10 ohms (for example), instead of a safety feature, we now have a potential hazard. We simply have another piece of live metalwork. It's not possible to tie any metelwork to to earth potential unless, the contact between them both is virtually resistance free. So to conclude.. if rod is of correct resistance its of great value, if its of high resistance its a potential hazard, adding to the problem rather than alleviating it.

 

[pc1966]

I notice your concerns about using rcd, s with electronics. Why not turn to rcd, s that don't include electronics? I like the arrangement best where we use the TNC-S, arrangement for ADS but also have an rcd for additional protection. In reality rcd, s are now becoming seen as, required protection, which in reality they are. They are single handedly responsible for reducing electrocutions massively in countries that use them as standard

Well you won't get a 30mA RCD in any practical manner without electronics or very delicate mechanical trip mechanisms!

I fully agree that RCDs have massively improved safety against shock and to some degree against fire, so that are very much a good thing.

But I always believe when it comes to safety you don't want a single point of failure. That is why we have double insulation, or an earthed object for ADS and single insulation. In both cases you need two failures for something to be at-risk.

Now you can say that an RCD on TT still needs double fault to be unsafe, the original live-CPC breach and the RCD to fail in detecting the fault current. But as such a fault makes the whole installation's CPC dangerously high in potential for long period you have a greater risk from any lack of supplementary bonding, or from odd external metalwork that is connected to the installation's CPC and touchable (hand railings or fences with lights mounted on them, etc).

If you can disconnect on OCPD or RCD then you need double-faults as well as the (far more likely) initial appliance fault to make things unsafe. As that is impractical with most TT setups I would always prefer a S-type 100mA/300mA RCD incomer and RCBOs for the "30mA additional protection" against shock.

Though I suspect here I am preaching to the choir!

 

[Julie.]

I think you have missed my point. What you state is correct PROVIDING as other posts mention the RA of the rod is very low. If the RA is above 10 ohms (for example), instead of a safety feature, we now have a potential hazard. We simply have another piece of live metalwork. It's not possible to tie any metelwork to to earth potential unless, the contact between them both is virtually resistance free. So to conclude.. if rod is of correct resistance its of great value, if its of high resistance its a potential hazard, adding to the problem rather than alleviating it.

No it isn't.

If there is a fault to earth somewhere; the potential at the ground at the fault would be say 230V (or 6.35kV, 19.05kV or whatever depending on the faulted circuit voltage) and would be zero at the substation earth - all points in between would have various potentials dropping as one gets closer to the substation.

This doesn't matter at any particular point - unless you contact another point a bit further down the route (such as a horse in a field etc) when the potential difference between the points matter.

However if something is directly connected to the earth point at the substation, such as the metalwork of street furniture then it would be very important!

Anyone or anything touching the metal whilst in contact with the ground could receive a considerable voltage (depends on how close to the fault or how close to the substation or nearest earth connection) .

If however there are lots of PME connections to the ground, then a couple of things happen - firstly the local ground is held towards the same as the substations earth (so reduced chance of a potential difference), and the fault current is likely to be larger , so will clear quicker, or in some cases it would actually be detected!

These connections should be as many as possible - not just PME rods in the distribution, but also the bonding to gas/water pipes at each house, to extraneous metalwork etc. (in fact as the move to plastic services increases the need for more distribution rods grows). Tying street furniture with rods does the same thing.

It is also important when you come to more rural areas as the number of distribution services and their PME/extraneous bondings fall.

This would be a cost impact, hence not all take-offs from the distribution system have rods, not all street furniture has rods etc.

The ideal would be for every point possible to have a rod (or rods), as cumulatively this would provide little chance for the local ground to raise potential in the event of a fault, but at a significant financial cost.

In reality there needs to be sufficient bonding to local ground everywhere to provide a safe potential in the event of a fault etc. Any further would be added cost, yet not provide anything.

Take any circuit in a home, if it has sufficient CPC to clear the fault in 0.4s should we all add another separate CPC to make sure?