Understanding the relationship between TNCS, TNS and TT

[HappyHippyDad]

I have never actually had to make an electrical system into a TT. I have added rods in order to get an effective Ra but never had to (for example) install a TT on a new garage or shed.

It's best I give an example with regards my question..

A new garage has a supply taken to it from the house. The house is PME and this has been extended to the garage, lets say 4mm 3 core SWA. The 3rd core is perfectly adequate as an earth. Also, the installer puts a copper rod in the ground and attaches this to the earth bar in the new garage CU. Is this OK? Is this actually better? Is it now dangerous?

I've got a few ideas but I'd rather not influence the line of thought and just leave it as above.

Cheers all.

 

[Designmen90]

...and the question is: If there is TN-C/S sistem with distrubuted PEN, or TN-S with distributed N and P conductor, why it's important do add your own earthing electrode (example 25x4), when you already have it from substation?
Why substations with such dstribution exist, why the all world doesn't use TT system.

 

[pc1966]

...and the question is: If there is TN-C/S sistem with distrubuted PEN, or TN-S with distributed N and P conductor, why it's important do add your own earthing electrode (example 25x4), when you already have it from substation?

Ideally you don't need earth rods with TN-C-S / TN-S as the supplier provides it. In the UK it is not a requirement, but the discussion has been made to add them to deal with the case of a PEN fault.

How successful that might be is debatable, a few rods will do nothing to divert a hundred or so amps of neutral current trying to find its way home via the earth options. But a few hundred rods or some connected to buried steel building foundations would make a difference.

Why substations with such dstribution exist, why the all world doesn't use TT system.

In the days before RCDs were common / cheap you would not be able to clear most earth faults via the OCPD. So it was much safer to have a reliable low-Z earth circuit available.

VOELCB were used for TT but were rather unreliable, and they could fail to trip if you had a parallel earth path, ironically enough

Now that RCDs are fairly cheap there is a stronger argument for TT, but not all locations are suitable for earth rods, and you might also have difficulties in multi-occupancy buildings around responsibility or routing for an earth rod system.

Even with RCD protection, it is less reliable than a MCB/fuse (due to the extra complexity of the electronics) in terms of definitely firing on a fault. Two RCDs in series such as a 100mA delay incomer and then RCBOs get round that issue, but at added cost (though now it is not too painfully expensive to do that).

So basically in most cases a TN provided earth is safer, but the rise of PME to save copper costs has undone some of that safety. TT avoids that aspect, but pushes reliable earthing & fault disconnection on to the consumer, and they generally can't be trusted to implement / maintain it to the same degree.

 

[LastManOnline]

It is only if most properties have rods to the total Ra for the section post-fault is low enough does it help to keep potentials down. Even then it would need a lot of rods.

But then again it also needs something to be monitoring this and disconnecting the segment / getting someone to fixing it as well!

It is only if most properties have rods to the total Ra for the section post-fault is low enough does it help to keep potentials down. Even then it would need a lot of rods.

But then again it also needs something to be monitoring this and disconnecting the segment / getting someone to fixing it as well!

Now that is an interesting point but again I would query whether the installing of many more rods at individual houses would have any effect on the touch voltage for those unfortunate enough to be caught on the wrong side of a broken neutral. Take an example of a housing estate with 20 houses, each having a rod with a res of 200 ohms. Neutral breaks, affecting first 10 houses. My calculations show the touch voltage at each rod still largely dependent on the individual resistance of each individual rod and unaffected by other rods. A question like that a little unfair on a Friday but am confident you are up for it?

 

[Simon47]

OK, if there are 10 houses after the break, then that's a combined Ze of 20 ohms (OK, there'll be some resistance in the connecting cables, but smaller by an order of magnitude). That's going to take 2.5 amps while keeping touch voltage to 50V. Now, what's the actual current going to be ? Well worst case is one or two houses on same phase have electric showers going while the oven's on for the Sunday roast - in practical terms, it's going to make the earth rods "live". A more realistic suggestion is that most houses are only taking a few amps - and being on different phases the neutral currents will largely cancel.

EDIT: Apart from touch voltage, they will also reduce the P-N imbalance seen by the properties. Even a modest improvement may make the difference between headline grabbing affects or more headline grabbing effects and "a few things get damaged".

So, a question for those of you with experience in this area, what IS a realistic expectation (or typical spread of values) of earth rod impedance ? Is it 200 ohms as here, or 2 ohms mentioned a few posts back - that's a very wide spread. Or is it a case of both of those (and worse) being the sort of values to be found in the wild ?

 

[LastManOnline]

OK, if there are 10 houses after the break, then that's a combined Ze of 20 ohms (OK, there'll be some resistance in the connecting cables, but smaller by an order of magnitude). That's going to take 2.5 amps while keeping touch voltage to 50V. Now, what's the actual current going to be ? Well worst case is one or two houses on same phase have electric showers going while the oven's on for the Sunday roast - in practical terms, it's going to make the earth rods "live". A more realistic suggestion is that most houses are only taking a few amps - and being on different phases the neutral currents will largely cancel.

So, a question for those of you with experience in this area, what IS a realistic expectation (or typical spread of values) of earth rod impedance ? Is it 200 ohms as here, or 2 ohms mentioned a few posts back - that's a very wide spread. Or is it a case of both of those (and worse) being the sort of values to be found in the wild ?

200 would be a good average

 

[davesparks]

200 would be a good average

Average of what? What areas have you taken your values from to get this average?
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OK, if there are 10 houses after the break, then that's a combined Ze of 20 ohms (OK, there'll be some resistance in the connecting cables, but smaller by an order of magnitude). That's going to take 2.5 amps while keeping touch voltage to 50V. Now, what's the actual current going to be ? Well worst case is one or two houses on same phase have electric showers going while the oven's on for the Sunday roast - in practical terms, it's going to make the earth rods "live". A more realistic suggestion is that most houses are only taking a few amps - and being on different phases the neutral currents will largely cancel.

EDIT: Apart from touch voltage, they will also reduce the P-N imbalance seen by the properties. Even a modest improvement may make the difference between headline grabbing affects or more headline grabbing effects and "a few things get damaged".

So, a question for those of you with experience in this area, what IS a realistic expectation (or typical spread of values) of earth rod impedance ? Is it 200 ohms as here, or 2 ohms mentioned a few posts back - that's a very wide spread. Or is it a case of both of those (and worse) being the sort of values to be found in the wild ?

It's not going to be as simple as that, the current flow could be all over the shop depending on the distubution of the loads. Also the balance between the phases will be having an effect on how much neutral current there actually is.

Different areas of the UK will vary greatly for their soil reisistivity and also ease of installation of earth rods.

We could probably get a massive spread of average values just from forum members experiences.

In my area I expect to see between 20 and 30 ohms from an 8' rod and I normally install at least 2 in parallel to achieve less than 20 ohms Ra.

 

[LastManOnline]

200 would be a good average

Average of what? What areas have you taken your values from to get this average?
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It's not going to be as simple as that, the current flow could be all over the shop depending on the distubution of the loads. Also the balance between the phases will be having an effect on how much neutral current there actually is.

Different areas of the UK will vary greatly for their soil reisistivity and also ease of installation of earth rods.

We could probably get a massive spread of average values just from forum members experiences.

In my area I expect to see between 20 and 30 ohms from an 8' rod and I normally install at least 2 in parallel to achieve less than 20 ohms Ra.

Use whatever values you normally get in your area. Don't forget the question which is basically trying to establish what effect on touch voltage will the sinking of rods in individual homes have. Or to put it another way if just one house had a rod and a broken neutral occurs what would be the touch voltage at his rod?
Now in another scenario imagine every house had a rod and the same broken neutral situation occurs, would the touch voltage at the rod of house number 1 be any different? I think not

 

[davesparks]

Or to put it another way if just one house had a rod and a broken neutral occurs what would be the touch voltage at his rod?
Now in another scenario imagine every house had a rod and the same broken neutral situation occurs, would the touch voltage at the rod of house number 1 be any different? I think not

As we have already established, many times, the touch voltage depends on the Ra.

It is most likely to be different if every installation has a rod and a different number of installations are downstream of the break in the PEN.
Without any rods or bonded extraneous parts at all the touch voltage from PEN to earth changes with every individual situation, except for a single installation, whne a break in the PEN occurs.

 

[pc1966]

10 houses, assuming spread over 3 phases, one might assume an average neutral imbalance of around 20-40A (i.e. 1-2 houses' worth of diversity assumption). To keep touch below 50V would mean something like 1.2 ohm from 10 houses' rod-sets, so around 12 ohms per rod-set.

In some areas that is feasible, other impractical.

But really we should not have to deal with the open PEN risk as there ought to be some more central arrangement to monitor connectivity and disconnect all phases at the substation, etc.

 

[davesparks]

ut really we should not have to deal with the open PEN risk as there ought to be some more central arrangement to monitor connectivity and disconnect all phases at the substation, etc.

Why shouldn't we have to deal with it? We put protective measures in place to minimise risks when faults occur, so why not for this type of fault?

 

[pc1966]

Why shouldn't we have to deal with it? We put protective measures in place to minimise risks when faults occur, so why not for this type of fault?

Just it would be far more cost-effective for the DNO to take additional steps, than for the UK's electricians to deal with it in many new and existing properties.

 

[static zap]

Why shouldn't we have to deal with it?

..Is it more about costing so the customers install can deal with it.
(Now the DNO have entered a more frugal phase )
How many years of dry weather for subsidence issues seen in the 70s

(Cross post similar to PC1966 ..sorry)

 

[davesparks]

Just it would be far more cost-effective for the DNO to take additional steps, than for the UK's electricians to deal with it in many new and existing properties.

And it would be safer if both the network and the installation contains provisions for protection against these faults.

Even if the DNO put measures in place to reduce the danger from these faults it won't change my opinion that installations should be taking steps to do the same.

 

[pc1966]

It would, but there are limits to what can be done in many cases for the open PEN risk. For example, if you haver external metalwork that will probably be connected (say metal outdoor lights, CCTV equipment, etc) or external plumbing that is bonded, then your options are:

• Make sure all external metalwork is isolated
• TT the supply
• Put in impractically low Ra rods
• Put in an EV charger-like system to disconnect the supply live and the CPC on a fault

The first option may be the easy one if carefully considered at the installation time, but how many existing properties will not be so isolated, and how many folk will consider the PEN fault case for fitting new stuff? And what of the DNO converting from TN-S to TN-C-S later?

Going TT is an easy option and the route in Europe, but has other down sides (as often covered discussing out building supplies), as covered above the local earth rod option is probably impractical in most cases, even were it mandated from the start (which in the UK it was not).

The last option, that of the EV charger style that opens the CPC with all live conductors is probably the simplest comprehensive fix, and would also avoid the fire/damage risk of, say, 300V supply instead of 230V if you are on a light-loaded phase when the PEN opens, but a major cost to change and not a product that is currently available for the CU.

In retrospect, smart meters could have done that and actually been useful, but that was not considered and cost saving on the design (coupled with general incompetence of the whole project management) mean it probably never will be.

This will be a debate that runs for years...

 

[LastManOnline]

It would, but there are limits to what can be done in many cases for the open PEN risk. For example, if you haver external metalwork that will probably be connected (say metal outdoor lights, CCTV equipment, etc) or external plumbing that is bonded, then your options are:

• Make sure all external metalwork is isolated
• TT the supply
• Put in impractically low Ra rods
• Put in an EV charger-like system to disconnect the supply live and the CPC on a fault

The first option may be the easy one if carefully considered at the installation time, but how many existing properties will not be so isolated, and how many folk will consider the PEN fault case for fitting new stuff? And what of the DNO converting from TN-S to TN-C-S later?

Going TT is an easy option and the route in Europe, but has other down sides (as often covered discussing out building supplies), as covered above the local earth rod option is probably impractical in most cases, even were it mandated from the start (which in the UK it was not).

The last option, that of the EV charger style that opens the CPC with all live conductors is probably the simplest comprehensive fix, and would also avoid the fire/damage risk of, say, 300V supply instead of 230V if you are on a light-loaded phase when the PEN opens, but a major cost to change and not a product that is currently available for the CU.

In retrospect, smart meters could have done that and actually been useful, but that was not considered and cost saving on the design (coupled with general incompetence of the whole project management) mean it probably never will be.

This will be a debate that runs for years...

Excellent analysis. Think you have pretty much summed up the whole issue we are facing with TNC-S as well as supplying the potential solutions. Undoubtedly not incorporating a mechanism to uncouple the DNO, s supply from the consumers installation under broken neutral conditions is an opportunity missed
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As we have already established, many times, the touch voltage depends on the Ra.

It is most likely to be different if every installation has a rod and a different number of installations are downstream of the break in the PEN.
Without any rods or bonded extraneous parts at all the touch voltage from PEN to earth changes with every individual situation, except for a single installation, whne a break in the PEN occurs.

Want to come back to you on this one. Am still of the view that the rods down stream of the break won't matter a jot but need to fully tease it out.