Another take on bonding for TNCS

[green-hornet]

This is very interesting but some people may have missed out on some very common faults with modern house building, that have a dramatic effect on earthing arrangements.

Years ago all services into the home was via metal pipes, even gas for you young plumbers out there.

Nowadays almost everything is in plastic.

Effective earthing with all TN systems had a backup with earth bonding to gas and water or any other services entering the property.

The problem with PME supplies are, that, not only does it carry a fault from your home, it does so for every home on the circuit, which are all sharing one cable back to the connection to earth at the distribution site. With connection through metal service pipes this would not be too much of a problem.

The way I like to look at it is like broadband the further away from the supply the slower your broadband speeds.

One word of warning though, you should never change one for another. Its up to the dno, you can always call them and discuss any faults you have found and they may reccomend a change to TT but THEY have to disconnect the link from neatral to earth, not you!

 

[steptoe]

APACHE,

well spotted that man,

to drive in a spike as well as leave the PME system in place is akin to attempted manslaughter,

may not be intended, but would do the same job.

dont people pay attention at school these days?

or does everyone just think cos the DNO gives you an earth (TNCS) it must be good, I'll just put in a rod as well and help them out a little.

I really hope that wasnt a spark said that,

something could be expected from a DIY bob of course, his ignorance could be excused except for in law where it is of no excuse.

PLEASE PLEASE PLEASE do not get involved with earthing of any kind unless you fully understand the implications of what you are about to do,

or are at the very least willing to seek and accept some advice,

it may save yours and someone elses' life someday.

ps, this includes removing/exporting a PME(stoopid system) from its natural habitat,(the standard house)

 

[steptoe]

Don't see the problem with PME if you are going to instal RCDs everywhere.There's nothing preventing you from using a rod as well.

if you have some time available there is a substantial post on the reasons as to why this is NOT a good idea,

I suggest you read it.

it may just save someones life someday.

 

[Andy™]

Its up to the dno, you can always call them and discuss any faults you have found and they may reccomend a change to TT but THEY have to disconnect the link from neatral to earth, not you!

but if you dont want to use the PME earth you dont have to

 

[Andy™]

Don't see the problem with PME if you are going to instal RCDs everywhere.There's nothing preventing you from using a rod as well.

except that if there is a fault, your earth rod will take fault current from more than just your house

 

[ianmacd]

Don;t cross the streams....erm...earths

 

[green-hornet]

but if you dont want to use the PME earth you dont have to

Unfortunately andy the dno have to disconnect the earth link before you can change it to TT.

I have found that most of the time they are very willing to do this, for instance I can order a cutout change which can take up to 6 weeks to get an appointment. I found a fault on a PME and asked if I could change it to TT they came the next day to disconnect the link to nuetral and reseal the main fuse.

Happy days.

 

[sarni31]

there are limiting factors tho, for example if you got plastic incoming water supply, and copper pipework within the house after the stop cock, then you could do with varifying that this water supply needs a bond or not, a reading below 22k ohms from metal work to MET would require bonding to be completed, if its above this figure then its not an ECP and does not require bonding. how many houses are potentially at increased risk with the neuytal going open, when they prob dont even need this water bond.

Is this correct and from where does the figure come from? ?:|

Guiness Drink

 

[sparkyork]

Is this correct and from where does the figure come from? ?:| Guiness Drink

im not great with numbers etc matey but ill have a go!

since 411.3.1.2. does not define a numerical criterion, we apply Ohm's Law: if RCP > ((UO/IB) - ZTL) then the conductive part is not liable to introduce an earth potential and does not constitute an ECP.

where RCP is the resistance between metalwork and MET. Typical values are: UO (phase voltage), 230 volts;

IB (current through the human (or livestock) body which should not be exceeded), 10 mA; and ZTL (human/livestock body resistance), 1000 ohms.

Substituting in the equation, RCP must be > ((230/0.01) - 1000), ie > 22 kilohms, for ECP bonding not to be required.

If water is involved (eg a bathroom) it may be safer to use IB = 5 mA and ZTL = 200 ohms, which requires bonding below RCP = 45.8 Kilohms.

hope that helps

 

[spinlondon]

APACHE,well spotted that man,

to drive in a spike as well as leave the PME system in place is akin to attempted manslaughter,

may not be intended, but would do the same job.

dont people pay attention at school these days?

or does everyone just think cos the DNO gives you an earth (TNCS) it must be good, I'll just put in a rod as well and help them out a little.

I really hope that wasnt a spark said that,

something could be expected from a DIY bob of course, his ignorance could be excused except for in law where it is of no excuse.

PLEASE PLEASE PLEASE do not get involved with earthing of any kind unless you fully understand the implications of what you are about to do,

or are at the very least willing to seek and accept some advice,

it may save yours and someone elses' life someday.

ps, this includes removing/exporting a PME(stoopid system) from its natural habitat,(the standard house)

So if having an Earth rod on a PME sytem is a no no.

Does that also mean the bonding to the incoming water is a no no?

 

[spinlondon]

except that if there is a fault, your earth rod will take fault current from more than just your house

As I said to Steptoe, if there is no rod, then the water main will take the fault current?

The difference being?

 

[spinlondon]

Quote about TNCS from BS7671

"The supply system PEN conductor is earthed at several points and an earth electrode may be necessary at or near a consumer's installation."

 

[Andy™]

Unfortunately andy the dno have to disconnect the earth link before you can change it to TT.I have found that most of the time they are very willing to do this, for instance I can order a cutout change which can take up to 6 weeks to get an appointment. I found a fault on a PME and asked if I could change it to TT they came the next day to disconnect the link to nuetral and reseal the main fuse.

Happy days.

why go to the hassle of that - just dont connect anything to the PME earth terminal and connect to your rod instead

or if earth leaves a sealed service head, stick it in a henley block

 

[Andy™]

As I said to Steptoe, if there is no rod, then the water main will take the fault current?The difference being?

fair point, but it depends if pipes are metallic or not. if they are, then they could take the fault current to earth (just like the neighbours will be too), or if they are plastic, then it cant

 

[AK-UK]

I agree with the idea of using RCD type protection, I use RCBOs since 2004 in all my installations, where it is possible, or if it is a new CU.

However - with TNCS if the neutral connection goes open circuit before the incomer - then if any appliance is on - even a low wattage light - the earth connection then effectively becomes live.

By the way, could be that I completely missed the point but I thought that TNC-S (PME) is the configuration where the earth and neutral are linked (and split) at the cut out. If so loosing the neutral basically will disconnect the main earth, therefore the bonding is so important. Can some one explain why the system will become live in case of a fault like this?

 

[steptoe]

I agree with the idea of using RCD type protection, I use RCBOs since 2004 in all my installations, where it is possible, or if it is a new CU.By the way, could be that I completely missed the point but I thought that TNC-S (PME) is the configuration where the earth and neutral are linked (and split) at the cut out. If so loosing the neutral basically will disconnect the main earth, therefore the bonding is so important. Can some one explain why the system will become live in case of a fault like this?

AK

you have redeemed yourself,

on a few of your post I almost replied,

but now you have grasped the most fundamenta of aspects ,

EARTHING

andyc and spinlondon seem to have forgotten the golden rule of

UNDER FAULT CONDITION

under normal condition NO earth is required.

 

[spinlondon]

I agree with the idea of using RCD type protection, I use RCBOs since 2004 in all my installations, where it is possible, or if it is a new CU.By the way, could be that I completely missed the point but I thought that TNC-S (PME) is the configuration where the earth and neutral are linked (and split) at the cut out. If so loosing the neutral basically will disconnect the main earth, therefore the bonding is so important. Can some one explain why the system will become live in case of a fault like this?

It depends where the break in the Neutral occurs.

The Earth connects to the Neutral, but it also connects to the main bonding on incoming services, and an Earth rod if used.

If a break on the supply Neutral occurs near to the installation, then the Neutral will use the main bonding for return, effectivly making the Earth live. As long as all the equipotential bonding is in place, there will not be a danger inside the house as you would effectively be inside a Farady cage.

However there will be no indication that there is a fault.

If there is no reliable source to Earth between the supply Neutral break and the installation (plastic incoming services and no Earth rod), then all the appliances in the house would cease to function.

This can be dangerous, as you could inadvertantly become a path to the real earth, taking the full load of all the appliances in the house, and any RCDs would fail to operate.

 

[spinlondon]

AK you have redeemed yourself,

on a few of your post I almost replied,

but now you have grasped the most fundamenta of aspects ,

EARTHING

andyc and spinlondon seem to have forgotten the golden rule of

UNDER FAULT CONDITION

under normal condition NO earth is required.

No, I have not forgotten about under fault condition.

I am however wondering what you mean by the term fault condition?

Are you refering to Phase fault, Neutral fault, Earth fault, Phase to Earth fault, Phase to Neutral fault or Neutral to Earth fault?

Or are you refering to overload as a fault condition?

 

[Apache]

If there is no reliable source to Earth between the supply Neutral break and the installation (plastic incoming services and no Earth rod), then all the appliances in the house would cease to function.This can be dangerous, as you could inadvertantly become a path to the real earth, taking the full load of all the appliances in the house, and any RCDs would fail to operate.

You've lost me on the last point there.

My understanding was that the RCD monitored L&N and looked for an imbalalce in the two. If there is nothing on the neutral and a person is the earth surly there is an imbalance and the RCD will trip?

No?

 

[spinlondon]

You've lost me on the last point there. My understanding was that the RCD monitored L&N and looked for an imbalalce in the two. If there is nothing on the neutral and a person is the earth surly there is an imbalance and the RCD will trip?

No?

Remember that the E&N are connected. The return path for the Neutral is via the RCD to the E&N connection, and then to earth via the person. Therefore no imbalance between the L&N.