Armoured Cable Query

[EFLImpudence]

Isn't the problem the required csa of bonding conductors - especially TN-C-S?

If there are other extraneous-c-ps. in an outbuilding the required bonding may be impractical and/or too expensive so people TT it and have to keep the earth separate.

 

[JohnW2]

Isn't the problem the required csa of bonding conductors - especially TN-C-S? If there are other extraneous-c-ps. in an outbuilding the required bonding may be impractical and/or too expensive so people TT it and have to keep the earth separate.

Well, yes, but there's still nothing hazardous or 'forbidden' about connecting an earth spike to a TN-C-S earth. Indeed, if a metallic connection from that spike enters the property (thus an extraneous-c-p), then the regs actually demand that it is connected to the TN earth.

In reality, a very modest conductor would be adequate for a connection to a domestic earth electrode. However, as bernard said, given that there are people out there who cannot think and/or don't understand the principles, it probably would not be safe to have a situation in which the 'required' CSA of a bonding conductor could vary according to the nature and characteristics of the extraneous-c-p being bonded. For such people "one size which would suit all" is the only safe regulation.

Kind Regards, John
Edit: missing word inserted

 

[PBC_1966]

In this part of the world where TN-C-S is the norm (and TT is prohibited), the National Electrical Code requires a local earth electrode to be connected to the supply neutral at each service entrance.

Where a supply is taken to an outbuilding, the ground is taken across from the main building and an additional earth electrode is also provided at the outbuilding.

The electrode in either case may be a purpose-driven rod (which must be at least 8 ft. in length), but other electrodes such as the building foundation's rebar in concrete may also be used.

 

[ban-all-sheds]

Sounds good. I was thinking when I read this that maybe we should have that requirement.

 

[EFLImpudence]

In reality, a very modest conductor would be adequate for a connection to a domestic earth electrode. However, as bernard said, given that there are people out there who cannot think and/or don't understand the principles, it probably would not be safe to have a situation in which the 'required' CSA of a bonding conductor could according to the nature and characteristics of the extraneous-c-p being bonded. For such people "one size which would suit all" is the only safe regulation.

Did the end of that sentence get lost?

 

[JohnW2]

In reality, a very modest conductor would be adequate for a connection to a domestic earth electrode. However, as bernard said, given that there are people out there who cannot think and/or don't understand the principles, it probably would not be safe to have a situation in which the 'required' CSA of a bonding conductor could vary according to the nature and characteristics of the extraneous-c-p being bonded. For such people "one size which would suit all" is the only safe regulation.

Did the end of that sentence get lost?

No - a little four-letter-word in the middle of the sentence (now inserted) 'got lost'! Apologies.

Kind Regards, John

 

[JohnW2]

In this part of the world where TN-C-S is the norm (and TT is prohibited), the National Electrical Code requires a local earth electrode to be connected to the supply neutral at each service entrance.

To me that makes very good sense.

I have to say that I got a big shock (not an electrical one!) when I discovered (from westie) what the 'M' of PME often meant. I had previously imagined/assumed that it meant that there really were 'multiple' connections to earth, relatively close, spread along the run of a network main. However, westie said that it very often means that, in addition to the connection of neutral to earth at the transformer, there is just one other connection to earth, at the far end of the main. Whilst I suppose that, literally speaking, two counts as 'multiple', that certainly disillusioned me!

Kind Regards, John

 

[EFLImpudence]

Not sure if I am repeating the obvious.

Isn't the problem the required csa of bonding conductors - especially TN-C-S? If there are other extraneous-c-ps. in an outbuilding the required bonding may be impractical and/or too expensive so people TT it and have to keep the earth separate.

Well, yes, but there's still nothing hazardous or 'forbidden' about connecting an earth spike to a TN-C-S earth. Indeed, if a metallic connection from that spike enters the property (thus an extraneous-c-p), then the regs actually demand that it is connected to the TN earth.
In reality, a very modest conductor would be adequate for a connection to a domestic earth electrode.

It would and there may be no reason NOT to connect an earth spike if it is the only part in the area - in effect bonding the ground.

However, as bernard said, given that there are people out there who cannot think and/or don't understand the principles, it probably would not be safe to have a situation in which the 'required' CSA of a bonding conductor could vary according to the nature and characteristics of the extraneous-c-p being bonded. For such people "one size which would suit all" is the only safe regulation.

True but...

...if there are other low impedance extraneous-c-ps or the regulations call for a minimum large csa bonding conductor and this is, for whatever reason, not possible then the electrode should not be connected to the supply earth.

For the people referred to above, it may be more misleading to say that there is nothing hazardous or forbidden about connecting the electrode to a TN-C-S earth as, in such a case, it would be hazardous to the installation.

 

[JohnW2]

Well, yes, but there's still nothing hazardous or 'forbidden' about connecting an earth spike to a TN-C-S earth. Indeed, if a metallic connection from that spike enters the property (thus an extraneous-c-p), then the regs actually demand that it is connected to the TN earth. In reality, a very modest conductor would be adequate for a connection to a domestic earth electrode.

It would and there may be no reason NOT to connect an earth spike if it is the only part in the area - in effect bonding the ground.

Although I suppose it is true, that is a slightly odd way of thinking about it - any main bonding could be described as "in effect bonding the ground".

However, as bernard said, given that there are people out there who cannot think and/or don't understand the principles, it probably would not be safe to have a situation in which the 'required' CSA of a bonding conductor could vary according to the nature and characteristics of the extraneous-c-p being bonded. For such people "one size which would suit all" is the only safe regulation.

True but... ...if there are other low impedance extraneous-c-ps or the regulations call for a minimum large csa bonding conductor and this is, for whatever reason, not possible then the electrode should not be connected to the supply earth.

I'm not sure that I completely follow. In real-world (domestic) electrical terms, there would never be a problem with using a conductor of 'modest' CSA to connect a local earth electrode to the supply earth. The only problem would be that some (stupid) people may extrapolate from this to using a conductor of inadequate CSA to bond a "low impedance extraneous-c-p".....

For the people referred to above, it may be more misleading to say that there is nothing hazardous or forbidden about connecting the electrode to a TN-C-S earth as, in such a case, it would be hazardous to the installation.

Again, I'm no really sure what you are suggesting would be "hazardous to the installation", and why?

Kind Regards, John

 

[JohnW2]

Where a supply is taken to an outbuilding, the ground is taken across from the main building and an additional earth electrode is also provided at the outbuilding.

As you are aware, the practical issue over here often relates to the fact that the regs do not distinguish between the required bonding conductors for extraneous-c-ps of low and 'medium' impedance. As far as the installation is concerned, the earth electrode at the outbuilding counts as an extraneous-c-p - which means that, with TN-C-S, a bonding conductor with a CSA of at least 10mm² would have to be run to the outbuilding if the TN-C-S earth were being 'exported' to there.

Kind Regards, John

 

[EFLImpudence]

I am saying that; for example, in a shed; because of the high impedance of an electrode, you may calculate the csa of the earthing conductor. It may be quite small and could be connected to the supply earth with no detrimental effect.

However, if there is also a low impedance extraneous-c-p present then this must be bonded (via local MET) to the electrode.
Depending on the impedance of this e-c-p the bonding conductor may have to be large.

If you then connect the electrode - and e-c-p - to a TN-C-S supply earth, the bonding conductor will have to be 10mm² all the way back to the supply as, if it is not, then fault currents in the conductor may be too high and damage be caused.

 

[ban-all-sheds]

No reason (apart from trivial cost), not to require a 25mm² or 35mm² bonding conductor, is there?

 

[ban-all-sheds]

As far as the installation is concerned, the earth electrode at the outbuilding counts as an extraneous-c-p - which means that, with TN-C-S, a bonding conductor with a CSA of at least 10mm² would have to be run to the outbuilding if the TN-C-S earth were being 'exported' to there.

So use 10mm² 3-core SWA to the outbuilding. Then you've got a choice of using one of the cores or the armour, or both. Since, as is often observed, the difference between an earthing connection and a bonding one is just naming, there's no reason why a conductor cannot have two names at the same time.

 

[EFLImpudence]

No reason (apart from trivial cost), not to require a 25mm² or 35mm² bonding conductor, is there?

No, but that is the reason (most?) people claim for TTing outbuildings.

Apart, that is, from those who think it compulsory.

 

[JohnW2]

I am saying that; for example, in a shed; because of the high impedance of an electrode, you may calculate the csa of the earthing conductor. It may be quite small and could be connected to the supply earth with no detrimental effect.

Electrically that's true. However, in terms of the regs, as written, since that earth electrode would count as an extraneous-c-p as far as the installation was concerned, the 'earthing conductor' connected to it would have to have at least the minimum CSA required for a main bonding conductor, at least until it left the shed. Somewhat ironically, it could then reduce in CSA outside the shed!

However, if there is also a low impedance extraneous-c-p present then this must be bonded (via local MET) to the electrode. Depending on the impedance of this e-c-p the bonding conductor may have to be large.

As previously said, the regs as written specify (certainly for TN-C-S) a minimum CSA for any main bonding conductor, regardless of the impedance to earth of the extraneous-c-p being bonded.

If you then connect the electrode - and e-c-p - to a TN-C-S supply earth, the bonding conductor will have to be 10mm² all the way back to the supply as, if it is not, then fault currents in the conductor may be too high and damage be caused.

If, as you have suggested, there is a "low impedance extraneous-c-p present", then (if you are exporting the supply earth) you are surely stuck with the 10mm² minimum all the way back to the MET, whether or there is a also a local earth electrode (at the shed) in the equation.

I'm starting to wonder if you're not simply saying that if there is a "low impedance extraneous-c-p" entering the outbuilding, then it may well be more convenient to not export the supply earth to the outbuilding - but, instead, to rely entirely on a local TT electrode. If so, we 'all' know that, but it's not what we're discussing here!

Kind Regards, John