The size of the main protective bonding connector. Absolute minimum for TNS is 6mm, absolute minimum for TNCS is 10mm. 544.1.1 refers.
Not neccessarily..........
Change to Earthing system
- Thread starter studentspark
- Start date
Not neccessarily..........
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For a 100A supply you are going to require 10.0mm² bonding conductors whether the supply type is TN-S or TN-C-S
544.1.1 Except where PME conditions apply, a main protective bonding conductor shall have a cross-sectional area not less than half the cross-sectional area required for the earthing conductor of the installation and not less than 6 mm². The cross sectional area need not exceed 25 mm² if the bonding conductor is of copper or a cross-sectional area affording equivalent conductance in other metals.
Except for highway power supplies and street furniture, where PME conditions apply the main protective bonding conductor shall be selected in accordance with the neutral conductor of the supply and Table 54.8.
Where an installation has more than one source of supply to which PME conditions apply, a main protective bonding conductor shall be selected according to the largest neutral conductor of the supply.
So for a TN-S supply, first we have to calculate the size of the earthing conductor.
This is calculated using Table 54.7
For this we need to know the cross-sectional area of the line conductor, which looking at Table 4D1A requires 25 mm² cable for a 100A supply.
Now back to Table 54.7
As the cross-sectional area of line conductor is greater than 16 mm² and less than or equal to 35 mm² the formula used to calculate the required size of the earthing conductor is
k1
---- x 16
k2
We get the value of k1 (the k value for the line conductor) from Table 43.1, which for a 70°C Thermoplastic insulated copper conductor k = 115
The value of k2 (the k value for the protective conductor) from Table 54.2, which for a for a 70°C Thermoplastic insulated copper conductor k = 143
By entering the figures into the equation,
115
----- x 16
143
= 0.8041958041 x 16
= 12.86 mm²
As the formula has produced a non-standard size, a conductor having the nearest larger standard cross-sectional area shall be used.
So, a 16 mm² main earth is required.
Now we have established the size of the main earth, we can go back to the first part of 544.1.1 (Except where PME conditions apply, a main protective bonding conductor shall have a cross-sectional area not less than half the cross-sectional area required for the earthing conductor of the installation)
Half of 16 mm² is 8 mm² which again is a non-standard size, so a conductor having the nearest larger standard cross-sectional area shall be used.
That is 10 mm²
544.1.1 Except where PME conditions apply, a main protective bonding conductor shall have a cross-sectional area not less than half the cross-sectional area required for the earthing conductor of the installation and not less than 6 mm². The cross sectional area need not exceed 25 mm² if the bonding conductor is of copper or a cross-sectional area affording equivalent conductance in other metals.
Except for highway power supplies and street furniture, where PME conditions apply the main protective bonding conductor shall be selected in accordance with the neutral conductor of the supply and Table 54.8.
Where an installation has more than one source of supply to which PME conditions apply, a main protective bonding conductor shall be selected according to the largest neutral conductor of the supply.
So for a TN-S supply, first we have to calculate the size of the earthing conductor.
This is calculated using Table 54.7
For this we need to know the cross-sectional area of the line conductor, which looking at Table 4D1A requires 25 mm² cable for a 100A supply.
Now back to Table 54.7
As the cross-sectional area of line conductor is greater than 16 mm² and less than or equal to 35 mm² the formula used to calculate the required size of the earthing conductor is
k1
---- x 16
k2
We get the value of k1 (the k value for the line conductor) from Table 43.1, which for a 70°C Thermoplastic insulated copper conductor k = 115
The value of k2 (the k value for the protective conductor) from Table 54.2, which for a for a 70°C Thermoplastic insulated copper conductor k = 143
By entering the figures into the equation,
115
----- x 16
143
= 0.8041958041 x 16
= 12.86 mm²
As the formula has produced a non-standard size, a conductor having the nearest larger standard cross-sectional area shall be used.
So, a 16 mm² main earth is required.
Now we have established the size of the main earth, we can go back to the first part of 544.1.1 (Except where PME conditions apply, a main protective bonding conductor shall have a cross-sectional area not less than half the cross-sectional area required for the earthing conductor of the installation)
Half of 16 mm² is 8 mm² which again is a non-standard size, so a conductor having the nearest larger standard cross-sectional area shall be used.
That is 10 mm²
It would have been a lot quicker to simply say
"Oh yes. there are differing requirements in BS7671 between a TN-S supply and a TN-C-S supply".
So let's consider the case where the existing TNS installation has 6mm bonding and the DNO upgrades to TNCS.
Should the DNO let the customer know that they might think about upgrading their bonding? Or put a leaflet through their letterbox saying if they have a shed, and it's got electrics, they need to check their earthing system to shed?
Because that's what the thread is about.
studentspark gets it...........
"Oh yes. there are differing requirements in BS7671 between a TN-S supply and a TN-C-S supply".
So let's consider the case where the existing TNS installation has 6mm bonding and the DNO upgrades to TNCS.
Should the DNO let the customer know that they might think about upgrading their bonding? Or put a leaflet through their letterbox saying if they have a shed, and it's got electrics, they need to check their earthing system to shed?
Because that's what the thread is about.
studentspark gets it...........
Well shouldn't they do that to every house that they go into that has undersized bonding or a garden shed then as there is no difference in the regs between the TN-C-S and TN-S systems? (well as far as a domestic supply is concerned)
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I've already said they should if they upgrade the supply from TNS to TNCS. This was also the suggestion of studentspark.
What do you think? It's OK to change your mind when provided with extra evidence, in fact it's a sign of strength in my opinion.
Where the garage contains an extraneousconductive-
part such as a metal water pipe
Among the options open to the installation designer
are to make the installation in the garage part of a TT
system or to provide main equipotential bonding to
the extraneous-conductive-part in the garage.
TT system
One possibility the electrical contractor may decide upon
is to supply the garage from a spare way in the consumer
unit and make the small installation in the garage part of
a TT system. A local earth electrode must be
provided at the garage. The metal water pipe must not be
used for this purpose. To achieve
protection against indirect contact for both circuits in
the garage an RCD must be employed. Main equipotential
bonding will need to be provided at the garage connecting
the metal water pipe and any other extraneousconductive-
parts to the earthing terminal in the small
distribution board. An exposedconductive-
part connected to one means of earthing
must not be simultaneously accessible with an exposedconductive-
part connected to another means of earthing
. Where the installation in
the garage is supplied by an armoured cable, the armour
or any protective conductor in the cable must not be
connected to and must not be simultaneously-accessible
with any exposed-conductive-parts in the outbuilding.
Main equipotential bonding
Another possibility is to include the garage within the
installation in the main dwelling and provide main
equipotential bonding,
(PME conditions apply), in practice this would mean a
10mm2 main bonding conductor would be
required to connect the water pipe in the
garage with the Main Earthing Terminal in
the dwelling.
Verify the existing installation
The electrical contractor must verify that
the rating and condition of existing
equipment, including that of the distributor,
should be adequate for the additional load
and that the existing earthing and bonding
arrangements are also adequate.
Therefore with ECP's in the detached outbuilding one of the above methods is neccessary with tncs supply. With a TNS system. it is not.
My concern is that the DNO can change a TNS Earthing system to PME without investigating how the supply to the shed is arranged.
They could put that in a leaflet.
part such as a metal water pipe
Among the options open to the installation designer
are to make the installation in the garage part of a TT
system or to provide main equipotential bonding to
the extraneous-conductive-part in the garage.
TT system
One possibility the electrical contractor may decide upon
is to supply the garage from a spare way in the consumer
unit and make the small installation in the garage part of
a TT system. A local earth electrode must be
provided at the garage. The metal water pipe must not be
used for this purpose. To achieve
protection against indirect contact for both circuits in
the garage an RCD must be employed. Main equipotential
bonding will need to be provided at the garage connecting
the metal water pipe and any other extraneousconductive-
parts to the earthing terminal in the small
distribution board. An exposedconductive-
part connected to one means of earthing
must not be simultaneously accessible with an exposedconductive-
part connected to another means of earthing
. Where the installation in
the garage is supplied by an armoured cable, the armour
or any protective conductor in the cable must not be
connected to and must not be simultaneously-accessible
with any exposed-conductive-parts in the outbuilding.
Main equipotential bonding
Another possibility is to include the garage within the
installation in the main dwelling and provide main
equipotential bonding,
(PME conditions apply), in practice this would mean a
10mm2 main bonding conductor would be
required to connect the water pipe in the
garage with the Main Earthing Terminal in
the dwelling.
Verify the existing installation
The electrical contractor must verify that
the rating and condition of existing
equipment, including that of the distributor,
should be adequate for the additional load
and that the existing earthing and bonding
arrangements are also adequate.
Therefore with ECP's in the detached outbuilding one of the above methods is neccessary with tncs supply. With a TNS system. it is not.
My concern is that the DNO can change a TNS Earthing system to PME without investigating how the supply to the shed is arranged.
They could put that in a leaflet.
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