Power point in external glasshouse

[cliffed]

Client wants 2g socket in greenhouse, obviously IP rated,will be supplied from Rcd, the greenhouse consists of metal frame work on the ground.
Does this require MPBC

 

[cliffed]

Where do you get this from?

If it's table 54.8 or 544.1.1 then as per your wording - those figures are "except where PME conditions apply "

Yea I know

 

[Julie.]

Yea I know

So if you believe that the PME note means that in any case where a PME is used to supply a property, the values in 54.8 and 544.1.1 are not correct , just where do you get the correct size from?

Wouldn't this be a glaring omission from the regs? - inability to size bonding if it's supplied via PME.


Or should we understand that "where PME conditions apply" refers to those situations where the Neutral and earth are combined (CNE) forming part of the PME configuration.

In which case, once they have been split at the service head then 54.8 and 544.1.1 apply in terms if bonding size.

 

[cliffed]

Your last sentence is exactly correct

 

[Julie.]

Your last sentence is exactly correct

Which would mean in accordance with 544.1.1 main bonding in sub-buildings could be sized at 50% of the conductors to that building or 6mm^2 whichever is the greater.

Just running a 1.5 or 2.5 SWA from the cu to this outlet and bonding the metal frame via 6mm^2 from the swa (+cpc if a 3rd core used) would be completely compliant with the minimum expense.

 

[cliffed]

Which would mean in accordance with 544.1.1 main bonding in sub-buildings could be sized at 50% of the conductors to that building or 6mm^2 whichever is the greater.

Just running a 1.5 or 2.5 SWA from the cu to this outlet and bonding the metal frame via 6mm^2 from the swa (+cpc if a 3rd core used) would be completely compliant with the minimum expense.

I shall be installing a TT System with the appropriate MPBC

 

[westward10]

So if you believe that the PME note means that in any case where a PME is used to supply a property, the values in 54.8 and 544.1.1 are not correct , just where do you get the correct size from?

Wouldn't this be a glaring omission from the regs? - inability to size bonding if it's supplied via PME.


Or should we understand that "where PME conditions apply" refers to those situations where the Neutral and earth are combined (CNE) forming part of the PME configuration.

In which case, once they have been split at the service head then 54.8 and 544.1.1 apply in terms if bonding size.

The way I view that Regulation is that if you have an installation where the source of supply is PME then it cannot be applied. If I have remote building from such a supply and the requirements for main protective bonding is 25.0 then this applies throughout and as such any distribution cable must be able to support it.

 

[cliffed]

The way I view that Regulation is that if you have an installation where the source of supply is PME then it cannot be applied. If I have remote building from such a supply and the requirements for main protective bonding is 25.0 then this applies throughout and as such any distribution cable must be able to support it.

Agree that’s my interpretation of the requirement

 

[Julie.]

The way I view that Regulation is that if you have an installation where the source of supply is PME then it cannot be applied. If I have remote building from such a supply and the requirements for main protective bonding is 25.0 then this applies throughout and as such any distribution cable must be able to support it.

If "except where PME conditions apply " means the same as 'except where the supply to the installation is PME"

then where is the size of bonding conductor defined within the regs?

It would appear to be totally wrong if the regs don't actually provide details for bonding size in the most common type of earthing arrangement used, yet do provide suitable guidance/rules for the less common versions.

For years most have used10mm^2, but if you do apply the "except where PME conditions apply " to mean 'except where the supply to the installation is PME" then there would be no justification for that, or any other size as both 54.1.1 and 54.8 both include the note that they do not apply where PME conditions apply.

Even using 25mm^2 wouldn't have any justification, especially as the temperature rise of CNE cable provides different loading compared to single core, hence DNOs have different cable ratings.

 

[mainline]

I shall be installing a TT System with the appropriate MPBC

If "except where PME conditions apply " means the same as 'except where the supply to the installation is PME"

then where is the size of bonding conductor defined within the regs?

It would appear to be totally wrong if the regs don't actually provide details for bonding size in the most common type of earthing arrangement used, yet do provide suitable guidance/rules for the less common versions.

For years most have used10mm^2, but if you do apply the "except where PME conditions apply " to mean 'except where the supply to the installation is PME" then there would be no justification for that, or any other size as both 54.1.1 and 54.8 both include the note that they do not apply where PME conditions apply.

Even using 25mm^2 wouldn't have any justification, especially as the temperature rise of CNE cable provides different loading compared to single core, hence DNOs have different cable ratings.

If the main supply type is a TNC-S (PME), then selection of main protective bonding conductor sizes is completed by reference to table 54.5 of BS7671.

To summarise this table, sizing is completed by reference to the size of the PEN (Protective Earth Neutral) conductor of the supply. PEN conductors of anything less than 35mm2 require a minimum of 10mm2 bonding conductors. PEN conductors between 35mm2 & 50mm2 require a 16mm2 bonding conductor to be used. The sizes of the required bonding conductors increase significantly as the size of the PEN conductor rises.

PME main bonding conductors are not subject the 25mm2 maximum requirement of any other supply type. The largest required bonding conductors on any PME supply is with an incoming supply PEN conductor of more than 150mm2 which requires a considerable 50mm2 bonding conductor to be installed!

 

[Julie.]

If the main supply type is a TNC-S (PME), then selection of main protective bonding conductor sizes is completed by reference to table 54.5 of BS7671.

To summarise this table, sizing is completed by reference to the size of the PEN (Protective Earth Neutral) conductor of the supply. PEN conductors of anything less than 35mm2 require a minimum of 10mm2 bonding conductors. PEN conductors between 35mm2 & 50mm2 require a 16mm2 bonding conductor to be used. The sizes of the required bonding conductors increase significantly as the size of the PEN conductor rises.

PME main bonding conductors are not subject the 25mm2 maximum requirement of any other supply type. The largest required bonding conductors on any PME supply is with an incoming supply PEN conductor of more than 150mm2 which requires a considerable 50mm2 bonding conductor to be installed!

That's exactly my point, if this table applies to PME (and other TN arrangements) then it does mean that "except where PME conditions apply " doesn't mean that just because the supply is PME that this exception applies.

To me it's simple

If the "except where PME conditions apply " means the same as 'except where the supply to the installation is PME" then there is no sizing provided for installations supplied via PME within the wiring regs. (Which doesn't make sense.)

On the other hand if "except where PME conditions apply " means exactly that, then you would use table 54.8/544.1.1 in their entirely for all TN supplies including PME.

If you were to deal with a "where PME conditions apply" situation, such as street furniture etc. direct from the utility supply, (i.e. before the CNE is split in the service head) then you would need to obtain that sizing from the utility - which is typically 35mm^2 for most applications, but perhaps 16mm^2 for street lighting and consumer services.

This latter aspect is not contained within the wiring regs.

 

[westward10]

That's exactly my point, if this table applies to PME (and other TN arrangements) then it does mean that "except where PME conditions apply " doesn't mean that just because the supply is PME that this exception applies.

To me it's simple

If the "except where PME conditions apply " means the same as 'except where the supply to the installation is PME" then there is no sizing provided for installations supplied via PME within the wiring regs. (Which doesn't make sense.)

On the other hand if "except where PME conditions apply " means exactly that, then you would use table 54.8/544.1.1 in their entirely for all TN supplies including PME.

If you were to deal with a "where PME conditions apply" situation, such as street furniture etc. direct from the utility supply, (i.e. before the CNE is split in the service head) then you would need to obtain that sizing from the utility - which is typically 35mm^2 for most applications, but perhaps 16mm^2 for street lighting and consumer services.

This latter aspect is not contained within the wiring regs.

Sorry I am not reading the way you are.

 

[loz2754]

Yet another debate caused by ambiguous wording in the book.
I think I may have changed my mind on this, and take it that the supplies to other buildings bit only applies to TN-S, not PME.
Err.....I think!

 

[mainline]

That's exactly my point, if this table applies to PME (and other TN arrangements) then it does mean that "except where PME conditions apply " doesn't mean that just because the supply is PME that this exception applies.

To me it's simple

If the "except where PME conditions apply " means the same as 'except where the supply to the installation is PME" then there is no sizing provided for installations supplied via PME within the wiring regs. (Which doesn't make sense.)

On the other hand if "except where PME conditions apply " means exactly that, then you would use table 54.8/544.1.1 in their entirely for all TN supplies including PME.

If you were to deal with a "where PME conditions apply" situation, such as street furniture etc. direct from the utility supply, (i.e. before the CNE is split in the service head) then you would need to obtain that sizing from the utility - which is typically 35mm^2 for most applications, but perhaps 16mm^2 for street lighting and consumer services.

This latter aspect is not contained within the wiring regs.

For pme it's not the c.s.a. of the meter tails that is used for Table 54.8, but the c.s.a. of the PEN conductor in the supply cable to the service head
For a supply on a standard cutout of 100A or less, the supply conductor will be 35mm or smaller. As such, the sizes for the earth conductor to the MET and onwards to the CU will be 16mm for a PME supply, and 10mm from the MET to where the gas / water enter (assuming the water is metallic). For a supply greater than 100A you should be asking the DNO

 

[mainline]

Yet another debate caused by ambiguous wording in the book.
I think I may have changed my mind on this, and take it that the supplies to other buildings bit only applies to TN-S, not PME.
Err.....I think!

Which is why suppliers frown on exporting pme

 

[brianmoooore]

This has become somewhat confused I asked in #22 if this applied to all TN systems and whilst TN-C-S does not necessarily mean PME I was answered yes it does but it doesn't?

More than somewhat confused!
Has there just been a change in the relevant regs, or is the argument that we have been applying them incorrectly all along?