Flats with 10mm twin and earth

[dokkan1080]

Seen loads of flats are fed from a 60amp isolator in 10mm twin and earth?

don't they need to be 16mm tails and at least a 10mm earth?

or is it just down to the adiabatic?

 

[Darkwood]

There is always a lot of confusion on this subject and many do get it wrong...

Say you have an HMO with one common supply into the building that all the flats are then taken from, this common supply is where you would do you Gas and Water equipotential bonding if needed in the appropriate size bonding cable.
Running sub-mains to the flats in say 16mm T&E would not require you to add additional earthing, it is a circuit and only need to have a earth that meets its own requirements, the point of bonding Gas and Water is to ensure the pipes do not bring in a different potential earth than that of the electrical earth and nothing more, it is not about earthing the internal pipe network.

If the flats however are supplied individually with their own cutout and meter then yes there may be a requirement to bond the gas and water as it enters each flat if required.

This set up applies equally to all sectioned buildings like factories where the gas and water is shared through different units, if they all have independent supplies then they need independently to be bonded if required, however if they are all sub mains off the same common supply then no they do not as any bonding requirements would be done at source with the incoming supply, if the units have independent water and gas in metal then that could warrants each unit to be bonded separately and can either be done to the local submains earthing size permitting or run individually back to the incoming supply MET.

 

[cliffed]

What would be different if it was tns?

So if a 16mm earth is added from the met to cu you can run the 10mm bonding back to the cu earth bar as opposed to the met

Yes it’s then the correct main earth for that cable.
Usually the 16.00mm is a sub main cable not anticipating any main bonding conductors connections.

 

[dokkan1080]

Yes it’s then the correct main earth for that cable.
Usually the 16.00mm is a sub main cable not anticipating any main bonding conductors connections.

16mm is not anticipating any main bonding conductors?

 

[dokkan1080]

There is always a lot of confusion on this subject and many do get it wrong...

Say you have an HMO with one common supply into the building that all the flats are then taken from, this common supply is where you would do you Gas and Water equipotential bonding if needed in the appropriate size bonding cable.
Running sub-mains to the flats in say 16mm T&E would not require you to add additional earthing, it is a circuit and only need to have a earth that meets its own requirements, the point of bonding Gas and Water is to ensure the pipes do not bring in a different potential earth than that of the electrical earth and nothing more, it is not about earthing the internal pipe network.

If the flats however are supplied individually with their own cutout and meter then yes there may be a requirement to bond the gas and water as it enters each flat if required.

This set up applies equally to all sectioned buildings like factories where the gas and water is shared through different units, if they all have independent supplies then they need independently to be bonded if required, however if they are all sub mains off the same common supply then no they do not as any bonding requirements would be done at source with the incoming supply, if the units have independent water and gas in metal then that warrants each unit to be bonded separately and can either be done to the local submains earthing size permitting or run individually back to the incoming supply MET.

So would both of the following be acceptable?

Or would the gas bond need to be by the meter or as the pipe enters a flat

 

[Darkwood]

Firstly your crap at art ?..

Let's start with ensuring we are clear on what we are discussing, we are discussing Equipotential Bonding here, this should not be confused with earthing requirements of supplementary bonding which the pipework may fall into, so the following info and questions only relate to equipotential bonding.

The drawing lacks a lot of info'

Is this a HMO with one common power source for all the flats?
Is there one common Gas feed to the building?
Is there one common water feed to the building?

What you have to remember here is we bond gas and water at source so it does not bring a different potential in from natural ground to that of the electrical earth, this thinking has to be applied to such setups if say the gas and/or water is one source but is buried again in conductive pipe before reaching the flats, the guidance cannot give every scenario but understanding why we do equipotential bonding goes a long way to knowing when it is needed.

The problem we have in this industry is if you are a member of a scheme then the advice you get may differ depending on the scheme you are a member of and this only fuels the confusion, some schemes simply don't want to get into reviewing every scenario so they play it safe and say you should bond it, if this is the advice of your scheme then you should follow it as it is there approval you need when they inspect your work, if the gas and water is deemed it needs bonding in each flat then an additional 10mm bond would be required as the sub-supply earthing is simply too small, you can take this to the flat consumer unit and then bond from there or run a direct bond to the met.

What you need to do regarding earthing of gas and water pipes in flats if they do not need equipotential bonding is establish if they are conductive parts that require supplementary bonding and if so then earth them in accordance with the guidance.

If you are still confused I suggest you purchase a copy of the guidance notes 8 which goes deeper with pictorial examples.

 

[dokkan1080]

Firstly your crap at art ?..

Let's start with ensuring we are clear on what we are discussing, we are discussing Equipotential Bonding here, this should not be confused with earthing requirements of supplementary bonding which the pipework may fall into, so the following info and questions only relate to equipotential bonding.

The drawing lacks a lot of info'

Is this a HMO with one common power source for all the flats?
Is there one common Gas feed to the building?
Is there one common water feed to the building?

What you have to remember here is we bond gas and water at source so it does not bring a different potential in from natural ground to that of the electrical earth, this thinking has to be applied to such setups if say the gas and/or water is one source but is buried again in conductive pipe before reaching the flats, the guidance cannot give every scenario but understanding why we do equipotential bonding goes a long way to knowing when it is needed.

The problem we have in this industry is if you are a member of a scheme then the advice you get may differ depending on the scheme you are a member of and this only fuels the confusion, some schemes simply don't want to get into reviewing every scenario so they play it safe and say you should bond it, if this is the advice of your scheme then you should follow it as it is there approval you need when they inspect your work, if the gas and water is deemed it needs bonding in each flat then an additional 10mm bond would be required as the sub-supply earthing is simply too small, you can take this to the flat consumer unit and then bond from there or run a direct bond to the met.

What you need to do regarding earthing of gas and water pipes in flats if they do not need equipotential bonding is establish if they are conductive parts that require supplementary bonding and if so then earth them in accordance with guidance.

If you are still confused I suggest you purchase a copy of the guidance notes 8 which goes deeper with pictorial examples.

???

Thanks for all that I will read gn8 im just trying to understand how my flat is earthed and such

Its a shares supply in respect that they share the same MET but all have separate isolators and separate consumer units

Seprate gas meters
Separate electrical meters
Not sure about the water its a converted house and shop so I imagine at one time it came into the house at a single point

So I guess though regardless of it is needed or not

If you bond in 10mm from the origins or have suitable earthing from you origins (16mm) to your consumer unit you can bond from that point. Irrelevant of ifs its already been bonded from the met.

 

[Darkwood]

Just reading up on my copy I dug out and it does actually express how it can get a bit complex to understand when we are dealing with HMO's and split premises etc but it does give good guidance on the matter.

 

[Rodrigo12]

If every circuit is RCD or RCBO protected 30mA to 150mA max will flow to earth why do you need 16mm earth cable?

 

[Deleted account]

If every circuit is RCD or RCBO protected 30mA to 150mA max will flow to earth why do you need 16mm earth cable?

Because you design the circuit with fault protection in mind the additional protection is “ additional” and “extra protection” to the fault protection.

 

[Darkwood]

If every circuit is RCD or RCBO protected 30mA to 150mA max will flow to earth why do you need 16mm earth cable?

RCD's/RCBO's can fail and do fail, you cannot rely on them solely for personal protection, other measures bust be implemented in design, I believe the reg's do stipulate this point.

 

[Rodrigo12]

RCD's/RCBO's can fail and do fail, you cannot rely on them solely for personal protection, other measures bust be implemented in design, I believe the reg's do stipulate this point.

Yes but really my question is why 16mm every time 10mm was always OK for 60 Amp fuse

 

[pc1966]

Yes but really my question is why 16mm every time 10mm was always OK for 60 Amp fuse

Short answer is the wiring regulations.

Longer answer is it depends on both the maximum fault situation (i.e. adiabatic limit) and the mechanical protection (or lack of) for the cables.

So if you have a 60A fused-switch feeding 16mm T&E for example, you know it is a 60A fuse and should not be changed without the installation being upgraded with it. Basically it is the installation's fuse and under control of the installation's designer/owner and they can check the cable is OK in terms of adiabatic limit. For example, by measuring Zs and looking up the OSG table B4, etc.

However, if you have the DNO cut-out it could be 60A or 80A or 100A at the DNO's discretion (even though it is unlikely the DNO will sneak in and change from 60A to 100A for a laugh) so any related wiring should meet the maximum expected there.

Finally we have the issue of TN-C-S bonding to extraneous parts and that is independent of the supply fusing, and the current limit there harder to establish as it depends on the location of any fault (so how much neutral imbalance current is available) along with the varios earth paths available both on any DNO electrodes as well and any metallic service pips of all properties on the faulted segment of the network.