Why are main protective bonding conductors sized as they are

[Pretty Mouth]

Hi all. Hope you are all well and virus-free.

This question is not how to select main protective bonding conductor sizes according to the regulations. That I understand how to do.
What I am asking is:

Why do the regulations require such a large CSA for Main Protective Bonding conductors?

My thoughts:

The regs don't state a maximum resistance for a MPB conductor, so I assume that the large CSA is not intended to minimise the resistance between the MET and the extraneous part to be bonded. Is this assumption correct?
The MPB conductor is sized according to the earthing conductor for the installation. Why? Is it possible that, in some instances, the bonding conductor may carry some of the current in the event of a fault?

 

[Ian1981]

Hi all. Hope you are all well and virus-free.

This question is not how to select main protective bonding conductor sizes according to the regulations. That I understand how to do.
What I am asking is:

Why do the regulations require such a large CSA for Main Protective Bonding conductors?

My thoughts:

The regs don't state a maximum resistance for a MPB conductor, so I assume that the large CSA is not intended to minimise the resistance between the MET and the extraneous part to be bonded. Is this assumption correct?
The MPB conductor is sized according to the earthing conductor for the installation. Why? Is it possible that, in some instances, the bonding conductor may carry some of the current in the event of a fault?

For PME conditions the bonding conductors May have to carry diverted neutral currents due to a open PEN conductor so are sized with the PEN conductor csa in mind , potentially taking some of the load of the installation, whatever is running at the time.
They do not carry fault current.
You actually should consult the dno on the required csa of the bonding conductors for PME and not solely rely upon table 54.8 but I’ll confess, if I’m the designer I just use the table.

 

[Pretty Mouth]

For PME conditions the bonding conductors May have to carry diverted neutral currents due to a open PEN conductor so are sized with the PEN conductor csa in mind , potentially taking some of the load of the installation, whatever is running at the time.
They do not carry fault current.
You actually should consult the dno on the required csa of the bonding conductors for PME and not solely rely upon table 54.8 but I’ll confess, if I’m the designer I just use the table.

Ian, thanks for the reply, and interesting to learn about diverted neutral currents due to an open PEN - certainly explains why a large bonding conductor is necessary in TN-C-S, and something I hadn't considered.

With regards to a bonding conductor carrying fault current - do you not think this is possible in some situations?

I'm thinking of perhaps 2 neighbouring terraced houses, TN earthing arrangement, sharing a lead water pipe which is bonded to the MET in each property. A short to earth in either house would have 2 low impedance paths back to the supply transformer - obviously via the DNO's earthing arrangement in the 'fault' house, but also via the bonded water pipe, neighbour's bonding conductor, and neighbour's earthing arrangement.

In such a case would you not expect a fair chunk of the fault current to make it's way home through the bonding conductor?

 

[davesparks]

In such a case would you not expect a fair chunk of the fault current to make it's way home through the bonding conductor?

You might expect a bit less than half of it to flow via the bonding, which is at least half the size of the main earthing conductor.

All these half's can't be coincidence.

 

[Pretty Mouth]

You might expect a bit less than half of it to flow via the bonding, which is at least half the size of the main earthing conductor.

All these half's can't be coincidence.

Thanks Dave. I guess it must have something to do with carrying fault current. I doubt I'll ever get right to the bottom of this one, and I don't suppose it matters really, but I like to know why things are so.

 

[davesparks]

Thanks Dave. I guess it must have something to do with carrying fault current. I doubt I'll ever get right to the bottom of this one, and I don't suppose it matters really, but I like to know why things are so.

I don't know the exact answer, I have always taken it as being half the earthing conductor as it may have to carry a little under half the fault current in worst case scenario.
However there will be more to it than that.
If I get chance I'll go over the books and think about it some more.

 

[Ian1981]

The half theory doesn’t always work when you think that a TT arrangement can have a 2.5 mm earthing conductor yet the bonding must be 6mm minimum.
The csa will be down to robustness.

Yet I appreciate that the regulation does state no less than half the required csa of the earthing conductor , no less than 6mm etc and being TT, the earthing conductor never usually needs to be a large csa.

 

[davesparks]

The half theory doesn’t always work when you think that a TT arrangement can have a 2.5 mm earthing conductor yet the bonding must be 6mm minimum.
The csa will be down to robustness.

Yet I appreciate that the regulation does state no less than half the required csa of the earthing conductor , no less than 6mm etc and being TT, the earthing conductor never usually needs to be a large csa.

It can be 2.5 in theory, but in practice I wouldn't normally go below 6mm for the same reason, robustness.

 

[Ian1981]

It can be 2.5 in theory, but in practice I wouldn't normally go below 6mm for the same reason, robustness.

Agree Dave, using 2.5mm just doesn’t seem adequate enough

 

[Pretty Mouth]

The csa will be down to robustness.

You might be right, but I suspect not. AFAIK the regs always specify minimum 4mm² for a protective conductor when not mechanically protected (or part of a cable), including supplementary bonding. It seems unlikely to me that a main bonding conductor would be treated differently in that respect.

543.1.1, 544.2, 705.544.2 iii

 

[Ian1981]

You might be right, but I suspect not. AFAIK the regs always specify minimum 4mm² for a protective conductor when not mechanically protected (or part of a cable), including supplementary bonding. It seems unlikely to me that a main bonding conductor would be treated differently in that respect.

543.1.1, 544.2, 705.544.2 iii

4 mm for a supplementary bonding conductor not mechanically protected, however a supplementary bonding conductor is not the same as a protective bonding conductor connected extraneous conductive parts to the MET

You have detailed the correct regulations for protective conductors but not main protective bonding conductors
544.1.1 is the regulation that must be complied with and states no less than 6mm for non pme conditions and table 54.8 for pme. If not for robustness then what?

Not sure why section 705 was mentioned as this supplements the requirements of parts 1-6

 

[Pretty Mouth]

4 mm for a supplementary bonding conductor not mechanically protected, however a supplementary bonding conductor is not the same as a protective bonding conductor connected extraneous conductive parts to the MET

It's not, but there is no reason I can think of that, generally speaking, main bonding conductors are any more or less susceptible to mechanical damage than supplementary bonding conductors, or any other protective conductor.

 

[Ian1981]

Protective conductors are specifically sized to deal with fault current only,But the main protective bonding conductors do not require mechanical protection At any size (excluding external influences that may warrant such protection) but protective conductors not integrated into a cable do,or be sized to 4 mm for added mechanical robustness if not mechanically protected.
It has to be for robustness, I can’t think of another reason other than in case fault current does flow through them as well.

I open to any other reasons as it’s an interesting discussion and my knowledge is limited to any other reasons.

 

[Pretty Mouth]

But the main protective bonding conductors do not require mechanical protection At any size (excluding external influences that may warrant such protection) but protective conductors not integrated into a cable do,or be sized to 4 mm for added mechanical robustness if not mechanically protected

I'm not sure I follow...

Protective conductors generally can be a minimum of 4mm² where not mechanically protected. As you say, this minimum is considered robust enough to survive normal wear and tear.

So why does the same not apply to bonding conductors? They are made of the same material, and are no more likely to receive knocks and other abuse from the installation users. Why would a 4mm² CSA not be robust enough for a bonding conductor, when it is fine for any other protective conductor?

There has to be another reason for the larger minimum CSA required for bonding conductors other than durability.

 

[Ian1981]

I'm not sure I follow...

Protective conductors generally can be a minimum of 4mm² where not mechanically protected. As you say, this minimum is considered robust enough to survive normal wear and tear.

So why does the same not apply to bonding conductors? They are made of the same material, and are no more likely to receive knocks and other abuse from the installation users. Why would a 4mm² CSA not be robust enough for a bonding conductor, when it is fine for any other protective conductor?

There has to be another reason for the larger minimum CSA required for bonding conductors other than durability.

Then Only the people who write chapter 54 could possibly give the answer?