MPB back to the MET or CU?

[studentspark]

If you have a TNCS supply coming into the property

In the down stairs meter cupboard you have a switch fuse protecting a 16mm2 T&E cable feeding a CU in an flat in the building.

I was just doing some Adiabatic sums on the 6mm2 CPC in the T&E ( Theoretical only, this is not a real example) to see if was a suitable size.

But then I started thinking about the MPB.

If the bonding in the flat is 10mm2, but that bonding has no connection back to the MET at the incoming position, apart from the 6mm2 cable, does that mean the sizing of CPC in the T&E is to small. Would I need at least a 10mm2 CPC back to the MET

WE have an equipotential zone in the flat, But does that need to come form the MET, and not the earth bar in the CU

Its OK an as far as the adiabatic, but would it fail on being to small for the MPB.

Thanks..

TNCS

• studentspark
• 4 Apr 2021

 

[EFLImpudence]

There should be satisfactory MPB from where the supplies enter the building to the building MET plus a MEC.

The supply to the flat is then just a circuit like any other - sized for the OPD.

Any bonding in the flat is not really a MPB but Supplementary Bonding yet should be 10mm² - just in case as you may not be able to check the actual MPB conductor.

 

[AndyPRK]

What’s mpb ?

As pipes come in downstairs,
Wouldn’t you bond them downstairs ?

 

[EFLImpudence]

What’s mpb ?

Main Protective Bonding.

As pipes come in downstairs,
Wouldn’t you bond them downstairs ?

Yes, that's what I said.

Not always able to inspect in flats, though, so it is done in the flat as well - just in case.

 

[333rocky333]

What’s mpb ?

Reg 544
Main Protective bonding conductors, I quess, though MPB is not a recognised Definition in the regs

 

[studentspark]

As pipes come in downstairs,
Wouldn’t you bond them downstairs ?

This is not a real situation, but based on lots of flats I've seen.
This is in London where you have many flats in one house.

Pipe work is often not accessible in hallways. Often you can see the gas pipework external to the building and entering at the flat.

 

[EFLImpudence]

I also put MEC for Main Earthing Conductor. This is not actually correct as it is THE Earthing Conductor.

Pipe work is often not accessible in hallways.

I covered that.

Often you can see the gas pipework external to the building and entering at the flat.

Yes, but the main supply should be bonded - if required.

 

[JohnW2]

... I was just doing some Adiabatic sums on the 6mm2 CPC in the T&E ( Theoretical only, this is not a real example) to see if was a suitable size. .... Its OK an as far as the adiabatic, but...

As a matter of interest, what value of I²t did you use for your adiabatic calculations?

WE have an equipotential zone in the flat, But does that need to come form the MET, and not the earth bar in the CU

An equipotential zone is an equipotential zone, regardless of what it is (or is not) connected to outside of that zone - which I guess means that the short answer to your question is "no".

Kind Regards, John

 

[studentspark]

OK thanks for the replies.

There should be satisfactory MPB from where the supplies enter the building to the building MET plus a MEC.

Sorry if I am not using correct terms or being a bit to abbreviatory if such a word exists.

So the gas pipe enters the flat at first floor level. Connected to that pipe via a BS 591 clamp is the 10mm2 main protective bonding conductor for the gas.

That MPB conductor does not go to the Main Earth Terminal downstairs, but to the earth marshalling point in the flats consumer unit.

The same situation exists with the water.


The Earthing conductor from the service head TNCS supply is connected to the Main Earthing Terminal.

From that MET is a circuit protective conductor to the switch fuse that feeds the upstairs flat..

That CPC is connected to the 6 MM CPC in the Twin and Earth cable that supplies power to the flat.

So the protective conductor for the flats connection to the main earthing terminal is only 6mm2.

As we have no MPB conductors at the Main earth terminal. Only a 6 mm2 connection from the flat.

That 6mm2 connection would not be adequate as far as MPB is concerned

 

[studentspark]

As a matter of interest, what value of I²t did you use for your adiabatic calculations?

I used If = Uo/Zs

 

[flameport]

Main bonding connects to the MET.
Where the MET is depends on how the building is configured and where extraneous conductive parts are located.

Could be one in each individual flat (very common) or one for the building as a whole (less likely)

If the ECPs such as gas/water enter each flat from outside and aren't accessible in the shared/common areas, there won't be anything to bond in the shared/common areas.

In any case, T&E is a poor choice for a submain for several reasons.

 

[studentspark]

Could be one in each individual flat (very common) or one for the building as a whole (less likely)

I thought there was only one MET in an installation, and everything off that was a Earth marshalling point. (Im talking about a house with individual flats, that share the same supply)

Its very common to see an Earthing terminal next to the intake. And from that all the individual Consumer units or Switch fuses for the various flats get there connection to the earthing system.

If the ECPs such as gas/water enter each flat from outside and aren't accessible in the shared/common areas, there won't be anything to bond in the shared/common areas.

Indeed, so the connections have to be in the individual flats. But If an installation requires 10mm2 MPB because its a TNCS. Does the fact that the connection to the means of earthing is only a 6mm2 connection. The equipotential zone in the flat is 10mm, but its connection to the means of earthing is only 6mm2

I see it like a lollypop 6mms stick and 10mm2 lolly.

Im probably making no sense

 

[flameport]

I thought there was only one MET in an installation

There is, but a single building can contain multiple installations.

A 6mm² earth conductor may comply with that theoretical calculation, but it won't comply with most DNOs requirements or where that same conductor is used as a bonding conductor, which it very often will be.

In reality it will need to be the same size as the neutral, or minimum of 16mm², which is why T&E for a submain is useless.
Unless that T&E is surface clipped or in conduit, it will need a 30mA RCD as well, which is also impossible.

 

[JohnW2]

I used If = Uo/Zs

That's the ('prospective') fault current.

I was asking about the value of " I²t " which you used for your adiabatic calculation - the value of that will depend upon the characteristics of the OPD as well as the fault current.

Perhaps it would be simplest if you showed us the 'working' of your adiabatic calculation?

Kind Regards, John

 

[studentspark]

There is, but a single building can contain multiple installations.

A 6mm² earth conductor may comply with that theoretical calculation, but it won't comply with most DNOs requirements or where that same conductor is used as a bonding conductor, which it very often will be.

That is the essence of my question. Is the 6mm connection also being used as a bonding connection.


And sorry to keep asking questions but leading on from that..
Where you find 16mm2 T&E as the supply from a switch fuse to a CU. On a EICR, what code would that be?


Many thanks