Calculate bonding sizes TT

[Bengaz]

Hi, searched but still niggling problem need help on...

Installation: -
4mm earth to earthing electrode (rod) Ra 42ohm
Zdb 33ohm
1361 60A main fuse
16mm tails
cant get acceptable result for using adiabatic as disconnection time too high (higher than 5 secs with that Zs reading)
no frontline RCD protecting tails.
What would the minimum bonding size be? (2mm therefore 4mm wiring?)

 

[pc1966]

You will not disconnect on the OCPD with practically all TT installations due to the significant Ra, you would be looking at having RCD here for fault protection, not just for additional protection against shocks from contact with L & Earth.

While you could attempt to compute the adiabatic limit from the RCD's disconnection time and the PFC to Earth, in reality you are looking at least at 4mm for this (as it is unlikely to be mechanically protected all the way, see 543.1.1).

But as you can see, this means you have no protection for a L-CPC fault before any RCD or RCBO, so you must have great care that cannot occur, for example not just sheathed tails (as normally used) but also a proper gland to clamp the cables as they enter the CU to reduce the risk of movement or strain on the tails causing them to come out of the incomer switch and/or cutting against the CU body.

Of course, you must have RCD protection on every circuit. Traditionally that was an up-front RCD (ideally something like 100mA delay) and then whatever instant 30mA RCD protection is needed. But a dual RCD board or all RCBO is also complaint. Just absolutely no non-RCD circuits as your Ra is too high for practically all common OCPD.

 

[Pretty Mouth]

It's easy to overthink these things sometimes.

What current would flow for a fault to earth?
What if that current flowed through the earthing conductor forever? Would the EC be able to handle it?

Now jump to 544.1.1 for the answer

 

[westward10]

You will not disconnect on the OCPD with practically all TT installations due to the significant Ra, you would be looking at having RCD here for fault protection, not just for additional protection against shocks from contact with L & Earth.

While you could attempt to compute the adiabatic limit from the RCD's disconnection time and the PFC to Earth, in reality you are looking at least at 4mm for this (as it is unlikely to be mechanically protected all the way, see 543.1.1).

But as you can see, this means you have no protection for a L-CPC fault before any RCD or RCBO, so you must have great care that cannot occur, for example not just sheathed tails (as normally used) but also a proper gland to clamp the cables as they enter the CU to reduce the risk of movement or strain on the tails causing them to come out of the incomer switch and/or cutting against the CU body.

Of course, you must have RCD protection on every circuit. Traditionally that was an up-front RCD (ideally something like 100mA delay) and then whatever instant 30mA RCD protection is needed. But a dual RCD board or all RCBO is also complaint. Just absolutely no non-RCD circuits as your Ra is too high for practically all common OCPD.

There is no absolute requirement in BS7671 that every circuit must have rcd protection with a TT earthing system.

 

[pc1966]

There is no absolute requirement in BS7671 that every circuit must have rcd protection with a TT earthing system.

True but pedantic.

Very few MCB will disconnect on that sort of Zs, even 3A B-curve (14.6 ohm) or 3A fuse (15.6 ohm), hence it becomes necessary.

 

[mainline]

You will not disconnect on the OCPD with practically all TT installations due to the significant Ra, you would be looking at having RCD here for fault protection, not just for additional protection against shocks from contact with L & Earth.

While you could attempt to compute the adiabatic limit from the RCD's disconnection time and the PFC to Earth, in reality you are looking at least at 4mm for this (as it is unlikely to be mechanically protected all the way, see 543.1.1).

But as you can see, this means you have no protection for a L-CPC fault before any RCD or RCBO, so you must have great care that cannot occur, for example not just sheathed tails (as normally used) but also a proper gland to clamp the cables as they enter the CU to reduce the risk of movement or strain on the tails causing them to come out of the incomer switch and/or cutting against the CU body.

Of course, you must have RCD protection on every circuit. Traditionally that was an up-front RCD (ideally something like 100mA delay) and then whatever instant 30mA RCD protection is needed. But a dual RCD board or all RCBO is also complaint. Just absolutely no non-RCD circuits as your Ra is too high for practically all common OCPD.

I don't think a dual rcd board is recommended on a TT Pete due to the internal tails.

 

[westward10]

True but pedantic.

Very few MCB will disconnect on that sort of Zs, even 3A B-curve (14.6 ohm) or 3A fuse (15.6 ohm), hence it becomes necessary.

In relation to the OPs readings true but nevertheless it is correct. A 5A BS3036 fuse can be pushed to 9.10 ohm and in the days of mixed disconnection times 16.8 ohm.

 

[westward10]

Are you an electrician because a 2mm conductor is not a common size.

 

[mainline]

Are you an electrician because a 2mm conductor is not a common size.

I think op is talking about his adiabatic calc coming back as 2mm

 

[westward10]

I think op is talking about his adiabatic calc coming back as 2mm

You don't use adiabatic to calculate bonding.

 

[mainline]

You don't use adiabatic to calculate bonding.

Protective conductor then.

 

[davesparks]

What would the minimum bonding size be? (2mm therefore 4mm wiring?)

As per 544.1.1 the minimum size is 6mm

 

[Bengaz]

Protective conductor then.

exactly then halved = obv not 2mm as I stated.
Guess I'm trying to understand how we can calculate the main earthing conductor size then half that for bonding.
Was trying to understand the way it could be calculated, I know the regs say 6mm FYI.
Maybe I worded things not too great...

 

[mainline]

As per 544.1.1 the minimum size is 6mm

That's for bonding conductors, o/p is asking about main earth to a rod.

542.3.1

 

[davesparks]

That's for bonding conductors, o/p is asking about main earth to a rod.

542.3.1

No they aren't, the thread title is about bonding size and the only question in their post is:

What would the minimum bonding size be? (2mm therefore 4mm wiring?)

 

[davesparks]

exactly then halved = obv not 2mm as I stated.
Guess I'm trying to understand how we can calculate the main earthing conductor size then half that for bonding.
Was trying to understand the way it could be calculated, I know the regs say 6mm FYI.
Maybe I worded things not too great...

Earthing conductor size can be calculated using the adiabatic equation.

Main bonding normally can be half that size, but not less than 6mm and not normally more than 25mm.

So if your earthing conductor needs to be 25mm you need a bonding conductor of 12.5mm minimum so would install 16mm as it's the next nearest size above that.

If your earthing conductor needs to be 6mm then your bonding conductor would also need to be 6mm due to the minimum size requirement.

 

[mainline]

No they aren't, the thread title is about bonding size and the only question in their post is:

Right at the start of his post he said "4mm earth to earthing electrode (rod) Ra 42ohm"

4mm earth to earthing electrode (rod) Ra 42ohm
Zdb 33ohm
1361 60A main fuse
16mm tails
cant get acceptable result for using adiabatic, as disconnection time too high (higher than 5 secs with that Zs reading)
no frontline RCD protecting tails.

I thought he got confused with Bonding and \Main earth, otherwise he wouldn't be quoting adiabatic calculations.

 

[Bengaz]

No, i ai'nt got confused, what I am asking is because you cant use the adiabatic equation for the main earthing conductor then what formula result makes the bonding size acceptable. Instead of just using the regs as 6mm minimum how can we measure the acceptability of the bonding size? ie if the ze was less than 1 then the 4mm main earth wouldnt be large enough then that would increase the bonding size respectively. Guess maybe the 6mm bond would always suffice in a TT system with the expected fault currents?

 

[mainline]

No, i ai'nt got confused, what I am asking is because you cant use the adiabatic equation for the main earthing conductor

Why ?

then what formula result makes the bonding size acceptable. Instead of just using the regs as 6mm minimum how can we measure the acceptability of the bonding size? ie if the ze was less than 1 then the 4mm main earth wouldnt be large enough then that would increase the bonding size respectively.

Half the size of the Main earth

Guess maybe the 6mm bond would always suffice in a TT system with the expected fault currents?

Yes

The bonding minimum is 6 mm

The main earth can be less.

 

[davesparks]

No, i ai'nt got confused, what I am asking is because you cant use the adiabatic equation for the main earthing conductor then what formula result makes the bonding size acceptable.

You can use the adiabatic equation, but the result will usually be less than the minimum permitted size for a protective conductor. So you will end up using the minimum permitted size.

Again with the bonding, you will often end up using the minimum size.