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.