Spot the Faults.

[Paul.M]

Out of interest, why would you want to do this?

Good question. Answer is... Due to the amount of bodging in the past and the amount of money that the customer is likely to spend fixing it, we need to know the options to resolve this matter.

 

[happysteve]

Good reason. I like it I have been in a similar position. When I am feeling brave enough, I will tell all.

 

[davesparks]

542.1.3.3 Where a number of installations have separate earthing arrangements, any protective conductors common to any of these installations shall either be capable of carrying the maximum fault current likely to flow through them or be earthed within one installation only and insulated from the earthing arrangements of any other installation. In the latter circumstances, if the protective conductor forms part of a cable, the protective conductor shall be earthed only in the installation containing the associated protective device.

So a protective conductor in a cable must be connected to the earthing system of the installation containing the protective device for the circuit. So both the armour and core in Paul's example must only be connected to the PME earth.

 

[telectrix]

542.1.3.3 Where a number of installations have separate earthing arrangements, any protective conductors common to any of these installations shall either be capable of carrying the maximum fault current likely to flow through them or be earthed within one installation only and insulated from the earthing arrangements of any other installation. In the latter circumstances, if the protective conductor forms part of a cable, the protective conductor shall be earthed only in the installation containing the associated protective device.

So a protective conductor in a cable must be connected to the earthing system of the installation containing the protective device for the circuit. So both the armour and core in Paul's example must only be connected to the PME earth.

i see your point, dave, but this is a single installation. the SWA must be earthed to the supply PME as fitting RCD protection to the SWA is not an option ( if the armour was TT). however, each sub main must be TT as it's caravans. the idea of looping all the TT rods in parallel, using the 3rd core of the SWA which is otherwise redundant, is to bring the TT Ra down. just looking for any reason why it's not acceptable.

 

[davesparks]

i see your point, dave, but this is a single installation. the SWA must be earthed to the supply PME as fitting RCD protection to the SWA is not an option ( if the armour was TT). however, each sub main must be TT as it's caravans. the idea of looping all the TT rods in parallel, using the 3rd core of the SWA which is otherwise redundant, is to bring the TT Ra down. just looking for any reason why it's not acceptable.

Electrical installation: an assembly of associated electrical equipment having coordinated characteristics to fulfil a specific purpose.

By setting up a separate earthing system you are creating a second installation which is seperate to the PME fed installation.

Can you not get a good enough Ra to make the whole lot TT?

GN7 says that the separation of the earthing system should preferably be effected at the main distribution board (page 53 plus the picture over the page) this is presumably so that the submain cpcs can be used to connect electrodes at each pitch together.

 

[telectrix]

problem is if the SWA is TT, then upfront RCDs willl be required. as each sub has it's own RCD, that would mean series RCDs. also a fault in 1 caravan could take out the upfront RCD thus killing the other 4 subs.

 

[davesparks]

problem is if the SWA is TT, then upfront RCDs willl be required. as each sub has it's own RCD, that would mean series RCDs. also a fault in 1 caravan could take out the upfront RCD thus killing the other 4 subs.

Not will be required, it only may be required. Decent electrodes at all 4 pitches and at the main DB may very well get you a good enough Ra to not need the RCD