TT systems and CCU

[BristolSpark]

I have been reading the regs but getting a little stuck. If you have a TT system and are gonna change the CCU, what is the deal regarding RCD's.

An Example there is a cut out on job wired through the meter then through an old RCD with live neutral and earth connections, then the supply goes to the CCU.

If I was to fit a new board with 100a main switch and a 80a rcd for all outgoing ways, where does this stand?

Also in same situation if a 100a main switch with Dual RCD at say 2 x 80a 30ma was installed in stead where does this stand?

What is the deal with front end RCD or S type RCD's do you have to install a 100ma RCD at the start then your CCU setup.

Some say treat a TT like A TNC-S just install a dual rcd board 30ma with earth rod in place job done. So just looking for clarity on this subject.

cheers

 

[Guest123]

Some say treat a TT like A TNC-S just install a dual rcd board 30ma with earth rod in place job done. So just looking for clarity on this subject.

cheers

The some are correct.

Only time you would need a 100mA S type up front would be if the CU was a metalclad one, to protect the supply tails.

 

[BristolSpark]

Cheers Lenny

So please correct me again.

The norm would be 100a main switch plus 2 x Rcd 30ma this should cover most domestic senarios and cover the regs all round regardless with TT TNs or TNcs

and if metal clad CCU then the s types comes into play.

so what happens if you got a 100a main switch and only one rcd in the board.

is it a recommendation to change this or can this be installed and comply

 

[Guest123]

So please correct me again.

The norm would be 100a main switch plus 2 x Rcd 30ma this should cover most domestic senarios and cover the regs all round regardless with TT TNs or TNcs and if metal clad CCU then the s types comes into play.

Correct.


so what happens if you got a 100a main switch and only one rcd in the board

Wouldn't comply with BS 7671 having the one RCD covering all circuits (minimise inconvenience in the event of fault....or do you mean split load type with 100A main switch with circuits on RCBO's and the other RCD protected side with MCB's???

 

[BristolSpark]

No I did mean one RCD and then the out going circuits with MCB's

So my first point is clarified. cool

its just that in the OSG it talked about s type. so I feel much more clarified now. cheers Lenny

Regarding earth rods, if you are in a property and there is a pavement outside the front door and you cant find the original earth rod but yet you can see the cable in the property. would you just bang another rod in the pavement outside or run the new earth towards back garden spike there then run cable back into house. and do away with the old cable.

 

[Guest123]

The rod termination would need to be accessible for inspection/testing so I would just install a new one, wherever is most convenient and least likely to cause a hazard.

 

[jimsky007]

Hi

The earth rod you put into the pavement has to have a Ze max 200ohms and it can be really difficult to get this value in some areas

So i would do a Ze on your earth cable at origin and if it is 200 or less it will be fine?

even though you should have access to the spike and the connection where your main earth cable is attached to it
In the past it has been know for this cable to have been attched to water supply pipes and you know what happens when new plastic water pipes are fitted

 

[wearethepeople]

I have just tested a TT system with a earth rod in pavement outside front door and is was 796 ohms!!!

Is it my responsibility or suppliers to sort!?

Can I route mai earth cable to rear of property approx 20m away for better earth reading?

---

 

[Spudnik]

As the suppliers would not have installed the rod, they have no responsibility to maintain it.

Down to you im afraid.

Generally it is good practice to install the rod as close to the origin of the installation as possible.


EDIT: Sorry, should have been 'rod' not 'road'!

 

[wearethepeople]

So if around the property I can't decrease the value is it still down to me!?

 

[Des 56]

So if around the property I can't decrease the value is it still down to me!?

Afraid so,if you alter or install it must be sorted


You can find another location or deeper rods or multiple rods or grids/tapes or soil treatment (piddle over it )
An idea that may save time and effort is to trail a lead around the house and bang in a temporary piece of rod or a screwdriver even, to get the best position before re routing
You may even find the existing is best


You could also check supplier for availability of Pme,it could become less bother than sorting the TT

 

[wearethepeople]

PME NOT AVAILABLE grrrrrrrrr.

Many Thanks anyway

 

[Spudnik]

You may find you will need to use more than 1 rod.

Not uncommon.

 

[Deleted member 9648]

The maximum permissable resistance on an earth electrode with 30ma rcd protection is 1667ohms.....the 200 ohms value quoted is only advice that anything over that value may be unstable...... anything under 1667 ohms will comply with the regulations.I'd only be worried if I had a very high resistance when the ground was already very wet and likely to dry out further.

 

[glasgowspark]

The maximum permissable resistance on an earth electrode with 30ma rcd protection is 1667ohms.....the 200 ohms value quoted is only advice that anything over that value may be unstable...... anything under 1667 ohms will comply with the regulations.I'd only be worried if I had a very high resistance when the ground was already very wet and likely to dry out further.

hear, hear. I was about to add the same comment until you beat me to it.