tt system

[Peter Woodford]

Guys,

been asked to do a cu change on a tt system i know it has to have a split load board but does it still need the main switch what i can remember it does'nt.

thanks

 

[Tag]

correct me if I' wrong, you can take the tails straight from the meter to the cu on any system can't you? Provided adequate protection?

 

[Mac]

Guys,

been asked to do a cu change on a tt system i know it has to have a split load board but does it still need the main switch what i can remember it does'nt.

thanks

Tails will need upgrading to 25mm.Main earth to stake i use 10mm and bonding to services,10mm as well.

 

[sonobit]

Tails will need upgrading to 25mm.Main earth to stake i use 10mm and bonding to services,10mm as well.

I wouldnt put a 10mm main earth to stake with 25mm tails!

 

[Mac]

I wouldnt put a 10mm main earth to stake with 25mm tails!

What would you use then on a TT earthing system to the rod

 

[spamwize2]

25mm sounds good to me, keep resistance down 10mm bonding is coming in now 16mm is next! they will be talking about static presence b4 you know it!

main switch is for disconnection keep it in

 

[Mac]

25mm sounds good to me, keep resistance down 10mm bonding is coming in now 16mm is next! they will be talking about static presence b4 you know it!

main switch is for disconnection keep it in

I was talking about 25mm tails and 10mm earth to the rod.What were you on about,mate.

 

[Des 56]

Guys,

been asked to do a cu change on a tt system i know it has to have a split load board but does it still need the main switch what i can remember it does'nt.

thanks

The main switch is required for all installations irrelevant of the earthing system
You may be refering to the time delay requirement that was often needed under the 16th edition

This requirement is obsolete because all circuits now require 30 m/ampRCD in a TT installation so it would be a waste of money puttiing in a time delay rcd
It doesn't have to have a split load board,you could use rcbos or do as I and use a split load with 30 m/amp rcd for sockets shower cooker and whatever else and rcbos for the lighti

Using large cables for the rod in a TT system is less of a worry than TN systems
The rcd will need only 30 m/amp to operate, so the earth cable is unlikely to carry a large fault current for any noticeable period unlike a TN systekm fault
4mm would probably be adequate for any TT installation, allthough the recomended will indicate larger sizes

 

[sonobit]

Is it me or do you need minimum main CPC not less than half CSA of incoming line conductor, but min of 16mm? table 54.7

 

[Pat Tester]

Ze with earth rod, the best I have managed is around 21 ohm, ohms law = 11.42A fault current
10mm is fine, you could use 4mm and still be within the regs, so I have been told.
I use 10mm

 

[Des 56]

Is it me or do you need minimum main CPC not less than half CSA of incoming line conductor, but min of 16mm? table 54.7

If you read 542.3 it tells you that the earthing conductor shall comply with section 543
Also if its PME then it shall also meet 544.1 so

Section 543 says it can be selected in accordance with 543.1.4 where it says it can be determined by table 54.7

The other option it gives is to use the calculation in 543.1.3
the adiabatic equation
For a system with a 30 m/amp rcd the size of cable to carry the very low required fault current is very small
It can be as low as 2.5mm if its protected or 4mm if its unprotected

If it is buried in the ground table 54.1 would apply where 16mm would be required for unprotected buried cable

Although these small conductor sizes can comply,I myself generally use 10mm for TT systems,but there are systems with smaller sizes being used out there and there is no problem people using them

So on PIRs do not be too hasty to condemn those installs

 

[sonobit]

thanks for the intel des but there was very little info on how guy was going to have a CPC to rod so i was basically being ballpark but safe, i was always told to have at leats half csa for your earth to line, regardless of of earthing system. but thanks anyway sorry Mac ive always just put in 16mm min.