TT system domestic CU upgrade

[nle]

im intending to carry out a c.unit upgrade on a domestic property, no earth provide by supplier so its a TT system,no electrode seems to be visible or present

but there is a reading there something like 5ohms could be a buried rod/rods somewhere or connected to old service pipe, going to install a new rod, but do i need a 100ma type s time delay main switch if im installing a dual 30ma rcd unit for discrimination? Obviously the Zs readings will be too high to achieve ADS with the mcbs so whats the crack with overcurrent protection? help....please...

 

[telectrix]

your 5 ohms could be from the water/gas bonding. you don't need a 100mA RCD as you are fitting dual 30mA RCDs. just a bog standard main switch. your |RCds will provide earth fault protection, max. ZS to be entered on the test results is 1667 ohms. but do try and fit rod/s to get the Ze as low as you can.

 

[nle]

but what about short circuit protection this wont come from the rcds or maybe the the thermal overload on the mcbs will still operate correctly?

 

[nle]

what about additional protection?

 

[IQ Electrical]

what about additional protection?

Additional Protection is provided by the 30mA RCDs

 

[Spudnik]

100mA is only required on the tails, unless you use a DI CU.

 

[Silly Sausage]

but what about short circuit protection this wont come from the rcds or maybe the the thermal overload on the mcbs will still operate correctly?

Remember your Zs values are only relevant for Line to Earth faults.
Short circuits (Line to Neutral) will generate quite sufficient currents for the breakers to operate within time, just like if it was a TNS or TNCS. The breakers would also behave as normal with overload currents (no earth involved).

Hope that's clear, I've got to be the world's worse teacher.

 

[Engineer54]

]im intending to carry out a c.unit upgrade on a domestic property, no earth provide by supplier so its a TT system,no electrode seems to be visible or present
[/I]
but there is a reading there something like 5ohms could be a buried rod/rods somewhere or connected to old service pipe, going to install a new rod, but do i need a 100ma type s time delay main switch if im installing a dual 30ma rcd unit for discrimination? Obviously the Zs readings will be too high to achieve ADS with the mcbs so whats the crack with overcurrent protection? help....please...

If after investigation, you find that there is no TT system electrode, then you MUST provide one. It is not permissible to rely on a bonded service to provide an installations means of earthing....


It may be a good idea to check with the local DNO to see if they are in a position to provide your customer with a PME connection, as conditions change all the time in previously designated TT areas.