Zs on tt system

[KNIPEX]

HI Guys, ze at the board on a tt system must not exceed 200 ohms i have 138 ohms lovely.

but when measuring zs at the sockets, lights, etc im getting 129 ohms 139 ohms etc, these readings are way over for max zs for 60898 breakers following table 41b of the regs.

is there another table for tt systems or are my reading just high???

or im i missing the point?? may sound simple question but hardly ever come across tt, and i believe its always best to ask if your unsure.

thanks.

 

[ron55]

Hi

The max ZE on a TT is 200 ohms but it can become unstable over 100 ohms so I treat this as the max. At 138 I would be putting in a new earth rod.

The whole system must be protected by an RCD - no exceptions.

Cheers, Ron

 

[Guest123]

Hey mate.

Max Zs permissable for circuits on a TT system is 1667ohms to achieve the safe touch voltage of 50V, should be under 200ohms though, so your values are ok but readings may be unstable.

Also all circuits must have RCD/RCBO protection

Table 41.5, P50 ol' red.

 

[KNIPEX]

so the zs reading dont have to comply with table 41b then for max zs on 60898 and 61009 mcbs then

 

[Guest123]

Not on a TT no.

If you look at P50, Reg - 411.5, this is all about TT systems.

 

[Spudnik]

so the zs reading dont have to comply with table 41b then for max zs on 60898 and 61009 mcbs then

No, but they should be no more than 1667.

However, as the others have said, Ze at the rod should be below 200 ohms, but this can be unstable and so should not really be above 100 ohms.

Possible the easiest thing to do is install another rod approx 1 rod length away from the other and loop the two together.

The above works in most cases.

 

[KNIPEX]

ok thanks so on the test sheet where it says max zs for the mcb everybox will be 1.667

 

[Guest123]

Not 1.667, but 1667 yes.

I still put the max Zs from T41.3 in the max permissable boxes though. Not sure why just always have.

 

[Spudnik]

ok thanks so on the test sheet where it says max zs for the mcb everybox will be 1.667

And in Max measured you put <1667.

Done.

 

[Des 56]

Hey mate.


Also all circuits must have RCD/RCBO protection

Table 41.5, P50 ol' red.

Using rcbos on a TT system without a front end s type rcd may in fact contravene the regs

I make this statement because many rcbos are single pole If the rcbos are double pole then no problem,but in a TT system the device for isolation must disconnect all live conductors and as single pole rcds do not disconnect both live and neutral they do not meet this requirement,so it is better when fitting rcbos in a TT system to make sure those rcbos are double pole

 

[Spudnik]

True Des, but the main switch is double pole so i would have thought this would be within regs?

 

[Guest123]

Using rcbos on a TT system without a front end s type rcd may in fact contravene the regs

I make this statement because many rcbos are single pole If the rcbos are double pole then no problem,but in a TT system the device for isolation must disconnect all live conductors and as single pole rcds do not disconnect both live and neutral they do not meet this requirement,so it is better when fitting rcbos in a TT system to make sure those rcbos are double pole

Good job I always fit one then.

 

[Des 56]

True Des, but the main switch is double pole so i would have thought this would be within regs?

Where I think the there may be a problem, a TT system which is derived from a TNC-S supply
In this case ,if there were a fault to earth and the rcbo disconnected only the live, then the neutral which is still connected woulld have a p d to true earth and become a possible danger

 

[slocm3105]

Using rcbos on a TT system without a front end s type rcd may in fact contravene the regs

I make this statement because many rcbos are single pole If the rcbos are double pole then no problem,but in a TT system the device for isolation must disconnect all live conductors and as single pole rcds do not disconnect both live and neutral they do not meet this requirement,so it is better when fitting rcbos in a TT system to make sure those rcbos are double pole

You are completely correct in what your saying Des. However if there is any means of double pole isolation i.e the main switch i'm sure that this will be suitable. However fitting a s type time delayed is favoured among many sparks I work with, including myself and this i term as "rolls royce service" .

Cheers




Matt