TT system consumer units

[soulman]

Hi,

I do a lot of work on domestic TT systems. Now that AMD 3 is in force, what are you guys using RCBO's or a 100ma S type main switch for TT earthing systems.

Cheers

 

[Leesparkykent]

If your relying on an rcd for earth fault protection I think its prudent to install a 100mA upfront as a bit of a fail safe. Obviously if you install an rcbo board with an upfront 100mA S type rcd then the rcbo's will either have to be double pole or single pole switched neutral.

 

[Nightmare1308]

hi I am interested in this subject due to people always have different views on this matter.
If you use a 100mA wouldn't the 30mA Rcd/RCBO trip it on x5?, or Would the time delay allow for this?
Also are you installing the RCD main switch from the white meter box (If applicable) to protect the tails before they enter a metal enclosure (Line+Neutral same hole, as this now been adapted?).
Is there any problem with a 300mA?
These are questions aswell not attacks.

Thanks

 

[Richard Burns]

A 100mA time delay RCD will not trip in the time it takes a 30mA RCD to trip because of the time delay, once the 30mA RCD has tripped the test current (or fault current) is no longer present so the time delay device will not trip at all.
However if there is a neutral earth fault causing an RCD to trip and that device is not double pole then the neutral earth fault will still be connected and so will be detected by the time delayed device which will also trip.

The 100mA Type S RCD is installed to protect the tails from an earth fault to the metal enclosure that would make the metal enclosure live, since the fault current to earth will be in tens of amps it will not blow the main fuse and would leave the enclosure live.

The problem is then that the enclosure for the 100mA RCD also needs to be metal and there is still no protection for an earth fault on the meter tails to this enclosure. This is where they now advocate ensuring that the meter tails cannot touch a metal enclosure, by using suitable tails restraint and glanding into the enclosure with a non conductive gland.

300mA would also work in the same way assuming (hoping) that you have an Ra <167Ω, however the RCD would be less available, more expensive and provide potentially less likelihood of tripping and higher fault current so why not use a 100mA.

 

[Deleted member 9648]

A 100mA time delay RCD will not trip in the time it takes a 30mA RCD to trip because of the time delay, once the 30mA RCD has tripped the test current (or fault current) is no longer present so the time delay device will not trip at all.
However if there is a neutral earth fault causing an RCD to trip and that device is not double pole then the neutral earth fault will still be connected and so will be detected by the time delayed device which will also trip.

The 100mA Type S RCD is installed to protect the tails from an earth fault to the metal enclosure that would make the metal enclosure live, since the fault current to earth will be in tens of amps it will not blow the main fuse and would leave the enclosure live.

The problem is then that the enclosure for the 100mA RCD also needs to be metal and there is still no protection for an earth fault on the meter tails to this enclosure. This is where they now advocate ensuring that the meter tails cannot touch a metal enclosure, by using suitable tails restraint and glanding into the enclosure with a non conductive gland.

300mA would also work in the same way assuming (hoping) that you have an Ra <167Ω, however the RCD would be less available, more expensive and provide potentially less likelihood of tripping and higher fault current so why not use a 100mA.

It's worth pointing out that it's not only the metal enclosure that will be live,but all conductive parts of the installation as well.

 

[Nightmare1308]

A 100mA time delay RCD will not trip in the time it takes a 30mA RCD to trip because of the time delay, once the 30mA RCD has tripped the test current (or fault current) is no longer present so the time delay device will not trip at all.
However if there is a neutral earth fault causing an RCD to trip and that device is not double pole then the neutral earth fault will still be connected and so will be detected by the time delayed device which will also trip.

The 100mA Type S RCD is installed to protect the tails from an earth fault to the metal enclosure that would make the metal enclosure live, since the fault current to earth will be in tens of amps it will not blow the main fuse and would leave the enclosure live.

The problem is then that the enclosure for the 100mA RCD also needs to be metal and there is still no protection for an earth fault on the meter tails to this enclosure. This is where they now advocate ensuring that the meter tails cannot touch a metal enclosure, by using suitable tails restraint and glanding into the enclosure with a non conductive gland.

300mA would also work in the same way assuming (hoping) that you have an Ra <167Ω, however the RCD would be less available, more expensive and provide potentially less likelihood of tripping and higher fault current so why not use a 100mA.

I see your point probably right with the 300mA.
We stock 300's because of a contract manager has specified it for all green field sites.
We use a gland which has 3 holes for each cable to enter then tighten onto, also use double insulated tails to prevent it as much as possible.
See a bit on NICEIC write up about it but they only spoke about a gland and double inulated tails.
Bit off topic but do you have any information or a book I can get about earthing systems Including LP (Lightening protection) as I see many site with it involved into the TT and TN-C-S systems and then times when they are not at all.?
thanks

 

[Midwest]

We use a gland which has 3 holes for each cable to enter then tighten onto, also use double insulated tails to prevent it as much as possible.
See a bit on NICEIC write up about it but they only spoke about a gland and double inulated tails.

I think most of the schemes/IET recommend the use of bespoke glands, of which you mention. If not just to maintain the particular IP rating on entering CU or enclosure, but also 'double insulate' or 'double prevent' the tails from shorting on entering the said CU (TT supply).

It would appear the apparent installation issues with reg 421.1.201, weren't given full consideration, at its inception and the 'grown-ups' have started to clarify their recommendations.