TT systems

[Dambo]

With TT systems having such a high Zs (in comparison to TNS and TNCS) would all the circuits need to be RCD protected?

 

[lilman]

Yes

 

[a1]

Yes

 

[M107]

Yes & in days gone by (as in pre 17th) thats where your 100mA S type rcd would have been used as the main switch/isolator, then a 30mA for the sockets.

 

[a1]

100 mA to protect the Whole Installation and 30 mA to protect individuals

 

[M107]

100 mA to protect the Whole Installation and 30 mA to protect individuals

You'd normaly find the lighting & submains on the 100mA S type & everything else on the 30mA

 

[Dambo]

You'd normaly find the lighting & submains on the 100mA S type & everything else on the 30mA

Why would the lighting and submains not be on the 30mA RCD?

 

[Lurch]

Why would the lighting and submains not be on the 30mA RCD?

Under the 16th edition there was no requirement for practically everything to be on the RCD so on a TN-C-S there would be no RCD for lights etc.... The RCD would only be for the sockets usually. On a TT system, as you say you need an RCD to meet disconnection times so a time delayed RCD would have been used in place of the main switch. Still meets the required standard but provides the minimal amount of protection to limit nuisance tripping.

 

[spinlondon]

Why would the lighting and submains not be on the 30mA RCD?

Sub mains and lighting circuits only require RCD protection if the cables are concealed in walls.

 

[Dambo]

On a slightly different subject (but still related) - When feeding a consumer unit in, for example, a garage, do the L and N feeding it come from the main switch in the main consumer unit or from an RCD? And would the E come from the MET?

 

[spinlondon]

On a slightly different subject (but still related) - When feeding a consumer unit in, for example, a garage, do the L and N feeding it come from the main switch in the main consumer unit or from an RCD? And would the E come from the MET?

I wouldn't do it off of the main switch.

Either come off a way in the CU, using the CU earth terminal, or break into the tails and use the MET.

 

[Lurch]

On a slightly different subject (but still related) - When feeding a consumer unit in, for example, a garage, do the L and N feeding it come from the main switch in the main consumer unit or from an RCD? And would the E come from the MET?

You would wire the supply to the garage CU as a final circuit from the main CU in most cases.

 

[Dambo]

I wouldn't do it off of the main switch.Either come off a way in the CU, using the CU earth terminal, or break into the tails and use the MET.

When you say 'come off a way' do you mean come off a separate MCB?

And what do you mean when you say 'break into the tails'?

 

[M107]

When you say 'come off a way' do you mean come off a separate MCB?yes, making the submain a circuit of the consumer unit

And what do you mean when you say 'break into the tails'?

Using henley (service connector) blocks you take tails from meter into henley block, then new tails from henley block to house cu & a second set of tails from henley block into switch fuse that in turn runs the submain to garage. Ofcourse the suppliers earth is also connected to as a cpc for the submain but I always TT out buildings

I've used somebody elses Idea & replied in red

 

[m4tty]

When you say 'come off a way' do you mean come off a separate MCB?And what do you mean when you say 'break into the tails'?

Yes, new MCB

Think he means put the existing tails into a henly block then two sets of tails coming out, one pair for original cu and other pair for additional cu.

 

[m4tty]

Replied same time and Mr :107 but his answer was better :|

 

[spinlondon]

When you say 'come off a way' do you mean come off a separate MCB?Yes.

And what do you mean when you say 'break into the tails'?

Depending on the size of the conductors you are using for the garage, you may not be able to fit the conductors in the CU main switch.

It is often the practice to split the feed from the meter into 2 by the use of Henly blocks. One feed to the CU, and the other to the garage.

I would advise the installation of a main switch at this point so as to be able to isolate both feeds for emergency purposes and compliance with Regulation 537.1.4.

 

[Lurch]

I would advise the installation of a main switch at this point so as to be able to isolate both feeds for emergency purposes and compliance with Regulation 537.1.4.

Bit more than a main switch would be required depending on the size of the feed to the garage.

I think the vagueness is confusing the issues.

If the supply to the garage is 16mm or smaller then it would fit into most modernish consumer units without too much swearing at it. If for some reason the garage supply can;t be fed from the consumer unit as a final circuit it will often be fed from it's own consumer unit that is connected into the existing tails via a Henley block, which is just basically a large connector block.

You would not just connect the cable to the garage CU straight into the Henley block.

 

[spinlondon]

Bit more than a main switch would be required depending on the size of the feed to the garage.I think the vagueness is confusing the issues.

If the supply to the garage is 16mm or smaller then it would fit into most modernish consumer units without too much swearing at it. If for some reason the garage supply can;t be fed from the consumer unit as a final circuit it will often be fed from it's own consumer unit that is connected into the existing tails via a Henley block, which is just basically a large connector block.

You would not just connect the cable to the garage CU straight into the Henley block.

Perhaps the vagueness is confusing.

However there are many ways that the garage can be supplied.

If for instance the garage is integral to the building, there may not be a separate supply, socket-outlets could be part of a downstairs ring, and lighting could be part of a downstairs light circuit. The same could apply with an attached garage.

If the garage is detached, it may be that it is quite close, and a feed could be taken at height using single conductors in containment. I f the garage is too remote, then a supply may have to be taken underground perhaps using SWA.

I do not want to apply specifics, untill such time as I know what those specifics are.

There are other considerations when stuffing extra conductors into a main switch in a CU.

The CU will have been type tested, to meet the requirements of a specific British Standard. Stuffing extra conductors to feed something not contained within the CU would perhaps mean that the CU would no longer meet that British Standard.

There is also the fact that the extra load could perhaps cause the Main switch to be overloaded.

As stated above, I do not want to apply specifics, untill such time as I know what those specifics are.

Your last statement is as bad as Niceguy stating that you can not use an RCD as a main switch. Both sweeping and incorrect.

 

[Lurch]

So basically no-one should answer anything then.

Your last statement is as bad as Niceguy stating that you can not use an RCD as a main switch. Both sweeping and incorrect.

I was on my way back to amend that with some qualifying statements.