TT systems

[PBC_1966]

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.

Or depending upon how far pre-17th you want to go back, you could find a single 100mA (non-delayed) RCD as the main feeding everything, which is what I have in my installation. Earlier still a 300mA or 500mA device was standard, or a voltage-operated ELCB.

 

[M107]

Or depending upon how far pre-17th you want to go back, you could find a single 100mA (non-delayed) RCD as the main feeding everything, which is what I have in my installation. Earlier still a 300mA or 500mA device was standard, or a voltage-operated ELCB.

Do you know what............. while I was typing that reply it was running through my mind about the amount of old wylex non TD 100mA rcd's you still come across on the tails in my neck of the woods, not too many of the old bakerlite VOELCB's now though

 

[PBC_1966]

Still a fair number of voltage ELCB's around this neck of the woods in my estimation - Mostly Crabtree E50.

 

[Dambo]

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

What do you mean when you say you always TT the out buildings?

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.

Regarding the Henley block way of feeding an out building, I just want to make sure that i've got the procedure right in my head:

The tails from the main CU would be taken out and fed into a henley block. Tails would then be taken back to the main CU and to the outbuilding CU.

But I can't see this being right as the L and N tails would then be fed into the same henley block causing a short circuit and even if both CU main switches were in the OFF position the henley block would remain live as the main tails are being fed directly into it. Please explain Pray headbang

I'm not about to carry out this procedure by the way, i'm in the process of training to become an electrician and just wanted to understand the procedure

 

[PBC_1966]

The tails from the main CU would be taken out and fed into a henley block. Tails would then be taken back to the main CU and to the outbuilding CU.

Correct.

But I can't see this being right as the L and N tails would then be fed into the same henley block causing a short circuit

L & N aren't connected together! You use a double-pole unit, like this:

http://www.tlc-direct.co.uk/Products/RDCB6.html

and even if both CU main switches were in the OFF position the henley block would remain live as the main tails are being fed directly into it.

Yes, the Henley block will be permanently live, unless you put another isolator in the tails from the meter ahead of it.

 

[Bolton spark]

You would be feeding 2 blocks 1 for live 1 for neautral not feeding all tails into same block ! Then the met for your earth.

 

[Risteard]

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

Strictly speaking a TT system does not HAVE to be RCD protected, but it would be nearly impossible to attain the disconnection times if it were not. Should the Zs be sufficiently low then an RCD would not be required.

 

[niceguy]

Strictly speaking a TT system does not HAVE to be RCD protected, but it would be nearly impossible to attain the disconnection times if it were not. Should the Zs be sufficiently low then an RCD would not be required.

Well i have done a lot of TT systems and i have never seen one low enough to do without an RCD on the circuits.

That aside, RCD's as a main switch were primarily used on TT systems to protect the tails as well.

If you use an all metal CU then one MUST be installed on the tails before it enters the enclosure OR use a plastic one.

 

[Lurch]

That aside, RCD's as a main switch were primarily used on TT systems to protect the tails as well.

An RCD won;t protect upstream equipment.

 

[Apache]

Calm down guys!