What size earth cable on a 100amp fused DNO TT supply

[Invisible-Man]

Hi,
I have a TT system with 100amp DNO fuse with 25mm tails, my question is what size earth wire/cable should be used to the earting rod? I beleive that anything over 16mm would not fit into the earthing connector block!

One other thing, the meter itself states 80amp, is this ok when there is a 100amp main DNO fuse protecting it?

Many thanks

 

[Taylortwocities]

I am assuming you are doing it yourself CU change? In which case you should be competent enough to know the answers!

However, in the spirit of replies on a DIY forum here goes.

I am assuming that your CU has a 100mA Time Delayed RCD as the incoming main switch? This will protect the general circuits such as immersion, lights, etc.
I am assuming that your CU also has a 30mA RCD and this protects other circuits incluing ALL socket outlets?

In a TT installation then, when properly installed as above, the maximum earth fault current that will flow in the wire to the earth spike will be 100mA so you could use a very weedy bit of wire to do this.
Table 10C in the On Site Guide gives you the CSA's and these vary depending on if your earthing conductior is buried or not. I usually run 10mm but 10C says (for conductors not buried and where EFLI is not less than 1 ohm):
Unprotected 4mm
Protected against corrosion 4mm
Protected against corrosion & mechanical damage (eg in conduit) : 2.5mm

Re the main fuse, did you really mean meter? You do not need to concern yourself with that, the DNO looks after this and they have their own ways of doing things Did you really mean your consumer unit? In any even you are OK with 100A main fuse, if your board is rated at 80A then ensure that your max assumed demand isnt more than 80A (like you should not have two 9.6KW power showers!)

TTC

 

[sm1thson]

there is a 100amp main DNO fuse protecting it? is there definately a 100Amp fuse inside the 100Amp fuse holder not an 80Amp one!?!

 

[securespark]

4mm2? 2.5mm2?

Look in your OSG.

However, there is nothing wrong in using 16mm2 if you want to...

And you DO know what the max earth loop for a rod on TT is, don't you?

 

[Invisible-Man]

Yes I know that the resistance should not be more than 20ohms.

One other question if I may... I know that being TT, all circuits have to be protected via RCD (main consumer unit is) but if running a submain in SWA via a switchfuse, can this come straight from splitting the tails from the meter then RCD protecting all circuits the other end via another consumer unit? I know that this would leave the SWA not protected by RCD but did not know how important this wasor acceptable?


Many thanks

 

[Spark123]

Nope, you will need to RCD protect the SWA cable at source, you risk livening the whole installation up if the RCD is ommitted and insufficient current can flow to operate the overcurrent protective device in the switchfuse in the event of a fault on the SWA cable.
You may use a time delay RCD at the source and a normal one at the remote end for sockets for discrimination purposes. iirc it the maximum external loop on a TT system should not exceed 200 ohms (or 100 ohms for NICEIC).

 

[Invisible-Man]

Many thanks for your advice, much appreciated !!!

 

[plugwash]

In a TT installation then, when properly installed as above, the maximum earth fault current that will flow in the wire to the earth spike will be 100mA so you could use a very weedy bit of wire to do this.

not true, 100ma is the most that will stay flowing for any signficant time but there could still be significant current flowing in the interval between a fault happening and the 100ma type S RCD tripping.

what limits this current is the EFLI, the figures in the table assume a min EFLI of 1 ohm which should be fine for most TT installs but i'm sure there are exceptions.

 

[Taylortwocities]

That's true Puggy, couldnt cover everything as this is supposed to be a DIY forum! Thought my extract from the OSG should have been enough.

Never know how deep to go on subjects - not enough info and you get picked up, too much info and folk shout because its too detailed for DIYNOT

Just trying to be helpful, but thanks for keeping an eye on me.........

TTC

 

[Invisible-Man]

As previously discussed, and If It exists, I would appreciate if someone could recommend a time delayed Rcd 100amp x 100ma, which I would like to also act if possible as an isolator straight after the meter, and if possible (don't think it is) one which can take double the tails on the exit side, this would save me from using a henley block to split the tails after in order to supply a fuseswitch to the submain mentioned earlier.


Many thanks !!!

Thanks for all your earlier replies (great forum) !!!

 

[securespark]

That's true Puggy,

Are you two an item??

 

[Taylortwocities]

Yup, all twitterpated.

Invis. The MK 6400S does the job @ 100A

You wont get 2 x 25mm tails into any domestic style cb or switch. max 35mm² I think. Henley block good & reliable and only a tenner.

TTC

 

[Invisible-Man]

Yup, all twitterpated.

Invis. The MK 6400S does the job @ 100A

You wont get 2 x 25mm tails into any domestic style cb or switch. max 35mm² I think. Henley block good & reliable and only a tenner.

TTC

Thanks !!!

 

[Invisible-Man]

Yup, all twitterpated.

Invis. The MK 6400S does the job @ 100A

You wont get 2 x 25mm tails into any domestic style cb or switch. max 35mm² I think. Henley block good & reliable and only a tenner.

TTC

Wow, thats a tad expensive !!! If supply could be changed to PME, that MK 6400S would not be needed... is that correct... if so I shall get onto the DNO !!!

Thanks