Earthing systems

[ezekiel]

the 2.5 on 40a is labelled shower - however i traced this to a 13a switched fcu feeding sockets in loft and a 3a fcu feeding light - there are 2 earth cables wich both appear to run downstairs and one is attached to incoming gas supply (cant find other) Cu is upstairs cables run into loft then go next door so i guess it may be overhead?

 

[Spark123]

It strikes me as odd why anyone would protect a piece of 2.5 with a 40A MCB though, the maximum for 2.5 on a radial would normally be 20A.
I'm thinking your supply is possibly a TT where you have an earth electrode, if this is correct then it means that your lighting circuits are not adequately protected against indirect contact i.e. there's no RCD protection for them.

 

[RF Lighting]

I'm not certain, but it looks to me like a TT fed in VIR singles.


If this is the case then your installation is in need of some urgent remedial work.


Can I be a pest and ask for another picture just of your cutout?

 

[ezekiel]

if this is so how would i better protect the light circuit (if possible) also would the earth electrode not be local to my property? many thanks

 

[RF Lighting]

Yeah you should have an earth electrode somewhere. If it is a TT then you really should have a time delayed RCD main switch. As things may be at the moment, a fault to earth is not forced to automatically disconnect the supply.

 

[ezekiel]

never noticed a earth electrode anywhere heres a close up of cutout
http://img134.imageshack.us/img134/1944/p1010668jf8.jpg

 

[luminaire]

also would it be safe to turn of at switch and rewire the cu as its a mess im quite confident at doing it as im not altering the circuits just tidying up cables?]

If you do decide to rewire the CU, I would be inclined to do PIR first, just to make sure you haven't got any other problems lurking beneath the surface.

 

[ezekiel]

thanx for all the help - planing on confirming if its a tt system then possibly change cu all together to recomended will need to do a lot of reading up first and will give it a go (will also get in touch with relevant bodies for certs etc...) however believe it will be a good a learning curve, will need to get a isolation switch added first - confident at the wiring but not with testing however i do know/work with some electricians who may help out - thanks again for help

 

[ban-all-sheds]

the 2.5 on 40a is labelled shower - however i traced this to a 13a switched fcu feeding sockets in loft and a 3a fcu feeding light

You should replace that MCB with a 16 or 20A one.

there are 2 earth cables wich both appear to run downstairs and one is attached to incoming gas supply (cant find other)

Hopefully the other goes to the incoming water pipe. But that accounts for the 2 10mm² cables in the earth/bonding positions - what about the 16mm² one?

Can you get one of the guys you work with to come and measure your Ze?

 

[plugwash]

what is the easiest wasy to tell what type of system you have TT is simple to tell but even after reading wikki cant tell between tnc or tncs any help appreciated as usual

You can't reliablly tell without opening the cutout.

 

[ezekiel]

gonna get it checked will post results asap

 

[Lectrician]

so i guess it may be overhead?

Guess?

Go and look!

 

[ezekiel]

yeah overhead supply spoke to spark at work at work he reckons 2 x 10mm gas & water bonding and 16mm to earth rod - tried to trace but cant find it ( spark gonna call in after work this week and check for me) would of liked to manage by myself but not that clued up yet

 

[ban-all-sheds]

Make sure you use that visit as an opportunity for learning - pref hands on.

 

[nozspark]

Going back to the original question (how do you tell between TN-S and TNC-S)

Isolate the installation, disconnect main earth from bonds & circuit earths, test EFLI (P-E) & note reading, repeat but this time (P-N)

For TNC-S your EFLI should be below 0.35 ohms
For TN-S your EFLI should be below 0.8 ohms

If your (P-E) & (P-N) readings are about or below 0.35 ohms and are the same then you have a TNC-S installation