What fault can turn a TT into a TNCS and disable the RCD?

[springcrocus]

My cat just digs a hole and pees in it . Would that get your Ra even lower?

 

[kme]

Steps, get us another can of lager when you go out to the fridge will you please, my glass is empty.

Tell us summat we didn`t know

And some bread well buttered please :slap

LMFAO - you`d have had a problem SAYING "bread" ; let alone spotting it - it travelled quite fast, as I recall....................

do you want a sleeping bag with your vino?????????

No, course he doesn`t mate - that would necessitate there being a definite demarcation between alert and Zzzzzzzzzzzzz - and, afaicr, there wasn`t one.

As the old quote goes...."Vini Rouge, Vini Mactop, Vini zzzzzzzzzzzz"

:slap

 

[sparkytim]

Ok I'll have a bash...TT to TNCS needs connection between cpc and N.

As RCD is balanced ,then a cross wire to keep both conductors balanced would disable it....my answer either short or crossed wiring on RCD circuit ?[/quote

No "crossed" wiring.

A short between L-N would have tripped the MCB.

Would not a short from N-CPC have tripped the RCD?

 

[remustered]

With the RCD was thinking cross wired as in shunt would effectively bypass RCD function while leaving internal test button working. Add TT to TNCS fault which is another connection....is this one fault doing both?

 

[sparkytim]

No shunt of RCD i.e. current still flows down thru L of RCD and back up N without an imbalance enough to trip.

Same single fault doing both.

 

[DRC]

I will have a go could it be possible that you have a L-E and N-E fault and they are balancing out the RCD Thus causing it not to trip? ray

 

[sparkytim]

I will have a go could it be possible that you have a L-E and N-E fault and they are balancing out the RCD Thus causing it not to trip? ray

Only one fault

 

[remustered]

Back again..hic...reduced impedance on TT...too much drink. IE water/flooding?

 

[sparkytim]

Ra 64 ohms (Stake earth)

No water or flooding involved.

Ok as Canoe hinted the answer is contained in the question

What connection does N have to Earth normally in a TT and this fault turned the install into a TNCS

 

[DRC]

Ok is there a fault at the star transformer or was the system previously a TNCS converted to a TT and someone forgot to remove the link to the neutral? Failing that I don't no, and it's time for football and alcoholGuinness

 

[steptoe]

Ok is there a fault at the star transformer or was the system previously a TNCS converted to a TT and someone forgot to remove the link to the neutral? Failing that I don't no, and it's time for football bike racing and alcoholGuinness

will you folks never get your priorities right?

 

[DRC]

Sorry steps should have been Alcohol/Football And then the bikes:slap

 

[sparkytim]

When you have all finished play and sobered up, have another try.

No external fault involved, just a basic common fault with dramatic consequences that it is good to aware of, especially those TT fans out there. ( No not as in Bikers)

(Just realised that explains the reason for Steps advocation of this system maybe? :slap )

 

[cm304eex]

The only thing i can think of that would make a TT system look like a TNCS is N-E fault, I am not sure why this has nullified the rcd though.

Am I getting closeray

 

[sammers230]

Little bit confused as to how one fault can cause such a big problem but as abit of a stab is any of the bonding involved as looping back eg fault goes down the rod and back up the gas/ water pipe but then there would still need to be the fault linking N-E and in such a way not tripping RCD and effectively rendering it inoperable?

 

[sparkytim]

Yup (cm)

The fauly was a N-E short on a wall light.

Now comes the interesting bit, how come the RCD was disabled?

We need to remember that current is Lazy, (Allways looks for and follows if possible the route of least Resistence)

I allways find as others, it helps to Draw out the circuit and route of Fault current, that should help in solving this one.

Sammers

Bonding not an issue

 

[cm304eex]

Fault current flowing down cpc so not causing imbalance in RCD

 

[sparkytim]

In a TN system a N-E Fault would trip the RCD as an imbalance would occur due to current flowing in the CPC to earth instead of back on the N thru the RCD.

So whats different in a TT system?

 

[sparkytim]

Well seems as if our students have given up on this one.

Open to ANYONE to explain how a N-E fault on a TT system can disable the RCD and what the potential damage can be under secound fault conditions.

 

[ProDave]

Nope.

I've drawn it out, and a N-E short at a light fitting, to me looks like some of the N current will flow via E, and so will cause an imballance and trip the RCD.

So I'm watching the thread to see what is magically different about TT to see a drawing of the installation of the circuit, the fault, and the current flow so see how it can disable the RCD.