Size of Conductor for TT Installation

[horsleya]

I am hoping to use my own house for my assessment to join either NICEIC/Elecs/NAPIT however I have a question over the size of the earth conductor.

Most people give me the stock answer of 16mm as the Tails are 25mm however I do not believe that this has to apply to a TT system although I am happy to be proven wrong.
542.3.1 refers to 543 which in turn states that I can calculate the CSA based on the adiabatic equation in 543.1.3. I am no expert on this equation so I would love to see someone work this through or let me know what they believe the values should be.

I have a Ze reading of 36 Ohms however when the Main Potential Bonding is connected my Zs drops to 26 Ohms (Parallel Paths). Therefore my PFC is 9amps and this checks out with my tester (I have rounded the figure up for this message).

As I currently have a 10mm cable about 3M long going to the Earth rod I believe that this is OK. The cable is above ground and although currently exposed I intend to enclose it in plastic conduit.

Am I missing something?
What should value t be in the equation? Should it be 0.2 from table 41.1 on page 46?

 

[Sintra]

Main Earthing Conductor


The minimum csa of the main earthing conductor must be determined by the adibiatic equation or selected from Table 54.7 543.1.1.
If the adibiatic equation is used, the minimum csa of the main earthing conductor must be 6mm 544.1.1.
Table 54.7 suggests a 16mm main earthing conductor for phase conductors with a csa of up to 35mm.

Adibiatic Equation


The c.s.a. of all protective conductors (excluding protective bonding conductors) can
be calculated using the adibiatic equation. 543.1.1

S is the minimum c.s.a. of the c.p.c. I is the fault current (230v / Zs) measured in Amps


t is the disconnection time of the protective device (see figs 3.1 - 3.6 p244 - 251).

k is a factor taken from the tables 54.2 to 54.6 p129, p130.

 

[horsleya]

At the risk of demonstrating a lack of knowledge on the adiabatic equation I will have an attempt:

I=9A (My PFC)
T=0.2 (Taken from table 41.1 on page 46)
k=143 (Taken from Table 54.2 on page 129)

Therefore square root of (9*9*0.2) = 4.0249
16.2 / 143 = 0.028 CSA

This looks far too low but I guess that this is due to the high resistance of an earth rod and hense the low PFC.

Any comments? Have I used the correct Values? Have I made a fool of myself?

 

[horsleya]

I posted my second post before seeing Sintra's reply so I will quickly check over what you have said and see if I was too far off.

Thanks for the quick reply

 

[Sintra]

Surely your PFC should be 900A not 9A

 

[Sintra]

Your calculation is however correct for the figures you have used.

 

[Spudnik]

I always bang in a 10 for TT.

 

[horsleya]

My prospectic short circuit current is naturally much higher but the prospectic Fault Current on the earth rod is definately 9A.

The Zs is 26 Ohms. 230/26 = 9 (Roughly).

As the Earthing conductor is only applicable under fault conditions I would have thought that this is what I should be using?

 

[Sintra]

My prospectic short circuit current is naturally much higher but the prospectic Fault Current on the earth rod is definately 9A.

The Zs is 26 Ohms. 230/26 = 9 (Roughly).

As the Earthing conductor is only applicable under fault conditions I would have thought that this is what I should be using?

Appologies I didn't read the values in 1st post correctly.