You are using an out of date browser. It may not display this or other websites correctly.

You should upgrade or use an alternative browser.

You should upgrade or use an alternative browser.

- Thread starter scouseneil
- Start date

Sponsored Links

If you're running an earthing conductor underground for TT and it is protected against corrosion & mechanical damage, then you can use 2.5mm².

see post above, but yes you can, for a supply with 2.5mm² tails..

not very likely..

again, see above but yes, as long as the supply is 2.5mm²

Table 4.2 in the OSG

Sponsored Links

the regs state quite clearly what I've stated above, if the guy writing the OSG can't interpret that then he shouldn't be writing it..

regs book..

fig 2.1 in definitions..

number 3 - earthing conductor

number E - means of earthing ( TN systems )

number T - earth electrode ( TT and IT systems )

number B - main earth terminal

so "earthing conductor" connects the MET to the source of earthing..

regulation 542.3.1 - Every

543 - Protective conductors - the cross section of a protective conductor, other than a

(i) calculated in accordance with Regulation 543.1.3 ( the adiabatic ), or

(ii) selected in accordance with Regulation 543.1.4 ( selected from table 54.7 the jist of which I posted above )

makes no discrimination between TT or TN systems..

so the OSG quite clearly has it wrong..

if you have a 2.5mm supply then you can have a 2.5mm burried earthing conductor, providing it's mechanically protected, 16mm if it's not protected..

S

ifyou follow the genaral rule of thumb which is

Main earth---16mm

Main Bond----10mm

Suplimetary---4

In a domestic propert

you cant go far wrong

The ONS does contain good information but it has to be used in conjunction with the big book

But by using the adiabatic, on a TT supply a 2.5mm² is sufficient to comply with section 543

go on then, prove it..

where do you get the I value from anyway? i can't remember...

where do you get the I value from anyway? i can't remember...

You CAN use table 54.7 - in which case the minimum size is generally half that of the line conductor

OR you can determine the minimum size by calculation. NOT both.

For a TT system, the worst possible scenario is a disconnection time of 1 second (411.3.2.4).

In reality it will be much less, since the protective device will be an RCD, and the disconnection time would have to be less than 0.2 seconds for most circuits anyway (Table 41.1)

This reduces the top half of the adiabatic equation to just I (since t is 1, and the square root of I squared is just I)

Table 54.2 gives the value of k as 143

Therefore you have S = I/143

Since the minumum value of S is 2.5mm, 2.5 = I/143

With simple rearrangement, 2.5 x 143 = I, so I = 357.5A

This is the maximum fault current which can be permitted using a 2.5mm protective conductor.

To get this, you would require an earth electrode with an impedance of about 0.64 ohms.

Clearly this is not going to happen, most earth electrodes will be many tens of ohms at best, and quite likely into the 100s of ohms.

With disconnection times of 0.2s, the required fault current is approximately 800A, or an electrode impedance of 0.28 ohms. Not going to happen.

and where do you get the value for I from? I can't remember.. it's been a while since I had to use it.. generally rule of thumb it.. it's easier..

This will not be happening on a TT supply, so 2.5mm is acceptable.

The value of I is the fault current (defined on page 128).

I'd just worked that out too

FYI, I=Uo/Ze

FYI, I=Uo/Ze

S

TN_C_S 0.35OHMS

TN-S 0.8OHMS

TT 21OHMS

If you electrode is 100 ohms then 230/100 = 2.3 amps.

If you electrode is 50 ohms then 230/50 = 4.6 amps.

Stick it into the adiabatic and you'll get a rediculously low minimum size! (not suggesting you should use anything less than those in the tables tho!)

TN_C_S 0.35OHMS

TN-S 0.8OHMS

TT 21OHMS

21 ohms is the maximum resistance to ground of their electrode, it doesn't include your electrode.

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Please select a service and enter a location to continue...

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links