TT earthing arrangement

In general there is no issue with adding extra electrodes, or indeed any extra metal that is not perhaps good enough to be the sole electrode - building steels come to mind. The better you can get a grip on the terra-firma is always good.

Unless the electrodes are massive, then 500m is more or less the same as 'infinite' or at least the resistance  to the far side of the planet would only be a few % more , so no concerns about being fooled by electrode near fields overlapping.

At the substation transformer there may be combined or separate HV and LV side earthing, the key thing is that if HV shorts to the transformer core or metal tank or whatever, there is not an excessive jump in the LV side neutral to true earth voltage. Which is done will depend on ground resistances and the impedance of the HV side supply, and possible fault to earth current.

What must be avoided is earthing zones that are not metallically interconnected coming within reach of each other - say 2m if accessible.

I am a bit puzzled by the 'armour disconnected' comment - is this to prevent a problem with circulating current - if it means there is no metal CPC at all between the TX and the suppliers main panel, then this is a TT substation, which would be unusual. Or there may be an N-E link at the suppliers main panel and there is in effect a PEN back to the transformer and it is TNC mains - can you clarify which is it please ?
M.

Hi mapj1,

There is no earth connection between the transformer and the main panel or N-E link in the main panel, as you say, it is a TT substation, the regional regulations identify TT as the standard earthing arrangement.

The Transformer earth electrode is approx10m from the main panel earth electrode providing an electrode to electrode resistance of approx 7 Ohms through the general mass of earth.

The installation designer has omitted to to provide RCD's for every final circuit (this, they advise, is by accident) as they are used to designing TN-C-S systems and were not aware that the regional standards were for TT.

I have advised if there is no problem with the local electrodes supplimenting the authority electrode, and if the measured EFLI values are below those provided in Table 41.3 then there maybe no need to add RCD's, where these parameters are not met then RCD's mst be installed.

The only issue then is that system voltages for the region are 415/240 volt and the Max EFLI should be adjusted to suit (max 264v), which i am currently working on.

Hi mapj1,

There is no earth connection between the transformer and the main panel or N-E link in the main panel, as you say, it is a TT substation, the regional regulations identify TT as the standard earthing arrangement.

The Transformer earth electrode is approx10m from the main panel earth electrode providing an electrode to electrode resistance of approx 7 Ohms through the general mass of earth.

The installation designer has omitted to to provide RCD's for every final circuit (this, they advise, is by accident) as they are used to designing TN-C-S systems and were not aware that the regional standards were for TT.

I have advised if there is no problem with the local electrodes supplimenting the authority electrode, and if the measured EFLI values are below those provided in Table 41.3 then there maybe no need to add RCD's, where these parameters are not met then RCD's mst be installed.

The only issue then is that system voltages for the region are 415/240 volt and the Max EFLI should be adjusted to suit (max 264v), which i am currently working on.

That sounds more or less OK. Of course when the numbers are processed, an RCD of some sort will  be needed in most final circuits - a 7 to 10 ohm loop is reassuringly low but even so only 24 - 30A of prospective maximum fault current.  That may fire a 6A b type breaker fast enough for safety of life concerns but anything much bigger (16 A, 32A whatever) will require an RCD somewhere to clear an earth fault. Common UK practice would be a 100mA or 300mA RCD (maybe time delay to discriminate with faster more sensitive RCDs further along) in place of the double pole main switch.

The one thing you must never ever have of course is a live to earth short in a location  that does not trigger  automatic disconnection - because all the metalwork on that earth, electrode, house plumbing etc. all becomes live and nothing opens to clear so it stays live indefinitely, and apart from a higher than normal electricity bill there is little to show for it.

Mike.

I have insisted that all submain breakers have ELR's however, as i said with two earth electrodes (authority and local) the fiinal curcuit EFLI measured values are compliant with Table 41.3 meeting the BS7671.411.5.2 Note 1 requirement.

the 7 Ohms measurement was tested with only the main board electrode stand-alone, the local electrodes and return paths were removed from the test, this in parallel with the supplimentary local electrodes reduces the earth resistance significantly.

For example the EFLI measured at the main incoming breaker of one of the installations was 0.22 ohms, while very low does not meet max EFLI for the breaker so an ELR will be installed.

EFLI measurement of a final circuit returned 0.24 Ohms for a 16A MCB well within the Table 41.3 max permitted EFLI of 2.73 Ohms.