Earth rod resistance

With a combined HV and LV electrode, you need to calculating the resistance needed to limit the rise of earth potential, and the relative step and touch potentials  - although a collective 1 Ohm isn't an unreasonable starting point - this is the collective resistance of the HV (or steelwork) and LV electrode, not just the rod banged in the ground


I would also check conductors E202 and E205 - it looks like you have both the transformer neutral earth and the switchboard neutral earth in place - which is a bit unusual. I would expect to see just E205 connected to the electrode (if you have REF functions on DB 02 incoming circuit breakers)


Regards


OMS

The earth resistance mentioned in the standard is for combined electrode resistance in that substation say for example if that substation consists of only four earth rods then each rod should not be greater than 4 ohms i.e R1= 4,R2= 4,R3= 4,R4= 4,  then the equivalent resistance is              1/Req = (1/R1+1/R2+1/R3+1/R4) = (1/4+1/4+1/4+1/4) = 1, therefore  Req= 1 ohm, but still this 1 ohm value should not be taken into consideration for all cases because the earth resistance totally depends on ground potential rise, step and touch potential which has to be calculated, you can refer IEEE-80 for calculation, but it is always advisable to maintain earth resistance to a very less value because in the distribution substation there will be frequent earth fault and due to this  high current will flow through ground which increases the ground potential rise and due to this step and touch potential also gets developed,if the earth resistance is very less then the ground potential rise also will be less and so the step and touch voltage,hence it is always good to maintain very less earth resistance.

Hi OMS

Much appreciated for the guidance, we are in the process of having fault level analysis, step and touch potential studies carried out.  Would we expect the required earth rod resistance to be an outcome of this type of study?

The E202 and E205 connections are both pre ct's at the moment and we are currently set for balanced rather than restricted earth fault protection, but looking at modding to REF in the near future. 

Many thanks

Al

Hi DAKSH

Much appreciated for the guidance, i have every other standard/guide apart from this, i will get a copy of IEEE-80 and hopefully provide a good resource

Many thanks

Al

The 20 ohm figure is sometimes seen as the limit for the LV earth electrodes, when HV side (transformer core and box) and LV side (neutral star point) earths are well separated - such a site could be "hot" in the sense that during an HV to transformer case fault, the rise of voltage may be quite a bit more than the LV network would reasonably tolerate during the short time while the HV side ADS operates.  This tends to be the case for rural pole pig transfomers supplying a few farms, not your situation. The HV earth is a wire running down the pole with the transfomer on it, the LV earth is the wire running down from the neutral line at the next pole along, or an equivalent.

The 20 ohm figure  appears in some of the distribution code guidance for DNOs.


The one ohm figure is about right for a 'cold' site, where HV side and LV side earthing are inter-connected or very close, and  for a moderate HV side prospective fault current.

However, if you are on a 33kV line, or the 11kV line is short and fat for some reason, so the prospective fault current into terra-firma while the HV side disconnects is higher than a few hundred amps, then less than an ohm may well be needed, or in some cases more may be OK - it is only a guide.

Note that underground HV feeds that bring their own earthed armour also need treating a bit differently, as the prospective fault current is then quite a bit higher.

HV feeders won't usually have high earth fault currents as these are limited by the earthing arrangements at the sending substation via the neutral earthing arrangements (earthing transformer, Liquid resistor, air cooled resistor etc or by using a resonant circuit (ie peterson coils)


As an example, with a 6.4 Ohm liquid or air cooled resistor earthing the undistributed 11kV system neutral, the earth fault will not exceed 1000A


If the HV is fully cabled, with earthed screens or armours, then it's almost certain the site is cold, as you are solidly connected to a "global" earthing system


All of that said, you still need to calculate the RoEP and contrast that with the speed of disconnection  - and then calculate the step potentials based on the assumed male body weight, boots and a layer of something vaguely insulating like gravel - something in the order of 2kV might be expected, but usually we are under 430V or 630V RoEP dominated by the HV protection speed. 


For the OP - noted the balanced EF rather than REF - it just looked odd on a first pass


Regards


OMS