Earth Resistance

You can't really connect them in series, but you can in parallel; in which case 1/R_total = 1/R₁ + 1/R₂ + 1/R₃.

Quite.

I have one question ...how do you measure earth resistance ? and how can we make sure the earth is OK?

i think it's not the earths resistance we are calculating but of your ground rods/probes, connection to the earth gave it a name "protective earth"..now as general practice, it is safe to have a net resistance of 1 ohm..and of course, probes/rods are connected in parallel....but i tried this experiment in college days (it was shear out of foolishness but we had an observation made), we tried testing the tables resistance (concrete table) as it turns-out..the greater the distance of the multi-meter test probes, the resistance value increased significantly..

Use an earth loop impedance tester and compare the reading to that in the requirements. Qualified electrical installers do it all the time.

Try this:

http://www.biddlemegger.com/biddle-ug/DET54R54D_UG.pdf

Refer IS 3043, Cl. 10.1 and 10.2.

BB Raina

Well, you don't want to connect them in series, and, in fact, I don't see how you could unless you had three different earths (i.e., planets) ;-)

If you think carefully about it, when you connect the three pits together, you are connecting them in parallel--one side of each pit is (connected to) ground (because it is in and part of the ground). I'm sure you then plan on connecting the other side of each pit to one common grounding connector, so this makes a parallel connection.

You could then use the formula for resistors in parallel to calculate the total earth resistance, but, not being a power engineer, I'm not 100% confident of the results--I think I'd connect the three together and then try to use the "fall of potential" method traditionally used for checking the actual total resistance of the three pits to ground.

But, the formula for resistors in parallel is like this:

1 / Rtotal = 1 / R1 + 1 / R2 + 1/ R3 + ...

You will find that the total is less than any of the three resistances, and the primary flow of current will be through the .78 and .97 ohm pit--the 68 ohm pit will not do very much to lower the resistance of the earthing system.

hi, actually earth pits are connected to earth mat and when u calculate individually u get only that pit resistance. actually all the earth pits are connected in parallel to earth mat . you can find by using earth megger

Homework again. If you can do the initial calculation you should be able to answer your own question.

BS 7430 or IEEE 80-2000, that all you need for your question. The answer is very simple, trust me. But you need to know by your self

You're playing with fire, literally, by connecting multiple earths.

It might be ok for most of the time but if you have a nearby lightning strike you are going to be in serious trouble. The problem stems from the way the several thousands of amps dissipates radially from the point the bolt strikes. This can create massive voltage differentials that are dependent on the distance you are from the touchdown point. Since with multiple pits you are almost certainly going to have them different distances from the touchdown point you could and likely will end up with massive voltage differentials between the separate earths that will then translate into massive currents flowing through the cables connecting the earth points. Not only could this damage any equipment connected to the earths but vaporize the cable connecting the earths to each other hence the playing with fire warning.

You could try to install some sort of overload protection to prevent the current from getting out of hand but I don't know what effect that would have on the overall earthing nor the legality of the installation.

If the earth points are very close (a couple of metres at most) then you may be able to get away with it but a better way would be to just have a single larger earth point.