Earth Resistance - Limits!

Hari

Hey cool guy, please elaborate your question.

In present form it makes no sense.

Hi current,

We are measuring the earth resistance & that seems to fall under the range of 1~ 3 ohm. We don't know whether it is a good earth to relieve from shock, hazardous ,etc.,

Is there any standard that define the good earth connection must have a resistance of this much ohm?????

Now I hope, that you got my point.

Hari

Is your supply multiply earthed neutral?

If not, the earth resistance must be low enough to reliably trip your protective devices (circuit breakers, fuses, earth leakage relay etc)'

I once installed a system for a house where earth resistance was 50 ohms! The supply authority said this was normal in the dry sandstone country of the installation.

The system only worked because the MEN effectively had a large number of similar resistances in parallel, giving an overall resistance low enough to be usable.

If you are solely dependent on the local earth then 3 ohms with a 240V supply will give a fault current of 80 amps, which will probably work although fuses may be a little slow to blow and if well cooled may not do so at all. 1 ohm would allow operation in most cases.

If your protection devices need higher current than the normal 15 - 20 amps of house circuits, you may need a better earth.

There is a good article in Wikipedia on earthing.

One of the ways of solving in principle the high earth resistance situation is to install a whole-house residual current device [RCD]. 100mA imbalance between live and neutral is a typical trip point for a house with a modern TT installation in the UK.

The fuses/breakers/whatever then protect the circuit conductors, and the RCD protects against high earth resistance.

Of course, there is great advantage to be had from lowering the earth resistance.

In all cases, comply with local wiring regulations.

Hi Friends,

FYI ::: I'm asking this for industry & not for home.

Hari

Hi Hari,

There are various recommended values of earth resistance. It depends on the type of usage.

1. If you require is for a sub-station it should be below 0.5 ohms.

2. For panels, UPS, distribution electrical system earth resistance should fall under 3 ohms.

3. Machines and body earth should fall below 5 ohms.

If the earth resistance value is high, there are various measures to drop its value.

As far as my understanding you are not using it for any sub-station. Thus. 1-3 Ohms is OK.

ashish

how can i get 0.5 homes for sub station

Hi Cool Guy

For industrial plants the following naximum earth resistance values are recommended:

For electrical equipment : 1 ohm for large generating stations and large substations

For electrical equipment : 5 ohms

For static discharge : 1 megaohms

For lightning discharge : 10 ohms

Please note that the objective from the point of view of safety consideration is not only to obtain a minimum value but to also keep in mind that there is adequate co-ordination between the value obtained and the setting of the protection relays/ devices.

For Overhead power lines pylons?

My ancient copy of the SAA wiring rules (Australia) states that an earthing system's resistance should be no greater than (Voltage to earth)/(5x current rating of largest fuse). Obviously "fuse" should be also taken to include "circuit breaker".

The standard I used is ancient, but the rule given is probably still relevant.

Haricool

1 -3 ohms is a very low earth resistance. What method are you using to measure it?

Hi Wareagle,

We measure the earth resistance using earth tester by following mode:

Normally ,three earth wire(covering a area of around 10 Sq.ft) are connected with the meter. And "Resistance of the earth" to be measured is connected to the earth terminal in the meter .

Hari

Hello Hari,

Your measurement of 1 till 3 ohms looks good I've to agree with ashuashi. But Wareagle makes also a good point. How was it measured. And more-over what is the soil-resistivity and how is that measured as well? For further reading I would like to refer to IEEE 80-2000, IEEE 142-1991 and BS 7430:1998. In these literature you can find solutions.

Regards,

Robert

You will find that if there is 5 ohms resistance to ground-you have a problem.

Michael Leiwig

Oil States Industries

Michael

Why does a 5 ohm resistance cause a problem? IN what type of enviornment? Please explain.

We do earthing to provide lowest resistive path to the fault current. If the earth resistance is high ~ 5 ohms or more & fault current receives path less than 5 ohms will travel to it and might cause serious issues.

regards,

Ashish

ashuashi

When you use the term "earthing" are you referring to bonding with a conductor or using EARTH itself as the fault clearing path? In the USA earth is never used as a fault current path. Equipment bonding conductors are installed for this purpose.

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