Difference between High and Low resistance earthing

1 megohm is effectively a short-circuit to static electricity, while providing a measure of protection against electric shock. 1 ohm is typically the maximum earth path resistance in equipotential bonding systems used in fault supression.

Is this what is required?

I'm talking about generator neutral earthing methods which could be selected high or low resistence type earthing system.

Sudh.

Sudh,

Again, let me inform you. When you make a question, please put in complete information so that our friends can make the proper response.

Here you have asked a question about grounding without saying that it is for generator earthing methods. It is only after someone has posted a reply that you give this piece of information.

This can be potentially dangerous because we might give you advice that is incorrect just because you didn't provide us with everything we needed to know.

Please be more forthcoming with your questions.

HOW TRUE !!!

Dear Mr Sudh 1)The high resistance earthing is used with an intention to reduce the earth fault currents,as the stator core or as a matter of fact any HT motor/Transformer core is quite costly and needs to be protected. 2)Reduced fault currents flow thru the core,which are detected and cleared by the E/F relays. 3)Effective earthing is used normally for LT systems,where the full fault currents are allowed to flow. The protection is thru HRC fuses. Pl let me know if u need any further information. regards Ravipra

Hello,

Thanks for the information really it will be more useful. can you pls explain the Low resistence type earthing method for generator grounding.what is design calculation for selection of low resistence type earthing.

Sudh.

I'm afraid I cannot quite grasp the question here. I worked in power stations for 15 years on generator and LNER units but I cannot associate the HV and LV generator earthing scenario which you mention.

A typical power station generator will have a star wound stator with the common end of each winding terminated at the star point termination in a special chamber under alternator or a termination box on top of it. There are no fuses or circuit breakers in the generator windings at this location and therefore they have no fault protection. The star/neutral point is earthed at the alternator in order to provide a zero reference with the mid point conductor being connected to the earth resistor. Again typically in the UK the earthing resistor is known as the Liquid Neutral Earth Resistor (LNER) which consists of a galvanised steel cylindrical tank and the metal body is terminated to the station earth mat system. The mid-point conductor is terminated on the top of the tank via an HV porcelain insulator. The tank is filled with distilled water to which sodium chloride was added to provide a conductive solution to the manufacturers specification resistance.

In the event of HV generator fault the current would flow in the mid-point conductor to the LNER and would be discharged through the resistor by heating the liquid in the tank, otherwise such a massive amount of energy would melt and destroy and destroy copper and metal conductors if were allowed to flow unrestricted.

Th only adjustment that could be made to the resistance was by changing the solution in order to maintain the cold temperature resistance. I could understand that things have possibly moved on a a bit and the LNER may well be replaced with resistance banks or inductors but I still cannot see the adjustable aspect of the resistor.

Obviously you would not want a HV generator or transformer earthing system to be the same piece of copper bar which the nearest fuse board is using for the earth path supplying the socket and metal kettle in the canteen, due to the danger of HV fault voltages being superimposed on the end user. In such cases a separate LV earthing system for the low voltage end user hand held equipment.

Dear Mr Sudh Please refer to "eatonelectrical.com" for further details rgarding generator grounding details. I am not very much sure of the design calculations.Hence not in a position to share the same.I wld be interested to note the design. regards

What size and aplication you are referring to?

This is important to select the adequate type of grounding.

Sudh,

Correct me if I'm wrong, I believe that your question is with regard to various "System Grounding" methods (as per IEEE Std 142) i.e. resistance (low or high), reactance, NEC/NER etc, is it? By definition, System Grounding in a 3-phase system refers to the intentional connection of a phase or neutral conductor to earth. NB:- this does not mean that the connection is done only at the generator i.e. it mainly refers to how transformers are earthed (and to be specific, their secondary sides)!

Going back to your question, low-resistance grounding results in ground-fault currents between 100A-1000A and in high-resistance grounding earth-fault current is in the order of 10A.

The main advantage of high-resistance grounding is therefore its ability to allow fault-current to exist for a longer period before clearing since the earth-fault currents are low. On the other hand, low-resistance grounding has an advantage of fast clearing and selectivity (grading between upstream and downstream). The former method as opposed to the latter is often employed in MV applications.

One of the the main draw-backs of these two systems is that they require surge-arrestors.

NB:- Read IEEE Std 142 for more info

Hope that this helps

Regards,

EddyM