In our plant We have MV 11KV power

[Mohsin Jameel]

In our plant We have MV 11KV power generation with NER of 40 Amps on each and 11/0.4 KV MV/LV trafo solidly grounded . We have earth grid through out our facility to sustain calculated short circuit current at each location. Have resistance of 2-5 ohm nearly at each point of 4sq KM facility for touch voltage and all equipment in site away from substation is directly connected to earth grid close to each equipment (donot have separate PE cable coming back to switchgear earth palte) and same earth grid of electrodes and cables network is connected to MV NER and LV trafo star point.

Want to clarify setting of earth relays for MV and LV motors outside in field, what percentage of Line-earth fault current should I consider in our case as return fault current is dependent on earth grid as no separate PE cable is available? Can we consider/name this arrangement as TT network? Should we need to install 300mA earth current setting for all motors to meet any standard requirement and limit touch potential?

 

[7anoter4]

As per IEC 60364-1 TT it is a grounding system type for up to 1000 V a.c. [or 1500 V d.c.]
As you already said the transformer low voltage windings -I think they have to be star connected -neutral is solidly grounded and you may use TN system.
The problem it is 11 kV system-I think it is delta connected. So you have not neutral and you have to create one-a grounding transformer-Z connected it is usually. In this case you may consult this article-for instance:
Neutral Point Connections in Mv Power Networks With Grounding Zigzag Transformers— Analysis And Simulations

https://hrcak.srce.hr/file/323757

If you intend to continue to function with one phase grounded you have to check if your equipment withstands the full line-to-line voltage. For instance, see IEC 60502-2 for cables.

 

[Kiribanda]

1) Your 11kV system is NOT low resistance grounded (400A) but looks like an high resistance grounded with a maximum allowable earth fault current of
40A. That means for your 11kV motors and other loads you should have a coordinated earth fault protection with 40A max. allowable. From the voltages
you have given (11kV & 400V) it seems that you are using IEC standards and therefore your both 11kV & LV cables donot have any bonding conductor similar to CSA/ NEC cables.
It means that you have to provide a good zero seq path for all earth faults on the 11kV system. Therefore, if 11kV cables do not have a metallic screen, you should provide
a separate earth conductor sized to 40A to each 11kV motors & other loads similar to TNS. This is in addition to the safety earth you have already provided to the motor enclosure.
Also you cannot depend on steel armour of these cables as the earth fault return path because it might corrode at any time and becomes high resistance.

2) Your 400V LV system seems to be solidly grounded. That means you have to have again a coordinated earth fault protection scheme for all your LV loads.
This is one of the disadvantages of solidly grounded systems but can't help. It also depends on whether your sc protective devices are fuses of circuit breakers.
Therefore, without knowing those details and also the single line diagram it is not possible to advise on earth fault current settings.

 

[7anoter4]

The grounding grid the main effect it is to reduce the touch and the step potential of the Earth when a short-circuit current flows through the ground, from or to an outside source or consumer. The protection clearing time has to be connected with the human withstanding to this potential. If the supplying source and the supplied equipment-including motors-are connected with the same grid
no EPR in the ground appears in a short-circuit case since the current flows through the grounding grid itself and not through ground[Earth].
The grounding grid is a mean to limit touch, step and transfer potential but for short-circuit current it has to be connected directly to the source. If the system is not provided with grounded neutral the phase- to- ground- to- phase is the possible short-circuit.
In my opinion, for medium voltage grounding installation you may follow the EN-50522 [or BS-50522/2010] recommendations. But for protection at 11 kV only IEEE Standard 242/2001 chapter 8.[8.2.4 High-resistance grounding, mainly]