This document provides guidelines for designing an earth mat for a metro station. It discusses standards for earth mat design, the design procedure, and provides an example of a design for the Joka Station. The key steps in design include determining the fault current, selecting conductor size, and using software to model symmetrical or asymmetrical mat layouts based on space availability and soil properties. The example shows the proposed mat layout for Joka Station, simulation results verifying touch and step potentials are below safety limits, and demonstrating the effectiveness of the design.

1. Earth Mat Design for Metro
Station
Subhendu Bikash Santra
SE/RVNL/KOLKATA

2. contents
 Introduction.
 Standards.
 Design Procedure.
 Example.

3. Introduction
 Earthing means connecting one conducting wire from
neutral point to ground [System Grounding] or from
equipment body to ground [ Equipment grounding].
 Earthing system is necessary to ensure safety of
equipment and personnel, continuity, protection and
reliability of overall system.
 Separate Earth or Clean Earth.
 Earth Mat.

4. Standard
 IS 3043:1989.
 IEEE 80:2000.

5. Purpose of Earth-MAT
 Provides easy and shortest path to flow the earth fault
current without adversely affecting the continuity of
service.
 Safe touch and step voltage for personnel nearby the
station area.
 Protection of costly devices from unbalanced voltage
during fault.

6. Design Procedure of Earth
Mat
 Determination of maximum L-G fault current and L-L-G
fault current at LT side. Take the larger one as fault
current for MAT design.
 Determine the grid current by using current split factor.
 Buried conductor size selection .
 Select Design according to the space availability.

7. Design Procedure of Earth
Mat
 Symmetrical [1:1 to 1:3 Mesh size]-IEEE Imperial
Formulae 80:2000 std.
 Non-Symmetrical- Finite Element Analysis using ETAP
software.
 For symmetrical-Minimum spacing between parallel
conductor is 2.5 m. and max-22.2m
 Perfect soil resistivity data is required for calculating
allowable step and touch voltage.

8. Design Procedure of Earth
Mat
 A high soil resistivity of crust layer is required where soil
resistivity is very high and fault current is large.
 Where soil resistivity is low there is no need of giving
high resistivity crust layer.
 Crust layer resistivity directly increases the allowable
touch and step potential without actually reducing
calculated touch and step voltage.
 Two layer soil model is required where soil resistivity is
non-uniform.

9. Design Procedure of Earth
Mat
 Increase in horizontal conductor length actually
reduces touch potential.
 Increase in no of down rod reduces step potential.
 Increase in no of mesh in mat reduces the touch
potential but beyond certain limit the MAT is said to be
saturated.
 So, we have to effectively use conductor and down-
rod to minimize the step and touch potential within
MAT zone and beyond of at least 2m from edge.
 IEEE 80: 2000 is particularly important for symmetrical
MAT design where space is not a problem.

10. Symmetrical Mat

11. Unsymmetrical MAT

12. Example : JOKA Station
 Space Availability : Non-Uniform
 Soil Resistivity: Non-Uniform. (7.60 ohm-m[6M] and
12.88 ohm-m (beyond 6M).
 Fault Current: 7.5 KA (max)
 Split Factor: 75%
 Crust layer treatment : Nothing is required as soil
resistivity is less and also fault current.

13. Example : JOKA Station
 Method : Finite Element Analysis Method.
 Analysis: ETAP software.

14. Proposed Drawing at JOKA

15. Simulation Result
Software
Window:
Software
Window:

16. Simulation Result
MAT Model
Corelated
with space
availability

17. Simulation Result
Result
Summery
Showing no
warning

18. Simulation Result
Step
Potential
Profile.

19. Simulation Result
Touch
Potential
Profile.

20. Simulation Result
Grid
Potential
Rise

21. Simulation Result
 Allowable Step: 231.8 V ; Calculated Step: 41.6 V
 Allowable Touch: 224.5 V ; Calculated Touch: 103 V
 Resistance : 0.09 Ohm (less than 1 ohm)
 GPR: 510.8 V which is considerably low for a remote
ended person. (transferred potential)
Thus the MAT design is safe and Acceptable.

22. Thank You