How To Verify Ground Connections and Fence Ground Installation


In Article " How to Verify Ground Rod and Grid Installation ", I explained the following points:

  • Practice#2: How to Verify Ground Grid Installation,
  • Practice#3: How to Verify Ground Rod Installation.


Today, I will explain the following:

  • Practice#4: How to Verify Connection / Interconnection Installation Ground Connection Requirements and 
  • Practice#5: How to Verify Fence Ground Installation.



You can review the following Articles For more information:








Practice#4: How To Verify Connection / Interconnection Installation Ground Connection Requirements











Use the following information in this work aid and figures in the Information Section to help in identifying connections to ground as follows:

1- Determine that buried connections are made by thermite welds, brazed or approved compression connectors and the grid interconnections are approved compression type or thermite welded.

2- Verify that no mechanical connections made are buried below grade.

3- Verify that grounding conductors from one equipment unit to another have not been looped.

4- Verify that all grounding conductors are as continuous as possible and that no unnecessary connections have been made.

5- Verify if ready disconnections for test purposes are provided in grounding connections to, generator neutrals, transformer neutrals, grounding electrodes such as groundwells or groups of ground rods, switchgear equipment ground buses, and switchgear neutral buses.

6- Verify that all grounding devices and conductors are installed so that they will be protected as much as possible against mechanical injury. In frequented areas, buried conductors, 120 mm2 (No. 4/0 AWG) or less, emerging from the ground should be protected by a non-metallic conduit for 6 to 12 inches (150 mm to 300 mm) above and below grade. Conductors should be ducted below concrete or where similarly inaccessible, preferably using non-metallic pipe. Metallic conduit and cable tray  shall be grounded at each end . Make sure that these systems are electrically continuous.

7- Verify that the grounding bus forms a closed loop so that equipment ground and system neutrals tee-connected to it have 2 current paths.

8- Verify that where a site includes ground buses and ground grids in combination, and their ground resistance area overlaps, they must be interconnected by at least two conductors per bus or grid.






Exercise for Verifying Connection / Interconnection Installation Ground Connection Requirements


Question#1: Buried connections are made by three different means. What are they?
Answer: Thermite welding, brazing, and compression.

Question#2: Mechanical connections are (choose one) 1- permitted, 2- not permitted to be buried below grade.
Answer: Mechanical connections are not permitted to be buried below grade.

Question#3: Looping of grounding conductors from one equipment to another is (choose one) 1- permitted, 2- not permitted.
Answer: Looping of grounding conductors from one equipment to another is not permitted.

Question#4: All grounding conductors should be as …………..  as possible and no unnecessary connections should be made.
Answer: All grounding conductors should be as continuous as possible and no unnecessary connections should be made.

Question#5: Ready disconnections for test purposes should be provided in grounding connections to what?
Answer: to Rods, ground wells, generator and transformer neutrals, switchgear equipment and switch gear neutral.

Question#6: All grounding devices and conductors should be installed so that they will be protected as far as possible against ………………. In frequented areas, buried conductors, sized ………………… or less, emerging from the ground should be protected by a non-metallic conduit for …..  To ……..  inches above and below grade.
Answer: All grounding devices and conductors should be installed so that they will be protected as far as possible against mechanical damage. In frequented areas, buried conductors, sized 4/0 AWG (120 mm2) or less, emerging from the ground should be protected by a non-metallic conduit for 6 to 12 inches above and below grade.

Question#7: The grounding bus forms a closed loop so that equipment ground and system neutrals tee-connected to it have how many current paths?
Answer: 2 Paths

Question#8: Overlapping ground buses and ground grid areas should have a minimum how many conductors per bus or grid interconnect?
Answer: 2 Conductors






Practice#5: How to Verify Fence Ground Installation












Use the following information in this work aid to help in identifying fence grounds as follows:

1- Verify that the following fences are grounded:
  • Fences that enclose or cross over a ground grid,
  • Fences that pass under a transmission line operating at 230 kV and above.


2- Determine whether the fence has been built to an SSD/1 type. If so, no grounding is required.

3- Verify that ground wells are located as shown on the drawing used for inspection and are connected to the grounding conductor.

4- Verify that the grounding conductor has a minimum of two connections to the ground grid.

5- Verify that the peripheral grounding conductor of the substation is
approximately 3ft (1 m) outside and parallel to the fence.

6- Verify that the fence is bonded to the peripheral grounding conductor at intervals of 50 feet (15 m) with a minimum size of 70 mm2 (2/0 AWG)

7- Verify that metal posts are used for the substation fence.

8- Verify that corner and gate posts are bonded to the parallel conductor and gates are bonded to gate posts with flexible connectors.






Exercise for Verifying Fence Ground Installation


Question#1: Determine if the following fences are to be grounded:
  • Fences that enclose or cross over a ground grid.
  • Fences that pass under a transmission line operating at 230 kV and above.

Answer:
  • Yes.
  • Yes.


Question#2: A fence has been built SSD/1 type. What type of grounding is required?
Answer: none.

Question#3: Ground wells are usually located where?
Answer: diagonally.

Question#4: How many connections should the grounding conductor located outside the fence area have to the ground grid?
Answer: 2 connections.

Question#5: How far should the peripheral grounding conductor be located from a substation fence?
Answer: 2 to 3 feet.

Question#6: A substation rated above 13.8 kV has a fence bonded to the peripheral grounding conductor at intervals of …………….. With a minimum size of …………   conductors.
Answer: A substation rated above 13.8 kV has a fence bonded to the peripheral grounding conductor at intervals of 50 FEET   with a minimum size of _2/0    conductors.

Question#7: What type of posts are used for a substation fence?
Answer: metal Post.

Question#8: To what are the corner and gate posts grounded to? How are gates bonded?
Answer: Corner and gate posts must be connected to the grounding conductor. Gates must be bonded to gate posts with Parallel conductor and flexible connectors.












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Types of Earthing System – Part Two


In Article " Types of Earthing System – Part One ", I listed the Different Types of Earthing Systems which can be divided according to the following factors:

  1. Function,
  2. System size,
  3. Neutral point connection to earth (Neutral Grounding),
  4. Neutral point connection to earth(Neutral Grounding) + the connection method of the electrical installations exposed conductive parts (Frame Grounding).


And I explained the first two categories in this Article, showing that:

  • Types of Earthing Systems according to its Function can be divided to Six types as follows:

  1. Static grounding, 
  2. Equipment grounding, 
  3. System grounding, 
  4. Lightning grounding, 
  5. Electronic (including computer) grounding, 
  6. Maintenance safety grounding. 

  • And Types of Earthing Systems according To Its Size can be divided to Two types as follows:

  1. simple.
  2. complex. 
Today I will explain the Last Two Types of Earthing Systems as follows.


You can preview the following Articles for more info:






Types of Earthing System (on LV Systems and Within Premises)


Different Types Of Earthing Systems

the Earthing systems can be divided according to the following factors:

  1. Function,
  2. System size,
  3. Neutral point connection to earth,
  4. Neutral point connection to earth + the connection method of the electrical installations exposed conductive parts.






Types Of Earthing Systems According To Neutral Point Connection To Earth (Neutral Grounding)

The Earthing systems can be divided to Five Types according to neutral point connection to earth as follows:

  1. The solidly (or directly) earthed neutral,
  2. The unearthed neutral,
  3. High impedance-earthed neutral,
  4. Resistance earthing,
  5. Reactance earthing,
  6. Petersen coil earthing.






Types of Neutral Point Connection To Earth






1- Solidly earthed neutral
An electrical connection is intentionally made between the neutral point and earth.

2- Unearthed neutral
There is no electrical connection between the neutral point and earth, except for measuring and protective devices.

3- High impedance earthing
A high impedance is inserted between the neutral point and earth.

4- Resistance earthing
A resistor is inserted between the neutral point and earth.

5- Reactance earthing
A reactor is inserted between the neutral point and earth.

6- Petersen coil earthing
A reactor tuned to the network capacitances is inserted between the neutral point and earth so that if an earth fault occurs, the fault current is zero.







Comparison between Neutral Grounding Methods

The performance characteristics of each neutral grounding method discussed above  can be compared as in below table:







Types Of Earthing Systems According To Neutral Point Connection To Earth + The Connection Method Of The Electrical Installations Exposed Conductive Parts.

According to Neutral point connection to earth + the connection method of the electrical installations exposed conductive parts, The Earthing systems can be divided to Five schemes as follows:

  • 3 Main Earthing Schemes,
  • 3 Sub-Main Earthing Schemes.







(3) Main Earthing Schemes


Each one of the 3 Main Types is defined by two letters as follows:

The First Letter:
It defines the situation of the neutral point in relation to earth, like:

T : solidly earthed neutral
I: unearthed or high impedance earthed neutral.

The Second Letter:
It defines the connection method of the electrical installation's exposed conductive parts, like:

T : the exposed conductive parts are interconnected and solidly earthed, regardless of whether the neutral point is earthed or not
N: the exposed conductive parts are directly connected to the neutral conductor.





So the three main earthing systems will be:

IT (Unearthed transformer neutral, earthed frame),
TT (Transformer neutral earthed, and frame earthed),
TN (Transformer neutral earthed, frame connected to neutral).







(3) Sub-Main Earthing Schemes


The Three sub-main earthing system are derived from the main  TN earthing system as follows:

TNC (If the N and PE neutral conductors are one and the same (PEN))
TNS (If the N and PE neutral conductors are separate),
TNC-S (Use of a TN-S downstream from a TN-C (the opposite is forbidden).

Where:

C : N and PE Combined,
S : N and PE Separate,
PEN: (Protective Earth and Neutral) conductor.


Note: 
Each system earthing can be applied to an entire LV electrical installation; however several system earthings may be included in the same installation.







IT earthing system

First letter I:
The neutral is unearthed or earthed via a high impedance, An impedance between 1000 and 2000 Ω is frequently used.

Second letter T:
The exposed conductive parts of loads are interconnected, either altogether, or in groups. Each interconnected group is connected to an earth electrode. It is possible for one or several exposed conductive parts to be separately earthed.






Notes:

  • Where possible, it is advisable to interconnect all the exposed conductive parts of the same installation and connect them to the same earth electrode. It is nonetheless possible for exposed conductive parts which are far away from each other, or located in different buildings, not to be. In this case, each group of exposed conductive parts connected to the same electrode and each individually earthed exposed conductive part must be protected by a residual current device.
  • The earth electrodes of the exposed conductive parts and the neutral may or may not be interconnected or the same.
  • It is not advantageous to distribute the neutral which results in the maximum length of wiring systems being reduced.
  • Installing an overvoltage limiter between the neutral point of the MV/LV transformer and earth is compulsory. If the neutral is not accessible, the overvoltage limiter is installed between one phase and earth. It protects the low voltage network against rises in voltage due to flashover between the transformer medium voltage and low voltage windings.







TT earthing system

First letter T:
The neutral point is directly earthed.

Second letter T:
The exposed conductive parts of the loads are interconnected, either altogether, or in groups, or individually, and are earthed. Protection is ensured by residual current devices. All the exposed conductive parts protected by the same protective device must be connected to the same earth electrode.






Note:

The neutral earth electrode and that of the exposed conductive parts may or may not be interconnected or the same. The neutral may or may not be distributed.







TN earthing system

First letter T:
The neutral point is directly earthed.

Second letter N:
The exposed conductive parts of the loads are connected to the neutral conductor.


the TN earthing system can be divided to 3 sub-main earthing schemes as follows:


1- TNC earthing system

Third letter C:
The neutral and protective conductors form a single conductor called the PEN.







Notes:

  • It is advisable to regularly connect the PEN to earth.
  • This system must not to be used for cross-sectional areas below 10 mm² for copper or 16 mm² for aluminium, as well as for mobile wiring systems. It is also forbidden downstream of a TNS system.




 2- TNS earthing system





Third letter S: The neutral conductor and protective conductor are separate.



3- TNC-S earthing system






In TNC-S earthing system, both TNC and TNS earthing systems can be used in the same installation. But the TNC earthing system (4 wires) must never be downstream of the TNS earthing system (5 wires).






In the next Article, I will explain the Selection Criteria for the Best Earthing System. Please, keep following.






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