Grounding vs Bonding - What you should know

Have you ever wondered what the difference was between bonding and grounding and its application in:

-    Overhead transmission lines
-    Transformer grounding
-    Substations

If yes … lets get into it.

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Grounding vs Bonding

If you are an electrical worker that actively engages with high-voltage equipment, which is anything amounting to more than 750 volts, then you should really understand high voltage grounding including the concept of grounding and bonding. 

The general difference between equipotential bonding and equipotential grounding is the event of an electrical fault or surge, lighting strikes or accidental contacts, electricity will flow through the bonded components. The key difference between bonding and grounding is that bonding distributes an electrical charge while grounding neutralizes it. 

With the differences established, let’s discuss the matter of grounding and bonding systems in equipment.  

Although structurally they vary for different equipment, the primary functions are:
-    Provide an essential connection to earth through the neutrals of generators, transformers, capacitors, and reactors.
-    Create a low resistive earthing path for fault currents arising from lightning rods, surge arresters, gaps, and related devices (unless using a grounding resistor, in which ground fault current is limited)
-    Limit the potential differences that appear between metallic objects or structures, and the ground (GPR), due to the flow of currents travelling through ground.
-    Aid relay operations to clear ground faults.
-    Increase the reliability and availability of the electrical system.
-    Provide a grounding path for de-energized equipment during (routine) maintenance.

Grounding and bonding history: Linesmen
Up until fifty years ago, parallel grounding was almost exclusively used as the main electrical safety grounding technique to "protect" the worker from dangerous shock hazards when working on high-voltage equipment. However, the issue was that it wasn’t working as there would still end up being some voltage drop across the person's body at the worksite which would ultimately lead to deadly shocks. 

From a personal protective grounding equipment perspective, if the connections between grounding cables, phase conductors, and earth were not perfect, due to oxidization for example, then achieving a system voltage near zero was next to impossible.
 

In the case of a lineman working on a wooden pole, the pole would act as a conductor to the ground. So effectively you had a set of parallel circuits to ground, one through the ground conductors and one through the worker and the pole. This lineman would be a light bulb that kept on shining.

 

From the diagram above, it can be observed that when a lineman is working on a wooden pole, the pole would act as a conductor to the ground. This is where the parallel grounding concept came from as you had a set of parallel circuits to ground, one through the worker and the pole and one through the ground conductors. This lineman would effectively be a shining light bulb.

So how does equipotential grounding solve this?

Combatting this issue is relatively simple. By ensuring the ground conductors and the pole are at the same potential through the utilization of a ground cluster. The ground cluster can be described as a chain that wraps around the pole and is tightened with a wheel binder. The cluster is placed just below the worker's feet and is then attach to the grounding conductor.

 

 

 

This effectively makes the worker a bird on a wire. Should an accidental re-energization occur in the nearby transmission line, there would be no difference in potential across the worker's body.

The overcurrent devices should trip during such an occurrence and the effective grounding will ensure the worker will stay safe, provided that the grounds were sized correctly, and proper installation practices were followed.

Grounding a Transformer

The concept of grounding and bonding can also be extended to transformers and is a requirement for the safety of the electrical system and personnel alike. The most common methods for grounding are as follows:

1.    Single-phase, 3-wire solidly grounded systems
a.    a line side connection combining the neutral and bonding conductor into a single conductor known as the system grounded conductor. 
b.    Fault current is not limited but has a path to ground.

2.    Three-phase, 4-wire solidly grounded systems
a.    A grounding system which also uses a system grounded conductor but is utilized per phase. 
b.    Fault current is not limited but has a path to ground.

3.    Three-phase, 4-wire impedance grounded system.
a.    A ground connection via high impedance to limit fault current and the neutral may or may not be distributed.
b.    Fault current is limited based on impedance value that’s in series with the ground.

Regardless of the implemented grounding and bonding method used, there are several key components present in the overall system which are:

1.    System bonding jumper
2.    Grounding electrode and grounding electrode conductor
3.    Bonding of metal water piping system(s) and exposed structural metal
4.    Supply-side bonding jumper 
5.    Equipment grounding conductor

Implementation of a proper grounding and bonding system is critical as improper installation or implementation may cause:
1.    fire hazards
2.    electrocution
3.    improper operation of protection devices,
4.    power quality problems. 

In order to prevent these hazards, its paramount to stop any neutral return current from flowing on electrical equipment, grounding paths, and bonding paths as required by the Electrical Code. To facilitate this, the grounded (neutral) conductor must be separated from the metal parts of equipment, except as required for service equipment and on transformers.

Furthermore, it is advised that the grounding conductor be properly sized to be able to effectively handle any fault current it may experience. For example, the conductor in the image below should be a higher gauge to distribute fault current effectively. Failure to do so may result in melting and possibly melting to other pieces of the equipment.

 

Image by ECM

Sub-Station Ground Grid

Substations are critical pieces of equipment that are utilized to distribute overhead power to close by consumers, be they residential or commercials.

These areas tend to have large currents flowing through the site which necessitates the need for proper grounding and bonding. A substation equipotential for bonding scheme can be seen below:
 
 

 

Where the bonding scheme bonds the switchyard, control room, and any key devices using physical connections between equipotential bonding busbars and metallic physical structures.

The bonding system is then connected to the substation grounding network via a parallel grounding conductor to an earthing busbar.  

From a grounding perspective, the most common configurations for substation grounding to earth are as follows:

•    Radial- a string of grounding electrodes with connections to each device in the substation. Cheapest solution but produces the biggest energized ground zone during a ground fault.

•    Ring- a set of conductors placed around the area occupied by the substation equipment and structures and connected to each through short links. The energized surface zone during a ground fault is reduced compared to the radial configuration, since the current travels through several paths.

•    Grid/Earth mat- As the name suggests it’s a configuration through a combination of horizontally laid out copper conductors slightly below grade (like a squared grid or mat). This system is connected to the bonding scheme of the substation equipment and metallic structures which in turn are connected to grounding rods which disperse the ground current deep into the earth. This configuration has the least energized surface area but is also the most expensive to implement.

The total grounding and bonding scheme of a substation should resemble something shown below.

 

Conclusion

I hope you found this article useful and if you did, please share it using the social media buttons at the bottom of the post!  

 

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