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Showing posts with label electrical grounding. Show all posts
Showing posts with label electrical grounding. Show all posts

Wednesday, November 18, 2020

How to Properly Test the Protective Bonding Conductor?

 

Electrical Installation Showing Main Protective Bonding 

BS 7671 strictly require that the protective bonding conductors are unbroken and have a resistance that is low enough to satisfy the standard. Protective bonding ensure that when fault occurs in the system, a dangerous potential will not occur between earthed metalwork (exposed conductive parts) and other metallic part of the building. When the protective bonding is visible in the entire location, a visual inspection will be enough. However, most of the installations have embedded wirings which make it difficult for visual inspections. Thus, testing is really necessary.  


The purpose of testing protective bonding conductor is to ensure a sufficient earth path, where in the event of a fault the exposed and extraneous conductive parts will have the same potential. That is why it is called ‘equipotential bonding’. In order to achieve this it is recommended that the resistance of the bonding conductors does not exceed 0.05Ω.


BS 7671 provide a recommendation for the maximum length of copper protective bonding conductor. 


Credit: Practical Guide to Inspection, Testing & Certification of Electrical Installations

 

As mentioned above, in many electrical installations, visual inspection is not possible since wirings are usually embedded. Protective bonding test can only be carried out during initial verification; this is because one end of the bonding conductor must be disconnected to avoid the measurement including parallel paths. We have to note that when disconnecting bonding, it is necessary that the installation is isolated from the supply. On larger installations it is often impossible to isolate the complete installation and therefore the conductor must remain connected. 


In this case, the low resistance ohm meter must be set on the lowest possible value of ohms and the leads must be nulled or the instrument zeroed. 

Step 1

Perform safe isolation procedure. Apply lock out device. 

Step 2

Disconnect one end of the protective bonding (see figure below). If possible disconnect at the consumer’s unit and test from the disconnected end and the metalwork which the bonding is connected to. This will test the integrity of the bonding clamp.



Step 3

Connect the leads of the multi-meter together and make sure that the reading is zero. 


Step 4

Connect one lead to the disconnected conductor. 


Step 5

Connect the other lead to the metalwork close to the bonding clamp



Step 6

Remember to subtract the resistance of the leads from the total measured resistance, if the multimeter has not been zeroed before the test. If the meter and leads have been zeroed then the value measured will be the resistance of the bonding conductor. 


Step 7

Ensure that the bonding conductor is reconnected upon completion of the test. 

The measured resistance must a value of not more than 0.05 ohms, as any parallel paths will result in the resistance measurement being lower. In case that the measured resistance of greater than 0.05 ohms, it must be considered as non- compliant and make a recommendation for improvement. 


Reference: 

  • Practical Guide to Inspection, Testing & Certification of Electrical Installations, by: Christopher Kitcher
  • City and Guilds
  • BS 7671
protective bonding conductor, earthing, BS 7671, IEC 60364, electrical installation

Wednesday, November 04, 2020

What are the Different Techniques to Ensure Effective Grounding?

 


Grounding is important in an electrical system since it provide the lowest resistance path to the ground. Circuit protection device such as circuit breakers have maximum allowable resistance to operate properly. In case that the required value of resistance goes beyond the allowable value, it will not trip in due time. The right value of earth fault loop impedance is needed.


Read: What is Earth Fault Loop Impedance?


For example, the table below shows the maximum value of overall earth fault loop impedance in order to comply with BS 7671 and IEC 60364 to disconnection time. 


In general, the following are the purpose of effective grounding. 

  1. To provide safety to personnel during normal and fault conditions by limiting step and touch potential.
  2. To assure correct operation of electrical/electronic devices. 
  3. To prevent damage to electrical/electronic apparatus. 
  4. To dissipate lightning strokes. 
  5. To stabilize voltage during transient conditions and to minimize the probability of flashover during transients. 
  6. To divert stray RF energy from sensitive audio, video, control, and computer equipment.

Techniques to Ensure Effective Grounding


1. Use of Ground Electrode (single electrode)

In this article, we refer to ‘ground electrode‘ or ‘grounding system‘ to describe these different methods of grounding. It should be noted that there are many different types of grounding systems available. 


The type installed will depend on the local conditions and the required function of the grounding system The simplest form of grounding element is the ground stake, this can take many forms with a variety of lengths from a few feet to many feet long made of materials such as brass, galvanized or stainless steel, the size and material as required locally The simple ground rod can be used for lightning protection on stand-alone structures such as pole mounted transformers or radio towers, it can also be used as a back up to a utility ground.





2. Multiple Grounding Electrode

A group of connected electrodes will have a more complicated interaction, typically configurations like this are present around substation sites and sensitive buildings. A slightly more complicated version of the rod system is the ground rod group, this is typically for lightning protection on larger structures or protection around potential hotspots such as substations.

This is typically for lightning protection on larger structures or protection around potential hotspots such as substations.



3. Ground Plates

This technique is used for areas where there is rock (or other poor conducting material) fairly close to the surface ground plates are preferred as they are more effective. Ground plates are used widely in telecoms applications.  They are particularly good where the deeper ground has high resistivity.
For areas where there is rock (or other poor conducting material) fairly close to the surface ground plates are preferred as they are more effective. 



4. Ground Mesh

A ground mesh consists of network of bars connected together, this system is often used at larger sites such as electrical substations. Ground meshes can be part of the foundations of structure.  At substations and generating site the metal parts of the foundations will all be bonded together and form part of the overall grounding systems.  At substation site an area of ground could be reserved at the start of the life of the substation with a ground mesh under the whole of the site.  As the site grows over a period of years new equipment can easily be installed and grounded by the mesh.  This ensures that the whole of the site remains at the same potential should a fault occur.



As a part of the design, we need to consider the soil resistivity so that we can determine the exact grounding techniques that we are going to use. Soil resistivity varies widely depending on soil type, from as low as 1 Ohmmeter for moist loamy topsoil to almost 10,000 Ohm-meters for surface limestone. Moisture content is one of the controlling factors in earth resistance because electrical conduction in soil is essentially electrolytic. 

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