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. 

Read: How to conduct polarity testing?

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. 

Read: What is Earth Fault Loop Impedance?

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

How to Select Overcurrent Protection Devices?

Circuit Protection Device

It is a standard rule that all electrical installations must be protected against overcurrent or short circuit by means of devices that will operate automatically to prevent injury to persons and livestock and damage to the installation, including the cables. As such, the overcurrent devices must be of adequate breaking capacity and be so constructed that they will interrupt the supply without danger. Also, the cables must be able to carry these overcurrents without damage. 

Fault currents

Fault currents arise as a result of a fault in the cables or the equipment. There is a sudden increase in current, perhaps 1O or 20 times the cable rating, the current being limited by the impedance of the supply, the impedance of the cables, the impedance of the fault and the impedance of the return path. The current should be of short duration, as the overcurrent device should operate.

Overload currents

Overload currents do not arise as a result of a fault in the cable or equipment. They arise because the current has been increased by the addition of further load. Overload protection is only required if overloading is possible. It would not be required for a circuit supplying a fixed load, although fault protection would be required.

For example, a circuit load supplying a 7.2 kW shower will not increase unless the shower is replaced, when the adequacy of the circuit must be checked against the new load criteria. A distribution circuit supplying a number of buildings could be overloaded by additional machinery being installed in one of the buildings supplied. 

Overload currents are likely to be of the order of 1 .5 to 2 times the rating of the cable, whereas fault currents may be of the order of 10 to 20 times  the rating. Overloads of less than 1 .2 to 1 .6 times the device rating are unlikely to result in operation of the device. 

British Standard 7671 and IEC 60364 requires that every circuit be designed so that small overloads of long duration are unlikely to occur. It is usual for one device in the circuit to provide both fault protection and overload protection. A common exception is the overcurrent devices to motor circuits, where the overcurrent device at the origin of the circuit provides protection against fault currents and the motor starter will be providing protection against overload.

Read: How to Select Proper Type of Miniature Circuit Breakers MCBs

Selecting protective devices

The type of protective device chosen will depend on a number of factors, including:

• the nature or type of load
• the prospective fault current P1 at that point of the installation
• any existing equipment
• the user of the installation, as a CB is easier to reset than a bolted­ type HRC fuse.

Breaking capacity

There is a limit to the maximum current that an overcurrent protective device (fuse or circuit breaker) can interrupt. This is called the rated short-circuit capacity or breaking capacity. BS 7671 and IEC 60364 requires the prospective fault current under both short-circuit and earth-fault conditions to be determined at every relevant point of the complete installation. This means that at every point where switchgear is installed, the maximum fault current must be determined to ensure that the switchgear is adequately rated to interrupt the fault currents.

Circuit breakers  have two short-circuit capacity  ratings. 

• Ics = is the value of fault current up to which the device can operate safely and remain suitable and serviceable after the fault. 
• Icn = is the value above which the device would not be able to interrupt faults safely . This could lead to the danger of explosion during faults of this magnitude or, even worse, the contacts welding and not interrupting the fault.

Any faults that occur between these two ratings will be interrupted safely but the device will probably require replacement.

circuit breaker, selection of protective devices, fuses, shot circuit, overcurrent, over-current, show to select overcurrent protection devices.