Published: 22 April 2008
Category: Q&A
Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This one, concerning extraneous-conductive-parts and whether to bond, or not to bond, is answered by SELECT:
Question: Recently, I've lost the plot on whether to bond, or not to bond, and whether a conductor qualifies as an extraneous-conductive-part, and therefore needs bonding.
The scenario: on the ground floor of a block of flats, there is a copper tube supplied tap, which can simultaneously be accessible via the distribution board in the same cupboard. The pipe disappears into the concrete floor directly below. The water service pipe stopcock cannot be seen for inspection and so I cannot confirm that it is bonded. The earthing arrangement is TN-S.
Using my 'wonder lead', the earth fault path measures 0.02 ohms from the tap to the MET.
Using the formula - Rcp > (Uo / Ib) - Ztl
Where:
Rcp = is the measured resistance between the conductive part concerned and the MET of the installation (in ohms).
Uo = is the nominal voltage to earth of the installation (in volts).
Ib = is the value of current through the human body (or livestock) which should not be exceeded (in amperes), and
Ztl = is the impedance of the human body or livestock (in ohms).
Example:
Rcp > 230 / (0.5 x 0.001) - 1000 = 459,000 ohms.
Therefore, as the earth fault path measures 0.02 ohms, this is not greater than the calculated value of 459,000 ohms and so, needs to be bonded.
Is my interpretation of this formula correct and that it simply is, black and white, where no main bonding can be viewed or the stopcock cannot be found?
Answer: As you indicate, main equipotential bonding is required to the services in the installation, such as water and gas service pipes. Therefore, if this is the main service pipe, then it should be provided with bonding. If, however, it is found not to be the main service pipe, then main bonding would not be required at this point. In addition, it should be noted that supplementary bonding is now generally only provided in special locations such as bathrooms.
The scenario: on the ground floor of a block of flats, there is a copper tube supplied tap, which can simultaneously be accessible via the distribution board in the same cupboard. The pipe disappears into the concrete floor directly below. The water service pipe stopcock cannot be seen for inspection and so I cannot confirm that it is bonded. The earthing arrangement is TN-S.
Using my 'wonder lead', the earth fault path measures 0.02 ohms from the tap to the MET.
Using the formula - Rcp > (Uo / Ib) - Ztl
Where:
Rcp = is the measured resistance between the conductive part concerned and the MET of the installation (in ohms).
Uo = is the nominal voltage to earth of the installation (in volts).
Ib = is the value of current through the human body (or livestock) which should not be exceeded (in amperes), and
Ztl = is the impedance of the human body or livestock (in ohms).
Example:
Rcp > 230 / (0.5 x 0.001) - 1000 = 459,000 ohms.
Therefore, as the earth fault path measures 0.02 ohms, this is not greater than the calculated value of 459,000 ohms and so, needs to be bonded.
Is my interpretation of this formula correct and that it simply is, black and white, where no main bonding can be viewed or the stopcock cannot be found?
Answer: As you indicate, main equipotential bonding is required to the services in the installation, such as water and gas service pipes. Therefore, if this is the main service pipe, then it should be provided with bonding. If, however, it is found not to be the main service pipe, then main bonding would not be required at this point. In addition, it should be noted that supplementary bonding is now generally only provided in special locations such as bathrooms.
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