F
firemanowl
Here is the second half of the long answer questions.
Q 24
a) Describe how an Insulation Resistance test would be carried out on the new lighting circuits (5 marks)
Identify the circuit to be tested and perform a safe isolation procedure ensuring that it is ‘dead’ and safe to test. Erect barriers and warning notices as required ensuring safety to all persons. Disconnect any voltage sensitive equipment that may be damaged by the test or that may give false readings, e.g. lighting control gear and neon lamps. Disconnect any loads such as lamps. Ensure all protective devices switched on and associated switches operated in all positions. Selecting an insulation resistance tester and suitable test leads (GS 38) and ensure in good condition and safe to use. Select 500 volt test range. Connect Line and Neutral together and test to Earth recording the result on the Schedule of Test Results. Now test between Line and Neutral, again recording the result. Note that on a new installation, a high reading, i.e. greater than 200 M ohms would be expected for L-E, N-E and L-N.
b) If an Insulation Resistance test between live conductors and earth on the whole installation resulted in a test value of 1.6Ω, explain, giving reasons, any action to be taken. (3 marks)
IEE Guidance Note 3 recommends that reading below 2 M-ohms should be tested / investigated further. This overall value of 1.6 M-ohms may be because all of the circuits have been tested as a block and due to parallel resistances an overall low reading results. Testing individual circuits would prove if this was the case and if it was, then the low reading could be ignored. However, if an individual circuit is found to have a low reading then this should be investigated and rectified by a fault finding process.
c) Describe how an external loop impedance test should be conducted. (4 marks)
As this is a ‘live’ test, suitable safety precautions should be taken to protect all persons. Barriers and warning signs could be used to achieve this. Select an Earth Fault Loop Impedance Tester tester and suitable test leads (GS 38) and ensure in good condition and safe to use. Select the appropriate range (0-20 ohms) and perform the test as required. The result should be recorded on the Schedule of Test and the checked for conformity with the Zs max values in BS 7671:2008 utilising the 0.8 ‘Rule of Thumb’ as required.
d) Describe how a prospective fault current test should be conducted at the origin of the installation (3 marks)
As this is a ‘live’ test, suitable safety precautions should be taken to protect all persons. Barriers and warning signs could be used to achieve this. Select an Prospective Fault Current Tester and suitable test leads (GS 38) and ensure in good condition and safe to use. Select the appropriate range and perform the test between Line and Earth (PEFC), Line and Neutral and Line to Line (PSSC) as required. The highest value of PFC should be recorded on the Schedule of Test Results (L-L value of PSSC in this case).
Q 25
The Maximum and measured values of loop impedance for four circuits are as follows:-
Max values Measured values
3.43Ω 2.7Ω 2.29Ω 1.7Ω 2.4Ω 1.9Ω 3.75Ω 3.2Ωa)
Explain the reasons for the use of the ‘rule of thumb’ (5 marks)
A ‘rule of thumb’ value of 0.8 x the Maximum Tabulated value of Zs is used to compensate for the increase in the value of resistance that occurs when the circuit is carrying its full design current Ib causing the conductor temperature to increase. This 0.8 value also allows for how readily the insulation allows the heat to be dissipated through it.
b) Using the above method, determine if the measured values are acceptable. (Show all calculations). (4 marks)
Method: Multiply Zs max by 0.8 and compare Zs measured with this corrected value. If the measured value is less than the corrected value then the circuit is acceptable. 3.43 x 0.8 = 2.744. 2.7 is just less so circuit acceptable. 2.29 x 0.8 = 1.832. 1.7 is less so circuit acceptable. 2.4 x 0.8 = 1.92. 1.92 is just less so circuit acceptable. 3.75 x 0.8 = 3. 3.2 is greater so circuit is unacceptable.
c) For any circuit that fails to comply, indicate two methods of overcoming the problem. (6 marks)
1)Reinforce the cpc by increasing the circuit cable size.
2)Down rate the value of In for the circuit protective device.
Q 26
a) Draw a labeled diagram of the earth fault loop path for any circuit in the installation. (8 marks)
See onsite guide
b) Explain why it would be inappropriate to use BS 3036 fuses in a distribution board at the origin of this installation (4 marks)
As the quoted value of PFC for the installation is 20kA, it would be inappropriate to use BS 3036 fuses in the distribution board at the origin as these fuses only have a short circuit breaking capacity of approximately 1.5kA and therefore would be unable to safely deal with the high value of available current.
c) Most standard PFC testers are designed for single-phase use. How could an appropriate value of short circuit current between phases be determined? (3 marks) Measure the value of PSCC between each Line and Neutral in turn. Multiply the highest obtained reading by 2 to simulate the Line to Line value. This will become the PFC value for the installation.
Q 24
a) Describe how an Insulation Resistance test would be carried out on the new lighting circuits (5 marks)
Identify the circuit to be tested and perform a safe isolation procedure ensuring that it is ‘dead’ and safe to test. Erect barriers and warning notices as required ensuring safety to all persons. Disconnect any voltage sensitive equipment that may be damaged by the test or that may give false readings, e.g. lighting control gear and neon lamps. Disconnect any loads such as lamps. Ensure all protective devices switched on and associated switches operated in all positions. Selecting an insulation resistance tester and suitable test leads (GS 38) and ensure in good condition and safe to use. Select 500 volt test range. Connect Line and Neutral together and test to Earth recording the result on the Schedule of Test Results. Now test between Line and Neutral, again recording the result. Note that on a new installation, a high reading, i.e. greater than 200 M ohms would be expected for L-E, N-E and L-N.
b) If an Insulation Resistance test between live conductors and earth on the whole installation resulted in a test value of 1.6Ω, explain, giving reasons, any action to be taken. (3 marks)
IEE Guidance Note 3 recommends that reading below 2 M-ohms should be tested / investigated further. This overall value of 1.6 M-ohms may be because all of the circuits have been tested as a block and due to parallel resistances an overall low reading results. Testing individual circuits would prove if this was the case and if it was, then the low reading could be ignored. However, if an individual circuit is found to have a low reading then this should be investigated and rectified by a fault finding process.
c) Describe how an external loop impedance test should be conducted. (4 marks)
As this is a ‘live’ test, suitable safety precautions should be taken to protect all persons. Barriers and warning signs could be used to achieve this. Select an Earth Fault Loop Impedance Tester tester and suitable test leads (GS 38) and ensure in good condition and safe to use. Select the appropriate range (0-20 ohms) and perform the test as required. The result should be recorded on the Schedule of Test and the checked for conformity with the Zs max values in BS 7671:2008 utilising the 0.8 ‘Rule of Thumb’ as required.
d) Describe how a prospective fault current test should be conducted at the origin of the installation (3 marks)
As this is a ‘live’ test, suitable safety precautions should be taken to protect all persons. Barriers and warning signs could be used to achieve this. Select an Prospective Fault Current Tester and suitable test leads (GS 38) and ensure in good condition and safe to use. Select the appropriate range and perform the test between Line and Earth (PEFC), Line and Neutral and Line to Line (PSSC) as required. The highest value of PFC should be recorded on the Schedule of Test Results (L-L value of PSSC in this case).
Q 25
The Maximum and measured values of loop impedance for four circuits are as follows:-
Max values Measured values
3.43Ω 2.7Ω 2.29Ω 1.7Ω 2.4Ω 1.9Ω 3.75Ω 3.2Ωa)
Explain the reasons for the use of the ‘rule of thumb’ (5 marks)
A ‘rule of thumb’ value of 0.8 x the Maximum Tabulated value of Zs is used to compensate for the increase in the value of resistance that occurs when the circuit is carrying its full design current Ib causing the conductor temperature to increase. This 0.8 value also allows for how readily the insulation allows the heat to be dissipated through it.
b) Using the above method, determine if the measured values are acceptable. (Show all calculations). (4 marks)
Method: Multiply Zs max by 0.8 and compare Zs measured with this corrected value. If the measured value is less than the corrected value then the circuit is acceptable. 3.43 x 0.8 = 2.744. 2.7 is just less so circuit acceptable. 2.29 x 0.8 = 1.832. 1.7 is less so circuit acceptable. 2.4 x 0.8 = 1.92. 1.92 is just less so circuit acceptable. 3.75 x 0.8 = 3. 3.2 is greater so circuit is unacceptable.
c) For any circuit that fails to comply, indicate two methods of overcoming the problem. (6 marks)
1)Reinforce the cpc by increasing the circuit cable size.
2)Down rate the value of In for the circuit protective device.
Q 26
a) Draw a labeled diagram of the earth fault loop path for any circuit in the installation. (8 marks)
See onsite guide
b) Explain why it would be inappropriate to use BS 3036 fuses in a distribution board at the origin of this installation (4 marks)
As the quoted value of PFC for the installation is 20kA, it would be inappropriate to use BS 3036 fuses in the distribution board at the origin as these fuses only have a short circuit breaking capacity of approximately 1.5kA and therefore would be unable to safely deal with the high value of available current.
c) Most standard PFC testers are designed for single-phase use. How could an appropriate value of short circuit current between phases be determined? (3 marks) Measure the value of PSCC between each Line and Neutral in turn. Multiply the highest obtained reading by 2 to simulate the Line to Line value. This will become the PFC value for the installation.
