Hi everybody, I currently study for the 2391 theory (for march 2012 exam) and have a bunch of 38 questions which cannot answer yet.
I wonder if anyone here would like to give it a go. I've highlighted my possible answers to some of them. Any little help is much appreciated. Thanks :smile5:
PS: My study materials are : Guidance Note 3(17-th edition by IEE), BS7671:2008, PRACTICAL GUIDE TO INSPECTION TESTING & CERTIFICATION OF ELECTRICAL INSTALLATIONS (by Christopher Kitcher) and EXAM SUCCESS 2391-10 (by IEE)
1. Explain why maximum values of earth fault loop impedance
given for agricultural and construction sites are lower than
those for ordinary installations ?
Under 17th edition every agricultural electrical circuit must now be protected by either a 30mA, 100mA or 300mA RCD. ???
2. List three parts of the earth fault loop impedance path external to a
TN-C-S system? (3 marks)
1) Line supply conductor.
2) Neutral supply conductor via (PEN)
3) Transformer line winding.
3. An RCD to BS EN 61008 is to provide additional protection for a 13A socket-outlets, for use by an ordinary persons, intended for general use. State
a)the maximum permitted operating current (I∆n) of the RCD
b)the maximum test current when testing a device in a)
c)the maximum permitted disconnection time for test in b).
a) 30mA b)150mA c)40ms
4. When testing SELV on the secondary side: you'll set your IR meter to 250V DC ?
5. Calculate the maximum resistance (R1+R2) of an oven circuit in a domestic installation 7m from ccu. (3Kw)
6. Is this a correct statement?
Supplementary bonding can only be omitted if the following are in place:
- suitable main bonding connection for all extraneous parts
- disconnection times are met
- 30mA RCD additional protection is in place
7. The maximum tabulated values of the earth fault loop impedance and measured values for each of the circuits are as shown :
Circuit.............. MaxTabulated Zs..............ZsMeasured Zs
32A......................1.07................................. 0.83
10A......................3.43..................................2.4
6A........................5.71..................................4.4
My Calculations
1.07x0.8=0.85x1.2=1.02>0.83=ok
3,43x0.8=2,74x1.2=3.28>2.4=ok
5,71x0.8=4.56x1.2=5.47>4.4=ok
a) If the temperature at the time of test was 20 C (no correction factor) and the cable is 70 C p.v.c. (factor 1.2) determine by calculation whether the measured values are acceptable (Ze=0.4 ohms).
b) i) if correction factors were unavailable, which method would be used to make a valid comparison of the measured and tabulated values?
ii) using this method indicate whether the results would be acceptable. ( Show all calculations)
8. The client informs you that the Regional Electricity Company is shortly to reinforce their network locally and change the existing supply to TN-C-S. Explain the effect of such a change on earth fault impedance values and the rating of existing protective devices.
9. State: a) the 2 IP codes relating to penetration tests on enclosures that have had insulation applied on site
b) which other test must be performed on the enclosures(a)
10. Why the rule of thumb is applied for Zs calculation of a circuit?
Surely NOT because of the temperature or is easier than other calculations !
11. State the first 3 tests in order for testing a new installed lighting circuit.
1. Continuity of Protective Conductors
2. Insulation Resistance
3. Polarity
12. State 2 locations which required to lower RCD disconnection times
from 0.4 sec to 0.2 sec. TT systems ?
13. Give an example of restrictive conductive earth location.
Could it be the so called earth-free location?
14. State the pre-test action required before an insulation resistance test.
15. State what general documentation and information are necessary before doing an initial verification to a new installation.
16. State the meaning of :
a) EEBADS What EEBADS stands for ?
b) indirect contact ???
c) one other method against indirect contact ???
17. What is the main requirement regarding the use of an installation whilst inspection and testing is in progress ?
18. Name the earth conductor between structural steel work and MET.
19. State 1 method of live polarity test
20. State 3 possible methods of overcoming the problem of a high measured value of earth fault loop impedance (Zs).
21. State the form of protection against electric shock provided by the following items:
a) class II equipment By supplementary Insulation
b) placing out of reach Protection against accidental touch
c) EEBADS What EEBADS stands for?
22. IP rating for an accessible horizontal top surface of an enclosure
IP2X or/and IPXXB (not sure!)
23. Meaning of 25 in the formula L Zs<25
24. State the 3 main technical reasons for conducting and ring final continuity test.
25. A live polarity test is to be conducted. State:
a) why such test is necessary To ensure the correct functioning of equipment?
b) how N-E polarity is checked. ???
26. Why the measured value of Zs of a circuit may not be the same as Ze+(R1+R2)
Due to parallel paths of protective conductors ?
27. State: a) type of earthing arrangement system that would enable PSCC to be calculated from UOC and Ze.
b) expected value of UOC. What UOC stands for???
c) maximum opening time for a 100mA RCD device Maximum 300 ms ?
28. 2 possible reasons why a P-N continuity test value to one sock of a ring final is higher than the rest
1) that socket is not part of the ring (is a spur connection)
2)...???
29. State a BS7671 special location which would:
a) use 500mA RCD for protection against fire. Construction Sites ?
b) prohibit the use of a socket outlet other than SELV socket Same as answer in a) ?
30. Explain why: a) certain Edison E17, E27 screw type lampholders do NOT require a polarity check ?
b) max. values of earth fault loop impedance given to fuses protecting circuits on construction sites are lower than those for ordinary installations
c) 2 reasons for correcting measured values of earth fault loop impedance so that a comparison with maximum values can be made
31. A test result indicate that the measured value of a circuit is lower than the calculated Zs=Ze+(R1+R2). Indicate 3 possible reasons
1) Due to parallel path of protective conductors
2)....................???
3)....................???
32. State 3 reasons for conducting a polarity test on a newly installed lighting circuit
33. State 3 different practical situations that require the sense of sight during the inspection of steel trunking containment system
34. State the minimum acceptable IR test values for the circuits supplying equipment at:
a) 38V ???
b) 135V 1M ohm
c) 400V. 1M ohm
35. An earth electrode test has the following results: 128 , 120 and 134 ohms
Determine the value of Ra to be recorded and show all calculations
36. Briefly explain the outcome if a fuse is placed in neutral conductor only and then the fuse operates under overload conditions
37. State the maximum operating time for a 200mA general purpose RCD to BS EN 61008 300ms ( GN3 page 56 ???)
38. State: 2 statutory documents relevant to inspection and testing process
