just started the 2391

[i=p/u]

this thread will go on for quite a while (14 WEEKS), to exams... ive nearly read gn3 for the first time and there is a few things i dont fully understand but im sure i will for when the time comes from help from you guys/dudes/punks who know just a little to much.

 

[i=p/u]

is that it

 

[i=p/u]

this one...the insulation resistance reading take between neutral and earth is 50M ohms(10m of 2.5mm twin cable) what would the resistance of 20m of the same cable? id say if 10m = 50Mohms then 20m = 25Mohms

 

[i=p/u]

calculate the maximum resistance(R1+R2) of a lighting circuit in a domestic installation with the last ceiling rose placed 32m from cu.switch drops are 3m.

1.5/1.0mm = 30.20/1000 x 35m= 1.057 ohms

 

[i=p/u]

calculate the maximum resistance (R1+R2) of an oven circuit in a domestic installation 7m from ccu.(3Kw)

would you just use the lowes cable you can get away with for 3Kw , 13.04A so 1.5mm

 

[i=p/u]

be back later... 2391 night , 40 miles away, im on course to success

 

[Richard Burns]

Q. periodic inspection involves?
Possibly:
Inspection and testing of an existing installation to provide for:
The safety of persons and livestock against electric schok and burns, regulation 131.1
the protection of property against damage by fir and heat from defects in the installation
Confirmation that the installation is not damaged or deteriorated so as to impair safety
Identification of defects/ non compliances which may cause danger.

what about this one... the resistance of 100m of 4.5mm cable is 0.36 ohms, what is the resistance of 600m of this cable, isit just a matter of 0.36x6=2.16 ohms directly proportional Yes

..this one...the insulation resistance reading take between neutral and earth is 50M ohms(10m of 2.5mm twin cable) what would the resistance of 20m of the same cable?id say if 10m = 50Mohms then 20m = 25Mohms Yes

calculate the maximum resistance(R1+R2) of a lighting circuit in a domestic installation with the last ceiling rose placed 32m from cu.switch drops are 3m.

1.5/1.0mm = 30.20/1000 x 35m= 1.057 ohms

calculate the maximum resistance (R1+R2) of an oven circuit in a domestic installation 7m from ccu.(3Kw)

would you just use the lowes cable you can get away with for 3Kw , 13.04A so 1.5mm
Not certain what they want on these two, there is not enough information, for C&G to ask you to guess cable size is not likely. Maximum resistance is a bit odd too, since you are calculating expected not maximum resistance.
A domestic cooker over 2kW would be wired in 6mm on a B32 breaker.

 

[i=p/u]

spoke to tudor about these questions and he said very likely wouldnt come up but he trying to show us the relationship between cables i.e

if the resisttance per 100m of 2,5mm csa cable is 0.06 ohms what is the resistance of 100m of a cable of the same conductor material with a csa of 6mm

R2 = R1 x A1 / A2
R2 = 0.06 x 2.5 / 6
R2 = 0.06 x 0.416 = 0.025 ohms



if the resistance of 100m of 4mm cable is0.08 ohms, what will be the resistance of 700 of 10mm cable of the same material?


i used the book values and got10mm=1.83m/ohms per meter /1000 x 700 = 1.28 ohms


R2 = R1 x L2 / L1 x A1 / A2

= 0.08 x 700 / 100 x 4 / 10

= 0.08 x 7 x 0.4 = 0.224 ohms

 

[i=p/u]

where supplementry bonding is provided as a dditional protection, it must include all simutaneously accessible exposed conductive parts, extrabeous conductive parts and the protective conductors of all equipment in the location being applied.
the effectivness of supplemenrty bonding can be verified by satisfying the following condition R<50/Ia

where R is the resistance between simultaneously accessible exposed conductive parts,
50 is the maximum safe touch voltage,
Ia is the current causing automatic operation of the protective devices within 5 secs or IAn raiting for rcds

now i understand if 30mA rcd is used you get 1666 ohms , now what happens if was no rcd what value for Ia would you use.., say was a bathroom with 40A shower R= 50/200=0.25 ohms, 6A lighting circuit R=50/30=1.7 , a radiator and towel rail
and a hotpress with copper heating tank

so does all the circuits need to be bonded together..

i know if main protective bonding is in place you dont need all this... just put me straight

 

[Richard Burns]

now i understand if 30mA rcd is used you get 1666 ohms , now what happens if was no rcd what value for Ia would you use.., say was a bathroom with 40A shower R= 50/200=0.25 ohms, 6A lighting circuit R=50/30=1.7 , a radiator and towel rail
and a hotpress with copper heating tank

so does all the circuits need to be bonded together..
so R must be less than 0.25 ohms and all extraneous / exposed conductive parts must be connected to the CPCs of the circuits involved.
i know if main protective bonding is in place you dont need all this... just put me straight

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

The Ia value used if an RCD is not in place is the current causing disconnection time of 5s for the protective device used (5x for B type MCB) or look them up in time current characteristics in BGB Appx 3.

 

[i=p/u]

in my kitchen, the water pipe has main protective bonding, and supplementry bonding..... in my bathroom the raidator has one pipe earthed, which i think is tied into the light and reason i think this is, is because yes the shower is on a rcd but the lights aint.... and the lights is within 2.5m of the raidator

 

[i=p/u]

and when you look at reg 701.415.2 it more less says if main bonding is in place water , heating etc there is no need for supplementry bonding

 

[lennox]

hi there

i have just started the 2391 also and my tutor has stated that one of the conditions to not install supplementry bonding is that there must be a continuity reading of within 0.05 ohms between all conductive parts and extraneous conductive parts.page 35 of guidance note 3,paragraph 4.aswell as it having a rcd protection on all circuits in the bathroom and also main protective bonding

 

[i=p/u]

so would you say if main bonding is in place , i.e water,central heating/gas, and rcd installed no need for supplementry bonding... what is the test you do with insulation resistance, is that to see if something needs bonded/in contact with earth potential

 

[Richard Burns]

Replied a bit fast there and forgot to say all accessible extraneous / exposed conductive parts.

There is some discussion that simultaneously accessible could be as low as 1.25m, but using 2.5m (within arms reach) is a safe approach.

It is important to remember that there are 3 requirements not just an RCD and main bonding, but also that the circuit meets the ADS requirements (even though it is unlikely that an RCD protected circuit would not meet these requirements).

The measurement of continuity <0.05 ohm between parts is a test to ensure that inaccessible supplementary bonding is in place. If the continuity is <0.05 ohm then it may be assumed to be in place.

To test to see if extraneous conductive parts are extraneous you do an IR test between the part and earth (CPC or earthed metalwork) and if the result is >22kohm (some say 16kohm for 30mA not 10mA protection) then the part is not extraneous as it will not be able to introduce an earth potential.

 

[i=p/u]

what is the significance of the value obtained and recorded from step 3 of continuity of ring final circuit conductor test

id say is part of you earth fault loop impedance value and has to be below a set value.. and can calculate voltdrop from this reading for the circuit..

maybe im saying this all wrong so if i am share you cool , il go cheeers

 

[i=p/u]

a ring final circuit continuity test revealed incorrect polarity on three socket outlets

socket L/N L/CPC
A open circuit correct

B correct open circuit

c open circuit open circuit

state wheather or not the reading for each socket are satisfactory and indicate which conductors have reversed polarity....

does this sound like im reading the question right... jeeees.... any pointers/help out there for a monkey tryin to worm his way intot the electrical world

 

[Richard Burns]

what is the significance of the value obtained and recorded from step 3 of continuity of ring final circuit conductor test

id say is part of you earth fault loop impedance value and has to be below a set value.. and can calculate voltdrop from this reading for the circuit..

maybe im saying this all wrong so if i am share you cool , il go cheeers

Step three is measuring the resistance of the figure 8 L-E loop. The value obtained is compared to the (r1+r2/4) result and should be within 0.05 ohms of this value.

The significance of the value obtained and recorded is that this value is your R1+R2 used in Ze+(R1+R2) for determining your Zs, earth fault loop impedance, for the circuit, so that you can ensure that the disconnection times for the protective device are met.

a ring final circuit continuity test revealed incorrect polarity on three socket outlets

socket L/N L/CPC
A open circuit correct

B correct open circuit

c open circuit open circuit

state wheather or not the reading for each socket are satisfactory and indicate which conductors have reversed polarity....

does this sound like im reading the question right... jeeees.... any pointers/help out there for a monkey tryin to worm his way intot the electrical world

Socket	L/N continuity	L/CPC continuity	Reversed polarity
A	open circuit	correct	Line CPC reversed
B	correct	open circuit	Line Neutral reversed
c	open circuit	open circuit	Neutral CPC reversed

All these results are not satisfactory, as they should all be correct.

For all sockets, because both tests involve the Line conductor and you know that reversed polarity exists, if there is a "correct" reading this must be because it is between these two conductors that the reversed polarity has occurred.
For socket C both readings are wrong and so the fault cannot involve the Line conductor, therefore it is a N/CPC reverse.

Sorry is this is not clear it is very difficult to describe.
This made me have to think, not part of the plan!

 

[i=p/u]

thanks did help,,,,

 

[ian65]

Hi just looking through posts and i am taking my 2391 (Written) again on 09/02/12 Thursday doing all the revision i can and looking to remember or should i say forget what i did wrong in Oct 22 last year.

I got baffled and new i had not got a good mark to pass, just thinking i have a better chance and feel more confident this time round..

Any one with a fail like my self just go for it again.

Thanks to the posts by people (members) on this forum i hold a better and full prospective on the testing side apart from doing it at work as a one off at our new factory with a 2391 holder.

Cheers to all and fingers crossed..

Ian.

 

[NickS]

I myself have found this thread major league helpfull, I should use the forums more often that's for sure.
Thanks

 

[onfire25]

2391-301 Written Exam Paper February 2012


Section A – All questions carry equal marks. Answer all questions. Show all calculations.



1 State three documents on which the value of prospective fault current (PFC) should be recorded.


2 State three items of information that should be made available to a person carrying out an initial verification of an electrical installation.


3 A new circuit is to be installed in a small factory. State one

a) statutory document that the installer must comply with

b) non statutory document that the installer should comply with

c) non statutory document which may need to be referred to during the initial verification.


4 A periodic inspection and test is to be carried out on a large commercial unit. State.

a) what must be agreed with the client

b) when the agreement in a) should take place


5 State three different practical situations that may require the sense of sight during the initial inspection of a steel trunking containment system.


6 State three GS38 requirements which apply to test probe leads.


7 State the

a) test instrument that would be used to carry for a continuity of a main protective bonding conductor

b) one reason for selecting the test instrument in a) above

c) the purpose of the conductor in a) above


8 State, in the correct sequence, the first three tests to be carried on completion of a newly installed lighting circuit.


9 A test is carried out on a lighting circuit to obtain R1+R2. Explain briefly the purpose of this test.


10 State the effect each of the following would have on the measured insulation resistance of a conductor.

a) decrease in length

b) decrease in CSA

c) decrease in temperature


11 State the effect each of the following would have on the measured insulation resistance of a conductor.

a) increase in length

b) increase in CSA

c) increase in temperature


12 State the minimum acceptable insulation resistance test values for each of the following circuits, supplying equipment at

a) 38V ac

b) 135V ac

c) 400V ac


13 State three reasons for conducting a polarity test on a newly installed lighting circuit

14 State the level of protection provided by equipment constructed to

a) IP4X

b) IPX4

c) IP2X


15 An earth electrode resistance test has been carried out, and the following results were obtained: 128Ohms, 120Ohms and 134Ohms. Determine the value of RA to be recorded on the certificate. Show all calculations.



16 BS 7671 requires fuses to be placed in the line conductor. Briefly explain the outcome if a fuse is placed in the neutral conductor only and the fuse then operates under overload conditions.


17 State, in which of the public distribution systems, each of the following components would be found.

a) earthed metallic sheath

b) PEN conductor

c) installation earth electrode


18 State, specifically, from the formula Zs=Ze+(R1+R2), what is represented by

a) R1

b) Ze

c) Zs


19 A 200mA general purpose RCD to BSEN 61008 is to be tested to ensure it operates within specification. State the

a) test instrument to be used

b) the maximum test current to be applied during the test

c) the maximum acceptable tripping time during the test in b) above


20 A prospective fault current test is to be carried out on a single phase supply. State

a) the two tests which are to be undertaken

b) which value from a) above will be recorded as the PFC

Questions 21-26 to follow shortly once typed up........

 

[vernam616]

How the hell did you get holdof that paper?

 

[onfire25]

Guy walked out of the room to have a smoke so i got the phone out

 

[tony mc]

Surprised you were allowed a phone I think my invidulator was trained by the gestapo.

Pens pencils calculator that was your lot!

Questions seem straight forward mind you!

 

[vernam616]

Very good i like it

Anyone talking to each other sharing answers? And how did you find it (section b) of course

 

[Dean]

Exam in Feb still refers to in as a PIR? I thought it would be EICR?

 

[tony mc]

Its still Periodic Inspection and Testing only the report has changed to EICR!

 

[onfire25]

2391-301 Written Exam Paper February 2012


Section B – Questions 21-26 refer to the following scenario. Answer all questions. All questions carry equal marks. Show all calculations.

The 400/240V three-phase, four-wire electrical installation within a three year old small industrial unit, forms part of a TN-S system and is to be inspected and tested during normal working hours.



The unit has a new tenant who requires an additional lighting circuit in the office area. The details of the protective devices utilised within the metal-clad TP&N distribution board are shown on Figure 1, attached.



The containment system within the workshop is surface mounted steel conduit and trunking. The office containment system is surface mounted PVC conduit and trunking.


All wiring is thermoplastic insulated single-core cables with copper conductors. The new lighting circuit is to be wired using the same installation method as the existing office.



The outside lighting circuits are controlled by a photo electric cell mounted on the roof of the installation.



21 State
a) the types of inspection and test that are required on completion of all of the works.
b) the documents that must be completed following this inspection and test.
c) two statutory documents that are relevant to the inspection and testing process
d) the title given in law to the inspector whilst carrying out the inspection
e) the legal status of the inspector
f) three items of information that must be available to the inspector which may be obtained from the DNO


22 a) identify two specific items on a new containment system which are to be inspected, using the sense of touch, before the cables are installed, stating what is being checked for in each case
b) identify three items on the existing containment system which are to be inspected using the sense of sight stating what is being checked for in each case


23 The existing ring final circuit in the workshop area is to be inspected. All socket outlets are connected correctly to the circuit. Step 1 of the test process produced the following results: Line loop 0.68Ohms, neutral loop 0.70Ohms and cpc loop 1.15Ohms.
a) state one possible reason why the cpc value is higher than the live conductors
b) calculate the expected measured values for steps 2 & 3 of the test
c) state all items that would be recorded following the successful completion of the test
d) under what circumstances would the ring final circuit continuity test not be required during the inspection and test
e) for polarity to be confirmed, what action is required if the tests have taken place at the terminals of the socket outlet and not from the socket fronts?


24 An insulation resistance test is to be carried out on the outside security lights. Describe the procedure for this test.
25 Figure 1, attached, shows the results from a previous inspection and test. State, using the information in Figure 1 attached
a) seven items which may require special consideration excluding equipment vulnerable to testing
b) the reasons why the items given in a) above are of concern
d) whether the equipment vulnerable to testing information is correct


26 Describe, with the aid of a fully labelled diagram, the earth fault loop impedance part for the new lighting circuit

 

[NickS]

onfire25 Brill thanks for the heads up. ;-)

 

[ian65]

The best part about asking the qestion about Tabulated values, there wasn't any real questions like it in the exam Cheers Ian.

 

[ian65]

My god word for word the exam i sat on Thursday is above lol. Thanks for the post mate i can now look through and see whats what cheers.. Ian.

 

[i=p/u]

when doing R1 and R2 on 3 phase do you test from each phase to earth and record all

 

[tony mc]

Test all 3 which should be the same and then record the highest value on you cert as the R1 + R2 for that cct.

 

[i=p/u]

so say we have 3phase switched fuse then maintenance isolater the specific 3 phase item , we test at all these ponts which confirms are polarity too..

 

[ovi4]

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

 

[tex431]

What answers did anyone get to 25a?