2391 questions

[brucelee]

Hi to all,
I am taking a 2391 course at minute written exam in june. I have a few questions iI am struggling with if anyone would be kind enough to share there knowledge i would appreciate it
1) when testing selv whats the insulation resistance value before further investigation cant find i gn3 tutor first said 5.0Mohm then 2.0Mohm so confused
2) when asked about first four tests for a new lighting circuit is it
. continuity of protective conductors inc main and supplementary bonding
. Insulation resistance
. polarity live test
.earth fault loop impedence
or would you include dead test polarity by continuity methods
3) when circuits dont meet max Zs two methods of overcoming this would you install rcd and supplementary bonding how would i answer this question or would you say shorten cable length ,increase csa
4) when carrying out continuity of ring final circuit conductors whats the reason you would select this instrument is it because it is capable of measuring the difference e.g.between a 1mm cable and a 10 mm
5) when asked to explain a test on a radial socket outlet circuit do i have to include testing Line-Neutral as well as Line-cpc so as to prove polarity?
6) three outcomes to be recorded on a schedule of test results after conducting ring test
i have more but i leave it at that for now
Think i am getting things mixed up because of all revising any help would be extremely helpful cheers

 

[Notsosmart]

Hi to all,
I am taking a 2391 course at minute written exam in june. I have a few questions iI am struggling with if anyone would be kind enough to share there knowledge i would appreciate it

1) when testing selv whats the insulation resistance value before further investigation cant find i gn3 tutor first said 5.0Mohm then 2.0Mohm so confused
0.5M is the minimum figure quoted by bs7671 but 2M is generally held as the minimum before investigation is required (Gn3 3.10.3b), & this is what you put on the answer sheet.

However most experienced testers would investigate if figures fall below 5-10Mohms which is what your instructor was probably getting at.

2) when asked about first four tests for a new lighting circuit is it
. continuity of protective conductors inc main and supplementary bonding Number1
. Insulation resistance Number2
. polarity live test ?
.earth fault loop impedence Number4
or would you include dead test polarity by continuity methods Number3 (part of number 1 but you include the switch + remember edison screw fittings)

3) when circuits dont meet max Zs two methods of overcoming this would you install rcd and supplementary bonding how would i answer this question or would you say shorten cable length ,increase csa

Shorteninhg cable length may not be an option so yes perhaps RCD, but how about changing the rating or type of protective device to increase the permissable Zs?


4) when carrying out continuity of ring final circuit conductors whats the reason you would select this instrument is it because it is capable of measuring the difference e.g.between a 1mm cable and a 10 mm ?
Not sure what instrument you are talking about


5) when asked to explain a test on a radial socket outlet circuit do i have to include testing Line-Neutral as well as Line-cpc so as to prove polarity?

No R1+R2 should be sufficient

6) three outcomes to be recorded on a schedule of test results after conducting ring test
i have more but i leave it at that for now
Think i am getting things mixed up because of all revising any help would be extremely helpful cheers

Bruce i hope this is helpfull I'm sure others will add to it.

 

[Risteard]

Further investigation required if under 1 Megohm for ELV is it not? 2 Megohms for LV.

 

[brucelee]

Bruce i hope this is helpfull I'm sure others will add to it.

thanks mate
first week i asked instructor he said it was 5.0Mohm for selv but this week he told class it was 2.0Mohm which is what i thought originally
Q4 was of a sample exam and all it says is
state
a) the instrument that would be used to carry out the test for continuity of ring final circuit conductors
b) the reason for selecting this instrument
c) the range of results that would typically be needed to confirm ring final circuit continuity in a domestic environment
if asked for legal status of inspector and tester is it a Competent person?

 

[brucelee]

some more questions for 2391
Can someone please tell me answers to any of the following

1. state three operational factors which must be considered when carrying out inspection and testing

2. When asked to state external fault path for different systems for a TT system would i answer consumers earth electrode, general mass of earth, suppliers earth electrode

3. When asked to explain why its necessary to carry out a continuity of protective conductors before an insulation test
would i answer with. to check cpc continous or full circuit not tested

cheers for any replies

 

[brucelee]

Bruce i hope this is helpfull I'm sure others will add to it.

cheers notsosmart mate for replies tutor has given me three answers for further investigation on selv he first said 5Mohm then 2Mohm ow he saying no figure stated so he has just caused confusion
only value i can find i GN3 is 2Mohm so im going with this
also mate on the question about first four tests on a new lighting circuit just to clear it up its
1 continuity of protective conductors inc main and supplementary bonding
2. insulation resistance
then getting mixed up with next two please help
sorry for confusion

 

[Risteard]

cheers notsosmart mate for replies tutor has given me three answers for further investigation on selv he first said 5Mohm then 2Mohm ow he saying no figure stated so he has just caused confusion

As I stated earlier the correct answer for ELV is a minimum insulation resistance of 0.5 Megohms with further investigation required if under 1 Megohm.

 

[brucelee]

As I stated earlier the correct answer for ELV is a minimum insulation resistance of 0.5 Megohms with further investigation required if under 1 Megohm.

sorry for my ignorance but where do you get that from it doesnt say it anywhere

 

[Risteard]

sorry for my ignorance but where do you get that from it doesnt say it anywhere

It's in BS7671.

 

[brucelee]

cheers risteard
dont suppose you could tell me what operational factors are when inspecting and testing

 

[Sintra]

sorry for my ignorance but where do you get that from it doesnt say it anywhere

SELV & PELV circuits must have a minimum insulation resistance of 0.5 Mega ohms when tested at 250v dc. with all current using equipment disconnected. 612.3.2

 

[Sintra]

cheers risteard
dont suppose you could tell me what operational factors are when inspecting and testing

Could possibly be along these lines but not sure.

the type of installation
its use and operation
the frequency and quality of maintenance
the external influences to which it is subjected.

 

[brucelee]

SELV & PELV circuits must have a minimum insulation resistance of 0.5 Mega ohms when tested at 250v dc. with all current using equipment disconnected. 612.3.2

cheers mate i know that its 0.5M for selv, my question was selv further investigation value which all i can find in Gn3 is for ir 2.0Mohm, risteard says 1.0M my tutor on 2391 course first said 5M then he said 2.0M this week he said there is no specification so much confusion.
the question on operational factors as you have stated thats all i could find the tutor didnt know if i get asked that in exam im going with what you have said

 

[Notsosmart]

cheers mate i know that its 0.5M for selv, my question was selv further investigation value which all i can find in Gn3 is for ir 2.0Mohm, risteard says 1.0M my tutor on 2391 course first said 5M then he said 2.0M this week he said there is no specification so much confusion.
the question on operational factors as you have stated thats all i could find the tutor didnt know if i get asked that in exam im going with what you have said

Ok, decided to investigate further & refresh my memory, in Guidance note 3 (mine is a 16th edition copy) pages 40-41, under the heading Protection by seperation of circuits (SELV) sub heading Testing of SELV circuits it states "Minimum IR as stated in table 2.2, however in practice readings of less than 5 megohms require further investigation"

Let me know if this is different in a 17th update GN3 and I will consider investing in a new copy.

 

[brucelee]

Thanks to all who are contributing in helping me with the answers to these questions,
I am doing the written exam in june and have been revising for last 16 weeks or more, reading everything i can, doing sample exam papers,course notes.
praying and all the rest.
but some of the info i have is contradicting each other.
for example
1). course notes say routine checks have to be carried out by a electrically skilled person
GN3 say it does not but they should be able top safely use the installation and be able to identify defects

2). what is the legal status of the inspector and tester
Is it duty holder or competent person
3). reasons for selecting instruments for the range of tests. i.e.
low resistance ohm meter
insulation resistance ohm meter
earth fault loop impedence tester
pfc tester etc
4). typical readings when carrying out the tests above

5) test procedures how will i know when to include safe isolation and proving dead with a approved voltage indicator

please can anybody clue me up on the above cheers