2391 exam question

[Jurasic Spark]

The guy on #40 asked why would you isolate to do PFC but no one took up the challenge.

As regards to all "relevent places" well thats for you as competent person to decide if any sub boards come into that category

 

[breakerman]

Page 74 3.10.3
Top of page 50 referring to 610.1

trebor, wat does this refer to...only i just got goin on the 2391 and dunno to much, thx

 

[WayneL]

trebor, wat does this refer to...only i just got goin on the 2391 and dunno to much, thx

Hi Breakerman,

Pg 74 3.10.3 is GN3 and referring to the fact that the installation must be isolated if you are to disconnect the main or supplemetary bonding conductors.

Pg 50 GN3 is referring to 610.1 of the regs - basically saying that the installation must be isolated in order to disconnect the Earthing Conductor.

GN3 doesn't, however, state that the installation should be isolated to measure PFC

 

[breakerman]

Hi Breakerman,

Pg 74 3.10.3 is GN3 and referring to the fact that the installation must be isolated if you are to disconnect the main or supplemetary bonding conductors.

Pg 50 GN3 is referring to 610.1 of the regs - basically saying that the installation must be isolated in order to disconnect the Earthing Conductor.

GN3 doesn't, however, state that the installation should be isolated to measure PFC

cheers for that Wayne,
getin all the more clued in...fab

 

[Baal]

But it doest state isolation for carrying out external earth fault loop impedance, on the live side of the main switch!

Then think about how your 'prospective fault current tester' operates, it is in effect an earth fault loop impedance tester and the same precautions should be observed-live side of isolated main switch and the same again for any distribution boards fed from distribution circuits.

Figure 2.7 on page 50 of GN3 illustrates this clearly.

 

[Risteard]

But it doest state isolation for carrying out external earth fault loop impedance, on the live side of the main switch!

Then think about how your 'prospective fault current tester' operates, it is in effect an earth fault loop impedance tester and the same precautions should be observed-live side of isolated main switch and the same again for any distribution boards fed from distribution circuits.

Figure 2.7 on page 50 of GN3 illustrates this clearly.

Earth fault loop impedance testers need a live supply. As do PFC testers.

Surely it is accepted that there is a degree of risk (which must be managed) with live tests. And live tests need a live supply.

The reason external earth fault loop impedance (Ze) needs the supply isolated is simply because the earthing conductor or alternatively main protective bonding conductors will have been disconnected from the supply and therefore it would be unsafe to energise the rest of the supply. This is not so with the testing of prospective fault current.

 

[Baal]

I agree with the point regarding disconnection of the earthing conductor but in City & Guilds land, you do not carry out a prospective fault test with the main switch or distribution board switch closed.

The NICEIC testing manual pages 229-234 also makes reference to the main switch and distribution board switches being opened "and isolating all final circuit protective devices, such as by switching circuit-breakers off or by withdrawing fuses"

 

[Jurasic Spark]

in City & Guilds land, you do not carry out a prospective fault test with the main switch or distribution board switch closed.

Sorry but I do not agree with this can you please explain why you feel the main switch needs to be OFF in order to measure PFC?

 

[novolts]

Hello to everyone new to the forum I am a 50 plus sparky who is nearly half way through the 2391 course but I am struggling with the course I would be greatful for any advice on how to study should I concentrate on Guide note 3 I have been in maintenance for years and I do not do much installation or inspection
thanks to all

 

[brucelee]

Hello to everyone new to the forum I am a 50 plus sparky who is nearly half way through the 2391 course but I am struggling with the course I would be greatful for any advice on how to study should I concentrate on Guide note 3 I have been in maintenance for years and I do not do much installation or inspection
thanks to all

yeah mate
mainly read gn3, but also section 6 and 7 of bs7671 , memorandum of gudance on the electricity at work regs 1989 . IP ratings . test procedures and writing them out etc
use correct terminology for everything . your course notes . have alook at Amberleafs stickys as he has included loads of stuff for 2391 that may come up inc sample tests and questions on the subject
exam success for 2391 is very useful shows you how to word things as c&g expect
practice calculations and practice writing them out formulae number calculation
ie Zs = Ze +(R1 +R2)
test sequences
ask on the forum for sample papers theres loads about
ask many questions as you need in the forums or type in the search bar for related topics
there is loads of stuff on here
also ask your tutor he should tell you what to revise and best way
all i can say is read all you can practice some sample papers as these will show you what to expect etc for written and some of the questions may come up for your exam
good luck mate hope thats a little help

 

[novolts]

thanks brucelee appreciate your advice
novolts

 

[MajorOak]

I am just doing my 2391 and working through all the questions I can get my hands on. Unless John H Owen typed it in incorrectly it says TEST not tests. The only reference I can find in guidance note 3 is in section 3.10.3 and the only test it mentions is "If an electrical installation is isolated from the supply, it is permissible to disconnect protective and equipotential bonding conductors from the main earthing terminal in order to verify their continuity"
This is a PIR question and Ze can be measure if the supply is still connected, however, it will include parallel paths. And yes while the supply is disconnected it is advisable to do Ze properly. But the only time you can disconnect the protective and equipotential from the main earthing terminal is when the power is isolated.
So I reckon this is the TEST that MUST be carried out?

Sorry, for the repeat, I didn't see how long the thread was until I posted

Cheers

 

[Guest 004]

Yep you need to measure the Ze with the main earth disconnected from the MET

 

[Guest 004]

Main reason for this is that you can tell how good the earth is upstream, and whether or not the install is relying on the downstream earth, as if it was it would be a failure

 

[trebor]

I am just doing my 2391 and working through all the questions I can get my hands on. Unless John H Owen typed it in incorrectly it says TEST not tests. The only reference I can find in guidance note 3 is in section 3.10.3 and the only test it mentions is "If an electrical installation is isolated from the supply, it is permissible to disconnect protective and equipotential bonding conductors from the main earthing terminal in order to verify their continuity"
This is a PIR question and Ze can be measure if the supply is still connected, however, it will include parallel paths. And yes while the supply is disconnected it is advisable to do Ze properly. But the only time you can disconnect the protective and equipotential from the main earthing terminal is when the power is isolated.
So I reckon this is the TEST that MUST be carried out?

Sorry, for the repeat, I didn't see how long the thread was until I posted

Cheers

The original question from the exam was you have been given a maximum of one hour to have the supply copletely isolated at origin, after that time when supply is restored you may isolate individual circuits if need for further tests,
which essential tests would you carry out within the allotted downtime?
Answer: continuity of main protective bonding conductors, continuity of earthing conductor, and Ze, all further tests can be carried with individual circuits isolated including the much misunderstood PEFC