An Awful Feeling This

[gap30]

Access dependant, your enclosure should be IP2X, therfore finger safe in theory. Add the line check and cover/shield live accessable areas. You should also be able to get your L and N probes onto the incomming terminal screws of the main switch, which will be live, therfore no need to close and open the switch to perform the test, making it a bit safer.

yes mate i have never done that i usually just open the main switch carry out the test and then reconnect earthing conductor and then close main switch

The question in full so you can get a better idea

a periodic inspection is to be carried out on a residence with gardens front and rear, which is to be used by a housing association for residential letting.

the original installation was carried out ten years ago, and a kitchen and bathroom extension was added five years later when mains gas was installed to the property. There are no forms of certification or circuit details available for the original installation or the additional work.

the installation forms part of a 230v, 50Hz TT system with a PFC of 1.3kA. A 100mA RCD which also serves as the main isolator, is installed in the consumer unit

 

[Graeme Harrold]

When answering the question, think safety, then BS, then technical, hence the shielding point I made earlier. This board may not have a busbar cover, so cover yourself in the answer by highlighting the safest method of working. The most valid point made when I did my 2391 3 years age was "you are not working live when testing, just in proximity of...."

 

[gap30]

bs 7671 requires fuses to placed in the line conductor only. State

a) two reasons why it is unacceptable to place fuses in the neutral conductor only
b) the test that would identify this error

had a look in the regs on this one and does not say why, just it cant be done - could anyone point me in the direction of the information relating to the answer of this question

the test i assume is polarity

 

[La Poste]

You just have to think what would happen in the event of an earth fault, the circuit breaker would not operate.
If a line and earth wire were to touch and cause a short circuit the fuse being in the neutral cable would not be part of this fault circuit and therefore would not operate, this could result in a fire.

Also if you had a short circuit line to neutral the fuse would blow but the line would still be at 230V so when you went to investigate you would probably get an electric shock.

Polarity is the correct test to use to ensure the fuse is in the line and not the neutral.

 

[gap30]

when working out cable resistance do i only add the 1.2 multiplier if told to do so ie 20C, i tend to add it all times and it can really change the final figures of Ze, Zs etc.

when working out questions in 2391 i often find myself second guessing on this

 

[La Poste]

Probably best to work out at 20C (R1+R2) and then compare to BS7671 but multiply BS7671 figures by 0.8. City and guilds seem to prefer it this way.
If they specifically ask for the 1.2 then use it.
City and Guilds tend to concentrate more on R1+R2 than Zs in my experience.
With this exam you really have to read the questions and figure out what they want to know.
That is why doing past papers is so important.

 

[gap30]

Probably best to work out at 20C (R1+R2) and then compare to BS7671 but multiply BS7671 figures by 0.8. City and guilds seem to prefer it this way.
If they specifically ask for the 1.2 then use it.
City and Guilds tend to concentrate more on R1+R2 than Zs in my experience.
With this exam you really have to read the questions and figure out what they want to know.
That is why doing past papers is so important.

i am more referencing if they want the zs and r1/2 for the circuit but no mention of cond temp that 1.2 can really be the diff on max zs

 

[La Poste]

i am more referencing if they want the zs and r1/2 for the circuit but no mention of cond temp that 1.2 can really be the diff on max zs

Post a question.
From an exam paper I mean.

 

[gap30]

using the information provided in the scenario, determine the PEFC at the origin of this installation

nominal voltage 400/230v TN-S System , Ze 0.3 ohms

now i have it in my head Ipf = Uo/Ze

but the sums dont add up to me 400/0.3 = 1333.3

plus i am right in thinking on 3 phase systems the prospective short-circuit current will always be higher anyway

right sod it i am going for 230/0.3 = 766 amps its the 400v thats knocking me

 

[brucelee]

Hi mate Uo is 230V U is 400v i think lol
Pfc will always be higher between the lines but you shouldnt measure that just measure the 3 lines to earth individually and multiply the highest by 2 this is the 3 phase pfc and this figure errs on the side of safety i think the real figure is 1.732 which is the square root of 3 but for 2391 multiply highest line to Neutral by 2 and record this

PSCC is always higher on 3 phase and for 2391 practical this was all we had to measure as it will always be higher than PEFC
L1 - N =516
L2- N = 505
L3 - N = 514
so L1 - N X 2 = 1032A or 1.032kA is the PFC

 

[electrician4u]

don't know what the statistics are but when i took the exam i was the only one not resitting out of 20-odd guys. i was surprised (and elated) i passed first time. i realised there were going to be no short cuts to success and read and re-read the regs, guidance notes and any other books i could find, especially the ones with pics-far more interesting! the regs make for an extremely boring read but if you're serious about passing, you've GOT to read them. you'll also need a good knowledge of them to register with Part P

 

[gap30]

using the information provided in the scenario, determine the PEFC at the origin of this installation

nominal voltage 400/230v TN-S System , Ze 0.3 ohms

now i have it in my head Ipf = Uo/Ze

but the sums dont add up to me 400/0.3 = 1333.3

plus i am right in thinking on 3 phase systems the prospective short-circuit current will always be higher anyway

right sod it i am going for 230/0.3 = 766 amps its the 400v thats knocking me

766 x 2 = 1532 so i assume it 1.53 ohms

 

[malcolmsanford]

No look at Bruce's post # 295

Do you actually read what posters write as an help to you ??

 

[tony mc]

Don't get the PSCC & PEFC mixed up.

The PEFC is what you are after so you do not multiply by x 2 or 1.73 as it is a single phase calculation between a phase and earth.

230/03 = 766A

So your PEFC is 766 Amps

I hope this makes sense.

 

[gap30]

No look at Bruce's post # 295

Do you actually read what posters write as an help to you ??

measure the 3 lines to earth individually and multiply the highest by 2

yes i try to but sometimes i get it wrong hence the original post i made