2391 practical

[treelectric]

Just had a go at the 2391 practical, and messed it up. have a lot of experience and did my theory home work, but time was the problem.
It was a periodic inspection and I thought this meant a 10% inspection, followed by all the usual instrument tests, then writing out certificates. Actually I was told it was a FULL periodic, whatever this is.( I haven't managed to find the description in GN3 or BS7671. Can any one help with this)?
Anyway I was informed by the examiner that this meant taking off ALL the covers from all circuit outputs, including final ring circuit, and then of course putting them all back on again before the live tests, which incidentally had to include Ze.
I actually spent a whole hour of my test taking covers off and putting them on again, which didn't seem to me like a test of my understanding and ablity.
However, that is the nature of it, so I would be pleased to hear from anyone who has succesfully done this, and especially would appreciate some tips on how to organise to get all done in the time allowed.
Is the order of tests really fixed? In this exam situation, starting with a live circuit, it would be more time efficient to start with Ze test, then at least most of the calculations could be done, even if there wasn't time to put everything back together. I would really welcome some feedback on this.

 

[Shakey]

Ok,

the PIR board is always a full inspection. So, as you found out that means ALL covers off (except the trunking!) The 10 percent is sampling, which you can do in the real world depending on the condition of the installation, but not in the exam.

You have 120 miniuts which terminates when you hand in your completed PIR, although the examiner does have a bit of tolerence with the time.

The tests are in order as stated in GN3. So its

Continuity (bonding and CPC's) so your R1+R2's and your ring tests
Insulation
Polarity
EFLI's
RCD,s and functionals

your EFLI's wil include Ze,
PEFC and PSCC (on both the main AND sub boards)
and your Zs' measurements

These can be done in any order, but i would generally go Ze, recconect the earth then straight into PEFC, then PSCC.

Note that you DONT have to mesure Zs on all the circuits, just the socket outlets on the ring, and socket outlet on the radial off the sub board. In accordance with the EAWR's, C&G state on all of the other circuits you can measure OR calculate Zs. So you have Ze, and R1+R2, so save time and calculate!

As a rough guide to timing, you should aim to complete your visuals, refit covers, and be nulling you meter for the bonding tests at around the 30 minuite point. Take one hour to do your tests, then aim to start your report with about 20-30 minuites to go

And remember:

No matter what, NEVER EVER switch on the main isolator on the three phase board with the busbar cover removed - this is an instant fail!!!!



hope this helps

 

[treelectric]

Yes it does help, especially as I wasn't sure if I could calculate Zs on non socket outlets or had to measure them. Your advice about finishing the inspection in 30 min does give me something to aim for, as that was the problem when I did it.
I was told I could calculate pscc and pefc if I wanted.

 

[Shakey]

Yes it does help, especially as I wasn't sure if I could calculate Zs on non socket outlets or had to measure them. Your advice about finishing the inspection in 30 min does give me something to aim for, as that was the problem when I did it.
I was told I could calculate pscc and pefc if I wanted.

Ok, if you have Ze, then you can calculate PEFC, but how are you gonna calculate PSCC? You dont know the impedence of the line to neutral loop so dont see how, (unless its TNCS of course!) but even then, only on the three phase board.

Rememebr you need PFC's on both boards, so if you are gonna calculate PEFC on the sub board, then its Ze plus R1+R2 of the submain cable supplying the sub board.- but even if it is TNCS you would need Rn of the submain cable to be able to calculate PSCC. Personally, I would always measure PFC's, it only takes two minuites!

With the Zs for all the non s/o circuits, C&G specifically say that you can calculate OR measure


and good luck!

 

[treelectric]

yes, I do see what you mean, but my old meter gives me Z between live and phase, as well as Ze so usually just calculate it. Maybe on the next try I'll use the meter they supply as I noted it did pscc and pefc.
Do you need Zs at the actual sub board? Or do you put the Ze for the main board into the schedule of test results for the 2nd board? To me it would seem much more logical to take another Ze at the sub board, as you could in a real situation have a long run of cable between the two, and that could make a significant difference to disconnection times. And for the sub main, the connection cable between the two would be part of the external
impedance loop. Am I right?
And thanks again.

 

[Shakey]

yes, I do see what you mean, but my old meter gives me Z between live and phase, as well as Ze so usually just calculate it. Maybe on the next try I'll use the meter they supply as I noted it did pscc and pefc.
Do you need Zs at the actual sub board? Or do you put the Ze for the main board into the schedule of test results for the 2nd board? To me it would seem much more logical to take another Ze at the sub board, as you could in a real situation have a long run of cable between the two, and that could make a significant difference to disconnection times. And for the sub main, the connection cable between the two would be part of the external
impedance loop. Am I right?
And thanks again.

Ok,

your meter actually measures Z anyway, so if you connect it line to Earth its Ze, line to neutral would be Z. And meters measuring PFC's do the same, between line and earth its PEFC and line and neutral its PSCC

now for the submain, take your measurement, but wether its Ze or Zs its the same reading at the point! Your Ze is only the external EFLI, and we disconnect the incoming earth so we are ONLY reading that part of the circuit which is external.

Now if you wanted to measure Ze at the sub board, you wouldnt disconnect the incoming earth to the suboard, and even if you did, you would still get parralel paths through your main and supplementary bonds etc etc.

And it wouldnt be Ze because its not external to the installation, because it includes R1+R2 of the submain. So actually when you measure the incoming to the sub board, you are measuring Ze + (R1+R2) , which of course is Zs!!!

and of course, the disconnection times (through the Zs measured at the submain circuits) ALREADY includes 'true' Ze + (R1+R2 submain) + (R1+R2 particular submain final circuit)

So, on both of your schedules of test results it is acceptable to put the same value of Ze (because you will only have one value of Ze)

although it is also acceptable to include R1+R2 (and the Zs value) of the submain cable as if it was another final circuit

i always tell people: its YOUR paperwork, and as long as you explain what you have entered, and it is logical, then you cant (or shouldnt ) be criticised for it

hey this getting good aint it!!!

 

[treelectric]

Yes, theres some useful stuff coming out of this. Thanks.

 

[Shakey]

Yes, theres some useful stuff coming out of this. Thanks.

no probs, any other queries you have, fire away

 

[flukey]

hi
noticed that you mention using your own meter during the pir??
when i took my 2391 we were told we couldnt use our own meters so had the added problem of using a meter your not familiar with

 

[Shakey]

hi
noticed that you mention using your own meter during the pir??
when i took my 2391 we were told we couldnt use our own meters so had the added problem of using a meter your not familiar with

well thats just the training centre then, there is nothing in C&G rules to stop you using your own meter

 

[flukey]

Hi shakey,
i said that at the time but was told part of the test is to ensure you choose the correct meter and select the correct setting??
when i said that whether they were my meters or the centres was irrelevant because i was choosing the meter and the setting and if it was wrong then quite rightly i would instantly fail.
i was told that this was a non negotiable c&g rule which seemed a bit unfair at the time but is a distant memory now seeing as i passed anyway

 

[charlie]

Have got my practical soon can anybody help.
Have been told there is a 3 phase starter + socket, is it o.k. to measure R2 for continuity of c.p.c. and when Zs is required to measure ( by using a plug and lead ) it at the socket ??

 

[Brutus]

Have got my practical soon can anybody help.
Have been told there is a 3 phase starter + socket, is it o.k. to measure R2 for continuity of c.p.c. and when Zs is required to measure ( by using a plug and lead ) it at the socket ??

Hi Charlie

basically no - GN3 says that motor circuits (which this is) then the loop imepodence is ONLY measured at the supply side of isolated motor control gear.

Do R1+R2 checks on the three phase supply to the contactor, and again on the other side to the socket.

Then either calculate Zs = Ze + (R1+R2)+(R1+R2)

or add the additional socket side R1+R2 to the measured Zs.

And rememebr, you ONLY have to measure Zs at the socket outlets, C&G state for all other points, measure or calculate.

 

[charlie]

Hi Charlie

basically no - GN3 says that motor circuits (which this is) then the loop imepodence is ONLY measured at the supply side of isolated motor control gear.

Do R1+R2 checks on the three phase supply to the contactor, and again on the other side to the socket.

Then either calculate Zs = Ze + (R1+R2)+(R1+R2)

or add the additional socket side R1+R2 to the measured Zs.

And rememebr, you ONLY have to measure Zs at the socket outlets, C&G state for all other points, measure or calculate.

Understand what you are saying but thought that applied for the dangers involved with testing on a live moving motor as the test rig as a socket and therefore danger as been reduced to a minimum i would have thought this to be o.k.

 

[Brutus]

Understand what you are saying but thought that applied for the dangers involved with testing on a live moving motor as the test rig as a socket and therefore danger as been reduced to a minimum i would have thought this to be o.k.

well the reasons are many, not least because the motor O/L contacts add a fair bit of resistance, however GN 3 says it pretty much as i have said it, therefore thats the way it is done during the test, and thats the way C&G mark it as well - I have been doing it all day!!