Discuss Lockdown Brain Food - 1 in the Electrician Courses : Electrical Quals area at ElectriciansForums.net

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SparkyChick

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To keep your trainee brains in tip top condition during lockdown, here's a few quiz questions....

1) A fault report is 'Circuit breaker keeps tripping'. What could be the cause of the tripping? How might those reasons manifest themselves with regards to how long the circuit breaker stayed on during the clients attempts to restore power (they have tried several times before phoning you)? What test would you carry out to try and establish which reason is at work? What results might you expect to see for each reason?

2) How can an RCBO make identifying the faults covered in question 1 more difficult?

3) You're carrying out an EICR and are currently testing a ring final circuit wired in 2.5mm/1.5mm T+E cable. The OCPD is a 32A Type B breaker to BS (EN) 60898 which is supplied via a 63A Type AC RCD with a rated tripping current of 30mA. During testing you establish that the maximum Zs of the circuit is 10.8 ohms. What reasonable assumption can you make about the earthing system of the installation? Can the circuit meet it's disconnection time requirements for the faults covered in questions 1 and 2? What additional test should you do to confirm it complies or not?

4) During the testing of the circuit in question 3, you tested 2 socket outlets to achieve a 10% sample size. How many sockets are on the circuit in total? As a result of the high reading, you test the remaining sockets and find that three of them do not give a Zs reading. You check the external earth fault loop impedance and get a measurement of 0.4 ohms. Does this confirm your assumption about the earthing system in question 3? What sort of faults could be causing high and non-existent readings (there may be more than one)? How would you go about finding them using a low reading ohm meter?

5) Does an RCD fused spur or socket outlet with a rated tripping current of 30mA manufactured to BS-7288 comply with the regulations for providing additional protection (assume that for Ideltan x 1 it trips in less than 300ms and for Ideltan x 5 it trips in less than 40ms)? If it doesn't, why doesn't it? If you installed one now, what is this considered to be with regards to the regulations? How should this be recorded? If you indulge in this kind of activity, what must you be able to do?

I realise not everyone is at the same stage and may not have a copy of the regulations, but have a go... do NOT post answers in the thread... start a conversation with me with the subject 'Lockdown Brain Food - 1 - Answers'. I'll post what I think the answers are next week (I'll be putting my tin hat on just in case I get one wrong).
 
Moved this into the public trainee area since we do seem to have a lot of peeps who don't have access to the private trainee only area.
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If anyone else fancies a go, feel free. Maybe you don't get a lot of fault calls? Maybe you're no longer a trainee but still quite new to the game?
 
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1, a L-N short or overload(euipment fault)

If the MCB keeps tripping, what is the interval?

I'd be getting them to unplug all appliances and switching off any SFCUs and seeing if the fault is still extant.

If not, process of elimination ith the appliances.

If so, after asking all the usual questions "where've you been drilling(you cretin)", and a ring, start testing the circuit. r1, r2, rn to make sure the subsequent tests are on the whole circuit.

Then IR looking for a low L-N reading.

Split ring and retest, repeat until found.

Suck air through teeth and quote a full rewire with AFDDs on all circuits.

Please don't post the answers in public... if you're just playing along wait until I post the answers... if you want to have a proper go, send your answer to me (as stated in the original post).

Thanks

SC
 
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Any trainees having a go at this and want more time? I was hoping I might have had some responses by now and I was going to publish my answers tomorrow (Friday) night.
 

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