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Discuss Fault Finding help in the Electrical Forum area at ElectrciansForums.co.uk.

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  1. danb86
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    danb86 EF Member

    Location:
    Bristol
    Hi, I'm being assessed for fault finding as part of my NVQ tomorrow and have just had a bit of a 'what the hell am I'm doing' moment!

    I'm building the fault in. It will be a loose neutral touching an earthed metal back box in a socket on a ring main, causing the RCD to trip.

    My plan of attack is to identify which of 5 circuits has the N-E fault by removing the neutrals/cpc of each individual circuit and doing insulation resistance until I narrow it down to the ring.

    It's the next part where I'm not sure of how you would go about it though.

    Assuming I didn't know what the fault was...

    Would you do end to end resistance of each conductor? It's TNCS so I guess the N reading might prove something

    or would you skip that and...

    Use the half split method, figure out what half the fault is on, and eventually discover it's a loose neutral in a socket when removing that socket?

    would you do none/both of the above??

    Once the fault is discovered and corrected, besides insulation resistance do I need to do any other tests to prove the fault has cleared and circuit is safe? e.g end to end resistance, maybe RCD test too?

    I know I've left this late and it probably all sounds stupid but I've got myself in a bit of a state.

    Any replies are much appreciated, Thanks.

    P.s. It has to be this fault, so dont suggest just putting in an open circuit (as much as I'd love to!)
     
  2. Pete999
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    Pete999 Forum Mentor

    Location:
    Northampton
    Business Name:
    None
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  3. danb86
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    danb86 EF Member

    Location:
    Bristol
    Thanks Pete999, makes sense that you should go through the complete procedure of ring continuity.

    With regards to the fault finding, would you do end to end and then the half split as I said above?

    Dan.
     
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  4. Pete999
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    Pete999 Forum Mentor

    Location:
    Northampton
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    Yes thats what I would do
     
    Last edited: Apr 21, 2016
  5. Richard Burns
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    Richard Burns Respected Member

    Location:
    Cambridgeshire
    Business Name:
    Richard Burns
    So from an external point of view you have an RCD tripping even when each individual circuit is switched off at the MCB, therefore this must be a neutral earth fault.
    So for each circuit, as you say, do N-E IR on the disconnected cores to identify the faulty circuit.
    Once you have the faulty circuit then testing end to end continuity could be worthwhile to see if there is a disconnection somewhere.
    (if there were no disconnection on either neutral or earth then the fault must be an earthed point contacting the neutral, could be an unsheathed cpc pushed against a neutral terminal or damaged insulation on a neutral core caused by an earthed part)
    Once you have found the disconnection in the neutral then you could approach this in various ways, e.g. leave the cores disconnected at the board and check IR on each socket along the ring until the IR is OK, then that socket or the one before it in the circuit has the fault, or you could do an R2 test from one neutral leg at the board to each socket neutral until you find the two sockets between which the continuity stops.
    (if there were a cable fault, nail through a cable type, then this would just narrow down the length of cable damaged)
    Or you can do the split the circuit down and narrow it that way.
    Physical inspection of the sockets would then identify the fault and once the fault is repaired it would be a good idea to perform the ring continuity testing, insulation resistance testing and RCD testing in case there is another fault that had been missed.
    Once the testing shows the circuit is OK then it can be reconnected and powered up.
     
    Last edited: Apr 21, 2016
  6. danb86
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    danb86 EF Member

    Location:
    Bristol
    Thanks so much Pete and Richard. I feel much happier going into this tomorrow having some feedback from you guys. Think I'll go with the split method as it's what I'm more familiar with. Just hope it all goes to plan!

    Dan.
     
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  7. Jay Sparks
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    Jay Sparks Forum Mentor

    Location:
    Bratford
    Just to add to the above. Don't forget to carry out safe isolation procedure using GS38 approved VI's, as this is one of the criteria for unit 318. Also, speak to the customer before hand, get as much detail as possible about the fault, this is also a criteria. And when you have found and fixed the fault, don't forget to do the IR test again, to make sure you have in fact cleared it, this is another criteria. Remember, if you are being assessed, your assessor will be looking for you to take a logical approach to the fault and will be trying to claim as many criteria as possible.

    I have copy and pasted all the different criteria you need for this 318 Fault diagnosis unit. (assuming you are doing the 2357 apprenticeship). Hope it helps,


    318-LO1 Confirm safety of the system and equipment prior to diagnosing and correcting electrical faults in accordance with statutory and non statutory regulations

    • 318-AC1.01 Carry out safe isolation procedures in accordance with regulatory requirements for electrical installations

    • 318-AC1.02 Ensure the health and safety of themselves and others within the work location during inspection, testing and commissioning

    • 318-AC1.03 Select and use appropriate warning notices and barriers

    • 318-AC1.04 Check the safety of electrical systems prior to the commencement of diagnosing and correcting electrical faults

    • 318-LO2 Carry out procedures to identify faults on electrical systems and equipment

    • 318-AC2.01 Use effective methods of communication to ascertain clear and detailed information about reported faults and any components which require replacing

    • 318-AC2.02 Identify and use appropriate system specification documents which relate to the electrotechnical systems and equipment being worked upon

    • 318-AC2.03 Report information about potential disruption that may be a consequence of fault diagnosis and correction work to relevant people, such as:
    ◊ 318-AC2.03a1 Other workers/colleagues 1
    ◊ 318-AC2.03b1 Customers/clients 1
    • 318-AC2.04 Assess the safe working practices which apply in the working environment to confirm that it is safe for fault identification work to take place


    • 318-AC2.05 Perform suitable diagnostic tests on the installed electrotechnical systems to successfully identify faults including:
    ◊ 318-AC2.05a1 Loss of supply 1
    ◊ 318-AC2.05b1 Overload 1
    ◊ 318-AC2.05c1 Short-circuit and earth fault 1
    ◊ 318-AC2.05d1 Transient voltage 1
    ◊ 318-AC2.05e1 Loss of phase/line 1
    ◊ 318-AC2.05f1 Incorrect phase rotation 1
    ◊ 318-AC2.05g1 High resistance joints 1
    ◊ 318-AC2.05h1 Component, accessory or equipment faults 1

    • 318-AC2.06 Use appropriate methods for locating faults on electrical systems and equipment, including:
    ◊ 318-AC2.06a1 Procedures and sequences - logical approach 1
    ◊ 318-AC2.06b1 Safe working practices 1
    ◊ 318-AC2.06c1 Interpretation of data 1

    • 318-AC2.07 Use appropriate tools and instruments correctly to complete fault diagnosis work, including:

    ◊ 318-AC2.07a1 Voltage indicator
    ◊ 318-AC2.07b1 Low resistance ohm meter 1
    ◊ 318-AC2.07c1 Insulation resistance testers 1
    ◊ 318-AC2.07d1 EFLI and PFC tester 1
    ◊ 318-AC2.07e1 RCD tester 1
    ◊ 318-AC2.07f1 Tong tester/clamp on ammeter 1
    ◊ 318-AC2.07g1 Phase sequence tester 1

    • 318-AC2.08 Confirm test instruments are fit for purpose, functioning correctly and are correctly calibrated

    • 318-LO3 Correct faults on electrical systems and equipment

    • 318-AC3.01 Confirm appropriate repairs, removals and replacements and their implications with relevant people including:
    ◊ 318-AC3.01a1 Other workers/colleagues 1
    ◊ 318-AC3.01b1 Customers/clients 1

    • 318-AC3.02 Perform fault correction procedures correctly and safely using appropriate tools, equipment and material

    • 318-AC3.03 Perform the removal and replacement of components and associated equipment from electrotechnical systems to ensure:
    ◊ 318-AC3.03a1 Ease of access to enable future maintenance 1
    ◊ 318-AC3.03b1 Accordance with: Relevant regulations 1
    ◊ 318-AC3.03c1 Accordance with: Manufacturer's instructions 1
    ◊ 318-AC3.03d1 Accordance with: Organisational procedures 1

    • 318-AC3.04 Apply appropriate procedures to ensure electrotechnical systems, equipment and components are left safe, in accordance with industry regulations, if the fault cannot be corrected immediately

    • 318-AC3.05 Perform appropriate inspection and testing procedures to confirm that systems, equipment and components are functioning correctly after completion of fault correction work

    • 318-AC3.06 Record test results and other appropriate information regarding the fault correction work clearly and accurately and report to relevant people, such as:
    ◊ 318-AC3.06a1 Other workers/colleagues 1
    ◊ 318-AC3.06b1 Customers/clients 1
    ◊ 318-AC3.06c1 Representatives of other services 1
     
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  8. Pete999
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    Pete999 Forum Mentor

    Location:
    Northampton
    Business Name:
    None
    Oh and don't call the circuit a ring main, it's a ring final circuit, I know I know I'm pedantic.
     
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  9. Simonslimline
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    Simonslimline Electrician's Arms

    Location:
    South West, Bath
    This bugs me as well. :biggrin5:

    A ring main feeds the substations @11kv.
     
  10. Pete999
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    Pete999 Forum Mentor

    Location:
    Northampton
    Business Name:
    None
    Pedan tic as well are we ????lol
     
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