AM2 Fault finding failure. Bonding splan motor circuit

[Pete286]

Hi everyone. I sat my AM2 last week and passed everything except the fault finding.
They just tell u pass or fail not what u got right or wrong etc. I struggled on the faults I'll explain below. I know the faults will be different next time but would still like to know how to approach these problems and what the faults actually could have been.

Bonding fault: 'engineer has been struggling to get continuuty on main bonding conductors'
Had sepearse gas and water main bonding. Was hidden behind trunking between the bird and clamps on pipes. I disonnected them and tested end to end. Had continuity. I tested from what was saying gas at the disconnected board to the clamp side of water and still had continuity. I said therefore it is a short circuit. Could it have been crossed polarity?

2nd fault: boiler and pump are continuously running.

3rd fault: motor will not start. I checked from board to starter end to end continuity N L1 L2 L3 and from starter to motor end to ends. All belled out ok. I wasn't sure how to test bewteen the contactors within the Dol.

All these faults have to be wiring faults and only found with continuity meter no IR.

If anyone has any other common faults for these 3 above please share

Cheers

 

[DPG]

I would say any fault resistor that they insert would be well above 0.5R. Others may be able to confirm this for certain.

 

[Powellm]

Fault 3. Complete half split technique. Spilt circuit in half and test if the start contactor, emergency stop and overloads all check out and you verify connection at this half you eliminate this side of the circuit. Approach the 2nd half of the circuit as a while new circuit and again half split this if there are normally open switches or contractors. If you find all normal readings to the motor the and all control switches and fuses arc are fine it's the motor. Impressive the examiner even more by pulling out your megger and complete a full winding test for insulation breakdown at 100v then rip out the stator and put a new squirrel in there. Good as gold... all the best next time.

 

[Powellm]

I would also like to apologise to all reading my above the grammar and spelling is horrendous

 

[sparky1]

unfortunately your not allowed an insulation tester (i.e megger) just a cheap DIY tester, this is to really test our knowledge and basic understanding i believe.

so any reading above 0.5 ohms is a high resistance fault is that correct????

 

[sparky1]

Bump

 

[sparky1]

 

[sparky1]

 

[sparky1]

Started to make test boards

 

[uksrevivor]

To the OP
What a brilliant thread this is.
To all that have replied i have to thank you all as it is most informative and this is exactly how us trainees learn on such issues. The replies have been very useful and this is why im still a member of this forum as you can learn a lot from it.
Sparky 1 could you post up the finished test board as I can see it would be very useful for other trainees maybe post it in the trainee section for us trainees.
Many thanks again I will watch this thread with interest.

 

[sparky1]

Ha thanks I should of really put it up in the trainee section put Pete had already started a post which tied in to my queries.

Didn't get chance today to finish the test boards I will try to complete tomorrow need to buy a resistor so can add a high resistance fault does anyone know where I could get one from I was thinking RS componemc's or FARNELL electronics both who are in Leeds does anyone know what type I need????

 

[sparky1]

Fault 3. Complete half split technique. Spilt circuit in half and test if the start contactor, emergency stop and overloads all check out and you verify connection at this half you eliminate this side of the circuit. Approach the 2nd half of the circuit as a while new circuit and again half split this if there are normally open switches or contractors. If you find all normal readings to the motor the and all control switches and fuses arc are fine it's the motor. Impressive the examiner even more by pulling out your megger and complete a full winding test for insulation breakdown at 100v then rip out the stator and put a new squirrel in there. Good as gold... all the best next time.

unfortunately they wont let you have a insulation resistance tester

 

[sparky1]

Boards built ready to get some practice

 

[davesparks]

so any reading above 0.5 ohms is a high resistance fault is that correct????

I think the better approach would be to look for an unusually high resistance. For example if the neutral of a circuit is 1ohm end to end and the live 100ohms end to end then it is a high resistance fault.

Taking a universal value of greater than 0.5 is high resistance may be misleading, fir example it is quite common for the end to end resistance of a real world ring circuit to be above this.

 

[sparky1]

Thanks Davesparks I agree, that's also helped me clear it up in my mind.

Got ten days both my resit so need to start using this multmeter a lot more