UK What sequence to do when doing fault finding (AM2)

[Mustard85]

Hi everyone just to let everyone know that I did my AM2 last week and I only failed on the fault finding part. I was very confident before going into the fault finding test but I lost my way and the pressure got to me. I'm so chuffed that I pass everything else.

Just to explain what the test is about in the exam they give you 7 faults and need to get 5 right all cable faults and you only allowed to used amulti meter (continuity and resistance testing), you have 2 hours to complete it.

My question is what sequence what you do the fault finding?
Would you check if your cores have a short circuit 1st Line to earth, line to neutral, neutral to earth if there no continuity then there isn't a short?

2nd would you check for open circuits using R2 method(end to end) this should all have a continuity if there isn't any you check for cross connection? Line 1 end neutral the other end

If there is no short or open circuit then you check for high resistance? Change setting on muti meter so you have Mega ohms on the screen. then test end to end for high resistance.

What I don't understand is that I had a fault on the FP200 and the discription they give me is the IR had failed while testing was done. I had to find the fault. I did my sequence In case of short and open circuit (all clear) then I did the resistance test end to end and that was reading low ohms!! The only way to get a reading between two cores but I thought you can only do that when you put 500v dc down the cores to test for IR.

Sorry for the long post if you can could you correct me if I did each test correctly please.

Really appreciate any help regarding this and thank you for your time to read this.

 

[radiohead]

I think possibly you are confusing continuity and insulation resistance. Think about a length of cable. If you test the continuity of each core from end to end you should get a low resistance reading, typically a fraction of one ohm. This test would be carried out with a low range ohmmeter,(continuity tester of a MFT).This is because you are measuring the resistance of a continuous copper conductor. Think again of that length of cable, with all conductor ends disconnected a 500v insulation resistance test between any of the cores should give a very high resistance reading, typically at the maximum range of the meter, >200/>500/>1000 megohms (million ohms) depending of the meter.This is because you are measuring the resistance between cores that are separated by a high resistance insulation designed to prevent contact between cores.

 

[telectrix]

i'd imagine that they had switched in a resistor between 2 conductors to simulate the fault . so as to make it findableusing a multimeter, maybe several Kohms.

 

[Mustard85]

Thank you for your reply. Would the fault be short circuit? What confusing is that I did resistance test end to end and that was clear. It never cross my mind to do resistance across 2 cores. Thought you could only do that using insulation resistance tester?

Just to clarify this. If the ohm meter is reading high, would the insulation resistance tester read low?

 

[telectrix]

imagine a 22k ohm resistor across 2 conductors. your multimeter set on ohms will read open circuit. your insulation tester would read 0.02 M ohms. however, if you set your multimeter to k Ohms range it will read 22K.

 

[Mustard85]

imagine a 22k ohm resistor across 2 conductors. your multimeter set on ohms will read open circuit. your insulation tester would read 0.02 M ohms. however, if you set your multimeter to k Ohms range it will read 22K.

I understand that now thank you. Would the fault be short circuit cos there is a resistor between 2 cores?

 

[telectrix]

a short circuit is zero ohms. a 22k for e.g. would read 22k on a multimeter, but almost a short circuit on a IR test.

 

[davesparks]

What I don't understand is that I had a fault on the FP200 and the discription they give me is the IR had failed while testing was done. I had to find the fault. I did my sequence In case of short and open circuit (all clear) then I did the resistance test end to end and that was reading low ohms!! The only way to get a reading between two cores but I thought you can only do that when you put 500v dc down the cores to test for IR.

With fault finding you need to think about what the fault is described as, what could cause it, and what tests could identify it.

In this example the information you have is that the circuit has failed an IR test.
So the first thing to do is think about what could cause this, and eliminate the simplest things first, start by checking that no loads were connected during testing and that there are no neon indicators, surge protection etc in the circuit.
Do they tell you what value of IR was measured and between which conductors? If not then ask them as this is relevant information.
Then carry out tests to identify where the fault could be based on the information.
Carrying out an end to end test on conductors when trying to find an IR fault is going to waste your precious time.

Remember these faults are always going to be simple text-book faults, this isn't a real world fault where anything is possible.

 

[Mustard85]

With fault finding you need to think about what the fault is described as, what could cause it, and what tests could identify it.

In this example the information you have is that the circuit has failed an IR test.
So the first thing to do is think about what could cause this, and eliminate the simplest things first, start by checking that no loads were connected during testing and that there are no neon indicators, surge protection etc in the circuit.
Do they tell you what value of IR was measured and between which conductors? If not then ask them as this is relevant information.
Then carry out tests to identify where the fault could be based on the information.
Carrying out an end to end test on conductors when trying to find an IR fault is going to waste your precious time.

Remember these faults are always going to be simple text-book faults, this isn't a real world fault where anything is possible.

Thank you for your reply. No they don't state what reading they had. Basically I just need to locate where the fault is. What the fault is and how to rectify it. There are only 4 faults it could be short circuit, open circuit high resistance or cross connection.

 

[davesparks]

Thank you for your reply. No they don't state what reading they had. Basically I just need to locate where the fault is. What the fault is and how to rectify it. There are only 4 faults it could be short circuit, open circuit high resistance or cross connection.

Did you ask for the reading and what it was tested between?
You should be able to work out which one of those four categories of faults this falls in to?

 

[Mustard85]

Did you ask for the reading and what it was tested between?
You should be able to work out which one of those four categories of faults this falls in to?

You have to test yourself. I think I had a reading of 22 Kohms between neutral and earth. I put down high resistance. I'm doubting myself now that it short circuit. I will phone the Assessor tomorrow to find out what faults I had wrong.

 

[radiohead]

You have to test yourself. I think I had a reading of 22 Kohms between neutral and earth. I put down high resistance. I'm doubting myself now that it short circuit. I will phone the Assessor tomorrow to find out what faults I had wrong.

You state in your OP that you were 'continuity and resistance' testing, and that an ohmmeter was used. An ohmmeter is not used for insulation resistance testing because it does not output a high enough voltage to break down and show up high resistance faults. A 500v insulation resistance tester is used which would read 0.02 Megohms on a fault of 22 Kohms. That would be an unacceptably LOW reading between N-E, not a high resistance. It would normally be termed a low insulation resistance to earth. A short circuit is normally a fault between live conductors, ie line to neutral.
I may have misunderstood your posts but it seems to me you were using the wrong meter to test insulation resistance.

 

[Mustard85]

You state in your OP that you were 'continuity and resistance' testing, and that an ohmmeter was used. An ohmmeter is not used for insulation resistance testing because it does not output a high enough voltage to break down and show up high resistance faults. A 500v insulation resistance tester is used which would read 0.02 Megohms on a fault of 22 Kohms. That would be an unacceptably LOW reading between N-E, not a high resistance. It would normally be termed a low insulation resistance to earth. A short circuit is normally a fault between live conductors, ie line to neutral.
I may have misunderstood your posts but it seems to me you were using the wrong meter to test insulation resistance.

This where I'm confused... If there a fault and the discription they give "there's an Insulation resistance issue when testing" my 1st reaction when read this was I can't do the IR on a Ohmmeter! Hopefully I'll get some answers tomorrow

 

[radiohead]

Ok, well hopefully you'll be able to come back with a bit more info tomorrow.

 

[Lucien Nunes]

If the insulation resistance is seriously low, such as the mentioned 22kΩ, you can find the problem using either an insulation tester or multimeter, as both will respond to that resistance. What you cannot do with the multimeter alone, for the reason that Radiohead clearly explains, is to subject good insulation to a stress test to prove that it is really good.

As an analogy, if water is gushing out of a faulty pipe joint, it doesn't take any special pressure test equipment to discover where the leak is. But, if the pipe appears watertight when there's some water standing in it, to say with certainty it won't spring a leak in use, you've got to put on as much pressure (or more) as it will have to withstand in use, and that is the specific function of an insulation tester.