Help with 24v fault-finding interview.

[osmosis88]

I am a recently qualified level 3 electrician with little practical experience outside of college.

Upon applying for a job I have reached the third and final round of interviews, this time it is a practical interview and have been told very little about it.

I do however know that it is likely to be fault finding on a 24v circuit, possibly LED's aren't lit, etc.

I have been trying to do some research online all week but have found very little help. The interview is drawing nearer and I feel extremely under-prepared.

If anybody has any advice or pointers (however trivial they may seem) I would very much appreciate it. Thanks

 

[sparkytim]

Same as any testing just 24V instead of 230.

If checking why something not working, then with Voltage indicator (Test meter or Test lamp type) check for 24 Volts at supply point, then at other points on the circuit to locate fault( break in circuit).

If not powered up then use continuity tests to prove circuit

A circuit is a circuit whatever the voltage

 

[Manator]

Same as any testing just 24V instead of 230.A circuit is a circuit whatever the voltage

I have quoted sparkytim with ommision, becase the above quote is very helpful when you consider any problem with testing "any circuit".

Thanks to sparkytim

 

[M107]

Same as any testing just 24V instead of 230.If checking why something not working, then with Voltage indicator (Test meter or Test lamp type) check for 24 Volts at supply point, then at other points on the circuit to locate fault( break in circuit).

If not powered up then use continuity tests to prove circuit

A circuit is a circuit whatever the voltage

Check it's 230v at supply point (as in the cu/mcb), then as sparkytim says move up the circuit in stages checking voltage at every accessible joint or connection, after the driver you'll be looking for ELV voltage (24/12v). If all is fine up to the lamp...change the lamp... if it still dont light up you'll be wanting to swap the driver.

With fault finding think easy things first.

 

[springcrocus]

I tend to work the other way.

If the lamp / LED / appliance isn't working, has it got power? (12/24/230v)

If yes, the item is faulty.

If no, then go back to the next point in the chain.

Check the switch (e.g. measure volts to earth both sides of switch) or driver (e.g. check the input voltage).

Good result here indicates faulty switch / driver.

Poor result means problems further back.

and so on...

Break the chain into small steps and investigate each step in sequence until an annomoly occurs. That is the fault point.

 

[Megaohm7]

You are lvl 3 qualified . Take your time and think about what You have learned . Each job is a new challenge you have the knowledge so really you will be prepared . As the replies that have been given before state, a circuit is a circuit and You will be just fine .... Good luck on the interview I know you will be nervous but it's a sign of being human

 

[osmosis88]

Thank you for the replies, the reassurane alone has been incredibly helpful.

I have learned that the circuit involves contactors and relays. As these are components that I haven't had experience with I can't help but wonder how to test them. I feel it's safe to presume that it's the same as any other component... Voltage indicator, or continuity test each side of the item, though I wouldn't forgive myself for not asking if I am wrong...

 

[Blue Fox]

A contactor or relay is basically just a switch, but instead of it being "turned on" by a finger pressing a switch it is switched on by a voltage across the coil of the relay or contactor. If your LED isn't coming on and is switched by the relay or contactor then see if the contactor is "in", they usually have a way of telling depending on the type, if it isn't then check if there is voltage on the coil, if there is and the relay isn't in then it is faulty, if theres no voltage then the fault is elsewhere in the circuit.

With regards to fault finding a circuit I usually find it easiest and quickest to check if the is supply at what is meant to come on is there or not and if not then go to roughly the half way point in the circuit and then go half way either forward or backward again. This then eliminates large parts of the circuit very quickly.

 

[SLIPSHOD & SLAPDASH]

I am going to guess that the test is being carried out at 24v for health & safety reasons. As already mentioned a circuit is a circuit whatever the voltage. Find out if you are expected to carry out the tests as if this was a 240v supply, ie prove tester working, safe isolation etc.

Most contactors can be manually pushed in to the on position by a raised plastic 'knob' or a hole ( which you could push a screwdriver in) on the top, for testing

 

[Sidewinder]

I am going to guess that the test is being carried out at 24v for health & safety reasons. As already mentioned a circuit is a circuit whatever the voltage. Find out if you are expected to carry out the tests as if this was a 240v supply, ie prove tester working, safe isolation etc. Most contactors can be manually pushed in to the on position by a raised plastic 'knob' or a hole ( which you could push a screwdriver in) on the top, for testing

One of the first tests I normally do as it splits the power and control sides straight away, and you then know where to search for the fault!

Not sure if it is frowned upon by "the powers that be" though!

ROTFWL

 

[Andy™]

One of the first tests I normally do as it splits the power and control sides straight away, and you then know where to search for the fault!Not sure if it is frowned upon by "the powers that be" though!

ROTFWL

just dont do what someone i was working with done a few years ago. star delta motor not starting. pushed in 2 contactors, got it running in star. and then pressed the wrong contactor in.... 1x big bang later, and 2x new contactors had to be fitted

 

[Sidewinder]

Regularly operate Y/D's manually.

They should have mechanical interlocks as well as electrical between the Y&D contactors.

Impressed a customer recently in diagnosing a Y/D timer fault, as I operated the starter manually to prove it was functioning OK as far as the load side went.

Knew the control was OK as it started in Y.

One new timer later with a 400V a.c. coil and all was well.

They are quite rare these days too!

 

[Andy™]

iirc, they were not mechanically interlocked

 

[Sidewinder]

OOPS!

Bit of a design failure there then!

System designer not up on his FMEA's.

 

[_chris_]

I saw a great one when i was an apprentice, working on a machine YD forward and reverse... electrician decided that the pen drawing written on the panel door would be good enough; all connect up... star forward, then delta reverse! That taught me a excellent lesson always work it out for yourself!

any way back to the 24v question. Make sure find the correct 0v i always start on the power supply then try and half the circuit (if there is a drawing) just keep halfing.

Chris