Brain-train question for you trainee's

[Darkwood]

I get called out to a customer who says they are getting shocks in the kitchen and they have pinned it down to when they touch the sink drainer and kettle at the same time.

Now just for this senerio we will skip safe isolation, testing sequence etc - its just hypothetical to get you imagining fault paths.

No rcd protection in the premises.
When the kettle is plugged in and boiling water you measure 70v between the drainer and metal work on the kettle.

Question 1 - give 2 examples of faults that could give these results and try to make your faults for both examples unrelated.




Again ignoring safe isolation and testing procedures for the hypothetical situe ...

The kettle has boiled and you switch it off at the socket but leave it plugged in, you still measure the 70v present.

Question 2 -What is most likely to be the fault route now with this extra info'?

PS there are no real correct answers here but I'm after you been able to interpret a fault condition when given certain information only, out in the real world faults don't always follow the examples you are taught in your text books.

 

[pennychew]

Very similar to Marvo's question about the baboon and the grill the first thing i would expect would be a L-E fault on the kettle again i would expect a blown element in the kettle also possibly a loose or not connected earth in the kettle or supplying socket as the reason its not tripping the MCB. Also if there is a difference in potential between the drainer and the kettle body the water pipes may not be bonded or there could be plastic pipe somewhere which has broken the bonding which should be grounding the taps/sink/drainer.

If theres still a difference in potential between the two when the kettle is switched off at the wall then it definitely sounds more like a drainer that isnt bonded and it or the supplying pipework are in contact with a live supply somewhere. This floating voltage would use the kettles earthing and the cpc to complete its path to earth through you.

The first thing i would have done is use an analogue tester or a multimeter with a low impedance setting to confirm it was actually 70v and not just a ghost voltage! as ive had this in the past with a fluke T5 showing 100v and it was actually 0.4v on a low impedance multi meter

 

[Darkwood]

Very similar to Marvo's question about the baboon and the grill the first thing i would expect would be a L-E fault on the kettle again i would expect a blown element in the kettle also possibly a loose or not connected earth in the kettle or supplying socket as the reason its not tripping the MCB. Also if there is a difference in potential between the drainer and the kettle body the water pipes may not be bonded or there could be plastic pipe somewhere which has broken the bonding which should be grounding the taps/sink/drainer.

If theres still a difference in potential between the two when the kettle is switched off at the wall then it definitely sounds more like a drainer that isnt bonded and it or the supplying pipework are in contact with a live supply somewhere. This floating voltage would use the kettles earthing and the cpc to complete its path to earth through you.

The first thing i would have done is use an analogue tester or a multimeter with a low impedance setting to confirm it was actually 70v and not just a ghost voltage! as ive had this in the past with a fluke T5 showing 100v and it was actually 0.4v on a low impedance multi meter

Totally happy with you replies and you already seem to have a nack at reading the situe' from info given and translating it into likely causes and and what the next step of action would be....
I just wish half my apprentices were like yourself.

 

[pennychew]

I wish i was as young as half your apprentices! i enjoy the challenge when someone posts up questions though so thanks for taking the time

 

[Darkwood]

I wish i was as young as half your apprentices! i enjoy the challenge when someone posts up questions though so thanks for taking the time

Don't knock it .. I have had an apprentice older than me and sometimes not a comfortable situe' when handing down the discipline ... but to his credit with him been a family man and needing to provide for them he was thus keen to learn and learned very quickly.. he now runs his own company and I do ask for his help when I'm busy so result there.

 

[malcolmsanford]

Very similar to Marvo's question about the baboon and the grill the first thing i would expect would be a L-E fault on the kettle again i would expect a blown element in the kettle also possibly a loose or not connected earth in the kettle or supplying socket as the reason its not tripping the MCB. Also if there is a difference in potential between the drainer and the kettle body the water pipes may not be bonded or there could be plastic pipe somewhere which has broken the bonding which should be grounding the taps/sink/drainer.

If theres still a difference in potential between the two when the kettle is switched off at the wall then it definitely sounds more like a drainer that isnt bonded and it or the supplying pipework are in contact with a live supply somewhere. This floating voltage would use the kettles earthing and the cpc to complete its path to earth through you.

The first thing i would have done is use an analogue tester or a multimeter with a low impedance setting to confirm it was actually 70v and not just a ghost voltage! as ive had this in the past with a fluke T5 showing 100v and it was actually 0.4v on a low impedance multi meter

Penny you do seem to work things through logically. Now playing the devils advocate.

As you know there is no need to bond drainers or pipes going to a drainer as to the regs.

You have noted quite rightfully that in order to get a voltage you need a potential. If there is a piece of plastic pipe in the system, how is this potential happening. A potential difference can only be achieved if a metallic service is in contact with earth. So though hypothetically there is a metallic main and copper internal system, if there is a piece of plastic pipe breaking the copper system, then where do you think you could also be getting a potential

 

[pennychew]

Penny you do seem to work things through logically. Now playing the devils advocate.

As you know there is no need to bond drainers or pipes going to a drainer as to the regs.

You have noted quite rightfully that in order to get a voltage you need a potential. If there is a piece of plastic pipe in the system, how is this potential happening. A potential difference can only be achieved if a metallic service is in contact with earth. So though hypothetically there is a metallic main and copper internal system, if there is a piece of plastic pipe breaking the copper system, then where do you think you could also be getting a potential

My first thoughts were a live conductor rubbed through and touching a metalic pipe connected directly to the sink after the plastic or maybe the sink drainer is directly screwed to the wall and a screw has nicked a buried cable... those would be my first suspicions!

 

[malcolmsanford]

Yes that is one.

Also if there is gas and that is metallic and bonded, where it connects to the boiler, so does the water supply, and if that connection is after the plastic insert, but before the kitchen drainer, then the copper water pipe will pick up the potential there.

 

[pennychew]

Yes that is one.

Also if there is gas and that is metallic and bonded, where it connects to the boiler, so does the water supply, and if that connection is after the plastic insert, but before the kitchen drainer, then the copper water pipe will pick up the potential there.

Never thought of that one!

 

[malcolmsanford]

Years ago I heard of a similar scenario that Dark highlighted. It was a top floor flat the couple said that every now and then they got a shock off the bath/sink in the bathroom.

They had about 3 or 4 guys in to check it and never got cured, and I know 2 of the sparks that went and they were good lads. In the end it was put down to imagination or static and basically left.

Then a few years later one of the sparks got a job of rewiring the basement flat. After a few days the top flat owners saw him and said they didn't know what he had done but they thanked him and said that since he had been there were no more shocks.

He was mystified and could not believe these guys were still having problems. Then once he got into the kitchen he twigged it. There was a single undercounter socket for a dishwasher/washing machine. Next to it was buried copper pipes for the equipment.

Over the years there was a soldered joint that started to weep and made the wall damp, where the socket was on. The old occupants used an under counter fridge the compressor must have been old and must have had a small leak to earth and so everytime it ran/kicked it a small leakage would track through the back box along the damp plaster, to the copper pipe, up the riser to the other flats.

Now there were 3 flats in this building,the basement and the top one, and also a middle grd floor, who never complained.

Find that one!!!

 

[pennychew]

Faults like that must be a nightmare to fault find! mind you fault finding my favourite part of the job, even then it can get a bit tedious when its something completely random like that example!

 

[Darkwood]

This is the mother of all faults ... Happened to my work mate..

Customer notice long duration tingling when standing in the room around the doorway, not touching anything just bare feet on floor and was painful if feet were wet -

Solid floor and dry as a bone.
No buried cables.
Average carpet and gripper arrangement.
No voltages ever detected when we were on site.

On his final visit he struck gold and the voltage was present when he stuck a probe in the carpet at the suggested point and measured to earth and yep 230V.

Turned out the TV had an integral fault and made the aerial socket live and no doubt the aerial itself but the last piece of the jigsaw was their was a foil tape lining the carpet edging to prevent fraying, it turned out the aerial cable with the live sheath had beed spiked by the carpet grip and thus made the whole foil trim live... as the TV was normally switched off when my mate arrived it was like chasing ghosts, not something you ask for when you turn up for a job 'can you turn the TV on' ....

 

[pennychew]

Its like when you watch 'air crash investigation' on the tv and its always a series of random little fauts that come together and bring a plane down!

 

[jimmyray]

Good effort Pennychew! These fault finding questions and answers are really helping me, so thanks guys. I'll try to get involved with answering them when I feel a bit more confident!

 

[Darkwood]

Good effort Pennychew! These fault finding questions and answers are really helping me, so thanks guys. I'll try to get involved with answering them when I feel a bit more confident!

I would suggest you dive in with answers ... the Training section is ideal for this as access is only to certain members who are willing to help and no matter if you mess up with the answer you won't be ridiculed here, you will be coached and guided to the correct solution by us prompting you and this will help you better than just sitting back and reading..

I and other members with access will continue to add these brain teasers as it seems to be welcomed in here and getting your support.