Fido's Getting Shocked!!!

[Marvo]

It's been a while since his last problem and up until recently Fido hasn't had a care in the world. That was until the other morning when disaster struck.

He just wandered outside to grab a bite to eat and as he came into contact with his personalised stainless steel food bowl he got an electric shock. The shock wasn't a massive one but it was enough to be uncomfortable. One thing that he did notice was that it seemed to happen every time his bowl came into contact with a steel conduit that runs down the wall behind it. He's figured that the conduit is carrying the cable that supplies the socket in the lounge.

He's pretty skittish at the moment because he read in the Readers Digest that 4 legged animals are very prone to death by high step voltages. He's not entirely sure what this means and wonders if someone can explain it in layman's terms for him.

Secondly he asked if I could organize a sparky to help him out with some testing. He asked what test equipment he might need for the job and also he wants to know what faults he should be looking for and what tests he should be doing to find them.

This whole electric shock thing has him pretty spooked at the moment and he's not going near his food until he's sure he's safe.




He thanks everyone in advance and says whilst he may have no money, anyone who helps him out is welcome to borrow his squeaky ball and tug-of-war toy to play with anytime they like.

 

[Silly Sausage]

By the look on his face, after the fact that the hairs on his balls have turned into wires, he's wondering why the ---- isn't my bowl bonded?

I want a new owner!

 

[pennychew]

Totally inexperienced at stuff like this but ill have a go...

I would guess my first step would be to check for voltage on the conduit surface down to earth? then do an IR test on the contained cables from the socket in the lounge, L-N N-E then E-L. After that id try an IR of the L and N against the steel conduit to see if there is any continuity.

If the steel conduit is being used as the cpc i dont know if it could carry a small current anyway?

I also dont know why 4 legged animals are prone to death from high step voltages but it gets the ball rolling!

Stu

 

[telectrix]

Totally inexperienced at stuff like this but ill have a go...

I would guess my first step would be to check for voltage on the conduit surface down to earth? then do an IR test on the contained cables from the socket in the lounge, L-N N-E then E-L. After that id try an IR of the L and N against the steel conduit to see if there is any continuity.

If the steel conduit is being used as the cpc i dont know if it could carry a small current anyway?

I also dont know why 4 legged animals are prone to death from high step voltages but it gets the ball rolling!

Stu

because a shock between front and back legs can be straight across the heart.

 

[telectrix]

get him a plastic bowl. bond with a bit of earth sleeve. no need for cost of copper.

 

[Marvo]

Things just went from bad to worse. Fido's bowl is a family heirloom, he inherited it from his late father so he doesn't want to swap it for a plastic one.

He took your advice though and got an electrician in who installed an earth spike and bonded the bowl.



He started eating his food after the sparky left and he got an even larger shock which caused him a bout of incontinence and nearly knocked him unconscious when he simultaneously touched the steel conduit at the same time. He's tried to get hold of the company (Cockburn Electrical) but they're not answering their phones.

He's now completely confused and teetering on the abyss of clinical depression. He wondered if one of the trainees can explain why the earthing which was supposed to make things safer has actually made matters worse.

 

[pennychew]

Things just went from bad to worse. Fido's bowl is a family heirloom, he inherited it from his late father so he doesn't want to swap it for a plastic one.

He took your advice though and got an electrician in who installed an earth spike and bonded the bowl.

View attachment 23403

He started eating his food after the sparky left and he got an even larger shock which caused him a bout of incontinence and nearly knocked him unconscious when he simultaneously touched the steel conduit at the same time. He's tried to get hold of the company (Cockburn Electrical) but they're not answering their phones.

He's now completely confused and teetering on the abyss of clinical depression. He wondered if one of the trainees can explain why the earthing which was supposed to make things safer has actually made matters worse.

Would i be right in saying if there was a short between Neutral and the metal conduit then bonding the bowl would only decrease the resistance to earth thus increasing the fault current? i reckon N-E fault as it isnt tripping the breaker/fuse as well.

 

[telectrix]

i'll send JRCPropery over to fix that fault for fido. might get it done for xmas.

 

[pennychew]

Does that mean i'm 'barking' up the wrong tree?

 

[trev]

This poor dog's got no bloody luck, maybe time for him to move home.

 

[Marvo]

Would i be right in saying if there was a short between Neutral and the metal conduit then bonding the bowl would only decrease the resistance to earth thus increasing the fault current? i reckon N-E fault as it isnt tripping the breaker/fuse as well.

This part is exactly right. If the steel conduit on the wall has a high 'touch voltage' then bonding the bowl is likely to actually increase the fault current flowing through poor old Fido if the bowl itself isn't in contact with the conduit. If the bowl is un-bonded then the bowl can 'float' at any voltage with respect to earth.

So if Fido was standing on a surface that has fairly good insulation and he leans against the live conduit whilst eating his food he won't get a shock if the bowl is not bonded. If the bowl is bonded then as soon as Fido touches the bowl and conduit simultaneously then he's going to get a nasty shock......so in this particular scenario bonding could result in a higher hazard by providing a path for a fault current which was never there previously.

I'll reply to the rest of your points when I have time this evening.

 

[pennychew]

Thanks marvo

 

[pennychew]

This poor dog's got no bloody luck, maybe time for him to move home.

Couldn't agree more, with luck like that he'll never win a lot.

 

[Marvo]

Totally inexperienced at stuff like this but ill have a go...

Good to see you're game, hopefully a few of the other trainees will have a go as well. It's nothing serious, just a few scenarios to get you thinking about different types of faults, fault-paths and testing possibilities.....and obviously help out poor Fido whilst you're at it

I've noticed there's lots and lots of threads in the main forums from guys who are blindly quoting test results without having much idea what they actually mean or in some cases why they're completely meaningless or irrelevant or entirely mis-leading. This comment isn't aimed the trainees in particular either, some of the supposedly experienced guys don't seem particularly clued up on testing either.

I would guess my first step would be to check for voltage on the conduit surface down to earth?

I like the idea, what you're suggesting is a test to establish what the actual 'touch voltage' the conduit is at.

Touch voltages are generally measured with respect to earth (not always but usually) because the poor sod getting the shock from the touch voltage usually has his or her feet planted on the ground. So, how would you do this test and what test instrument would be best to get a meaningful result? Also what is the maximum touch voltage allowed by the UK regulations? (my local regs allow <50volts)

then do an IR test on the contained cables from the socket in the lounge, L-N N-E then E-L. After that id try an IR of the L and N against the steel conduit to see if there is any continuity.

Okay, what you're suggesting here is testing the actual circuit that's contained inside the conduit. In the interests of being methodical lets do the touch voltage tests first to establish if the conduit is actually at a dangerous voltage then later we can start to investigate the circuit after that if it's necessary.

If the steel conduit is being used as the cpc i dont know if it could carry a small current anyway?

Absolutely it could. The CPC is both functional and protective.

Protective, because under fault conditions it's designed to be sufficiently low resistance to earth and sufficiently large (ie it has a cross-sectional area or CSA big enough) to allow fault currents to flow to earth without it rising to a dangerous voltage and sufficient current will flow in these circumstances to cause the 'over-current protective device (MCB or fuse) to disconnect the circuit within the specified time limits according to the regs. Out of interest what is the required disconnection time required by the UK regs?

Functional, because it also allows for surge currents to safely flow to earth. Many electronic devices have power supplies with surge arrestors. When a surge occurs the arrestor starts conducting to earth to prevent the high voltage entering the electronic equipment and causing damage. These arrestors also have a small amount of standing leakage so it's not uncommon to see a few miliamps of current flowing down a CPC even if there's no faults present and this standing leakage could become a few miliamps more if there's surges present on the supply.

I also dont know why 4 legged animals are prone to death from high step voltages but it gets the ball rolling!

Stu

As Tel mentioned, four-legged animals, especially livestock, are particularly prone to death because any current flowing between their legs will flow through their heart causing defibrillation. Also the distance between the front and back legs is greater which means the voltage difference will be higher if a step voltage is present.

i reckon N-E fault as it isnt tripping the breaker/fuse as well.

There's different protective devices to protect against different things.

A fuse or MCB is there to prevent the cabling of the circuit being overloaded which would cause the wiring to overheat and damage the cabling or even a fire being started.

An RCD is there to protect the user of the installation. It looks for an imbalance (even a very small one) between the current flowing down the live and the current flowing down the neutral. The reason for this is that any current flowing into the circuit and not flowing out of the circuit is 'leaking' somewhere to earth. If current is leaking from the circuit it means the insulation is compromised which could potentially be hazardous to a person in the vicinity of the circuit. Even 30mA (30 thousanths of an Amp) is considered a dangerous leakage current that could cause injury or death to someone if it were to flow through them.

You rightly point out that a N-E fault probably won't trip an MCB or blow a fuse but would a N-E fault cause Fido to get a shock if the neutral was fully functional?
What is the max permissible N-E voltage allowable by UK regs?

 

[Marvo]

The questions in blue aren't a test as such, I'm genuinely unsure of the answers.

 

[Simonslimline]

UK Touch voltages <50volts AC. <120volts DC.

Disconnection times below

 

[pennychew]

Cracking post Marvo, appreciate you taking the time to answer my questions.

You rightly point out that a N-E fault probably won't trip an MCB or blow a fuse but would a N-E fault cause Fido to get a shock if the neutral was fully functional?
What is the max permissible N-E voltage allowable by UK regs?

My thinking behind the N-E fault was if the containment was being used as the cpc then would this cause it to become a live conductor?

Stu

 

[pennychew]

i'll send JRCPropery over to fix that fault for fido. might get it done for xmas.

Come on Tel be a Pal, a spark of your obvious pedigree must be able to shed some light on this?

 

[Baker1988]

Cracking post Marvo, appreciate you taking the time to answer my questions.



My thinking behind the N-E fault was if the containment was being used as the cpc then would this cause it to become a live conductor?

Stu

I always thought they wouldn't be a voltage difference between neutral and earth as the earth is connected to the star point of the generator where the neutral also comes from so they should be at zero volts.

A neutral to earth should not give him a shock as the neutral and earth should be at the same potential current will still be in the neutral but there is no potential difference between neutral and earth so nothing should happen.

I have touched a neutral bar (not on purpose) while the circuit was energised and I didn't get so much as a tingle as I was on earth which was 0 volts and the neutral bar was at 0 volts.

 

[Baker1988]

Also I many wrong again but step voltages is the difference in potental between two point which in this case is Fido's front and back legs which because of the difference in potential and the resistance of fido would create a current which could kill him.

I'm sire this happend to a horse a few years back at a race track when a electrician was doing a ir test.

 

[happysteve]

Thanks for setting up this scenario, Marvo, and for your explanation of some things. I haven't had time to do more than read it (didn't see it until yesterday), but when I get a moment I'll ponder it.

Anyway, your time's appreciated.

 

[Marvo]

Thanks Happysteve.

I spoke to Fido this morning and he's in fairly good spirits all things considered but he's getting a bit peckish. He's been avoiding his food bowl like the plague after the bonding incident.

I had a look at the lounge socket as well, it appears the house has been rewired some time ago in PVC singles. They've reused the old steel conduits and steel backboxes for the new all-plastic sockets they've installed. I also saw the socket circuit is a 20A radial circuit with 2.5mm wires. There's no RCD VOELCB's or RCBO's in the CU, just MCB's.

Just to reiterate, the first test we were going to do was to establish what touch-voltage the conduit is at. Any thoughts or ideas on what tester or what method just throw them in the ring and we can discuss some pros and cons.

 

[trev]

FFS man! just dip your hand in your pocket and get this place fixed before someone gets killed.

I'd be happy to give an off plan price but would have to agree board and lodging, safari trip allowances etc

 

[pennychew]

Thanks Happysteve.

I spoke to Fido this morning and he's in fairly good spirits all things considered but he's getting a bit peckish. He's been avoiding his food bowl like the plague after the bonding incident.

I had a look at the lounge socket as well, it appears the house has been rewired some time ago in PVC singles. They've reused the old steel conduits and steel backboxes for the new all-plastic sockets they've installed. I also saw the socket circuit is a 20A radial circuit with 2.5mm wires. There's no RCD VOELCB's or RCBO's in the CU, just MCB's.

Just to reiterate, the first test we were going to do was to establish what touch-voltage the conduit is at. Any thoughts or ideas on what tester or what method just throw them in the ring and we can discuss some pros and cons.

I reckon an analogue volt meter as I keep reading that digital meters can give odd readings with touch voltages

 

[pennychew]

FFS man! just dip your hand in your pocket and get this place fixed before someone gets killed.

I'd be happy to give an off plan price but would have to agree board and lodging, safari trip allowances etc

Can I go as your apprentice? You'll pass my house on the way to the airport!

 

[trev]

Can I go as your apprentice? You'll pass my house on the way to the airport!

Nee botha bonny lad. You got your jabs up to date?

 

[pennychew]

Nee botha bonny lad. You got your jabs up to date?

Are we driving through Sunderland first like?

 

[trev]

Flying over it is just as bad mate.

 

[Marvo]

I reckon an analogue volt meter as I keep reading that digital meters can give odd readings with touch voltages

Good answer...any ideas why you're likely to get different test results depending which one you use?

Also any idea why an analogue tester might give you a more meaningful result?

 

[pennychew]

Good answer...any ideas why you're likely to get different test results depending which one you use?

Also any idea why an analogue tester might give you a more meaningful result?

Ive read an explanation for it a couple of times in threads on the general forum but i cant remember why off the top of my head.

 

[Marvo]

No probs, at least you're aware there's a difference between analogue and digital testers (other than the way they actually display the results using a needle or digital display). That already puts you ahead of most of the pack.

The thing with testing in general is you usually want to take a measurement without actually having any effect on the circuit under test whilst you're doing it. If the instrument (tester) is connected into a circuit the instrument itself can actually alter the circuit characteristics which means just by connecting the tester you've altered the thing you want to measure. The difference the actual test instrument makes to the circuit is often called the 'burden'. Here's an article that explains some more.

Sometimes though to get an accurate test result you actually need to intentionally 'burden' the circuit to simulate a real-life event. With most of the 'safety' type tests such as touch voltage and leakage current tests we deliberately want to simulate the human body that would normally be in the circuit if leakage current was flowing. We often go to ridiculous lengths to make these human body simulations, often we add networks of resistors and capacitors to pretend there's an actual person in the test. Other times we cut up pieces of tin foil into scale models of human hands to simulate the area of contact that would be involved if a real person touched something and got a shock. Sounds pretty far fetched for middle aged men to be making tin foil hands but it happens quite regularly I can assure you.

Measuring touch voltage is definitely one of those tests where you'll get the most accurate or relevant result if you burden the circuit the same as a human body or person would burden the circuit if they were coming into contact with it. So....you'll get the most relevant result if the resistance across your test probes is the same as the the resistance (impedance actually) of the person who would be touching the same points in a real-life scenario.

TBH I'm going to come unstuck here again because I don't know which 'electrical models' the UK regs use for human body equivalents so anyone in the know feel free to chime in with info.

As an electrical engineer we use the UL medical electrical human equivalent models. Generally we consider the human body to look like this as an electrical circuit;



Okay...let me know if I've lost anyone with the explanation so far, I'll probably finish off tomorrow evening.