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.