Never assume - stupidity strikes again to accident prone A

[AJB Temple]

Just had a fair old belt from the mains, which woke me up somewhat. Needed to replace the switch on a lighting circuit. Two gang two way. Plastic switches previously and the previous person just snipped off the CPC (earth) which was unhelpful rather than taking ten seconds to connect it to a terminal block. I tested the live wires and neutrals with a Megger, then switched of the circuit at the consumer unit RCCB and tested again for dead. Twas dead.

Put my finger in plastic socket to pull the earth out so I can use it and "BANG" I get an electric shock. Rubber soled shoes on dry stone floor and only one hand in contact, but this was my first electrical shock in 35 years as I am generally super careful and do the prove/test/prove again routine always with a meter of Megger probe. So I was VERY surprised. I was also alone.

In this case I had not been able to get the probe to the "earth" wires and assumed that they were genuine CPC. In fact one of them was live and connected at one end to the socket circuit in the same room (as I now have proved) and someone had used a yellow cable and re-sleeved the last few inches as green and yellow. I can't imagine why someone would put a stray single wire down to a socket, sleeve it as CPC then bury it in a wall void.

So that is the lesson for today - never assume. Still buzzing......

 

[fiveeyes]

Glad that you fared well..is not your voltages higher than we have here?

 

[AJB Temple]

Not sure where "here" is for you five eyes as your profile doesn't say. It's nominally 230v AC at 50 Hertz. Can vary a bit either way.

 

[AJB Temple]

Just for interest the last time I had a shock was a really big one. It pre-dates part P building regs. We had a large farmhouse at the time, which had been a rebuild so nearly everything was new. I was doing some electrical work in a bathroom. We had a Philippina nanny (Mary Lou - she was great, qualified midwife) to help look after baby offspring. I had turned off the circuits at the CU and specifically told her NOT to turn them back on. I was lying on the floor in the bathroom doing whatever beneath the floor for the room below and after about half an hour got the most almighty shock.

I went downstairs, shaking like a leaf, and Mary Lou had turned the power back on because she needed to do something. Taught me a big lesson, which is never trust anyone else when working on dangerous stuff. Since then I use circuit lockout padlocks, label flags and proper test gear.

My wife finds CUs confusing because ON is up for MCBs and RCCDs whereas a socket or light switch is down for on. She has a point really.

 

[StevieB]

Yellow used to be (possibly still is) the 4th wire for 2 way switching. Is it possible someone has swapped an old 2 way switch set-up for a one way 3 wire set-up and simply left the 4th wire connected at the other end?

 

[AJB Temple]

Yes, yellow is for 2 way switches. However, a 1mm yellow sleeve should not have been connected into the live supply of a 13amp ring socket circuit and just snipped off inside a light switch box, and there is no logical reason I can think of for sleeving it CPC. All I could see with the wire pushed back into the void was green and yellow. It's my own fault - most of the original wiring here was bodged and I should have expected stupidity.

 

[HappyHacker]

I had a near mis when disconnecting a wall light circuit. I removed the one lighting fuse, tested the circuit for dead, is was but I wondered why the live had not been jumpered across to the second switch on the plate. When finished customer asked me if I could replace the 2 gang switch with a single one. I got one from the van but some second sense made me check the live of the second switch before disconnecting the switch, it was live and I still had the lighting fuse in my pocket. Someone had wired the wall lights to the power circuit.

One cannot be too careful, despite testing for dead, things can become live from shared neutrals and even if the main breaker is off from the supply earth if there is a fault in adjacent premises. The standard advice is test for dead work as if live.

 

[Mike G]

Blimey, Adrian, that was unlucky. Glad you're OK.

I have a light switch which controls lights on 2 different circuits (ie 2 different breakers). I once switched the circuit off for that room to do something or other, and luckily tripped the live circuit with my sleeved screwdriver. If that had been my finger I would not have been a happy bunny......

 

[SamQ aka Ah! Q!]

Adrian, it would do no harm at all to have yourself checked out, by which I mean a twelve-lead in the hands of (at least) a paramedic trained to read the resultant print out.
Electrical shocks directly interfere with the heart's internal pacemaker, the S.A.N..The problem is, said interference may not be immediately evident to either the shock's recipient, nor a quick 'once over' from a concerned family member or First Aider. The problem then lurks and gestates, to reappear - with attitde - later on.

I've been directly involved in health and safety (N.B. small letters) since1974 and there is no way now, after all that experience, that I would let an electrical shock pass without post-episode examination. I have, as a teenager, taken two "good belts" from open 240V mains, but at our more mature stage in Life, I have little faith that we are as resilient as we once were.

 

[RogerS]

Glad you're OK, Adrian.

There was a BBC Transmitter engineer up a step-ladder with his head in between two transmitter parts. They were supposed to be off. One of them wasn't. His right ear touched it, we're talking kV ...not piddly domestic..he reacted and his head banged into the other - dead - transmitter part. His head then reacted to the bang, which touched his right ear again.....his life was saved because his body literally rattled it's way clear as he fell in a heap.

 

[AndyP]

Shocking!
I’ve never quite felt comfortable with the lecky here. Sockets running off lighting circuits were an early warning.
Salutary lessons there for all of us.

 

[Malc2098]

A highway engineer I knew of was measuring the road environment on a railway bridge in preparation for some changes. We all used steel tape measures for anything up to about five metres. The wind caught the tape and blew it over the parapet on to the live overhead conductor below. He was killed instantly.

 

[Tiresias]

Whooo. Poor you.

Probably had half a dozen of 240v pings in my life. Twice because of my own doziness, others because of some one else’s stupidity (you’ll note the distinction).

Certainly, does liven you up. Although as I get older I do seem to have got a bit more risk averse.

A long, long time a go (in a galaxy far, far away) I used to work for British Gas. The switches in the compressor stations (I think) had two massive carbon spikes as contacts (about 70mm dia) that plunged into the sockets with an hydraulic over centre thing, all under oil (for the sparks). Now obsolete. I can’t remember the voltage, but it was probably quite a bit, many kV. Even I would shirk from that.

 

[Pete Maddex]

I used to fix TVs, we tested the EHT by seeing how long an arc you could get the final annode to the chassis, they ran about 25KV.


Pete

 

[AJB Temple]

Thanks for the good wishes chaps. I was perfectly OK but a bit shocked. I simply couldn't believe that I had touched a live wire when I had tested the circuits. I've also got more risk averse as I got older and normally I would turn the power off completely for that part of the house (There are sub CUs). But didn't because my wife had washing and drying running whilst she was out, and I thought I had the circuit isolated. I did have the lighting circuit isolated but not the sockets that this one was connected to unknown to me.

Malc's story is super shocking. We have quite low overhead power lines running across a corner of our plot (it's rural and most houses in the lane are supplied this way) so the threat is not far away. We had one line brought down across our drive last winter when a branch came off an oak tree. We were all a very wary of it, not knowing if it was live or not, and not being keen to find out.

 

[SamQ aka Ah! Q!]

Thanks ....... We had one line brought down across our drive last winter when a branch came off an oak tree. We were all a very wary of it, not knowing if it was live or not, and not being keen to find out.

Especially if it was subsidiary, on its way to a step-down transformer....

 

[Sheffield Tony]

I used to fix TVs, we tested the EHT by seeing how long an arc you could get the final annode to the chassis, they ran about 25KV.

My school friend and I - must have been 14 at the time - were grafting together 2 broken TVs in order to watch the launch of the first space shuttle. One had a duff tube, the other had a different fault, but they were the same sized tube. Swapped the CRT over, turned it on, not working. Forgot to put the (ground ?) connection onto the back of the tube. Turned it off, diligently discharged all the big looking capacitors, went to put the missing lead in place - forgot the tube itself holds a fair bit of charge. Made me jump quite bit - enough to fail my other hand and hit my friend, who got the shock second hand.

Worse was changing an internal chassis mount 1 1/4" cartridge fuse in a lighting controller, having pulled the wrong plug. One hand on the case, one finger on the live fuse clip. I was checking for a pulse to see if I was still alive !

 

[RogerS]

I used to fix TVs, we tested the EHT by seeing how long an arc you could get the final annode to the chassis, they ran about 25KV.


Pete

The dad of my old schoolchum also repaired Tvs but of the valve vintage. He'd diagnose whether or not a valve was working properly by jabbing his finger on the various electrodes, anode etc and judging whether or not the voltage seemed OK.

 

[Pete Maddex]

I did valve tvs, lots of high voltage and the risk of X-rays.

I was left on my own in the workshop fixing the tvs, what could go wrong?

Pete

 

[Eric the Viking]

Dad was repairing the chassis of our Pye "Luxury 19-Inch" TV on the dining table. He emphatically told me, aged around five, not to touch anything

I didn't.

I did, however, point at the undone anode connection on the side of the tube, most carefully from about three inches away, and just got out, "Dad what's that bit?", when the spark jumped the gap.

It threw me across the room (-ish), and all he could do was laugh, saying, "I told you not to."

To the day he died, he insisted I touched it, and I still insist I didn't.

 

[jimmy s]

Glad you were OK. We had a situation some time back where heavy dc cable was getting removed in below ground cable subways. The guys were cutting the cable out in about 3 foot sections as it was really heavy cable - used for dockside power on submarines at one time, and was taking an age to remove. They were cutting it into bits with a hydraulic cable cutter which was foot operated. The lighting in the tunnels wasnt great and one night they inadvertently pulled the 11,000 volt ring main out of the cable brackets and cut into it with the cutter by mistake. One almighty bang later and the whole cable cutter head just evaporated, luckily the guys were OK probably because the cutter was foot operated and the fault path was direct to ground or they would have had very poor odds of surviving I would imagine. I understand with HV supplies you can still get a potential difference across the ground known as step voltage which can result in a fatal shock with earth faults on HV systems, caused by variance in potential between your feet.

 

[Eric the Viking]

Step voltage: I think that's a common cause of death of quadrupeds - mostly cattle and sheep - which are unfortunately close to lightning strikes.

 

[SamQ aka Ah! Q!]

Could an experienced electrician or physicist please comment on the following?
I was taught to discriminate between voltage and current: At uni, the E.M. electrode 'gun' operated at 100k Volts, but only microamps of current. Our E.M.techs maintained "you'd barely notice a tickle from that!". Teaching Junior Science with Van de Graf generators reinforced that belief.
My sparky - talking about National Grid electricity - said: "mains voltage is only 240/260V, but up to 16 Amps current. 0.30-0.45 Amps kills".
So, not being conversant with AC vs DC, for years I have had a mantra something along the lines: 'voltage (to some extent) is unimportant, but current can be deadly'. T? F?

I am also aware of capacitors (and old CRT tubes it seems) having a high discharge capability, is that D.C. or high voltage, or both, and is it potentially more harmful because it is D.C.?

 

[NickM]

My dad tells a story about how my grandpa used to light his cigarette at home by tearing a small piece off the newspaper and lighting it on the bar of the electric fire. My dad, who I guess was in his late teens, thought this was a good idea so did the same thing. However, he used the inside of a cigarette packet which was foil covered paper and he got a boot off the fire!

My grandpa thought it was hilarious and my dad is convinced he watched him do it knowing exactly what was going to happen!

The thought of them all sitting around at home smoking is actually the most shocking part of the story for me!

 

[Dr.Al]

Could an experienced electrician or physicist please comment on the following?
I was taught to discriminate between voltage and current: At uni, the E.M. electrode 'gun' operated at 100k Volts, but only microamps of current. Our E.M.techs maintained "you'd barely notice a tickle from that!". Teaching Junior Science with Van de Graf generators reinforced that belief.
My sparky - talking about National Grid electricity - said: "mains voltage is only 240/260V, but up to 16 Amps current. 0.30-0.45 Amps kills".
So, not being conversant with AC vs DC, for years I have had a mantra something along the lines: 'voltage (to some extent) is unimportant, but current can be deadly'. T? F?

I am also aware of capacitors (and old CRT tubes it seems) having a high discharge capability, is that D.C. or high voltage, or both, and is it potentially more harmful because it is D.C.?

I'm neither an electrician nor a physicist, so you'll have to put up with a lowly electronics engineer

It is the current that kills, but you need voltage to drive that current. Your body is effectively a resistor and Ohm's Law says V = I × R or I = V / R.

A given voltage source will have a (fixed under normal operation) voltage output and a maximum current output. Stick a power supply with 100 kV voltage and maximum current of 100 μA across a 1 kΩ resistor and rather than delivering the "Ohm's Law" current (100 A) it'll clamp at 100 μA and the voltage will drop accordingly.

Put a 240 V, 16 A supply across that same 1 kΩ resistor and you'll get 240 mA flowing (enough to kill, assuming that 1 kΩ resistor is your body).

As a result of all of that, you'll get a shock off a lower voltage if that voltage is across a smaller (and hence lower resistance) route across your body (e.g. from one finger to the other) than you will if it's across all of your body (e.g. from one hand to the other). The latter shock is more likely to be deadly though as it might go though your heart on the way.

To put it simply then, you need volts to drive amps through the body, the amps driven through the body will do the damage.

Capacitors store DC voltage. It isn't necessarily that much more harmful to have DC vs AC, but there can be some side effects - DC can cause (for example) your hand to close, gripping the thing that is giving you the shock. AC might cause it to close / open / close / open etc (but too fast to actually have an effect) so you're more likely to be able to withdraw your hand when you realise you shouldn't have put it there. In that way, AC is a bit safer, but any high voltage with a reasonable amount of current-delivery-capability is dangerous.

 

[Pete Maddex]

I had with some 9V batterys 10 cliped together woudn't give you a shock through dry fingers (90v) but 20 would.

Dry fingers increase the resistance of your skin.

Pete

 

[Windows]

Great question Sam.

I’m not qualified, but my mental model was this: electricity kills in (at least) two ways. One is high power causing heat. The other is electrical disruption of nerve signals. Power is determined by voltage x current (and heat produced is further determined by resistance) so both voltage and current play a part in high power injuries. For nerve signal disruption, power seems less important except to note that a low power signal applied at a limb is less likely to reach brain or heart which would have particularly bad effects if disrupted. Further, the frequency of the power supply is related to its potential to disrupt the body’s natural signals so AC power (at low frequencies as used in domestic transmission) is worse than DC

Could experts tell me if this model makes sense or where it breaks down?

 

[HappyHacker]

The following gives a good description of the effects of current on the body.

Conduction of Electrical Current to and Through the Human Body: A Review - PMC

Objective: The objective of this article is to explain ways in which electric current is conducted to and through the human body and how this influences the nature of injuries. Methods: This multidisciplinary topic is explained by first reviewing ...

pmc.ncbi.nlm.nih.gov

 

[Windows]

AC vs DC as I understand what’s described in that article:
1. Skin passes more current when voltage is changing
2. Nerve & muscle cells pass current and/or contract when voltage changing
3. Heart is vulnerable to fibrillation during particular period of the cycle
4. DC electrocution has one chance to mess up the rhythm of the heart.
5. AC electrocution has 50 or 60 chances per second.

Result: much greater likelihood of AC zapping the heart during its vulnerable period than DC and at a higher effective current.

“With alternating current, a shock duration of longer than 1 cardiac cycle will definitely give stimulation during the vulnerable period.“

 

[Windows]

Feel like it would only be responsible of me to reiterate SamQ’s sage advice to get checked out by a medical professional following an electric shock.

 

[Sheffield Tony]

AC vs DC as I understand what’s described in that article:
1. Skin passes more current when voltage is changing
2. Nerve & muscle cells pass current and/or contract when voltage changing
3. Heart is vulnerable to fibrillation during particular period of the cycle
4. DC electrocution has one chance to mess up the rhythm of the heart.
5. AC electrocution has 50 or 60 chances per second.

Result: much greater likelihood of AC zapping the heart during its vulnerable period than DC and at a higher effective current.

“With alternating current, a shock duration of longer than 1 cardiac cycle will definitely give stimulation during the vulnerable period.“

Interestingly, the conclusions from this article are quite the reverse of everything I've been told elsewhere - which is that DC is more capable of producing a lock-on situation, whereas AC - especially high frequency AC - is beyond the bandwidth of muscles to respond to.

One related anecdote on this is relates to the electric chair in the US; Edison's involvement in its development meant AC was chosen, as Edison wished to defame the Westinghouse AC system by associating it with death, to the advantage of his own "safer" DC system. Except it didn't work well. the first victim was only made unconscious by 700v, and finally killed by over 1000v basically by thermal effects (he started smoking).

What's right ? I don't know anymore.

 

[jimmy s]

I'm neither an electrician nor a physicist, so you'll have to put up with a lowly electronics engineer

It is the current that kills, but you need voltage to drive that current. Your body is effectively a resistor and Ohm's Law says V = I × R or I = V / R.

A given voltage source will have a (fixed under normal operation) voltage output and a maximum current output. Stick a power supply with 100 kV voltage and maximum current of 100 μA across a 1 kΩ resistor and rather than delivering the "Ohm's Law" current (100 A) it'll clamp at 100 μA and the voltage will drop accordingly.

Put a 240 V, 16 A supply across that same 1 kΩ resistor and you'll get 240 mA flowing (enough to kill, assuming that 1 kΩ resistor is your body).

As a result of all of that, you'll get a shock off a lower voltage if that voltage is across a smaller (and hence lower resistance) route across your body (e.g. from one finger to the other) than you will if it's across all of your body (e.g. from one hand to the other). The latter shock is more likely to be deadly though as it might go though your heart on the way.

To put it simply then, you need volts to drive amps through the body, the amps driven through the body will do the damage.

Capacitors store DC voltage. It isn't necessarily that much more harmful to have DC vs AC, but there can be some side effects - DC can cause (for example) your hand to close, gripping the thing that is giving you the shock. AC might cause it to close / open / close / open etc (but too fast to actually have an effect) so you're more likely to be able to withdraw your hand when you realise you shouldn't have put it there. In that way, AC is a bit safer, but any high voltage with a reasonable amount of current-delivery-capability is dangerous.

Yes volts jolt, current kills - as out physics teacher at school used to say.

 

[RogerS]

......

Capacitors store DC voltage. It isn't necessarily that much more harmful to have DC vs AC, but there can be some side effects - DC can cause (for example) your hand to close, gripping the thing that is giving you the shock. AC might cause it to close / open / close / open etc (but too fast to actually have an effect) so you're more likely to be able to withdraw your hand when you realise you shouldn't have put it there. In that way, AC is a bit safer, but any high voltage with a reasonable amount of current-delivery-capability is dangerous.

That paragraph is wrong. It is AC that is much more dangerous. DC does not make you grip the object. You recoil with the shock.

But in a hand, for example, the muscles to grip an object are much stronger than the ones to release. So when they are being triggered at AC vols at 50Hz, the more powerful ones win and you don't let go.

And thank you for giving me the opportunity to remind folks about being in a hotel on fire. When you arrived, you should have remembered to count the doors between you and the fire exit. The fire alarm goes off, you open the door .... having checked it's not hot 'cos there's flames the other side so not a good idea ..... to find a smoke-filled corridor. So, blinded by the smoke, you get down on your hands and knees and start scrambling for the fire exit and running your hand along the skirting to count the doors. You MUST use the back of your hand because very often utilities like electric wiring are down at that level. You do NOT want to be feeling along with your fingertips.....AC....see above !!!

 

[Windows]

Interestingly, the conclusions from this article are quite the reverse of everything I've been told elsewhere - which is that DC is more capable of producing a lock-on situation, whereas AC - especially high frequency AC - is beyond the bandwidth of muscles to respond to.

One related anecdote on this is relates to the electric chair in the US; Edison's involvement in its development meant AC was chosen, as Edison wished to defame the Westinghouse AC system by associating it with death, to the advantage of his own "safer" DC system. Except it didn't work well. the first victim was only made unconscious by 700v, and finally killed by over 1000v basically by thermal effects (he started smoking).

What's right ? I don't know anymore.

I have recently seen some text that does distinguish between high and low frequency AC, but I didn’t know where I might encounter high frequency AC so I failed to remember any lessons like what frequencies are considered high or where I saw the reference.

 

[SamQ aka Ah! Q!]

My grateful thanks to all who responded to my query, especially Dr. B. for a straightforward explanation and HappyHacker for some pertinent researching - I'm slowly trying to digest that wide-ranging review.
What I am taking away from this is a refreshed respect for all things electrical.
Essentially: it's the current that kills, but the voltage can provide the penetrative force to permit entrance to the body's more aqueous (electrically vulnerable) tissues. D.C. can.be nastier than A.C. because of the inability to let go. Your respiratory and cardiac tissues get physiologically compromised by relatively small electrical supplies; possibly beyond recovery. Edison was a sadist.
I've skipped the nuances. I'm not an A and E operative, so no need for them. That said, I re-iterate: "Lecky is dangerous and always assume a wire is 'live' until a Megger tells you otherwise".

 

[Lurker]

What’s Edison got to do with it.
He was successful at making money from ideas that he nicked, nothing more.

 

[RogerS]

My grateful thanks to all who responded to my query, especially Dr. B. for a straightforward explanation and HappyHacker for some pertinent researching - I'm slowly trying to digest that wide-ranging review.
What I am taking away from this is a refreshed respect for all things electrical.
Essentially: it's the current that kills, but the voltage can provide the penetrative force to permit entrance to the body's more aqueous (electrically vulnerable) tissues. D.C. can.be nastier than A.C. because of the inability to let go. Your respiratory and cardiac tissues get physiologically compromised by relatively small electrical supplies; possibly beyond recovery. Edison was a sadist.
I've skipped the nuances. I'm not an A and E operative, so no need for them. That said, I re-iterate: "Lecky is dangerous and always assume a wire is 'live' until a Megger tells you otherwise".

Mains AC does not let go. Not DC

 

[SamQ aka Ah! Q!]

What’s Edison got to do with it.
He was successful at making money from ideas that he nicked, nothing more.

Read the review paper linked to.

 

[SamQ aka Ah! Q!]

Mains AC does not let go. Not DC

Sorry, Roger is quite right. I had reached saturation when I read/tried to take in that point.
I was confusing the tetanic grip with the D.C. 'pain on contact, pain on breaking contact' information in an adjacent entry
Like I said, saturation.

 

[Guineafowl21]

Essentially: it's the current that kills, but the voltage can provide the penetrative force to permit entrance to the body's more aqueous (electrically vulnerable) tissues.

To the very good replies above, I’d just add another thing: time.

To get a cardiac arrest/ventricular fibrillation fatal shock, a certain amount of electrical energy must be transferred to the heart. Given by: E=VIT. Volts, amps and time. A Van de Graaf generator, at 100 kV, can put kA through your heart, but only for an insignificantly brief period of time.


So, you can’t really pick out one factor from VxIxT and say it’s the significant one. It’s a complex and inter-dependent relationship:

V - about 50V will penetrate the skin barrier, depending on thickness/dampness. As V rises, it can punch right through into the quite conductive tissue beneath. Language here is loose, because humans are non-ohmic, ie our resistance depends on the voltage applied (and other things.)

I - about 30-50 mA through the heart will stop it, but much higher currents can cause ventricular clamping, which can have a protective effect.

T - T needed depends on I, which in turn depends on V in complex fashion.

One last practical point: if you do get a belt, check your pulse and get off the ladder. You may be in VF and not realise it, but will have time to call for help. There are stories of electricians found dead, sitting on their toolboxes, a lit cigarette in their hands. What happened was:

Shock (VF!) - feel woozy - sit down - light fag - die.