longjohnsilver
Well-known member
How did you get a shock off a cable that was disconnected at both ends? What is induced voltage?
Never mess with 240V
- Thread starter mariadz
- Start date
How did you get a shock off a cable that was disconnected at both ends? What is induced voltage?
RupertW
Well-known member
Induced voltage is voltage that is transferred by induction when you have an AC current. It's what is used for example for Apple Watch charging where no actual contact is made between charger and watch. Induction coils can be used to transform electricity to different voltages or current.
It seems unlikely but possible that sufficient induction could be built up by laying two long lenghs of three core cable together, but what makes it seem odd to me is that I would only expect it to be effective if there were separate lengths of single core cable.
It seems unlikely but possible that sufficient induction could be built up by laying two long lenghs of three core cable together, but what makes it seem odd to me is that I would only expect it to be effective if there were separate lengths of single core cable.
coopec
N/A
In the dark old days of my life when our syndicate was trying to get money off a crooked builder something went wrong behind the house switch/circuit breaker board. I couldn't afford to get an electrician in so I did the work on live wires myself! (Of course that was highly illegal)
RupertW
Well-known member
Not always - the law was changed after a friends Dad did the wiring and killed her, so her mother got the law changed so nobody could ever do AC stuff unless professional. Understandable but I think wrong given the low number of deaths caused by so many of us doing our own wiring.
Aurai
Active member
Should anyone like to read the HSE version?
https://www.electricalsafetyfirst.org.uk/mediafile/100117573/Best-Practice-Guide-2.pdf
https://www.electricalsafetyfirst.org.uk/mediafile/100117573/Best-Practice-Guide-2.pdf
longjohnsilver
Well-known member
Thanks for the explanation.
coopec
N/A
Aurai
I looked at it and it is 20 pages. Can you highlight the relevant sections for us please?
I looked at it and it is 20 pages. Can you highlight the relevant sections for us please?
coopec
N/A
As far as I'm concerned that is how a transformer works. Am I correct?
Aurai
Active member
Coopec
The 20 pages are full of guidance for working on low voltage AC systems. Safe isolation is drummed into trained persons and pretty much standard in industry, the risk occurs for boat owners without training. The guide, for instance, mentions retaining control of both ends of your supply cable. Effectively mentioned by contributors to this thread, similarly advises against using multi-meters for checking for live voltages. Isolation tape is advised against and a few pages on locking arrangements so a circuit cannot be energised inadvertently. In the absence of an apprenticeship or professional training, I suggest the document is read by anyone working on AC on their craft, when they cannot control both ends of their supply cable. I appreciate that common sense goes a long way, but not necessarily all the way. Cheers
The 20 pages are full of guidance for working on low voltage AC systems. Safe isolation is drummed into trained persons and pretty much standard in industry, the risk occurs for boat owners without training. The guide, for instance, mentions retaining control of both ends of your supply cable. Effectively mentioned by contributors to this thread, similarly advises against using multi-meters for checking for live voltages. Isolation tape is advised against and a few pages on locking arrangements so a circuit cannot be energised inadvertently. In the absence of an apprenticeship or professional training, I suggest the document is read by anyone working on AC on their craft, when they cannot control both ends of their supply cable. I appreciate that common sense goes a long way, but not necessarily all the way. Cheers
Lon nan Gruagach
Active member
Mebby, mebby not.
There are 2 mechanisms by which electrickery can act at a distance. One is capacitance, the other is inductance.
Capacitance uses the attractions of opposite charges to work, if you attach a plate conductor to a high voltage then a similar, but isolated plate nearby will (briefly) act as if there were a connection. Charges in anything attached to the second plate are attracted to it by the charge on the first plate.
Inductance uses electromagnetic principals. A current in a conductor creates a magnetic field. A changing magnetic field will induce a current in a conductor. So an alternating current in one conductor will generate an alternating current in an adjacent one.
Transformers have many turns and an iron core in order to improve efficiency.
GeorgeLlewellin
Member
Boy pliers, electric wires,
Blue flashes, boy ashes.
Blue flashes, boy ashes.
RupertW
Well-known member
I read that as yes to the question which I would have said. But some transformers also have capacitors added to their circuits for various reasons from smoothing spikes to particular voltage changes.
thinwater
Well-known member
Some good advise, some bad advise, and some Darwinian advise.
An electrician will always use a meter, because you never know what sort of miss-wiring exists. It's not an extra step, it is the first step. He will also do some investigation to confirm he understands the circuits. It's all part of getting the job done right the first time.
Also remember that with 240V arc flash begins to become an issue. I saw a man badly injured by 480V (lots of 2nd and 3rd degree burns) when just replacing a fuse. You don't have to be shocked to be injured.
An electrician will always use a meter, because you never know what sort of miss-wiring exists. It's not an extra step, it is the first step. He will also do some investigation to confirm he understands the circuits. It's all part of getting the job done right the first time.
Also remember that with 240V arc flash begins to become an issue. I saw a man badly injured by 480V (lots of 2nd and 3rd degree burns) when just replacing a fuse. You don't have to be shocked to be injured.
PaulRainbow
Well-known member
A pretty foolproof way of checking to see if it's live (with 240v) is to stick your fingers on the terminals :encouragement:
This doesn't work with 12/24v DC, to check those you have to put the negative and positive terminals on your tongue
This doesn't work with 12/24v DC, to check those you have to put the negative and positive terminals on your tongue
PaulRainbow
Well-known member
On a more serious note than my previous post, i reckon DC on a boat has much more potential to cause serious problems. Everyone knows that 240v will give you a shock and 12v won't. People that might be cautious about working on mains circuits will often have a go with 12/24v circuits. There are not many weeks that go by where i don't see some DC wiring that by some act of God hasn't quite burst into flames :ambivalence:
Stuff wired up with speaker wire and no fuses, devices wired up with vastly under rated cable, no fuses, no isolators etc Faults by-passed dangerously, leaving the faulty cabling in place and still live. The list goes on........
Stuff wired up with speaker wire and no fuses, devices wired up with vastly under rated cable, no fuses, no isolators etc Faults by-passed dangerously, leaving the faulty cabling in place and still live. The list goes on........
VicMallows
New member
Paul,
Couldn't agree more about 12v. It still to this day scares me rigid whenever I see someone working on a high current DC supply (such as a car or boat) with a wedding ring on. I once saw the consequences
Couldn't agree more about 12v. It still to this day scares me rigid whenever I see someone working on a high current DC supply (such as a car or boat) with a wedding ring on. I once saw the consequences
Jamesuk
Active member
I’m like that with the bow thruster oil
Change... even if everything is off and I’ve volt meter checked the bow thruster I still think it’s going to turn on and cut my hands off
penberth3
Well-known member
Paul,
Couldn't agree more about 12v. It still to this day scares me rigid whenever I see someone working on a high current DC supply (such as a car or boat) with a wedding ring on. I once saw the consequences
Also steel watch straps!
thinwater
Well-known member
Paul,
Couldn't agree more about 12v. It still to this day scares me rigid whenever I see someone working on a high current DC supply (such as a car or boat) with a wedding ring on. I once saw the consequences
My grandfather didn't wear a wedding band. While working as a mechanic for the US Navy during WWII he got the ring between a starter stud and ground and nearly melted the ring. They had to cut it off and he had a major scar. I stopped wearing a ring after several near misses in industry. My first boss lost a finger when his ring caught on a ladder rung.
No rings.
coopec
N/A
A pretty foolproof way of checking to see if it's live (with 240v) is to stick your fingers on the terminals :encouragement:
This doesn't work with 12/24v DC, to check those you have to put the negative and positive terminals on your tongue
My Father told me if you were unsure about something being live and you wanted to check if 240V was alive you should always check with the back of your hand as your muscles contract with an electric shock and it would break the contact if it was alive.