Why has DIYNOT got a electrics forum?

[kendor]

The 50Hz frequency i'm given to understand from my college days is close to the natural frequency that can affect muscles in such that it is a possibility of the heart muscle rythmn being affected or even stopped.

An example at work it was deemed far safer to use high frequency(4khz) neon invertors (drivers) as the worst case injury from touching the HT lead would be a burn as opposed to HT transformers which of course were mains frequency.

 

[Big_Spark]

If low voltage and high current were fatal then you'd have to wear insulated gloves when welding. Ergo, low voltage and higher current is safer.

Gedditt?


joe

Joe, I suggest you short out the ends of your welder then...

 

[kendor]

Joe, I suggest you short out the ends of your welder then...

And his guitar amp

 

[Big_Spark]

Seriously, reducing the voltage to 115V would not make things safer, as it would increase the current being drawn on a given circuit, and until you get to a certain point, it is CURRENT that kills, not voltage.

You're separating current and voltage, and they intrinsically cannot be separated.

If we take a model of the human body as one of fixed impedance (which it isn't, but it'll do for now), then half the voltage will induce half the current. Which is safer.

Take a 1KW electric heater in varous voltages. At 230V it's 4.35 A. At 115V it's 8.7A. At 5V it's 200A. But where's the harm in touching a 5V live wire? You're not going to pull much current out of 5V, irrespective of what else is connected to it.

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Lower volages and their corresponding higher currents create bigger voltage drops in cables; so the cables get hotter. But if we had less volts then the cables would be sized bigger to cope with that, so it probably wouldn't actually present an increased fire risk either.

But your forgetting that electrical circuits do not exist in a state of zero current. Usually when people recieve a shock, the circuit is under load and when you short that circuit you recieve a full shock equivilent to the load on the circuit and some.

The average human (dry) resistance is about 1500 Ohms, so at..

50V = 0.033 A
70V = 0.046A
100V = 0.066
120V = 0.08
150V = 0.1A
180V = 0.12A
200V = 0.133A
220V = 0.146A
230V = 0.153A
240 = 0.16A
415V = 0.2766A

Now as 20mA or 0.02A is potentially fatal, you can clearly see that the human body is more than capable of generating a fatal shock all by itself in any electrical circuit with enough potential difference present.

As far as I am aware, most welders seem to operate at something like 24V (feel free to correct this is wrong) and at this voltage the human body will generate about 0.016A, however if the welder is being used at say 80A, a shock from one could prove fatal.

It would we wrong to imply that all or even most electrical shocks are fatal or even serious, the vast majority are just very painful experiences, but that does not mean we should ignore the reality of the potential dangers that exist in all electrical systems, especially when we are dealing with mains voltages.

 

[pickles]

So the consensus seems to be voltage irrelevant, hz not irrelevant as it is at a naturally dangerous frequency at 50/60hz which is the way I understood it

I'm not a sparky so I bow to superior knowledge but always understood use of lower voltage on building sites to be for safety reasons.

Next question how practical is a frequency change in the uk network

 

[Big_Spark]

So the consensus seems to be voltage irrelevant, hz not irrelevant as it is at a naturally dangerous frequency at 50/60hz which is the way I understood it

I'm not a sparky so I bow to superior knowledge but always understood use of lower voltage on building sites to be for safety reasons.

Next question how practical is a frequency change in the uk network

The lower voltage used on Building sites is for safety as the lower voltage does reduce the chances of a fatal shock, however they do occur, although very rare.

The idea that you could change the Freq of the supply in the UK is about as practical as changing the side of the road we drive on...

 

[slippyr4]

But your forgetting that electrical circuits do not exist in a state of zero current. Usually when people recieve a shock, the circuit is under load and when you short that circuit you recieve a full shock equivilent to the load on the circuit and some.

No you don't! I can't even begin to explain what crap that is. When you touch a live thing, an entirely new circuit is created, the current in which will be related to the impedance of the circuit. It has nothing to do with any other circuit.

In fact, lets forget the human body for a minute, and consider that what you're saying is that at first connection a circuit conducts the full load current of the circuit it joined on to.

Lets take the phase that supplies my street. It's probably running in the hundreds of amps. When I flick the switch on my consumer unit, does my house suddenly draw 100's of amps? does it *******s.

 

[Thermo]

Ladders. In 1999, about 28,000 people were killed or injured falling from ladders and steps in the home. We could probably save quite a few lives, and a lot of economic costs, if we banned the use of ladders & steps. Would anybody be up for that? Would preventing people from changing their own lamps, painting their own houses, using their lofts etc be a price worth paying in order to save lives, or would it be an unwarranted intrusion?

better look out for the working at hieght regs then

ps are you sure about your numbers, hse reckon it was 67 fatals and 4000 injuries from falls in 2003 (still far too many)

 

[pickles]

A bit of topic now but my neighbour, who is usually reliable and didn't appear to be winding me up, says he knew an electrician who used to check 3 phase supplies by putting his forearm across the bus bars.

Is this possible or a wind up/urban myth

 

[Big_Spark]

But your forgetting that electrical circuits do not exist in a state of zero current. Usually when people recieve a shock, the circuit is under load and when you short that circuit you recieve a full shock equivilent to the load on the circuit and some.

No you don't! I can't even begin to explain what rubbish that is. When you touch a live thing, an entirely new circuit is created, the current in which will be related to the impedance of the circuit. It has nothing to do with any other circuit.

In fact, lets forget the human body for a minute, and consider that what you're saying is that at first connection a circuit conducts the full load current of the circuit it joined on to.

Lets take the phase that supplies my street. It's probably running in the hundreds of amps. When I flick the switch on my consumer unit, does my house suddenly draw 100's of amps? does it ****.

Slippery, your missing the point, and perhaps I didn't phrase my comment accurately enough...

What I should have pointed out is that the majority of fatal shocks occur not from touching the phase conductor, but from creating a circuit across the neutral or acroos the load of an appliance, so that the person recieves the full load shock....

It is often not realised by many that the shock hazard of the neutral conductor is far higher than that of the phase conductor. Whilst it is true to say in a perfect world the electrical potential between the neutral and earth or protective conductors would be zero volts, however in the real world this is often not the case. This difference may only be a few volts, but it does open up the possibility for electric shocks.

More importantly if the neutral becomes broken, and this is then shorted across by a person, so completing the electrical circuit, you will recieve a full load shock, and in many cases these are fatal.

 

[Big_Spark]

A bit of topic now but my neighbour, who is usually reliable and didn't appear to be winding me up, says he knew an electrician who used to check 3 phase supplies by putting his forearm across the bus bars.

Is this possible or a wind up/urban myth

Bullshiite..he may get away with it once or twice, but if he went across all three phases he would certainly be killed, probably instantly as his heart would be fed three different frequency supplies 120 degrees out of phase, it would stop his heart DEAD...

I recieved a 415V shock once due to someone knocking into me as I worked on something live that could not be shut down (It was in a hospital)..I spent several days in bed, had a nasty burn on my hand and was not right for a couple of weeks...

 

[ban-all-sheds]

But your forgetting that electrical circuits do not exist in a state of zero current. Usually when people recieve a shock, the circuit is under load and when you short that circuit you recieve a full shock equivilent to the load on the circuit and some.

How on earth can someone with 236 Part 1, 2 and C, HNC Elec Eng, 232 Supp Electronics & 2380 come out with such nonsense?

 

[ban-all-sheds]

What I should have pointed out is that the majority of fatal shocks occur not from touching the phase conductor, but from creating a circuit across the neutral

Across from the neutral to where/what?

or acroos the load of an appliance, so that the person recieves the full load shock....

So you're saying that if I were to put my hands across the supply terminals of my shower when it was on, I'd get 45A flowing through me?

That is nonsense.

I'm not going to respond to any more of the examples in your post, but I'm sure we would all be very interested if you would draw a diagram representing any of the scenarios you mention, showing the resistances of the load(s) and the body parts involved, and mark on it the currents that would flow in each part of the circuit thus formed, ensuring of course that you conform to the relevant laws of Messrs Ohm and Kirchoff.

 

[Big_Spark]

But your forgetting that electrical circuits do not exist in a state of zero current. Usually when people recieve a shock, the circuit is under load and when you short that circuit you recieve a full shock equivilent to the load on the circuit and some.

How on earth can someone with 236 Part 1, 2 and C, HNC Elec Eng, 232 Supp Electronics & 2380 come out with such nonsense?

BAS...I said in my subsequent post that I did not explain my meaning correctly...that comment by me was misleading...

And read my subsequent post that you have selectively quoted from...If you complete a circuit across a broken Neutral you will recieve a shock, especially on circuits with inductive or capacitive loads.

I suggest you try it...

 

[slippyr4]

Slippery, your missing the point

No, i'm not. You have a disturbing lack of understanding of the basic laws of electrics. I think you should go and read up on ohms law. It's GCSE science.