Why has DIYNOT got a electrics forum?

[Big_Spark]

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.

Slippery..your now talking total rubbish..read the rest of my post...

If you really want a technical discussion on electricity I could lose you in one post..however as I have been at work for 13 hours and am fed up with it for one day..it won't happen tonight...

 

[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?

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

I'd not seen that subsequent post when I made mine.

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...

If you complete any circuit, in the sense of putting part of your body between two points between which there is a potential difference you'll get a shock, if the p.d. is high enough and the source can supply enough current.

Like I said, please draw us a diagram....

 

[Big_Spark]

BAS..the point I was making is that there is a significant difference between a shock from Phase to earth, where the current flowing in your body will be determined by your body's resistance, as for any electrical circuit, BUT if you complete a circuit by making the connection ACROSS A BROKEN NEUTRAL, then the shock you recieve will be the current flowing in the circuit at that time..you will effectively become part of the circuit rather than a seperate circuit..(as with a Phase to neutral or Phase to earth shock)

 

[slippyr4]

Slippery..your now talking total rubbish..read the rest of my post...

Ok, lets analyse it in detail.

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....

Ok, there's no such thing as a partial circuit. Either you have a circuit, or you don't.
From "across the load of an appliance": I'm assuming you mean that, if we take for example, an electric fire, you mean a shock where I hold each end of the element, thus i'm across / in parralel with the load. The wires supplying the imaginary electric fire are of negligable resistance.

The electric fire element can be represented as a 50 ohm resistor. The electric fire is thus 1KW ish. The current in the unfettled circuit is V/R = 4.6 Amps.

The human is 1500 ohm.

You will agree that, the phase end of the element is at a potential of 230v compared to the neutral end of the element. Therefore, we can use ohms law again. V=230, R = 1500, I = V/R = 230/1500 = 153 mA. As you can perhaps see, the electric fire has absolutely nothing to do with it.

It is often not realised by many that the shock hazard of the neutral conductor is far higher than that of the phase conductor.

That's not true, and even you know it. I don't dispute that neutral is a potential danger, it's simply not as dangerous as the phase, except when the neutral is broken and it comes up to phase potential

And let's look at that. The neutral is now broken on our electric fire. The "neutral" end of the element is at 230V potential, although no current is flowing because the circuit is broken. I now take hold of the neutral end of the element (at 230V) and grab hold of the real neutral.

Picture the circuit now. I am now in series with the electric fire. You'll remember from school, that ohm's law still applies. Two resistors in series can be thought of as one resistor whose value is the sum of the two resistors. So, the full circuit has a resistance of the 50 ohm element plus the 1500 ohm person, total 1550 ohm. The current can be worked out with ohms law. V/R = I, 230/1550 = I = 148 mA. That's less current than the heater drew before the neutral broke.

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.

The only thing you've said in this thread which is correct.

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.

This repeats the broken neutral in the electric fire scenario above. It's just plain wrong.

If you really want a technical discussion on electricity I could lose you in one post..however as I have been at work for 13 hours and am fed up with it for one day..it won't happen tonight...

No you couldn't. I know more about electricity than you learn in GCSE. You don't.

 

[Big_Spark]

slippery..you have some knowledge, that I do not dispute, but your actual understanding is flawed..I accept that I do not always explain my meaning as well as I probably should, but your not correct in some of your assertions...

I simply cannot be a**sed to get into this now...I'll post something on this tomorrow as I am going to bed now..got to be up at 5am..

 

[slippyr4]

slippery..you have some knowledge, that I do not dispute, but your actual understanding is flawed..I accept that I do not always explain my meaning as well as I probably should, but your not correct in some of your assertions...

I simply cannot be a**sed to get into this now...I'll post something on this tomorrow as I am going to bed now..got to be up at 5am..

If you can explain how a person with a resistance of 1500 ohms could ever get a shock of more than 154 mA from a 230V supply, then I will salute you, along with thousands of physicists, electrical engineers, and electricians who's very foundation of knowledge has been proved wrong.

But no hard feelings- this is only a discussion on a forum; please do not be ****ed off or offended, i'm certainly not. I'll await tomorrows revelation!

 

[joe-90]

I was always led to believe that the body resistance was 1K ohms. Not that it really matters, just curious.


joe

 

[pickles]

Is this the electricians version of who can p*&s the furthest

 

[AdamW]

I'm a bit late to the fray, but isn't 60Hz more efficient for transmitting electricity over long distances?

I have a vague recollection of a lecture where we were told Tesla worked out the most efficient ac frequency was about 60Hz.

Dunno why we went to 50Hz though...

As to the Lib Dem Daughter who got electrocuted on the shelf... what are you meant to do before drilling any hole into any wall? Test for wires. I have only forgotten that once, and that was the one time I drilled through a cable. So really her death was caused by improper shelf fitting.

And if you expect they will ever force building regs on us for fitting shelves... well you'd probably be right But I think we can agree it would be met with much hostility and be flouted 99.99% of the time.

Banning DIY electrical work because of such incidents would be like banning forks and kitchen sinks because someone might jam the first in a toaster whilst gripping on to the pipes of the latter.

However, I'm totally with anyone who is of the opinion that Fred Uckwitt should not be allowed to do any DIY at all. I'm sure you've all moved house and then found some DIY shockers. It's not just electrics!

 

[oilman]

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...

A bad comparison. Sweden used to drive on the left.

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...

Exactly. Bullshite.

The frequency of the phases is the same for each phase.

If it was a 240V 3-phase supply, the phase-phase voltage is 415V. THAT is what kills, it does it by almost instant cooking. The phase angle is irrelevant. Please describe how he could get across 3 phases at once.

Big -spark, slippyr4 has brought up several inaccuracies in your assertions. For heavens sake read through your posts before you submit them.

 

[pickles]

No idea, like i say I'm not a sparks so don't all go into one if the following is rubbish. The thing is firstly the guy is alive, secondly the person who told me obviously thought he was mad when he saw him do it and just said it in passing. If it was a wind up he wouldn't have said it that way. that's why I posted because I find it hard to believe but apparently he did it all the time

My theory is that if for the sake of argument he's got rubber soles on and it's dry there's no earth path so it's just the current flow between phases. By definition this can only take place along his arm so there is no shock risk to his heart. If the resistance of his skin is high enough, and remember it is the outside of his forearm not his fingers there won't be any significant current flow. What do you all think

Frankly if its true what other explanation is there. I have heard it said that a lot of sparks tap live cables with the back of their knuckles to see if it is live. Iv'e heard this from so many different sources it has to have some foundation. it' s usually followed by "if you do this all the time you get used to it"

Personally I think anyone who does this is certifiable. I stuck my finger in light socket age 5 and it left a lasting impression

 

[ban-all-sheds]

BAS..the point I was making is that there is a significant difference between a shock from Phase to earth, where the current flowing in your body will be determined by your body's resistance, as for any electrical circuit, BUT if you complete a circuit by making the connection ACROSS A BROKEN NEUTRAL, then the shock you recieve will be the current flowing in the circuit at that time..you will effectively become part of the circuit rather than a seperate circuit..(as with a Phase to neutral or Phase to earth shock)

No you won't. No you can't. It is simply impossible for that current to flow through you, a fact which was initially recognised by Georg Ohm.

And anyway - if the neutral had been broken, and therefore the circuit interrupted, the current flowing "at the time" would have been zero, so even if your theory wasn't rubbish it would be rubbish.

If you become part of the circuit then you are an impedance of a few hundred to a few thousand ohms in series with the overall load on the circuit.

So if the load had been 5 ohms, and the current therefore would have been 46A, if you put, for argument's sake, 500 ohms of you across the break in the neutral, the load on the circuit is now 505 ohms, and therefore the current that will flow will be 455mA.

For your theory that 46A would flow, the potential difference across the break in the neutral would have to have risen to 23kV. How is it going to do that?

You seem to be recycling ignorant old wive's tales without any thought to the physics behind them.

 

[ban-all-sheds]

Is this the electricians version of who can p*&s the furthest

No - sadly it's another example of Big_Spark coming on here and spouting complete b*llsh*t, trying to back it up with a number of big "I am" statements, and then melting away when pressed to prove it.

A situation which I'm sure is all too familiar.

 

[ban-all-sheds]

I'm a bit late to the fray, but isn't 60Hz more efficient for transmitting electricity over long distances?

If it is, it won't be by much.

I have a vague recollection of a lecture where we were told Tesla worked out the most efficient ac frequency was about 60Hz.

I don't know if he actually worked it out. He may well have established that it was in the ballpark where transformers became reasonably efficient. It was high enough to avoid lights flickering, and low enough for generating equipment not to have to turn at silly speeds.

That he went for 60Hz rather than 50 or 55 or 70 is often said to be because it made it very useful for clocks. Works for me.

Dunno why we went to 50Hz though...

See above. 60 offended the Germans.

Interestingly (or not) at the start of this year Consolidated Edison, the NYC power company, still had 1600 DC supply customers in Manhattan.

 

[Spark123]

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

I think there maybe an element of truth in it, not checking the supply is on though (as that is a bit too daft), an old electrician once told me that when they had to prove to another person that the supply was isolated they had to touch the busbars, needless to say it made them do their isolations properly (and touch the busbars with the back of their hand).