Safe Speed Forums

We were always taught that "hand to hand" resistance is on average about 2.5K ohms, thus you will draw approx 100mA if you grab hold of a mains cable. A current flow of 100mA for half a second is enough to kill you, which is why typically, "earth leakage" protection devices (aka RCD's) trip at 20 or 30mA, which is usually survivable.

But either way, buckmac is bang on with his comment about the difference between lighting and ring mains being irrelevant, as there ain't no way you are going to blow a 1 amp circuit breaker or fuse, let alone a 5A or 10A amp one as typically used for lighting circuits! It's kind of like the difference between falling out of the 33rd floor instead of the 37th...

Though paradoxically, these days many domestic consumer units have earth leakage protection on any circuits where there is a risk of a shock, ie a socket ring main, whereas lighting circuits often don't have this protection to avoid spurious "nuisance" tripping when bulbs blow. So it may well be the case that you are actually better off grabbing hold of the ring main rather than the lighting circuit!

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I'd been given to understand (by an older, more experienced and much better DIYer than me) that I'd have got a worse shock off the mains ring than the lighting ring, though TBH I really didn't understand why when they're both 240V. Just took his word for it, though like I said with only one fuse to pull to kill all the mains sockets it's pretty unlikely the same thing would have happened with a socket and I'm not about to stick a wet coat hanger in one to find out. But if an electrician an an electrical engineer are telling me that he was talking out of his bum then I know who I'm going to trust on that point. I stand corrected, which is a whole lot better than smouldering in a heap on the floor corrected.

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Our box has earth leakage on all circuits, so every time a bulb blows we lose the complete circuit and occasionally the main trip goes too. PITB, but nothing we can do about it now.

I can also confirm that while potentially on a fuse limited system you might get 5A+ down a lighting circuit and 30A+ down a mains circuit, it is just a differentiation in how much you cook - I suppose in the occasional and unlikely event of you completing the circuit for a long period of time and surviving the initial shock then you might be less burnt, but there is little in it. Unlikely because with AC your muscle spasm will disconnect you very fast, and secondly as the practical instructor I had said "It's volts that jolt, it's mills that kills" - 10,000 volts at low current will give one hell of a shock but not kill (as all lightning strike survivors will attest), but 20 or 30 milliamps of current from even a 50v source will stop your heart.

Now not being pedantic -
Slight safety mod to that statement Rewolf - before you pull fuses/switch off - check that indicator( screwdriver etc) works - me , i prefer a meter with leads certified to minimum of 600 v , test both wires to earth with earth leg secured - so you only use one hand with no path through heart.
Test meter etc to show it works, then after isolation.
I sometimes also earth both wires , but then all my domestic circuits are RCD protected - old habit from working on telecomms circuits - megger a working telephone circuit at 250 v and charge the bell cap --or circuits where power fed down cable
Now this poses a thought - if you rent and prior to this reg you fitted lights etc - and the landlord requires you to make as on entry - can you do the work??

Most of the boxes I've come across recently have a series of trips which are effectively earth leakage trips. I found that installing one with a slightly higher rating usually stops the trip when the bulbs go.



At the end of the day it's the power that kills. The amount of power that is delivered to your body is dependant on two things: the voltage behind it (pressure if you like) and the resistance of your body. (This is assuming that there is unlimited power capacity behind the shock)

The lightning strike will take the path of least resistance (usually over the surface of the body which is why there are generally wide scale burns involved) so there will be little current going through the body (i.e. little power). This is generally the case as you are wet outside due to the rain and are in contact with the ground.

To get 20 to 30 milliamps of current at 50V you're resistance would need to be down to 2000 ohms (2Kohms) which is have VERY wet skin and grip the wires real tight. Incidentally, if you are in contact is such a manner that the current flow doesn't pass across the chest, the chances are you'll survive.

Leastways, that's always been my understanding, but I stand to be corrected.

Cheers, Chris.

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I guess that to stick with the letter of the regs you would have to employ a P qualified contractor to come and remove your previous work, though having said that, surely removing circuits wouldn't quite fall within the remit of the new regs.

On the subject of testing circuits, one thing that I remember well from the wise old spark that first trained me is that no matter what tester or meter you use, and no matter what means of isolation you have used, the only true final test of whether a wire is dead is to touch it.

So even when you *know* you've isolated and tested a circuit, that first touch is still the final acid test, and should be made with care. An old trick is to make the first touch with the back of your hand - if it's live you may well find that the hairs stand up on your hand before you actually touch it, and if you get past that, then when you subsequently do get a "belt" the muscle spasms it causes will move your hand away, minimising the duration of the shock you receive. If you touch it with the inside or tip of a finger there is a chance that a live cable will cause your hand to clamp around it, prolonging the shock and maybe turning it into a fatal one.

The actual "mechanics" of death by electric shock are usually not a "frying" process as everyone tends to assume, as the 100mA that typically flows from a domestic shock just won't do that much cooking! No, the villain in the piece is the fact that we use alternating current for our domestic supplies. If you connect yourself such that the current path flows across your heart, then it mis-interprets the alternating waveform as a signal telling it to beat. Thus your heart tries to catch up with the frequency of the current, which at 50 cycles a second it is not going to do. What happens is that the heart gets up to a rate of 2 or 3 beats per second then goes into a state of "fibrillation", which means it half-beats without actually pumping blood.

The irony is that lower voltages don't cause a big enough current flow to cause fibrillation, whilst higher voltages tend to cause such a jolt that they "throw you off" before fibrillation happens. Oh and Direct Current doesn't cause it at all.

So if you were charged with the task (pun intended ) of devising an electricity system best suited for killing people receiving casual shocks, 240V 50Hz AC is probably what you'd go for! It's just a shame that it's also about the most efficient and practical for domestic use.

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CSCP Latin for beginners...
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Couple of points my little brother always told me:

DC is more dangerous than AC because DC will 'pull' you onto it and 'suck' you to stay there whereas AC will pull and push and pull and push meaning you have literally a half chance of pulling yourself off when it is pushing you away.

Voltage and Amperage. For those who are still a little confused a good way of thinking about this is akin to water pipes.

If you imagine 2 pipes that are both for arguement's sake 10 metres long but 1 is an inch across, the other 5 inches across.... VOLTAGE can be expressed by saying that if a tissue (or dye) were introduced at one end of both pipes at the same time to emerge from the other end of the pipes at the same time, then both pipes have the same pressure behind them pushing the tissues/dyes along.

HOWEVER, the 5 inch pipe can carry a much, much larger volume of water than the 1 inch pipe at the same pressure (voltage), therefore the volume (measured in electricity in AMPERAGE) can be much larger.

Using the water pipe analogy once more, if you imagine a bung in the ends of the pipes holding the water back - as the bung breaks, the larger pipe is likely to give you the biggest soaking BUT in order to get soaked first the bung must break.

This is why a 12 volt car battery won't kill because 12 volts is not enough to overcome the resistance in the skin. Stick 4 batteries in sequence, however (48 volts) and touch the + one end and the - at the other and you would be in very serious trouble - just think of how much energy goes through your starter motor when you turn the ignition key: Enough to move your stalled car from a dangerous position with the car in 1st gear.

We can't really adequately describe the relationship between voltage and current without a concept of resistance. People often don't appreciate that a 100W bulb takes more power than a 40W bulb precisely because its filament has a lower resistance. Less resistance means more current and power is (generally) the product of voltage and current.

Ohms law shows us how resistance and voltage together contol current:

I = V/R (I being current, V being Voltage, and R being resistance).

(The astute may notice that as resistance approachs zero, current tends to approach infinity which is why 'short circuits' are damaging and why we have fuses)

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Paul Smith
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Quite good not too technical ideas on this -
http://www.purchon.com/physics/ohmslaw.htm

Even gives a little thought to power - but thats a different topic specialy between AC and DC - shades of leading/lagging currents to complicate the calculation in AC.