[Gllug] OT: Mains electricity colour coding.

[James Courtier-Dutton]

On 26 October 2010 23:16, Chris Bell <chrisbell at chrisbell.org.uk> wrote:
> On Tue 26 Oct, t.clarke wrote:
>>
>> I suppose through which bit of you 30mA at 230V goes makes a big difference!
>> Reminds me, incidentally, of the old saw:
>> "Its volts that jolts, bit mills that kills"
>>
>> Tim
>
>   Its volts that drive the current, but the damage depends on the current.
> I have heard that if the contact resistance is low enough, perhaps because
> it is wet, the damage caused from 110 volts can be much worse because your
> muscles do not push you away quickly enough. I was told that if I needed to
> check whether something is live by touching it always use the back of the
> hand because it is more sensitive and the natural reaction is to pull away,
> never the palm side because you may grab and lock on. DC volts are even more
> likely to cause lock-on.
>

I knew it as "Volts that jolts, Current that kills.". You might
remember to large glass globes that when you touch them, they make you
head stand on end. Those use very high Volts, but next to no Current
so they are harmless.

During an electric shock, your muscles contract. First use a test
device like a live detection device like those screw drivers with
lights on them. You should never really have to test for live by
touching it. The advice about test touching with the back of you hand
it correct safety advice. If it was live, your fingers would form a
fist and so, if you used a finger or the palm of your hand, you would
grip the item that is causing the shock that is not a good thing to
do.
If the wiring is protected by an RCD, you should not get a fatal shock
because the RCD should cut in.
RCD detects the current in the live wire and compares it to the
current in the neutral wire.
If there is a difference, the current is escaping to somewhere,
probably via an electrocuted person, and cuts the circuit at a very
low "leakage" current.
This is far more affective than the fuse in the power plug.
Power plug fuse: 3A, 5A or 13A.
RCD cut off: 30mA.

The main protection that the Power plug fuse protects against is it
stops house wiring from melting due to too much current. E.g. A short
between live and neutral.
The earth is present and is connected to the shield or outside of a
bit of equipment e.g. brass light switch. So, if a live wire gets
loose, and touches the brass light switch surround, the current goes
to earth instead of via the next person to turn the light on.
This leakage to earth will trip the RCD and render the faulty circuit safe.
So, in conclusion, the only device that actually protects you from
electric shocks in the home is the RCD.
This is why it is an extremely good idea to make sure your house
wiring contains RCD devices.
Some older fuse boxes do not contain RCD devices, but luckily might
contain a global RCD for the whole house, but this RCD would have to
have a cut off higher than 30mA. The Global RCD also makes it really
difficult to diagnose where the problem is.

My house has 4 rings per RCD device. (quite normal for modern fuse boxes)
If the RCD trips, one simply switches each ring on in turn until it
trips again. One then has it narrowed down to which ring is the
problem one and then switch that one off, leaving all the others on,
and then wait for a qualified electrician to come and fix the fault.
If the ring is a power plug ring, it is also a good idea to unplug all
devices to find out which causes the problem. For safety, only go to
unplug all the devices with the power to the whole house off.
If the ring is a lighting ring, qualified electrician is the only safe
way to go.

Kind Regards

James
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