On 27/12/2011 04:56, Lieutenant Scott wrote:
On Tue, 22 Nov 2011 20:17:22 -0000, John Rumm
wrote:

On 22/11/2011 16:15, Lieutenant Scott wrote:
On Tue, 22 Nov 2011 15:22:32 -0000, John Rumm
wrote:

On 22/11/2011 11:16, Lieutenant Scott wrote:
On Tue, 22 Nov 2011 03:25:49 -0000, John Rumm
wrote:

On 21/11/2011 03:21, Lieutenant Scott wrote:

Light fittings are exceedingly rarely a cause of electrocution.

Except when some health and safety ****t insists the fitting gets
earthed. Hold the fitting with the left hand and change the bulb with
the right....

With BC lamp fittings its pretty much a non issue, since you won't be
able to make contact to the pins for adequate duration to sustain a
serious shock. With a ES fitting, then there is a slightly larger risk
if it is wired incorrectly with the outer ring live.

Besides, electrocution isn't all it's made out to be. I've had
loads of
shocks off the mains, never given me anything more than a fright and a

Much depends on how good your connection to earth is at the time of the
shock.

I've had shocks within one hand and across my body from one hand to the
other. I guess I don't have a weak heart.

Weak or otherwise, the wrong set of circumstances will either stop it,
or more likely result in ventricular fibrillation.

The heart has approximately 5 signals to keep it going. It's not easy to
stop it.

The danger is not stopping it, but upsetting one of those signals enough
to disrupt the normal synchronisation of muscle contractions that make
an effective heartbeat. Electric shocks have a tendency to leave it
contracting out of sequence - the result is that it basically jiggles
about but does not pump effectively. This situation is as life
threatening as a stopped heart, but can be fixed (ironically) with an
electric shock of significant magnitude to effect resynch all the nerve
pulses simultaneously.

warm hand. However I do know someone who fell off a ladder because he
replaced a light when the circuit wasn't off like he thought it
was. RCD
didn't trip, "normal" breaker didn't trip. 5 amps live to neutral
through him was enough to make him jump off the ladder.

Until recently, the use of RCDs on lighting circuits was uncommon. So
there is a fair chance that it was not even supplying that circuit.
A 6A
MCB will need 30A to trip "instantly", and there is little chance of
passing that through a body at 240V, hence it offers no protection from
direct contact at all (and is not supposed to either)

Kinda puts a damper on the belief by some that they are safer than
fuses. More convenient yes, but not safer.

Depends on the type of fuse... compared to BS3036 rewireable ones they
perform better, and hence one does not need to design circuits with a
current derating on the cable for installation. Also there is no chance
an ill informed user can rewire them with the wrong wire / tin foil etc.

What's with no amperages matching up? 5 amp fuse, 6 amp breaker, and the
"lighting" cable in Wickes is 13 amps!

Rewireables are more of a concern on power circuits than lighting. They
can still cause problems with circuit design even then though if there
are other de-rating factors that need to be considered. The main danger
is abuse - i.e. a halogen goes pop and takes out the fuse, and it then
gets rewired with the wrong wire - or a paper clip - because that is all
that is to hand. You then have a fire risk since the cable no longer has
adequate fault protection.

Compared to a cartridge fuse, then they are no better (and in some cases
once could argue not as good (i.e. they are more likely to trip on a
incandescent bulb failure than a fuse). However they are certainly more
convenient.

I'd prefer them to trip when an incandescent fails. My fuse didn't and
the buggered halogen spot took out the automatic light switch circuitry.

Mains halogens have a habit of doing that even when the fuse goes.

I have fitted MCBs ONLY to my lighting circuit. This because I have
automatic lights, the sensors are very susceptible to surges, and
putting a fuse carrier back in is not a clean switch on.

You ought not be re-energising under load anyway... not good for fuse or
MCB contacts.

Very inconvenient not to. And impossible for me. My light switches are
automated.

but do pose a significant safety risk when interrupted so having
them sharing a RCD with many other circuits is not ideal.

Safety risk?!? Do you have circular saws in your living room or
something? Darkness isn't the end of the world.

Depends on what you are doing when it occurs. Fewer than 20 people will
be killed by their fixed wiring in any give year, however thousands
will
die as a result of trips and falls in their own home.

(note there is a mistake in what I said above... the 20 deaths/year are
from *all* sources of electrocution including misuse of portable
appliances - those from fixed wiring typically used to amount to one or
two a year (although there is evidence that this is now increasing
following the introduction of part P of the building regs (as you would
expect)))

IN creasing?

Indeed. The stats had been falling year on year as older places were
updated. Part P has put a slight damper on people doing rectification
work, and hence they living with substandard installation rather than
fixing them. So the injuries actually rise.

And the majority of those have nothing to do with unexpected darkness.

Probably true, however it was found that there were sufficient number of
serious falls in darkness that resulted from nuisance RCD trips to cause
a revision of the guidance on RCDs originally included in the 15th
edition of the regs.

We need to get rid of these paper pushers.

Depends on which ones. If you mean those in government adding complexity
and red tape to people carrying out remedial electrical work, then yes I
agree. However if you mean the proper engineers who design and specify
our electrical standards, then I disagree. Due to our standards we have
one of the safest electrical systems in the world, and a vastly smaller
injury rate than most countries.

Some appliances typically exhibit high leakage currents, and are
hence
likely to sensitise an RCD without gaining much safety benefit.

Which is why I'd never fit an RCD to my house.

You would be monumentally stupid not to, since should you ever be
exposed to a life threatening shock, there is a fair chance that one
would save you.

As stated earlier, 240 volt shocks are not life threatening to healthy
people.

That is complete and utter ********.

For several reasons....

Firstly 240V shocks can and do kill perfectly healthy people every year.

All 20 of them. Big deal.

Would you like me to pass that message on to my former next door
neighbour? Her son (in his 20s) was electrocuted on a building site a
few years ago. Oddly her response was not "big deal".

So why am I still in one piece?

Because in spite of what you believe to be the case, you have never had
a serious shock.


Secondly there is a much larger class of people who will be seriously
injured and suffer permanent after effects as a result of shocks.

There are an even greater number of will suffer serious but recoverable
injury, and still more that will be hurt to various degrees.

Research conducted by the National Safety Council shows that in the UK
there are something like 1 million hospital visits resulting from
electrical shocks per year. (obviously these include minor burns cuts
and bruises, right up to cardiac damage, severe burns / disfigurement
etc, and deaths).

990,000 of them because their finger is a bit sore.

Yup, like this chap:

http://t1.gstatic.com/images?q=tbn:A..._yvZZsFjA0IoT9


I tell you what would be safer though - not earthing so many things.
Imagine this - you touch something live, you get a tingle. Now imagine
you are leaning against an earthed sink, washing machine, etc, etc. Now
you are a circuit. Why did people think an earth was a good idea?

You seem to be misunderstanding the whole purpose of earthing. It is not
there to reduce the shock potential difference you might be exposed to.
It is designed to operated in conjunction with a circuit protective
device to de-energise a circuit in the event of a fault.

If a wire falls off in some appliance and makes the metal case live, the
earthing connection to the case will ensure enough fault current flows
to blow a fuse or trip a MCB - thus limiting the maximum shock duration
to something survivable.

The argument that you are safer with no earths around is true, but only
when you can ensure there is *no* access to any independent earth. If
there is, then you need to make sure that the earth is "good" or else
you may not get adequate fault current flowing to operated the CPD while
at the same time having an earth good enough to kill you.

If you want to seek to reduce the magnitude of potential difference that
you might be exposed to, you need equipotential bonding. This is
typically used in location where people are highly vulnerable to
electric shock (e.g. when wet / naked / barefoot etc as in bath and
shower rooms).

Er..... if everything is earthed, they ARE at the same potential.

Alas not necessarily. Earthing as required to pass adequate current to
trip a circuit protective device does not have conductor sizes adequate
to ensure limitation of touch voltages. Also note that earthing can be
fully functional in some cases with a loop impedance of 100 ohms or
more. That will have little or no effect reducing touch voltages but
would still pass adequate current to trip a RCD.



--
Cheers,

John.

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