Once again I've managed to chop through the flex on my hedge cutters.
They're rated at 400 watts, so I guess a 5amp fuse should have been used
in the plug instead of 13 amp. Even so, I don't understand how the fuse
survived what was a clean cut.
Yes, I did unplug it instinctively, but I didn't see any urgency to do
so because I assumed the fuse had blown.

stuart noble wrote:

Once again I've managed to chop through the flex on my hedge cutters.

Was it on an RCD? Did that trip?

They're rated at 400 watts, so I guess a 5amp fuse should have been used
in the plug instead of 13 amp. Even so, I don't understand how the fuse
survived what was a clean cut.
Yes, I did unplug it instinctively, but I didn't see any urgency to do
so because I assumed the fuse had blown.

I suppose its possible (but unlikely) the individual conductors are cut
one by one without shorting them together, more likely the cut was done
quickly enough that a short-duration short-circuit didn't have chance to
blow the fuse, and the cut was clean so didn't leave a short after the
cut was done.

On 27/07/2012 13:39, stuart noble wrote:
Once again I've managed to chop through the flex on my hedge cutters.
They're rated at 400 watts, so I guess a 5amp fuse should have been used
in the plug instead of 13 amp. Even so, I don't understand how the fuse
survived what was a clean cut.

At the moment of the cut the voltage must have been close to zero and/or
the cable did not make contact for long enough to zap a fuse.
Or did an ELCB or RCD somewhere else trip first?

Yes, I did unplug it instinctively, but I didn't see any urgency to do
so because I assumed the fuse had blown.

It is unwise to ever assume that a cut power cable is not still live.

--
Regards,
Martin Brown

On Fri, 27 Jul 2012 13:46:58 +0100, Martin Brown
wrote:

On 27/07/2012 13:39, stuart noble wrote:
Once again I've managed to chop through the flex on my hedge cutters.
They're rated at 400 watts, so I guess a 5amp fuse should have been used
in the plug instead of 13 amp. Even so, I don't understand how the fuse
survived what was a clean cut.

At the moment of the cut the voltage must have been close to zero and/or
the cable did not make contact for long enough to zap a fuse.
Or did an ELCB or RCD somewhere else trip first?

Yes, I did unplug it instinctively, but I didn't see any urgency to do
so because I assumed the fuse had blown.

It is unwise to ever assume that a cut power cable is not still live.

Both the above. Also 400 watts may be the usual running power but start up
might use more. Suggest you retain the 13a fuse or use a 10a if you have
one. A RCD or ELCB should be used if the circuit is not protected by one.

In article ,
Martin Brown writes:
On 27/07/2012 13:39, stuart noble wrote:
Once again I've managed to chop through the flex on my hedge cutters.
They're rated at 400 watts, so I guess a 5amp fuse should have been used
in the plug instead of 13 amp. Even so, I don't understand how the fuse
survived what was a clean cut.

At the moment of the cut the voltage must have been close to zero and/or
the cable did not make contact for long enough to zap a fuse.

Yep - done the same with the lawn mower a few weeks back.
It was on a 10mA RCD, and that didn't have time to trip either.

Yes, I did unplug it instinctively, but I didn't see any urgency to do
so because I assumed the fuse had blown.

It is unwise to ever assume that a cut power cable is not still live.

Indeed.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

stuart noble wrote:
Once again I've managed to chop through the flex on my hedge cutters.
They're rated at 400 watts, so I guess a 5amp fuse should have been used
in the plug instead of 13 amp. Even so, I don't understand how the fuse
survived what was a clean cut.
Yes, I did unplug it instinctively, but I didn't see any urgency to do
so because I assumed the fuse had blown.
more likely to trip an RCB these days.


--
To people who know nothing, anything is possible.
To people who know too much, it is a sad fact
that they know how little is really possible -
and how hard it is to achieve it.

On 27 July, 12:39, stuart noble wrote:
Once again I've managed to chop through the flex on my hedge cutters.
They're rated at 400 watts, so I guess a 5amp fuse should have been used
in the plug instead of 13 amp. Even so, I don't understand how the fuse
survived what was a clean cut.
Yes, I did unplug it instinctively, but I didn't see any urgency to do
so because I assumed the fuse had blown.

not enough i squared t to blow it.

On 27/07/2012 13:39, stuart noble wrote:

Once again I've managed to chop through the flex on my hedge cutters.
They're rated at 400 watts, so I guess a 5amp fuse should have been used
in the plug instead of 13 amp. Even so, I don't understand how the fuse
survived what was a clean cut.

Much will depend on the prospective fault current... this will be
dictated by the supply impedance to your house, the circuit impedance
from your CU to the socket in use, and then more significantly by how
many metres of relatively skimpy flex were on the trimmer (and any
extension lead). You would probably find if you did the sums, it all
adds up to something north of a couple of ohms. That means that even
under short circuit conditions you are limited to around 100A of
prospective short circuit current.

Have a look at:

http://wiki.diyfaq.org.uk/index.php?...FusingTime.png

Note that to open a 13A fuse in 0.1 secs, could take getting on for 110A
- and that assumes the short was there for a full tenth of a second.

Yes, I did unplug it instinctively, but I didn't see any urgency to do
so because I assumed the fuse had blown.

Never a safe assumption (and another reason why using a RCD protected
circuit is a good idea)



--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On Fri, 27 Jul 2012 15:29:54 +0100, John Rumm
wrote:

On 27/07/2012 13:39, stuart noble wrote:

Once again I've managed to chop through the flex on my hedge cutters.
They're rated at 400 watts, so I guess a 5amp fuse should have been used
in the plug instead of 13 amp. Even so, I don't understand how the fuse
survived what was a clean cut.

Much will depend on the prospective fault current... this will be
dictated by the supply impedance to your house, the circuit impedance
from your CU to the socket in use, and then more significantly by how
many metres of relatively skimpy flex were on the trimmer (and any
extension lead). You would probably find if you did the sums, it all
adds up to something north of a couple of ohms. That means that even
under short circuit conditions you are limited to around 100A of
prospective short circuit current.

Have a look at:

http://wiki.diyfaq.org.uk/index.php?...FusingTime.png

Note that to open a 13A fuse in 0.1 secs, could take getting on for 110A
- and that assumes the short was there for a full tenth of a second.

Yes, I did unplug it instinctively, but I didn't see any urgency to do
so because I assumed the fuse had blown.

Never a safe assumption (and another reason why using a RCD protected
circuit is a good idea)

Erme - wouldn't it also be a good idea to never assume that an RCD has
tripped?

Nick

On Jul 27, 1:39*pm, stuart noble wrote:
Once again I've managed to chop through the flex on my hedge cutters.
They're rated at 400 watts, so I guess a 5amp fuse should have been used
in the plug instead of 13 amp. Even so, I don't understand how the fuse
survived what was a clean cut.

Because it was exectly that - a clean cut.

Yes, I did unplug it instinctively, but I didn't see any urgency to do
so because I assumed the fuse had blown.

Foolish asumption.

MBQ

On 27/07/2012 15:33, Nick Odell wrote:
On Fri, 27 Jul 2012 15:29:54 +0100, John Rumm
wrote:

On 27/07/2012 13:39, stuart noble wrote:

Once again I've managed to chop through the flex on my hedge cutters.
They're rated at 400 watts, so I guess a 5amp fuse should have been used
in the plug instead of 13 amp. Even so, I don't understand how the fuse
survived what was a clean cut.

Much will depend on the prospective fault current... this will be
dictated by the supply impedance to your house, the circuit impedance
from your CU to the socket in use, and then more significantly by how
many metres of relatively skimpy flex were on the trimmer (and any
extension lead). You would probably find if you did the sums, it all
adds up to something north of a couple of ohms. That means that even
under short circuit conditions you are limited to around 100A of
prospective short circuit current.

Have a look at:

http://wiki.diyfaq.org.uk/index.php?...FusingTime.png

Note that to open a 13A fuse in 0.1 secs, could take getting on for 110A
- and that assumes the short was there for a full tenth of a second.

Yes, I did unplug it instinctively, but I didn't see any urgency to do
so because I assumed the fuse had blown.

Never a safe assumption (and another reason why using a RCD protected
circuit is a good idea)

Erme - wouldn't it also be a good idea to never assume that an RCD has
tripped?

Indeed it would.

I was not intending to give the impression that the RCD would cut off
the power as a result of the cut - in fact its quite likely it would not
since the tool is almost certainly of a double insulated design with no
earth connection. So unless the blades also had a decent connection to
earth at the time of the cut, the RCD would be unlikely to see enough
imbalance to trip.

However the key point is that if as a result of assuming the cable is
dead when its not, you then handled the live end, the RCD would keep you
alive.


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On 27/07/2012 15:53, Man at B&Q wrote:
On Jul 27, 1:39 pm, stuart noble wrote:
Once again I've managed to chop through the flex on my hedge cutters.
They're rated at 400 watts, so I guess a 5amp fuse should have been used
in the plug instead of 13 amp. Even so, I don't understand how the fuse
survived what was a clean cut.

Because it was exectly that - a clean cut.

Yes, I did unplug it instinctively, but I didn't see any urgency to do
so because I assumed the fuse had blown.

Foolish asumption.

MBQ


Thanks everyone. We live and learn

On 27/07/2012 13:46, Andy Burns wrote:
stuart noble wrote:

Once again I've managed to chop through the flex on my hedge cutters.

Was it on an RCD? Did that trip?

No, it wasn't. Just a normal extension lead straight into a socket. As
it happens the fuse box in this property is an old fashioned type with
no circuit breakers, so I will continue nagging them to get that upgraded.

They're rated at 400 watts, so I guess a 5amp fuse should have been used
in the plug instead of 13 amp. Even so, I don't understand how the fuse
survived what was a clean cut.
Yes, I did unplug it instinctively, but I didn't see any urgency to do
so because I assumed the fuse had blown.

I suppose its possible (but unlikely) the individual conductors are cut
one by one without shorting them together, more likely the cut was done
quickly enough that a short-duration short-circuit didn't have chance to
blow the fuse, and the cut was clean so didn't leave a short after the
cut was done.

stuart noble wrote:

On 27/07/2012 13:46, Andy Burns wrote:

Was it on an RCD?

No, it wasn't. Just a normal extension lead straight into a socket. As
it happens the fuse box in this property is an old fashioned type with
no circuit breakers, so I will continue nagging them to get that upgraded.

If you're ahemprone/ahem to pruning cables you could get a plug-in
RCD, or extension reel with one built-in for outdoor use ...

http://toolstation.com/shop/p12514
http://toolstation.com/shop/p94537

On Fri, 27 Jul 2012 16:01:17 +0100, stuart noble
wrote:

On 27/07/2012 15:53, Man at B&Q wrote:
On Jul 27, 1:39 pm, stuart noble wrote:
Once again I've managed to chop through the flex on my hedge cutters.
They're rated at 400 watts, so I guess a 5amp fuse should have been
used
in the plug instead of 13 amp. Even so, I don't understand how the fuse
survived what was a clean cut.

Because it was exectly that - a clean cut.

Yes, I did unplug it instinctively, but I didn't see any urgency to do
so because I assumed the fuse had blown.

Foolish asumption.

MBQ


Thanks everyone. We live and learn

Don't know whether there is a better way but I always run the cable over
my shoulder to my hedgecutter. So far I have not hit my cable. It makes it
easy for me to see and the cable does not hit the ground and cause the
slack to go all over the place.

On 27/07/2012 16:16, Andy Burns wrote:
stuart noble wrote:

On 27/07/2012 13:46, Andy Burns wrote:

Was it on an RCD?

No, it wasn't. Just a normal extension lead straight into a socket. As
it happens the fuse box in this property is an old fashioned type with
no circuit breakers, so I will continue nagging them to get that
upgraded.

If you're ahemprone/ahem to pruning cables you could get a plug-in
RCD, or extension reel with one built-in for outdoor use ...

http://toolstation.com/shop/p12514
http://toolstation.com/shop/p94537


Twice in 40 years, and both in the last month. Oh dear

stuart noble wrote:

Andy Burns wrote:

If you're ahemprone/ahem to pruning cables

Twice in 40 years, and both in the last month. Oh dear

Let me rephrase that then, if you're /becoming/ prone :-P

After serious thinking John Rumm wrote :
I was not intending to give the impression that the RCD would cut off the
power as a result of the cut - in fact its quite likely it would not since
the tool is almost certainly of a double insulated design with no earth
connection. So unless the blades also had a decent connection to earth at the
time of the cut, the RCD would be unlikely to see enough imbalance to trip.

However, had the OP then touched the still live cable end, the RCD
would have served its purpose.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk

On 27/07/2012 18:05, Harry Bloomfield wrote:
After serious thinking John Rumm wrote :
I was not intending to give the impression that the RCD would cut off
the power as a result of the cut - in fact its quite likely it would
not since the tool is almost certainly of a double insulated design
with no earth connection. So unless the blades also had a decent
connection to earth at the time of the cut, the RCD would be unlikely
to see enough imbalance to trip.

However, had the OP then touched the still live cable end, the RCD would
have served its purpose.

Did you really need to snip the bit where I said:

"However the key point is that if as a result of assuming the cable is
dead when its not, you then handled the live end, the RCD would keep you
alive. "

just to make that comment? ;-)


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On 27/07/2012 16:05, stuart noble wrote:
On 27/07/2012 13:46, Andy Burns wrote:
stuart noble wrote:

Once again I've managed to chop through the flex on my hedge cutters.

Was it on an RCD? Did that trip?

No, it wasn't. Just a normal extension lead straight into a socket. As
it happens the fuse box in this property is an old fashioned type with
no circuit breakers, so I will continue nagging them to get that upgraded.

In which case change the plug on the end of your extension lead to a RCD
type, then use that for anything that needs power outside.

Don't wait for someone else!


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

In article , John Rumm
wrote:
On 27/07/2012 16:05, stuart noble wrote:
On 27/07/2012 13:46, Andy Burns wrote:
stuart noble wrote:

Once again I've managed to chop through the flex on my hedge cutters.

Was it on an RCD? Did that trip?

No, it wasn't. Just a normal extension lead straight into a socket. As
it happens the fuse box in this property is an old fashioned type with
no circuit breakers, so I will continue nagging them to get that
upgraded.

In which case change the plug on the end of your extension lead to a RCD
type, then use that for anything that needs power outside.

I prefer the in-line RCDs since some sockets are mounted so that you can't
get an RCD plug into them

--
From KT24

Using a RISC OS computer running v5.18

On 27/07/2012 18:45, John Rumm wrote:
On 27/07/2012 16:05, stuart noble wrote:
On 27/07/2012 13:46, Andy Burns wrote:
stuart noble wrote:

Once again I've managed to chop through the flex on my hedge cutters.

Was it on an RCD? Did that trip?

No, it wasn't. Just a normal extension lead straight into a socket. As
it happens the fuse box in this property is an old fashioned type with
no circuit breakers, so I will continue nagging them to get that
upgraded.

In which case change the plug on the end of your extension lead to a RCD
type, then use that for anything that needs power outside.

Don't wait for someone else!



Good advice. Thanks

On Fri, 27 Jul 2012 18:57:43 +0100, charles wrote:

In article , John
Rumm wrote:
On 27/07/2012 16:05, stuart noble wrote:
On 27/07/2012 13:46, Andy Burns wrote:
stuart noble wrote:

Once again I've managed to chop through the flex on my hedge
cutters.

Was it on an RCD? Did that trip?

No, it wasn't. Just a normal extension lead straight into a socket.
As it happens the fuse box in this property is an old fashioned type
with no circuit breakers, so I will continue nagging them to get that
upgraded.

In which case change the plug on the end of your extension lead to a
RCD type, then use that for anything that needs power outside.

I prefer the in-line RCDs since some sockets are mounted so that you
can't get an RCD plug into them

I put an RCD socket by the back door.



--
Use the BIG mirror service in the UK:
http://www.mirrorservice.org

*lightning protection* - a w_tom conductor

stuart noble wrote

Once again I've managed to chop through the flex on my
hedge cutters. They're rated at 400 watts, so I guess a 5amp
fuse should have been used in the plug instead of 13 amp.
Even so, I don't understand how the fuse survived what was a clean cut.

The cut wont necessarily short anything and even when
there is a short, it may not have lasted long enough to
blow the fuse.

Yes, I did unplug it instinctively, but I didn't see any
urgency to do so because I assumed the fuse had blown.

charles wrote:
In article , John
Rumm wrote:
On 27/07/2012 16:05, stuart noble wrote:
On 27/07/2012 13:46, Andy Burns wrote:
stuart noble wrote:

Once again I've managed to chop through the flex on my hedge
cutters.

Was it on an RCD? Did that trip?

No, it wasn't. Just a normal extension lead straight into a
socket. As it happens the fuse box in this property is an old
fashioned type with no circuit breakers, so I will continue
nagging them to get that upgraded.

In which case change the plug on the end of your extension lead to
a RCD type, then use that for anything that needs power outside.

I prefer the in-line RCDs since some sockets are mounted so that you
can't get an RCD plug into them

Mounted too low to the skirting/worktop etc?

--
Adam

In article ,
ARWadsworth wrote:
charles wrote:
In article , John
Rumm wrote:
On 27/07/2012 16:05, stuart noble wrote:
On 27/07/2012 13:46, Andy Burns wrote:
stuart noble wrote:

Once again I've managed to chop through the flex on my hedge
cutters.

Was it on an RCD? Did that trip?

No, it wasn't. Just a normal extension lead straight into a
socket. As it happens the fuse box in this property is an old
fashioned type with no circuit breakers, so I will continue
nagging them to get that upgraded.

In which case change the plug on the end of your extension lead to
a RCD type, then use that for anything that needs power outside.

I prefer the in-line RCDs since some sockets are mounted so that you
can't get an RCD plug into them

Mounted too low to the skirting/worktop etc?

correct. You win a prize.

--
From KT24

Using a RISC OS computer running v5.18

"charles" wrote in message
...


Mounted too low to the skirting/worktop etc?

correct. You win a prize.

Turn them upside down.

charles wrote:
In article ,
ARWadsworth wrote:
charles wrote:
In article ,
John Rumm wrote:
On 27/07/2012 16:05, stuart noble wrote:
On 27/07/2012 13:46, Andy Burns wrote:
stuart noble wrote:

Once again I've managed to chop through the flex on my
hedge cutters.

Was it on an RCD? Did that trip?

No, it wasn't. Just a normal extension lead straight into a
socket. As it happens the fuse box in this property is an old
fashioned type with no circuit breakers, so I will continue
nagging them to get that upgraded.

In which case change the plug on the end of your extension lead
to a RCD type, then use that for anything that needs power
outside.

I prefer the in-line RCDs since some sockets are mounted so that
you can't get an RCD plug into them

Mounted too low to the skirting/worktop etc?

correct. You win a prize.

That's a bad installation then:-(

And where do you charge all the electrical appliances with the transformer
plugs that suffer the same proplem?

I always thought that 2 double sockets on the landing would be enough for
recharging purposes. It's not.
--
Adam

On Friday, 27 July 2012 13:39:12 UTC+1, stuart noble wrote:
Once again I've managed to chop through the flex on my hedge cutters.

Fuses take time to respond, and this time is (roughly) inversely proportional to the square of the current. If you cut the cable quickly, you can beat the fuse - although this is really rare for small fuses. If you cut the cable by tearing it, so that the "short circuit" through the blade either doesn't happen, or only happens briefly, then the energy in the fuse is insufficient to break it.

John Rumm wrote:

However the key point is that if as a result of assuming the cable is
dead when its not, you then handled the live end, the RCD would keep you
alive.

And thus able to learn the lesson that it's a bad assumption that
cutting the cable will ensure that it is no longer live.

On 30/07/2012 00:27, Mark Evans wrote:
John Rumm wrote:

However the key point is that if as a result of assuming the cable is
dead when its not, you then handled the live end, the RCD would keep you
alive.

And thus able to learn the lesson that it's a bad assumption that
cutting the cable will ensure that it is no longer live.

Well to be fair, your would probably learn that lesson even without the
RCD, it would just hurt a whole lot more, and the amount of time you
have available to benefit from what you have learnt might be
considerably shorter! ;-)


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

"John Rumm" wrote in message
...
On 30/07/2012 00:27, Mark Evans wrote:
John Rumm wrote:

However the key point is that if as a result of assuming the cable is
dead when its not, you then handled the live end, the RCD would keep you
alive.

And thus able to learn the lesson that it's a bad assumption that
cutting the cable will ensure that it is no longer live.

Well to be fair, your would probably learn that lesson even without the
RCD, it would just hurt a whole lot more, and the amount of time you have
available to benefit from what you have learnt might be considerably
shorter! ;-)

There appears to be a fair few people here that think an RCD stops all
people getting killed.
Its not that simple.

The current/time limit is set so most people will not be killed.
There is still a small group of people that can be killed before the RCD
trips.

Those people won't know who they are so never treat a cable as off even if
you have an RCD.

It will protect the majority of people most of the time but there will still
be deaths.

On 30/07/2012 11:31, dennis@home wrote:


"John Rumm" wrote in message
...
On 30/07/2012 00:27, Mark Evans wrote:
John Rumm wrote:

However the key point is that if as a result of assuming the cable is
dead when its not, you then handled the live end, the RCD would keep
you
alive.

And thus able to learn the lesson that it's a bad assumption that
cutting the cable will ensure that it is no longer live.

Well to be fair, your would probably learn that lesson even without
the RCD, it would just hurt a whole lot more, and the amount of time
you have available to benefit from what you have learnt might be
considerably shorter! ;-)

There appears to be a fair few people here that think an RCD stops all
people getting killed.
Its not that simple.

It is a good working approximation in most cases it works. Far better
than being fried to a crisp or seriously burned which is what happens if
you grab live and neutral in the same hand. Most people that have never
had a shock assume that you can let go - you can't.

A hapless friend fell for exactly that scenario in a physics lab where
someone had wired a mains cable with a plug on each end!

The current/time limit is set so most people will not be killed.
There is still a small group of people that can be killed before the RCD
trips.

Those people won't know who they are so never treat a cable as off even
if you have an RCD.

That is good advice.

It will protect the majority of people most of the time but there will
still be deaths.

You would have to be incredibly unlucky and have the current across your
heart to get a fatality inside the proper RCD profile. I would be
interested to see any credible evidence of a fatality where a working
RCD had failed to protect a vulnerable adult. Defective RCDs may
sometimes fail to trigger leading to death but that is a different
matter (you should always test them first).

I don't count any articles in magazines that also cover UFOs, alien
abductions, spontaneous combustion, Elvis is alive etc.

--
Regards,
Martin Brown

"Martin Brown" wrote in message
...
On 30/07/2012 11:31, dennis@home wrote:


"John Rumm" wrote in message
...
On 30/07/2012 00:27, Mark Evans wrote:
John Rumm wrote:

However the key point is that if as a result of assuming the cable is
dead when its not, you then handled the live end, the RCD would keep
you
alive.

And thus able to learn the lesson that it's a bad assumption that
cutting the cable will ensure that it is no longer live.

Well to be fair, your would probably learn that lesson even without
the RCD, it would just hurt a whole lot more, and the amount of time
you have available to benefit from what you have learnt might be
considerably shorter! ;-)

There appears to be a fair few people here that think an RCD stops all
people getting killed.
Its not that simple.

It is a good working approximation in most cases it works. Far better than
being fried to a crisp or seriously burned which is what happens if you
grab live and neutral in the same hand. Most people that have never had a
shock assume that you can let go - you can't.

A hapless friend fell for exactly that scenario in a physics lab where
someone had wired a mains cable with a plug on each end!

The current/time limit is set so most people will not be killed.
There is still a small group of people that can be killed before the RCD
trips.

Those people won't know who they are so never treat a cable as off even
if you have an RCD.

That is good advice.

It will protect the majority of people most of the time but there will
still be deaths.

You would have to be incredibly unlucky and have the current across your
heart to get a fatality inside the proper RCD profile. I would be
interested to see any credible evidence of a fatality where a working RCD
had failed to protect a vulnerable adult. Defective RCDs may sometimes
fail to trigger leading to death but that is a different matter (you
should always test them first).

How about this from http://www.bassengineering.com/E_Effect.htm



The current is the controlling factor for Electrocution and Electrical
Shock. The threshold for perception is about 100 microamps (0.0001 Amps).
Also See Microshock Electrocution Hazards for currents less than 100
microamps. The National Electrical Code (NEC) considers 5 milliamps (0.005
Amps) to be a safe upper limit for children and adults hence the 5 milliamps
GFI circuit breaker requirement for wet locations. The normal nervous system
reaction to any perceptible electrical shock may cause a person to injure
themselves or others, therefore the so called safe limit does not assure
freedom from injury.

The more serious electrocution and shock hazards occur above the let go
limits. 99% of the female population have an let go limit above 6 milliamps,
with an average of 10.5 milliamps. 99% of the male population have an let go
limit above 9 milliamps, with an average of 15.5 milliamps. Prolonged
exposure to 60 Hz. currents greater than 18 milliamps, across the chest
causes the diaphragm to contract which prevents breathing and causes the
victim to suffocate. No data is available for females or children but
suffocation is presumed to occur at a lower current level.



If you believe it 99% of women can die from suffocation with less than 20
mA.
And as you say they can't let go.


The whole thing is based on statistics and 30 mA is not an absolute safe
limit that would have to be down in single mAs.

On 30/07/2012 12:01, Martin Brown wrote:
On 30/07/2012 11:31, dennis@home wrote:


"John Rumm" wrote in message
...
On 30/07/2012 00:27, Mark Evans wrote:
John Rumm wrote:

However the key point is that if as a result of assuming the cable is
dead when its not, you then handled the live end, the RCD would keep
you
alive.

And thus able to learn the lesson that it's a bad assumption that
cutting the cable will ensure that it is no longer live.

Well to be fair, your would probably learn that lesson even without
the RCD, it would just hurt a whole lot more, and the amount of time
you have available to benefit from what you have learnt might be
considerably shorter! ;-)

There appears to be a fair few people here that think an RCD stops all
people getting killed.
Its not that simple.

It is a good working approximation in most cases it works. Far better
than being fried to a crisp or seriously burned which is what happens if
you grab live and neutral in the same hand. Most people that have never
had a shock assume that you can let go - you can't.

A hapless friend fell for exactly that scenario in a physics lab where
someone had wired a mains cable with a plug on each end!

Potentially nasty - but not as bad as a hand to hand shock...

The current/time limit is set so most people will not be killed.
There is still a small group of people that can be killed before the RCD
trips.

Those people won't know who they are so never treat a cable as off even
if you have an RCD.

That is good advice.

It will protect the majority of people most of the time but there will
still be deaths.

You would have to be incredibly unlucky and have the current across your
heart to get a fatality inside the proper RCD profile. I would be
interested to see any credible evidence of a fatality where a working
RCD had failed to protect a vulnerable adult. Defective RCDs may
sometimes fail to trigger leading to death but that is a different
matter (you should always test them first).

I don't count any articles in magazines that also cover UFOs, alien
abductions, spontaneous combustion, Elvis is alive etc.

As with most cases in this area, its hard to be certain about things
since the actual numbers are so low.

With the possible exception of the toddler playing with the unterminated
flex and plug, I don't recall reading of any deaths that happened in
spite of an RCD operating (and in that particular case I don't know if
there was a RCD at all, and if there was, if it operated - I suspect the
answer to both those is no)

You actually need a confluence of a greater set of circumstances than
just someone being particularly sensitive. The points of contact make a
huge difference, as does the quality of the independent earth connection
through the person getting the shock.

In (the far more rare) cases where one has made direct contact to both
line and neutral, then a low impedance connection to earth would be
beneficial since it would maximise detectable leakage. However the
current profile of the shock could potentially be way above the trip
threshold of the RCD since it can only limit the duration, not the
current. In the more common case of direct or indirect contact to only a
live, a high impedance connection to earth will serve to limit the shock
current, which along with the RCD imposed time limit is usually adequate
to protect even sensitive individuals.


--
Cheers,

John.

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