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Tingle from metal lamp = dangerous?

D

Dave Stanton

Jan 1, 1970
0
That depends. Actually, it is so, UNLESS you have an RCD/GFI, which places
a small voltage between neutral and ground as they exit the RCD device.
After all, the ground and neutral are connected together in the service
panel, so there's no difference between them unless the RCD, connected

SBH

Sorry, you don't know much about UK mains distribution do you ?.

The neutral wire in the UK is connected to ground only at the sub station
under normal situations and nowhere else. Therefore it can be at a
different potential to true ground.

You really should not begiving advice in a UK ng about a electrical system
which you clearly have no first hand experience of.

Dave
 
A

Alistair Riddell

Jan 1, 1970
0
Sorry, you don't know much about UK mains distribution do you ?.

The neutral wire in the UK is connected to ground only at the sub station
under normal situations and nowhere else. Therefore it can be at a
different potential to true ground.

Actually, in a TN-C-S supply, which is pretty common nowadays, they are
connected together in the service head (grey box with big fuse in it
belonging to the electricity co. next to the customers' meter).
 
B

Bob Eager

Jan 1, 1970
0
The neutral wire in the UK is connected to ground only at the sub station
under normal situations and nowhere else. Therefore it can be at a
different potential to true ground.

I agre with your comment about him not knowing much about UK mains. But
in a PME (TN-C-S) situation, neutral and earth are connected together
much closer than that - at the cutout (incoming cable connection).
 
B

bz

Jan 1, 1970
0
an RCD/GFI, which
places a small voltage between neutral and ground as they exit the RCD
device. After all, the ground and neutral are connected together in the
service panel, so there's no difference between them unless the RCD,
connected after the panel, puts it there.

You are so wrong!!!!!
The purpose of this small
voltage is so the RCD can detect a ground-neutral "fault" (one where
the neural wire has touched the gound inside your lamp or other
appliance) and trip when that happens. The reason RCDs do this is
because having the neutral touch ground makes the major feature of the
RCD (the ability to tell if more current is flowing in the hot wire
than in the neutral, signaling an extra path to ground which may be you
being electrocuted) inoperative. So the RCD is designed to shut off the
power if its sensing function (which requires a separate neutral and
ground path as they exit the device) is compromised and it becomes
"blind" and unable to perform its function. But again, if you don't
have an RCD in the first place, none of this applies. YOU don't care if
the ground touches neural in your old lamp UNLESS you have an RCD/GFI
to trip in the first place.

The GFI does not work that way. The ones I have seen use a small toroidial
transformer to sample the current flow in the hot and neutral lines. (Some
also sample the safety ground and trip if there is any flow there).

The circuit is a balanced bridge as long and the current in the hot line is
equal to the current in the neutral line, the GFI does not trip.

If there is an imbalance, it implies that some current is flowing through
something OTHER than the designed load and that current is going to a
different neutral or to earth ground.

You can still get electrocuted if you have one lamp that has the hot
shorted to the frame and you touch that and another device that has neutral
connected to the frame. If both were on the same GFI protected circuit, you
would NOT trip the GFI.

NEVER connect the neutral to the frame of any device because you defeat the
GFI when you do this. Frame always connects to safety ground.

Your advices would also make the device deadly if it is ever used in an
older house with two wire plugs and someone plugs it in backwards.

Never connect neutral to the frame.

--
bz

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

[email protected] remove ch100-5 to avoid spam trap
 
R

Richard Tobin

Jan 1, 1970
0
You really should not begiving advice in a UK ng about a electrical system
which you clearly have no first hand experience of.

The article was posted to several newsgroups, only one of which is
uk-specific. It's quite likely that readers won't notice that.

-- Richard
 
E

ehsjr

Jan 1, 1970
0
bz said:
You are so wrong!!!!!




The GFI does not work that way.


A GFCI works, in part, as you describe below.
In addition, it detects a ground/neutral short
as described at the following url:
http://www.codecheck.com/gfci_principal.htm
I believe that function is what the poster was
referring to.

Ed

The ones I have seen use a small toroidial
 
D

Dave Stanton

Jan 1, 1970
0
I agre with your comment about him not knowing much about UK mains. But in
a PME (TN-C-S) situation, neutral and earth are connected together much
closer than that - at the cutout (incoming cable connection).

Yes I know I was'nt 100% correct, but did'nt want to cloud what is
becoming quite a serious case of incorrect and dangerous info.

Dave
 
J

Jeff Wisnia

Jan 1, 1970
0
Lars said:
On Wed 27 Apr 2005 23:35:40, Jeff Wisnia wrote:



I am standing on a carpet on the third floor of a building. The
carpet is 80% wool and presumably the 20% is polyester.






I myself am touching the "cold end" of the screwdriver mains tester
with my finger.





Could I not use an ammeter inthe first place rather than a test bulb
given that a fully-lit 100 W bulb would have a current flow of approx
0.4 A (= 100 watts/240 volts).


The reason I suggested using a small bulb rather than a meter is this.

Just in case you DO have a dead short between the "hot" side of your 240
volt supply and the metal body of the lamp, then connecting that
grounded wire I suggested directly to the lamp body through an ammeter
would cause one heck of a lot more current than the 0.4 A you mention,
possibly enough to damage the meter before the fuse or breaker opened,
unless the meter was set to something like a 400 amp range. Capiche?

My experience is the same as yours in that I have many 2-core only
appliances with metal bodies which seem designed specifically to be
2-core. Often their label refers to "double insulated" - whatever
that may be.

And I've recently read that common consumer "pop-up" toasters don't have
three wire cords and grounded cases because people often stick tableware
in them trying to remove a stuck piece of bread. If those metal
implements shorted a toaster element to a grounded case, the element
could burn out.

They rely instead on the presence of GFCI protected outlets to keep
foolish people from electrocuting themselves while doing that. GFCIs
have been required by code for kitchen outlets in the US for many years now.
Jeff
 
C

Chris Lewis

Jan 1, 1970
0
According to Jeff Wisnia said:
The reason I suggested using a small bulb rather than a meter is this.
Just in case you DO have a dead short between the "hot" side of your 240
volt supply and the metal body of the lamp, then connecting that
grounded wire I suggested directly to the lamp body through an ammeter
would cause one heck of a lot more current than the 0.4 A you mention,
possibly enough to damage the meter before the fuse or breaker opened,
unless the meter was set to something like a 400 amp range. Capiche?

The meter would probably _still_ explode.

I saw the result of someone mistaking a 100A DC ammeter for an AC
voltmeter, and sticking the probes in a 120V socket. Not pretty.
Yup, the breaker tripped. But the meter innards had vaporized,
and the industrial grade receptacle was destroyed.

Shouldn't put an ammeter across anything that could even remotely
have line potential across it unless there's some sort of limiter
(ie: lightbulb) in the way. Intermittent short => kaboom!
And I've recently read that common consumer "pop-up" toasters don't have
three wire cords and grounded cases because people often stick tableware
in them trying to remove a stuck piece of bread. If those metal
implements shorted a toaster element to a grounded case, the element
could burn out.

True, but more importantly, if the toaster is grounded, and you stick
a knife into it to clear out a piece of bread, where's your other
hand? On the toaster!

-> Toasted user, not toasted bread.
 
B

Bob Eager

Jan 1, 1970
0
What's in a name? Ahem, I called it a "service panel," and you two call
it a "service head," or a "cutout." But it's the same place in the
system.

Fine. But then it wasn't you I was disagreeing with.
 
B

bz

Jan 1, 1970
0
Look you simply don't understand the things completely. Rather than
argue with me, I recommend you educate yourself.

Educate yourself.

Your explanation of how the RCD/GFI works "an RCD/GFI, ...places a small
voltage between neutral and ground ....The purpose of this small voltage is
so the RCD can detect a ground-neutral "fault"...." is wrong. Dead wrong.

The GFI does NOT depend on a voltage difference to sense the fault, it
depends on unbalanced current flow, as I said. See the current transformer
in the drawing in your reference?

See the two current carrying wires going through the core? As long as the
current in those is equal, they cancel each other out and there is no
current induced in the secondary that feeds the 'sensor circuit'.

You gave dangerous advice when you said:

..... But if the casing is connected to neutral at least, then you have
a chance that a hot wire contacting it will trip your fusebox, which is
a poor man's GFI.
[unquote]
even with the caveats you gave.

Never wire the neutral to the case.
Case to safety ground or be sure to float the case.

Neutral to case can kill you even with a GFI!!!!
1) If a hot case on one item and the neutral case on the other are plugged
into the same GFI protected circuit, there will be NO ground fault if you
contact both.

2) If a an old, non polar outlet is encountered or a miswired outlet (they
do happen) and 'neutral' is actually 'hot'

--
bz

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

[email protected] remove ch100-5 to avoid spam trap
 
B

bz

Jan 1, 1970
0
In addition, it detects a ground/neutral short
as described at the following url:
http://www.codecheck.com/gfci_­principal.htm
I believe that function is what the poster was
referring to. <<

Correct. Modern GFI's really have two parts. The main one keeps you
from killing yourself, by cutting off current when you have a hot to
ground current. The active SECOND part (a second toroid) allows the
device to detect ground-neutral faults, so it can trip then, when its
sensing function has been compromised.
Look again. See the two wires going through the core? [ignore the test
line]

Each of those wires serves as the primary of a transformer.

As long as the SAME current flows in BOTH of those wires, they cancel each
other out. No current is induced into the secondary.

An imbalance in the current in those wires can be due to MORE current
flowing in EITHER of them than in the other.

An imbalance will trip the GFI.

Notice, there is NO second circuit to detect ground/neutral shorts. It is
an imbalance in current that trips the GFI.

The GFI depends on the current through BOTH wires being equal.




--
bz

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

[email protected] remove ch100-5 to avoid spam trap
 
B

bz

Jan 1, 1970
0
Educate yourself.

My appologies. I read a bit further in the article and see the second
circuit. The GFIs I have taken apart did NOT have two torroids.

I was wrong. Sorry. Please accept my humble appology.

(I still think that connecting neutral to case is dangerous, however)



--
bz

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

[email protected] remove ch100-5 to avoid spam trap
 
E

ehsjr

Jan 1, 1970
0
bz said:
In addition, it detects a ground/neutral short
as described at the following url:
http://www.codecheck.com/gfci_­principal.htm
I believe that function is what the poster was
referring to. <<

Correct. Modern GFI's really have two parts. The main one keeps you
from killing yourself, by cutting off current when you have a hot to
ground current. The active SECOND part (a second toroid) allows the
device to detect ground-neutral faults, so it can trip then, when its
sensing function has been compromised.

Look again. See the two wires going through the core? [ignore the test
line]

Each of those wires serves as the primary of a transformer.

As long as the SAME current flows in BOTH of those wires, they cancel each
other out. No current is induced into the secondary.

An imbalance in the current in those wires can be due to MORE current
flowing in EITHER of them than in the other.

An imbalance will trip the GFI.

Notice, there is NO second circuit to detect ground/neutral shorts. It is
an imbalance in current that trips the GFI.

The GFI depends on the current through BOTH wires being equal.

He does not need to look again. He has it
right. Go read the url.

No one is disputing the imbalance detection.

The poster is discussing the *additional* circuitry
that is used to enable a GFI to detect a
downstream short between neutral and ground.

Ed
 
B

bz

Jan 1, 1970
0
ehsjr said:
bz said:
A GFCI works, in part, as you describe below.

In addition, it detects a ground/neutral short
as described at the following url:
http://www.codecheck.com/gfci_­principal.htm
I believe that function is what the poster was
referring to. <<

Correct. Modern GFI's really have two parts. The main one keeps you
from killing yourself, by cutting off current when you have a hot to
ground current. The active SECOND part (a second toroid) allows the
device to detect ground-neutral faults, so it can trip then, when its
sensing function has been compromised.

Look again. See the two wires going through the core? [ignore the test
line]

Each of those wires serves as the primary of a transformer.

As long as the SAME current flows in BOTH of those wires, they cancel
each other out. No current is induced into the secondary.

An imbalance in the current in those wires can be due to MORE current
flowing in EITHER of them than in the other.

An imbalance will trip the GFI.

Notice, there is NO second circuit to detect ground/neutral shorts. It
is an imbalance in current that trips the GFI.

The GFI depends on the current through BOTH wires being equal.

He does not need to look again. He has it
right. Go read the url.

No one is disputing the imbalance detection.

The poster is discussing the *additional* circuitry
that is used to enable a GFI to detect a
downstream short between neutral and ground.

You are correct.
I was wrong.

I missed seeing the other drawings, further down the page.
My mistake.



--
bz

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

[email protected] remove ch100-5 to avoid spam trap
 
D

Dimitrios Tzortzakakis

Jan 1, 1970
0
Lars,
to check the leak resistance of mains circuits you need a special
"megger"meter.This is usually in the range of Mohms, so usually a normal
multimeter shows infinite resistance (sometimes modern digital ones are
capable of measuring this resistance).It's very dangerous to use something
that gives you (even) a slight shock.If you find it's too much money to have
it professionaly repaired (maybe a worn out cable that touches the frame?)
then better trash it, than put your life in danger.
 
B

Bill Woods

Jan 1, 1970
0
On Thu 28 Apr 2005 09:44:56, s--p--o--n--i--x wrote:
If it's the sort of anglepoise lamp I'm thinking of it will be
double insulated.

What does "double insulated" mean in actual practice?
 
P

Pop

Jan 1, 1970
0
Basically, Double insulated is a little misleading, but
in UL/CSA parlance, it means that the internal wiring
is such that it would take TWO faults to occur, either
of which would stop the product from working, before
anything could become electrically dangerous to the
user. Usage conditions are also taken into account.
Among other things it also means that there cannot
be any exposed metal on the product. ALL exposed metal
must be grounded, which become a non-essential in
double insulated products.

It also means it doesn't need a third "prong", and the
blades do not have to be polarized on the two-blade
plug, and it does not need a 3-hole outlet.

Pop
 
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