This morning I am decorating the bedroom and in order to do so I have to
take down the shelf holding LCD TV, VCR and DVD player. As I was unplugging
everything I noticed whilst holding the aerial lead (metal part of the plug)
I brushed the inside of my forearm against the DVD player case (metal) I got
a tingling sensation, it didn't hurt but just a sensation. I have checked
the earth connection in the DVD player 3 pin plug and it appears OK. I
tried the 'setup' in other sockets with the same result. I haven't taken
the cover off the DVD player to investigate if the cover is grounded inside
yet. Anybody got any ideas where or what the problem could be. We have
other metal cased items in the house and there doesn't appear to be a
problem on those so I think it is unlikely to be in the mains wiring but
then again I am not a Sparkie!
Lots of devices use high value resistors between line and ground for RF
suppression. These are of such a value to limit any current to a safe
figure. But if the conditions are right - like no ground connection at
the time - you can get a tingle. It's quite normal.
*Why are a wise man and a wise guy opposites? *
Dave Plowman email@example.com London SW
This puzzles me also.
We were given a Panasonic VCR last week. It has a metal
cover (fixing screws touchable), lots of 'earthed' screens
inside, all connected together.
But the mains lead to the VCR is only 2-wire?
Ok, so it might earthed via the coax cable to the TV.
But the mains lead to the TV is also only 2-wire?
There's no chance that it could be earthed by the aerial.
So I seem to have mains-powered boxes with metal bits
that are touchable, with none of those metal bits
Well, yes, we seem to all agree on that.
However, it does beg the question*why* is it normal.
We don't accept minor tingles from anything else, and accept it as 'normal'.
Why should we accept it in this case?
Recently, we criticised a householder who electrocuted himself by coming
between between a live kitchen shelf who's mounting screws had punctured a
wire in the wall, and a nearby earthed appliance. He was criticised for
not realising something was up when they'd been getting a tingle from the
I don't know what 'class2' 'double insuated' actually means, ( Yes, I know I
could google it... ) but in this case, it doesn't appear to amount to much
if there are exposed metal parts which routinely deliver a shock!
Just seems fundamentally wrong to me.
All it would take is single failure of a filter cap for the
partially-exposed chassis to become properly live, and with no earth,
over-current protection would not trip.
A class2 device is unlikely to give you a tingle unless you can feel
some form of inductive coupling to the casework...
With a normally earthed bit of eletronic kit then you can get a tingle
if you have a relaively high earth loop impedance (e.g. as would be
typical in houses with TT (local earth rod) earthing). The RFI filter on
the input can then in effect hold the casework at half mains voltage
(although with a very high impedance).
I would expect with a double isolated device the casework would not be
connected to the RFI filter.
Or touch the aerial inlet connector, as discussed in this thread...
I can see how that could happen.
However, for the purpose of this thread, I'd like to focus on what's
acceptable practice in the design of a double-insulated appliance with a
2-core mains lead, and why it's acceptable. Here, someone made the actual
choice to design the high ( infinite! ) earth impedance, with the
I'd expect that too.
But how about the shell of connectors which are externally accessible?
Is it regarded as acceptable for those to give you a tingle?
If so, why is it acceptable?
Getting shocks off other kit is not normally regarded as acceptable.
Is it OK for an electric shower to give you a little tingle, so long as it's
just the pipe outlet and nothing more, and the tingle is high-impedance so
not many mA flows?
Is there a rule about how much externally-touchable surface is OK to be
Or how big those shocks are allowed to be?
It just seems wrong to me that *any* device should be, by design, giving
*any* shocks, however mild.
it means metal cased equipment that is to a fair extent single
you havent had a shock. A tingle is not a shock.
its acceptable to have single insulation, mark it as double insulated,
and its ok if you get a slight bite off the aerial.
why not? Why would something be prevented if it isnt causing a problem?
yes, the consequences that it cost a little less.
true. But not the point. The point is not what runs through someones
imagination, but what actually happens in the real world. The reality
is that the safety record is excellent.
double isolated? I dont think so.
So you expected wrongly. Thats all.
no. A shower needs secure earthing not to be dangerous, so tingling
isnt an option.
There are insulation requirements for both class I and class II goods.
Whether theyre always followed is open to debate I suppose.
there you go assuming again. I'd agree its not best practice, as its
long been known that touching an aerial that bites when youre standing
on a roof, no matter how slight the shock, can be enough for a person
to fall off and die.
cos its not killing people, as far as we know, and its cheaper.
quite, if a metal shelf gives you electricity its fairly obvious
you mean its not the way you'd assumed. Every aspect of life is like
that. Single insulated goods are called double insulated to inspire
public confidence in safety, and to distinguish them from goods with
none of the requirements of so-called double insulation. Theres nowt
new about political spin and public miseducation.
As others have said - it's normal.
It's caused by one of the devices ( TV / Digibox / VCR etc ) connected to
the antenna circuit using only a 2-wire mains lead and also having internal
filters which by design cause small amounts of leakage current to flow to
their internal chassis. Since the internal chassis is not grounded (
2-wire mains lead ), the chassis floats at around half mains voltage. This
slightly-live internal chassis is not exposed anywhere except the aerial
inlet, and this is is where you get the tingle from.
You get the tingle when you touch both the slightly-live chassis, and
something with a true ground, at the same time.
Also, this 'floating' voltage will be back-fed from the offending appliance
to your RF distribution, meaning that quite possibly all the aerial outlets
in your house will also have this voltage present on them. The first many
people know about this is when they get a tuner card for their PC, and as
they plug the aerial in, they feel the tingle between the aerial wire and
the PC case ( which is grounded via a 3-pin plug. ) They often assume the
PC is faulty and 'live', but usually this is not the case. The PC is
correctly earthed, it is the aerial cable screen that it 'live'.
I know someone who has run a short earth wire between the screen connection
in the back of the UHF wall-plate to the earth lug in an adjacent 13A
outlet, basically earthing these floating chassis devices via their aerial
wires. This was intended to allow this leakage to bleed away, since he
thought this may be causing issues with the tuner card in his PC. I don't
know whether he has had any issues with ths or not.
We've got a Panasonic TV that does this and as said consequently
'livened' up the RF distribution system. At the time I thought that it
was possible that line and neutral had been swapped in the TV and I put
it on an extension lead with the supplies reversed, and the leakage
I meant to chase it up with the manufacturers but that never happened
and when some time later and the warranty was over, and I was fitting a
digibox, I took the back off and changed the mains connector to the pcb
Remember that with many countries electrical systems their sockets do
not preserve polarisation. So the devices are designed to not be phased
(pun intended) by reversal of the supply. This is also why they normally
insert a capacitive filter to earth from both phase and neutral.
It may suggest on your set there is only a filter between one leg of the
supply and earth.
seems no one has picked up on the fact that john held the aerial lead if it
was still attached to the tv then there is your answer .
The tv sends a small radio carrier out through the aerial in order to better
capture the radio signal ,if its sat' ie sky then the LNB power is sent via
As any radio guy will tell you you can get a heck of a belt of an aerial .
This also manifests itself in aluminium coax plugs as the plug itself
greying and becoming very flakey also if dark sometimes a small spark can be
seen when fitting/removing.
None of this is dangerous in normal cases and as with any electrical
appliance always unplug before moving .
Well, sort of. Actually, the local oscillator signal leaks out of the
aerial - it's not designed to do so, but it's not worth designing it
out in cheapo domestic equipment like TVs.
Look up "superhet receivers" as to why.
"Other people are not your property."
[email me at huge [at] huge [dot] org [dot] uk]
Basically it means you buy an invention of the Devil called a PC with
a TV card in it. You extend the aerial to go in the PC and while
holding the aerial, you touch the PC case and get a shock.
The immediate reaction is
So you do the next best thing and drag it in here. :))
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