- - - - - - - -Why do I get a shock or Tingle when I touch the VCR's RCA connectors ?- - - - - - - -
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Mains powers Power Supplys ......
Back in the days of "REAL" transformers, all electrical and electronic appliances were internally "Earthed" or "Grounded" at their internal metal chassis via the Green Earth wire
within the power cord to the mains GPO, (General Purpose Outlet). Back in those days, the wiring colours were RED (Active) , BLACK (Neutral) and GREEN (Earth).
Therefore, in accordance with international trends and standards, the Australian wiring code of colours became Brown (Active), BLUE (Neutral) and GREEN/YELLOW stripe for the Earth wire.
Three wires entered into the appliance via a "cable entry device" which facilitated good mechanical support to the wires and an "Energy Authority" approved 240 Volt 3 wire cord .
The earthing wire which when traced all the way back to the meter box was in fact earthed inside the consumers "fuse box" or "power box" then down to an earth stake driven
into the soil, usually near the power " fuse box " installation.
This should be typically a moist area to facilitate good grounding into the soil. Now, having said that, we here in Australia, use what is termed M.E.N. system, or Multiple Earth Neutrals.
which is a fancy way of stating that the neutrals all end up in the meter-fuse-power box all connected to the earth wire to ground. The water utility added in a plastic isolator between the street
main water supply and the domestic or business user thus resulting in incoming the water pipe being totally "isolated" from the street except by the conductivity of the water within.
The appliance, both electrical and electronic would have the earth wire connected directly to its chassis, thus forming a safety earth return should any problems occur with that appliance.
This concept still is a pretty good system which is why professional equipment manufacturers have adhered to it !, thus, if something happens to cause any sort of conductive path from the Live
(or Active) side of the 240 Volts mains supply to the metal chassis, then that " fault " current is conducted directly to earth or ground.
If the impedance of the fault current is low enough a large current will flow and usually cause that particular equipment's "protective" fuse to blow, disconnecting the active leg of the 240 Volts mains
supply and thereby signaling that something is very wrong, by the absence of appliance's indicator LEDs, Neon indicator or pilot lights or just plain not working at all.
Looking at Hi-Fi set-ups and P.A. (Public Address) systems, it is not uncommon to have a noticeably audible 50Hz hum ever-present. This can be very annoying to say the least.
In dedicated distributed video systems a 50Hz hum can present itself as a visible light and dark horizontal bar moving slowly up and/or down the video or television screen.
If the hum is too severe, it can disrupt the synchronisation signals and, in some extreme cases, cause major distortion of the TV or video image.
It is a common practice for "totally ignorant" persons to disconnect the mains earths from various items of equipment until the hum ceased.
One of the dumbest examples is that many Rock n' Roll singers and performers have suffered severe shocks and in some cases electrical burns to their lips due to the total ignorance of their
"technician" who removed the earth connection from their guitar amp thinking this will stop it humming..... lame, very lame indeed and very dangerous !.
What they fail to realise is that there is often a mains filter typically consisting mainly of two 400V or 630V rated polyester capacitors, one from the Active 240 Volts side of the mains to the chassis
and the other from the Neutral side, connected to the chassis.
These are fairly effective at preventing RF interference and in some cases, spikes from entering or leaving the amplifier, however when the earth connection is broken they form a simple voltage
divider with the centre-tap on the chassis !
Here in Australia, as well as New Zealand and many other countries where the mains voltage is around the 240 Volts AC mark, this places about 120V ~170 Volts AC on the chassis and the
capacitors are usually of such a value that roughly 1mA to 18mA of current can flow from the electrified, unearthed chassis to anything that is properly earthed.
So when the singer holding their guitar which is solidly connected to the unearthed amp touches their lips momentarily to the microphone which is just as solidly earthed by the PA system,
they get a major shock in a very sensitive part of the body - their moist lips !....Zap !.....The current flows from their hands through their chest and heart to their lips.
The natural assumption is to blame the Microphone or the PA system, but all tests of that equipment show no problem.
If the guitar amp suffers a serious electrical fault or the capacitor in the Active line shorts out, which incidentally they do "short-out" sometimes, the performer could easily be killed.
Double-insulated equipment is not connected to the mains earth and most of the Hi-Fi and Video gear intended for the domestic user is double-insulated.
Typically, the cable (below) whereas only two wires is used to power the appliance, the neutral is used as a "pseudo-earth" as in the "meter box, where the neutral is in fact connected to the earth.
In double-insulated gear the metal chassis is connected only to the "common" side of the circuitry to achieve some shielding to prevent RF interference from radiating into or out of the item.
There can be no mains filters which connect to the chassis, so at least the chance of a shock from this cause is eliminated or at least minimised.
It is easy to identify double-insulated equipment because the mains power lead will often be flat instead of round and the mains plug will only have two pins and there may also be a symbol
on the chassis - two concentric squares or put simply, a square within a square molded into the appliance's case indicating double-insulated.
movemeTHE DOUBLE INSULATED LOGO
So what can go wrong with double-insulated gear ??????........ Plenty,...... as we shall now see. All electrical equipment contains a mains powered PSU (Power Supply Unit).
The PSU (power supply unit) converts the raw mains into low voltages for the equipment's circuitry and isolates the mains from the low voltage side... almost.
You see, all power transformers, whether they are designed for a switch-mode supply or linear supply, they all leak a "small" amount of current from the primary to the secondary board earths due to stray capacitance.
In double-insulated items there is no earth as in earth to "GROUND" per se available to use as a shield between input and output, so the power supply is designed as far as possible to minimise the leakage due to
this unavoidable effect.
Unfortunately, switch-mode supplies run at very high frequencies, so the smaller capacitance in the smaller transformer ends up leaking just as much as the bigger capacitance in the larger
transformer running at 50 or at 60Hz.
LEAKAGE ? uh ?......
The leakage current is usually less than 1mA and is often as low as 10uA, but even at such low current, it can still present and still can cause problems.
Generally as a rule of thumb, the more power an appliance uses, typically the higher the leakage will be.
Things that use an external power supply typically plugged into the wall's 240V AC GPO (general purpose outlet) are not exempt either.
These power supplies are usually double-insulated and leak just like anything else connected to the mains.
Insects, dust, moisture, food particles and all sorts of miniscule bugs find their way into most electrical equipment.
We have noted over the years that some "silicone based" sealants use to "hold" electronic parts actually attract ants, black small ants that devour the set silicone.
Electrolytic capacitors and batteries can leak their corrosive contents and components can overheat or be damaged in a million other ways.
Insulation gradually loses it's plasticiser due to age, exposure to light and thermal cycling to mention a few causes and becomes brittle.
Insulation in transformer windings degrades over time due to vibration and heat.
All these things can conspire together to cause electrical leakage (or even short-circuits) to the unearthed chassis of double-insulated gear and, guess what ? All of a sudden it's not insulated at all !
The introduction of an earthed item into a system made up of double-insulated gear is not unlikely, but it is likely to be the start of some "interesting" problems.
Let's say you have a DVD player, two VCR's, a TV/Monitor, a CD player, a cassette deck, a Pre-amp, a Graphic Equaliser, .......you get the idea.
All these things are double-insulated and they are all connected to each other either directly or through one of the others. The leak current all adds up.
It would not be unreasonable to expect any where from 0.1mA to 1 mA ( or even more ) of leakage from the combined equipment because the leakage currents simply add together.
If you were to connect a digital multimeter between this pile of gear and the chassis of your new item, let's say it's a big brute of a surround sound amplifier which just happens to be earthed,
you will measure a very significant ac voltage, probably at least half the mains supply or even higher.
( Don't try to measure the current, you might blow up your multimeter if there's a bad fault or worse, Kill yourself ).
Now let's say you have the metal shell of an RCA on an audio cable which is connected to an output on the unearthed gear in one hand and you steady the amplifier with your
other hand as you start to plug it into an input. As soon as you complete the circuit between them you will get a mild electric shock.
Depending on how moist your skin is and how high the leakage is this might be unnoticeable or it might cause you to say a bad word!.
Unless you have a bad heart it is unlikely to hurt you physically, but it can give you quite a sting, especially when you do not expect it.
Once the first connection is made and the leakage is flowing to ground, there will be no more tingles.
MOVEMEHUMMING ALONG......
You might notice though, that your previously hum-free system is now humming.
The hum might be affected by the volume control on your amp or not or it might only record on tape or it might be visible on your TV or ... What do you do?
That's a very good question and one which deserves a really good answer.
Unfortunately, there is no easy answer, but now that you know a bit more about where the hum comes from, an orderly and logical disconnection and re-connection of
the whole system will locate the major culprit.
If you have been unlucky, you might also notice that that first input you plugged in is now noisy or distorted or not working at all.
If all else fails, read the Manual. Ever wondered why they say in the instruction manuals that you should connect or disconnect the power when making or breaking connections to anything?
Well now you know! It's to avoid shocks, huge hums, loud crackles and damage caused by high voltages being connected to sensitive inputs before the common/earth/ground side
manages to short-out the dreaded mains leakage (or possibly your own static-electricity build-up).
It might be a nuisance, but it does avoid all these problems, and could save your life (or at least that of your other gear) if one of your double-insulated ' what-ever' happens to
have a serious electrical fault or a bit more leakage than your other equipment can tolerate.
Lastly, this tale, only one of so many, it has to be told...........so one day, one of our newer customers rang us one day to order some special length VIDEO and AUDIO cables.
Nothing strange about that, however his next request had me indeed quite puzzled - the customer wanted the cables totally insulated, that is, with no exposed metal anywhere.
I questioned him as to the thinking behind this and asked why, his quick reply was "Because he gets really bad tingles and mild shocks from exposed connectors from his video
editing suite and his TBC (Time Base Corrector) , when he re-configures the Video or Audio wiring to other video equipment he wishes to utilise".
So, upon further questions, I was informed that he had 4 ordinary VCR's, a TBC (Time Base Corrector), a Genlock and several Philips 13" Audio/Video monitors as well as his computers.
The computer and the Genlock were the only items that were earthed during manufacture, all other equipment he used was of the "double insulated" type.
Sparks may fly....
At one stage, a "bright spark" suggested that he remove the earth wires from both genlock and computer, which of course would do nothing and make the whole setup totally "floating"
above earth potential, I conveyed to him that this suggestion made me truly horrified and explained that the results may contribute to the deaths of some-one later down the track.
I arranged for "house-call" by a colleague to visit him, he is a technically competent electronics technician friend of mine who's job it was to go over his setup and in consultancy
with myself, we ended up attaching 0.22uF Class X2 400V AC poly caps to the rear of each VCR's lid temporarily as shown in the two drawings below (undo the screws and add in the caps
with an "earth lug" ), this also preserved his 4 VCR's warranty (nothing needed to be added inside) simply a "lug" under a screw on each device.
This rectified his zapping problem, the AC present was to the order of 127 volts measured by the CRO (Cathode Ray Oscilloscope) with about 27 mA (milli Amps) of current flow.
This certainly assisted in getting rid of the considerable leakage of the 4 VCR's and the TBC contributing to the mild electrical shocks and some of the hum.
You will note that for simplicity, I have drawn 2 VCRs with bare components shown, this is NOT how the completed item should be wired.
It would be electronically better to place all parts in a very small "jiffy" box with double sided tape or velcro all from either Jaycar or Dick Smith/Tandy.
The 0.22uF class X2 capacitor should be wired up as shown in the small drawing below and it is a good idea to connect all the earth bound wires into on 240V plug.
Parts list...............
1 x 3 pin mains plug (wire all earth wires in last of all)
1 x 22nF ( 0.22uF ) Class X2 polyester capacitor rated at 400V ~ 630V AC,
1 x plastic box,
1 x crimp eyelet per appliance,
1 x 2 way mains block per appliance,
1 x strip of double sided tape (or velcro tape),
1 x small nut & screw 10mm to hold the terminal block.
1 x strip of 10 terminal blocks, cut into two per box.
1 x length of 240V mains rated flexible green earthing wire,
1 x Mains surge protector power board to suit all your devices.
4 x small cable ties.
NOTE: You may want to consider the additional circuit concept (below) as a adjunct protection.
Above you will note ( Fig.3 ) the simple representation of a RCA connection, note at the video cable entry (on left) it is connected to the "hot" tab of the
rear of the RCA female connection along with a 5.6 Volt 400 Milli-watt zener diode, this also we have found to be an excellent clamp against those nasty
"bities" that are often encountered with connecting to RCS connectors on "switched mode" powered electronic equipment used these days. It works.
Parts list...............
1 x 400 Milli-watt 5.6Volt zener diode.
1 x Small amount of solder.
1 x Time to carefully do the job.
Note: The addition of several 5.1 Zener diodes on all RCS sockets did in fact clamp and stop the "Biteys" from sparking when connecting other switched-mode devices.
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