Proudly making Quest AV equipment in Australia since 1988
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abn 99 064 323 255  also t/a Quest Electronics ®  and Quest AV ®
Phone and SMS ... 0431 920 667 Mail ... POB 348  Woy Woy  NSW   2256
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In    the    "good    old    days"    all    electrical    appliances    were "earthed"   (aka   "grounded")   which   means   that   all   exposed metal   parts   were   connected   to   the   ground   pin   of   the   mains cable   attached   to   the   equipment.   This   was,   and   still   is,   a pretty     good     scheme     which     is     why     the     professional equipment   makers   have   stuck   to   it!   If   something   happens   to cause   any   sort   of   conductive   path   from   the   Live   (also   known as   Active)   side   of   the   mains   supply   to   the   metal   chassis, then    that    "fault"    current    is    conducted    to    ground.    If    the impedance   of   the   fault   current   is   low   enough   a   large   current will   flow   and   usually   cause   the   equipment's   protective   fuse to    blow,    disconnecting    the    mains    supply    and    thereby signalling that something is very wrong. In    HiFi    setups    and    Public    Address    systems    it    is    not uncommon     to     have     an     audible     hum     ever-present.     In distributed   Video   systems   such   a   hum   can   become   visible as   a   light   and   a   dark   horizontal   bar   moving   slowly   up   and/or down   the   video   screen.   If   the   hum   is   severe   it   can   disrupt   the synchronisation   signals   and   cause   major   distortion   of   the image.   It   was   (and   unfortunately   still   is!)   common   practise   for ignorant    persons    to    disconnect    the    mains    earths    from various   items   of   equipment   until   the   hum   ceased.   This   is   a very dangerous, potentially lethal, thing to do! An   example:-   Many   Rock   n'   Roll   performers   have   suffered shocks   or   electrical   burns   to   their   lips   due   to   the   ignorance   of the    "technician"    who    removed    the    earth    connection    from their   guitar   amp   to   stop   it   humming.   What   they   did   not   know was   that   there   is   often   a   mains   filter   consisting   mainly   of   two capacitors,    one    from    the    Live    side    of    the    mains    to    the chassis   and   the   other   from   the   Neutral   side   to   the   chassis. These   are   fairly   effective   at   preventing   spikey   interference from   entering   or   leaving   the   amplifier,   but   when   the   earth connection   is   broken   they   form   a   voltage   divider   with   the centre-tap on the chassis! In   Australia   and   the   UK   where   the   mains   voltage   is   around the   240V   mark,   this   places   about   120V   on   the   chassis   and the   capacitors   are   usually   of   such   a   value   that   roughly   1   to 10mA    of    current    can    flow    from    the    electrified,    unearthed chassis   to   anything   that   is   properly   earthed.   So   when   the performer,     holding     a     guitar     solidly     connected     to     the unearthed   amp,   touches   their   lips   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   lips!   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   (and   they   do   short   out sometimes,   by   the   way)   the   performer   could   easily   be   killed. DON'T EVER DISCONNECT A MAINS EARTH!!! Double-insulated   equipment   is   not   connected   to   the   mains earth   and   most   of   the   HiFi   and   Video   gear   intended   for   the domestic   user   is   double-insulated.   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.      So      what      can      go      wrong      with double - insulated gear? Plenty, as we shall now see. All   electrical   equipment   has   a   power   supply   which   may   be internal    or    external.    The    power    supply    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 linear   supply   or   a   switch-mode   supply,   leak   a   small   amount of   current   from   the   primary   to   the   secondary   due   to   stray capacitance.    In    double-insulated    items    there    is    no    earth available   to   use   as   a   shield   between   input   and   output,   or   to drain    the    leakage    to    ground,    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   bigger   transformer   running   at   50   or   60Hz. The   leakage   current   is   usually   less   than   1mA   and   is   often   as low   as   10uA,   but   it   is   still   present   and   can   cause   problems. Generally,   the   more   power   something   uses,   the   higher   the leakage   will   be.   Things   that   use   an   external   power   supply (also   known   as   a   wall-wart   or   plug-pack)   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   detritus
finds     it's     way     into     electrical     equipment.     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,    a    notebook    computer,    a    media    player,    a tv/monitor,   a   CD   player,   a   cassette   deck,   a   graphic   equaliser, ...you   get   the   idea.   Let’s   also   say   that   all   these   things   are double-insulated.   Now   they   are   all   connected   to   each   other either   directly   or   through   one   of   the   others.   It   would   not   be unreasonable    to    expect    0.1    to    1mA    (or    even    more)    of leakage   from   the   combined   equipment   because   the   leakage currents   simply   add   together.   If   you   were   to   measure   the voltage   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 directly, you might damage your multimeter. 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   be   quite unpleasant,   especially   when   you   don't   expect   it.   Once   the first    connection    is    made    and    the    leakage    is    flowing    to ground,   there   will   be   no   more   tingles   -   from   that    item   -   the others are still ‘floating’! 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    be recorded   or   it   might   be   visible   on   your   TV   or...      What   do   you do?   That's   a   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,    but disconnect    all    the    gear    from    the    mains    first,    otherwise    if you've   been   unlucky,   you   might   also   notice   that   that   first input you plugged into is noisy, distorted or dead. Ever    wondered    why    they    always    say    in    the    instruction manuals   that   you   should   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   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    'whatevers' happens    to    have    a    serious    electrical    fault    or    a    bit    more leakage than your other equipment can tolerate. Lastly,   a   tale   of   excess.   A   customer   of   ours   rang   one   day   to order   some   audio   and   video   patch   cables.   Nothing   unusual about   that,   but   the   next   request   had   me   spinning   out   -   they wanted   them   totally   insulated,   no   exposed   metal   anywhere. "Why?"   I   asked.   "Because   we   get   really,   really   bad   shocks from   exposed   connectors   in   our   duplicating   setup"   came   the answer.   Upon   further   investigation   I   was   informed   that   they had   no   less   than   sixty   domestic   vcr's   connected   to   a   rack   of distribution amplifiers. 'Domestic'   was   the   key   word.   "Are   they   earthed?"   I   asked. "No,   nothing   is,   we   had   to   un-earth   everything   to   try   and   get rid   of   all   the   hums".   I   declined   to   make   the   leads   as   I   had   no desire     to     be     a     party     to     the     electrocution     of     some unsuspecting    operator.    An    electrician    colleague    checked over   their   system   and   ended   up   having   to   attach   earth   straps to   all   the   vcr's   to   get   rid   of   the   considerable   leakage   of   60 vcr's.   He   also   re - attached   all   the   lifted   grounds   and   made their   system   safe.   A   few   relocated   mains   plugs   and   some isolating   transformers   for   audio   and   video   lines   (our   VIT1 ) from   other   rooms   (to   break   earth   loops)   and   the   system   was clean   and   safe.   You   really   can't   beat   the   results   of   doing things properly (and safely)! (c) 2000 Quest Electronics abn 99 064 323 255
I'm Shocked - Why Am I Getting A Tingle?