Right. The GFI dectects only an imbalance in neutral and hot currents
on the device it's protecting.
This means that EVERYTHING on the stage area needs to be on a GFI, and
that includes backline. If an amp is popping the GFI, you need to fix it.
Now, I know that outdoor festivals it's very common to defeat GFIs in
order to deal with backline equipment that has ground fault issues. I
don't have a real problem with this, IF the power system grounds are
good enough and the crew is watching out.
Note that he was electrocuted by a circuit which BY LAW needs to be on
a GFI, and has been for more than a decade now.
See, in a perfect world, there are no electrical leakage issues. But
the ground is there to protect you when things go wrong. And the GFI is
there to protect you when the ground goes wrong. It's a belt-and-suspenders
thing.
Maybe you'll never need the ground to be there. Probably you'll never
need the GFI. But if you SHOULD need it, you'll be really glad you have it.
Starting in the 60's I've enjoyed playing guitar and singing in
various garage bands. Nothing serious, loads of fun and we even made
a bit of money at it. I know this situation well: playing guitar
with the fingers of your left (fretting) hand wrapped around the neck
and strings and leaning into the mike to belt out the
chorus...Suddenly your lips brush against the metal wind screen of the
mike and you feel the jolt which hasn't killed you but now tells you
that this is going to be one of those "electrical evenings" with one
more thing to worry about other than harmonies, lyrics and timing!
Over the decades, I've noticed this phenomenon becoming much less
frequent and today it's virtually a non-issue. This of course is due
to the fact that nowadays everything is grounded. Even vintage gear
can be made safe such that no performer or tech should be in the
position of becoming the return path for two conductors that are at
significantly different potentials.
But what was the cause of this event? It was actually fairly common.
Every musician who played amplified instrument in the 60's and 70's
experienced it and very few of the amplifiers involved were actually
faulty.
I refer you to the schematic of a Fender Bassman model AA864
http://www.freeinfosociety.com/electronics/schemview.php?id=859 This
model is an example of the type of amplifier used in the 60's for
everything from bass to organ to guitar and yes, vocals. It was
common practice (although not ideal) to run all instruments and mikes
off 2 to 4 instrument amplifiers all of which had non-grounded and non-
polarized power plugs. Even when you graduated to a real PA system
the amplifier's design looked basically the same. Note the primary
(mains) circuit of the power supply. No earth ground, no polarization
for the plug or accessory socket.
Note the .047mfd 600 vdc cap attached to the "ground switch". This
allowed you to switch the chassis ground through the capacitor to
either the hot or neutral side of the mains. Other amplifiers lacked
the switch but still had a cap of similar value hard wired to one side
of the mains. In practice, you flipped the switch or unplugged and
reversed the "polarity" of the the power cord to the position which
gave you the least hum. It is ironic that the "least hum" position
was sometimes the one where the cap was connected to the hot side.
For this reason or simply because the band couldn't care less about a
bit of hum during the quiet passages their was a good chance that one
or more chassis ground was actually connected through a cap to the hot
side.
Now the reactance of a .047 ufd capacitor at 60 Hz is 56.4 Kohm. If
shunted across 117 vac the resulting "leakage current" is 2.074 ma
definitely enough to feel as a shock but probably not enough to kill
you unless you had other medical issues. In fact it would not trip
most GFCIs. Unless you had very sweaty hands and very moist lips, the
actual current flowing from one "hot" amp through your body to another
"cold" amp would be considerably less especially when the second
series connected cap is taken into account. This is probably why most
of the aforementioned musicians of the 60's are still alive though
perhaps a bit impaired.
So what was the problem? Well consider what the consumer-grade
capacitors of that area were made of. Was it mylar or polypropylene
or polystyrene? Did Jimi Hendrix use a capo onstage? Well the
typical cap of that value and voltage rating was a paper/foil/wax
device. If you've ever taken one of these apart (which is easily
done), you'll appreciate how low-tech they were and how flimsy was the
paper barrier which could short reducing the reactance to ZERO. If
you were unlucky enough to be standing on wet ground or lying in a
bathtub or brushed up against a genuinely grounded PA system you could
be in for a really bad day!
Fortunately what I've described above was a rare event but I sure am
happy to use properly grounded gear so that I can concentrate on
harmonies, lyrics and timing!
David
