Re: Hum, Ground Loops and Safety
- From: "Mike Rieves" <mriev@xxxxxxxxxxx>
- Date: Wed, 2 Jul 2008 22:47:07 -0500
"Brian Running" <brunning@xxxxxxxxxxxxxxxxx> wrote in message
news:OrOak.16051$Ri.5243@xxxxxxxxxxxxxxxxxxxxxxx
Electricity is a little bit like water, enough so that we can use water as
an analogy. If you have two reservoirs of water, one higher than the
other, and the physical block between them is removed, then the water in
the higher reservoir will flow to the lower reservoir, until they are at
equilibrium. If you have two electrical conductors that are charged with
electricity, and one has more charge than the other, then if you remove
the blockage between those two conductors, the electricity will flow from
the conductor with the higher charge into the one with the lower charge,
until they are at equilibrium. The difference between the charges in the
conductors is what we call "voltage." It's also called "potential,"
because there's potentially a current flow there, it's just waiting for a
conductor path to a place where there's a lower charge.
Imagine that you have a very, very sharp butcher's cleaver, and several
shelves that are uneven, slanted, and covered with slippery grease. If
you set the cleaver on one of the shelves, it has the potential of sliding
off, and if it hits someone on the way to the next shelf down, it could
hurt them very badly. The highest shelf has the greatest potential to
hurt someone, because if the cleaver slides off that shelf, it could fall
all the way to the floor, and by the time it hits the floor, it's going to
be moving very fast -- it could chop through a lot of things, including a
finger. You don't want that. Assuming we have to put the cleaver on one
of the shelves, what's the best way to ensure that the cleaver will not
fall and hurt someone? Put all the shelves on the floor. In that way,
there's absolutely no possibility that the cleaver will fall if it does
slide off a shelf, it just slides harmlessly right onto the floor.
There's no potential energy in the cleaver if the shelf it's on is already
down on the lowest level in the room.
Follow the analogy, now: With electricity, the lowest level in the
room -- the floor below which nothing can fall any farther -- is "ground."
If everything is at ground level, there's no potential, there's no
voltage, and no current will flow. No cleaver will fall. If any
conductor is "raised off the floor" at all, then current will flow to the
floor -- to ground -- if it's given a path to do so.
With electrical gear, the way to put all the shelves on the floor is to
connect them all with a conductor, so all the barriers to current flow are
removed, and the electricity seeks its own level. There will be no
voltage among the pieces of gear. Then, to make sure that the shelves
really are "on the floor," and there's no lower level to which the current
could go any farther, we connect the conductor that connects all the gear
to the earth. The earth is absolutely the lowest level, the basement
floor, when it comes to electrical potential. No electrical cleaver can
fall any farther than that.
Your water analogy is much better than your cleaver analogy. The water
analogy allows the use of different size "pipes" that each allow a specific
amount of water (electrical current) to flow. The higher the water tank, the
higher the static pressure (voltage), the bigger the pipe, the more water
(current) will flow through it at a given pressure. The copper wire in the
electrical cord is like a very big pipe, it allows lots of water (current)
to flow through it when a voltage is applied. A human body by comparison, is
like a very, very small pipe, as long as the big pipe is there, very little
water (current) is going to flow through it, certainly not enough to cause
damage, not even enough to feel. Current always flows through all paths from
higher to lower potential, the idea is to keep the pipe with the intentional
flow very large in comparison to the other pipes where unintentional current
is flowing. There simply isn't a comparable analogy for the cleaver. Simply
stated, voltage by itself won't hurt you, current will. You can be exposed
to a hundred thousand volts and as long as there is no different potential
for the current to flow to, you won't be harmed in the least, on the other
hand as little as 40 to 60 milliamps flowing through your heart can cause to
go into fibrillation and you'll die unless something happens to re-regulate
your heartbeat, so fifty volts or even less can be enough to kill you under
the right conditions.
Shocks resulting in serious injury or death to musicians while playing are
nearly always caused, not by equipment failure, but by faults in the AC
line, checking the AC line for faults before hooking up equipment eliminates
virtually all danger of serious injury. Since virtually all audio
electronics, especially amps, are transformer coupled to the AC line, it's
extremely rare for a fault to develop that will result in the chassis going
electrically hot with respect to the AC ground, either damaged insulation on
the AC line wire with the wire coming in physical contact with the chassis,
or a primary to secondary short in the transformer are about the only ways
for it to happen, and both are extremely rare in modern equipment. Modern AC
line transformers have built in protection that will result in the fuse or
breaker blowing in the event of an internal short. If you own an older amp,
it's a good idea to periodically check and if necessary, replace the AC line
wire because the insulation can break down or be damaged over the years. In
over forty years of using and working on amps, I've never seen or heard of a
primary to secondary transformer short resulting in a hot chassis, that
doesn't mean it has never happened, but it does mean it's extremely rare. As
far as musicians being electrocuted by their equipment, unless you're
extremely careless and mistreat your equipment, you're probably more likely
to get struck by lightning than you are to be electrocuted by your
equipment.
Be safe!
If you really want to be safe, check the AC outlets for faults before
hooking up the equipment, carry a non-conductive mat to stand on while
playing, and never, ever, ever touch a mic and your strings or any other
metal on your bass at the same time. Even then you could be struck by
lightning while playing...
.
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