3 phase question - Page 2

 



William wrote:


The GFI should have tripped out.  They work by sensing how much
current is flowing through the hot and neutral wires, and comparing
the difference. If the difference is large enough, they trip.  Many
poorly maintained sound systems will trip GFIs, as they are, in fact,
dangerous in various ways.  ( the most common is the so-called
death diode in older guitar amplifiers. If it failed closed, it put mains
AC power on the chassis of the amplifier, raising the entire guitar
to line voltage.  Guitarist is playing, current runs up the guitar cord,
energizing the metal of the guitar and the guitarist, then grabs the
microphone to sing or speak, which completes the circuit to
ground through the guitarist.  )
    I suspect that the sound guy either bypassed the GFI entirely,
or had a ground/nuetral swap someplace that he inadvertently
corrected by his ground/nuetral swap.   There shouldn't be a
"normal" earth leak large enough to trip a GFI in a system.  That
leakage is enough to kill someone, and needs to be repaired.

What is the name of the sound system company?   I'd like to
know who are the more dangerous ones out there.

    --Dale

 



Dale Farmer wrote:


Capacitor not diode.  *thwaps self on forehead*  Must remember to
type when fully awake.

 

Dale,
          Your description of the operation of GFI devices matches what I
have heard elsewhere.
     I have also read or heard that these devices do not depend on the
existence of a separate earth ground to operate. Because of this, they could
be used to improve the safety of outlets on older 2 wire house hold outlets.
It makes sense to me that the devices could be designed that way.
    I was suprised therefore to find that the installation instructions for
the one I purchased, was explicit in that it was only for use on circuits
that have an independent safety ground. I doesn't say so in so many words,
but there seems to be a strong implication that it won't work.

    I can see that all else being equal, a seperate ground would be better.

    The question is, do all, some, or none of these devices work on the
older 2 wire wiring?

 Steve Thomas

 



Steve Thomas wrote:


They work for certain values of work.
    They are allowed for retrofit work only without a ground connection
under the US National Electrical Code.  They do not create a ground
where one did not exist before.  So anything that actually depends on
a good ground connection, such as AV systems and surge protectors,
to name two of the most common, will not work correctly, and may
make your gear more susceptible to damage.
    They are there pretty much only for life safety purposes when you
retrofit them onto a two wire power circuit.   They will give you a
false sense of security, when you plug in a three wire power cord,
thinking that there is actually a protective electrical ground present.
    As for the manufacturers instructions, unfortunately this is one of
those things that qualified people only should be installing, but lots of
unqualified people install.  So they put in a lot of weasel words into
the instructions.  Damn lawyers and the pathetic lawsuits they bring.


    --Dale

 

GFCIs consist of line and neutral conductors passing side by side or
coaxially through the center of a toroidal transformer.  Any current
imbalance in the two conductors causes a small voltage to be produced in the
secondary and a circuit board amplifies this voltage and uses it to energize
a solenoid thus tripping the internal circuit breaker and cutting off the
power to the outlet. The circuit safety ground is not used by the device but
it should be present for added safety.