[Jukebox-list] "Grounding" ?

[Jay Hennigan]

Ron Rich wrote:
> Hi All,
>   Due to the discussion of "3 wire cords", I have remembered that I have no idea how this concept works!  If someone has the answers, please inform me---
>   A "3 wire" 117VAC ( also called a "U ground" ) socket having a black (or red), a white, and a green wire is said to be "grounded". The black is said to be the "hot", the white is said to be "the neutral", an the green is said to be "ground" (in the USA--colors--er, colours, differ in Europe, and elsewhere).
>   The fact is that the "neutral" and the "ground" are ALWAYS tied together, somewhere, either at the building's power distribution box (Now required here, along with a "grounding rod" at least six feet long, pounded into the earth, and connected to the "green" wire), or by the power company at some point. To my way of thinking, this makes the green wire, in a "plug", un-necessary, except for the fact that the ground "pin" of all 3 wire plugs is longer, and in theory, will "make" contact prior to the hot contact.  
>   I become even more confused (only possible for me !), when I look at what Seeburg did from the model 100"B",  thru at least the STD model. They used non-polarized, 2 wire, line cords in the B thru the LPC-1 models. However the internal power sockets were polarized. From the LPC 480, on up, 3 wire line cords, and polarized sockets were used. However, in all cases, the power cords to the "control center(s)" were non-pols ! Some of the amplifiers in the later models, when a "line cord" was used, had polarized plugs on them??? Also, the lamp cords used in the later products, tended to be the pol type.
>   So my questions are--What good does the 3 wire cord do, since the white and green wires are tied together?  And, Why did Seeburg ( and probably others) use the non-pol 2 wire line cords for components??

The difference in terms of the National electrical Code relates to a 
groundED and a groundING conductor.  A grounded conductor is one that 
should be at ground potential, a grounding conductor is one which is 
intended to ensure that equipment is at ground potential.

The neutral is indeed tied to earth ground at one point and one point 
only within a building, usually at the main service disconnect.  There 
are obscure exceptions but this is close enough for this discussion.

Early on, little attention was paid to hot vs. neutral in terms of 
equipment wiring.  There were two wires with 110 volts between them. 
The plug connected the appliance to the line, a switch was put in series 
and things just worked.

The problem was that insulation would fail, things would get wet, and 
faults would allow exposed metal parts to come in contact with one of 
the two line wires.  A person coming in contact with one of the wires 
alone wasn't a problem.  A person coming in contact with the hot wire as 
well as a ground (water pipe, wet ground, etc.) would complete a circuit 
and be shocked.

Polarized plugs were an early attempt to introduce some degree of 
safety.  The switch could be placed in the hot side of the line so that 
when a device was off its internal wiring was connected only to the 
neutral.  Portable lamps could be wired so that the shell of the lamp 
socket was at neutral and the button at the base was hot and switched, 
but the wide neutral prong was NEVER intended to be connected to parts 
that people would normally touch while operating the device, such as the 
metal case of an electric drill.  Such parts were supposed to be 
insulated from all AC wiring.

But insulation fails, people get shocked and/or killed, and they or 
their surviving relatives sue.

Three different solutions to this problem have been implemented, each 
with its good and bad points.

"Double insulated" appliances.  Here the appliance is designed with 
extra protection so that if a primary insulation fails, a second 
insulator prevents contact with live parts.  Good but still not 100% 
foolproof.

Grounded appliances.  A third wire in the cord, green, is wired to a 
groundING conductor.  This connects to the exposed metal parts of the 
appliance.  Now if a short circuit occurs inside the appliance, the 
groundING conductor ensures that the case doesn't become energized, and 
the short circuit trips the breaker without shocking the operator.  Also 
good, but now the appliance is a ground waiting to complete the circuit 
to some other defective appliance.

Ground-fault circuit interrupters (GFCI).  This is a relatively new 
development.  They work on the idea that the current flowing out the hot 
(live, phase) wire should all go through the load and return through the 
neutral wire.  Both the live and neutral wires are passed through a 
toroid transformer core.  A secondary winding goes to a solid-state 
amplifier and trip relay.  If the line and neutral currents balance, the 
magnetic fields cancel and no voltage is applied to the amplifier.  If 
there is a ground fault such as a person touching the live wire and a 
ground OTHER THAN THE NEUTRAL such as a water pipe or another grounded 
appliance, then the current going out the hot wire will be more than the 
current returning through the neutral, a voltage appears on the 
secondary, the trip circuit is operated, and the circuit shut down.  The 
advantage is that a GFCI can be used with any appliance regardless of 
polarized plug, grounding wire, etc.

You'll see GFCIs as an outlet in kitchens and bathrooms with a TEST and 
RESET button, as well as built in to the plugs on hair dryers.  There is 
also a style that replaces a conventional breaker in a breaker box.

Getting back to jukeboxes, it is not practical to go through and 
double-insulate a jukebox made in the 1950s or before.  Adding a ground 
wire makes sense only if care is taken to ensure that the ground from 
the groundING wire (the green one) is securely connected to all of the 
exposed metal parts of the box, which can be difficult.  And, in a 
game-room situation you have now created a big grounded object right 
next to the old pinball which may not be as carefully restored and has a 
fault bringing its case to 120 volts.  So the guy who touches the 
pinball and the now-grounded juke gets shocked (and possibly blames the 
juke if he touched it last.)

A GFCI works all the time and doesn't require any modification to the 
box.  For Seeburgs that have a cord going to an outlet on the inside of 
the cabinet like an extension cord, the GFCI can be placed inside the 
cabinet, but you can also just change the outlet in the room to a GFCI 
type.

Personally, I always install a grounding 3-wire cord on amplifiers when 
I rebuild them.  In virtually all cases, the amplifier chassis is 
already bonded to the cabinet and mech, and if not it is fairly easy to 
do so.  A GFCI is always a good idea as well.  Doing both doesn't hurt 
anything.

As to your other question, the white neutral wire shouldn't be connected 
to exposed metal or anything not part of the active circuit.  Even 
though it should be groundED, it isn't used for groundING.  Treat it as 
a live wire from a safety standpoint.  The green wire should ONLY be 
connected to metal parts that are NOT part of the active circuit, and 
should be used only for groundING as a safety measure.


--
Jay Hennigan - CCIE #7880 - Network Engineering - jay at impulse.net
Impulse Internet Service  -  http://www.impulse.net/
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