Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: About plugging things.



At 18:08 8/20/00 -0700, NR wrote:
Why is there a ground in electric appliances. Why not just having a
positive and a negative wire. I would also like to know if both the black
and red wire are above ground or if one or both are below. Thanks in
advance to anyone who will answer.


It is not hard to accept that an appliance ground wire is intended
to be protective. In US usage, that (usually) means protection from
fires, and in Europe it means protection from electrocution.

It is instructive to examine the intended mechanism.
Wires can fail open or fail by shorting to some other metallic
surface.

If the live wire fails open, the device is dead and the power switch
may be left on, so the device may be put in damp conditions because
it is thought to be disconnected from line power, which is unhealthy.
This is not a protected scenario.

If the neutral wire fails open, the device may also be left in damp
conditions, which is unhealthy.
This is not a protected scenario.

If the ground wire fails open, there is no visible effect, but
protected scenarios are now unprotected.

If the live wire fails short, it can short to the neutral or to the
ground path. In either case there is an electrical flash, and often
a report, but a fuse blows so the overcurrent is interrupted.
However users in conditional balance may be startled off a ladder
etc., with unfortunate results. This is the 'protected' scenario.

In efforts to improve the statistics of the protection, some national
authorities now specify a fuse per appliance arrangement, in the
appliance, or in the plug. This allows the minimal size fuse to be
specified for a particular appliance. Low current fuses of 1 to 5 amps
are often benign in failing.

There is another mode of protecting the user. it relies on
1) insulating all user accessible parts by two shields from
shorting live wires
2) making user accessible parts from wood, plastic, ceramic etc.,
which will not support a lethal current.
3) restricting the size of apertures and making them daedelian
in order to prevent the curious, innocent and young from probing
with a conductive rod of reasonable size, such as a pencil, nail,
finger etc.
These appliances may dispense with a ground wire.

These means offer reasonable protection, though it must be said
that the US has possibly the worst domestic electrical fire
rate of the developed countries. US residents should undoubtedly
find solace in the comparitive rarity of US domestic electrocutions.

(A part of this fire hazard is traceable to the US propensity
for rapid house building techniques on strong wooden frames,
with plywood roof skins, sometimes covered with wood shingles.
Bottle rockets are outlawed, but twist together wiring
techniques are permitted by code.)

Workers who are obliged to work in severe conditions like
wet, muddy foundations etc., are usually provided with isolated
supplies: either a 1:1 isolation transformer, or a step down
transformer and low voltage devices.

brian whatcott <inet@intellisys.net> Altus OK
Eureka!