(Doug Miller) writes:

In article , Nate Nagel wrote:
Doug Miller wrote:
In article , Nate Nagel
wrote:


but the receptacles installed on the first and second floor are
grounding type and it appears that the ground is provided by a jumper at
each receptacle between the ground terminal and the neutral. I realize
that *theoretically* this is functionally identical,


No, it isn't, not even theoretically. This makes it possible for the chassis
of any piece of equipment plugged into the outlet to become electrically
live,
and it's not at all safe.

It is functionally identical, so long as the neutral isn't broken.

No, it is *not*, as I just pointed out. You seem to be unaware that the
neutral wire carries current.

That's true.

If the ground and neutral are interconnected at
the receptacle, anyone simultaneously touching the metal chassis of any
equipment plugged into that outlet, and anything else that's grounded (e.g. a
water pipe or faucet, or simply standing on a concrete floor) makes a parallel
path to ground for the current in the neutral conductor -- and enough current
can pass through that person's body to cause a significant danger.

In normal cases there are not enough current that can pass
through the person body to cause a significant danger.

Even though the is a lot of current going on the neutral
wire, the potential difference between the neutral wire and
the "real" ground is quite low. Typically in normal operation
(all wiring OK) up to few volts, transients to higher voltages.

When the voltage difference between ground and the neutral
is just few volts, not much current can flow between a person
that touches them both. The human body and skin has
considerable resistance, especially on low voltages.

The dangerous voltage differences exist and dangerous currents
can only flow when the neutral gets broken...

Wiring where grounded outlets gets only two wires
(live and combinet neutral+ground) used to be quite common
practice in several European countries (for example Finland)
for a quit long time. At some point the regulations changed
demanding separate neutral and ground wires to outlets in
most cases.

--
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at
http://www.epanorama.net/