27 Aug
2015
27 Aug
'15
2:27 a.m.
Paul Koning wrote:
I can't vouch for the truth of the story; I heard it a long time ago from a
fairly reliable source.
The story seems plausable but I disagree with the theory of how it could have
happened.
But consider this theory. Suppose you have two service drops, fed from
transformers off the utility high voltage line. The neutral is simply
defined by the ground rod at the transformer and at the service entry to
the building.
The neutral is defined by being connected to the star point on the transformer
secondary as well as being grounded. If it is not (at least intended to be)
connected to the star point, it's a ground, not a neutral.
It is possible to get a three phase power feed with no netural provided but
this is only suitable for use with balanced three phase loads, not a typical
building load which includes single phase lighting and power outlets.
If the building is a steel frame and all service entries are bonded to the
steel, and the steel is generally conductive, you have a single neutral.
This is a ground, not a neutral.
But if some aren't bonded, or the building isn't conductive, then you have
two separate ground references. Also, the green wire (protective ground)
is connected to the neutral at the service entry.
Ok.
Now suppose that you have unbalanced phases, which will generally be the case.
That produces a neutral current,
Ok.
which dissipates through the ground rods.
No. The neutral current returns to the transformer star point. There will only
be significant current flow through the ground rods if there is a break in the
neutral between the transformer star point and the point where it is grounded at
the service entry and the transformer star point is also separately grounded.
The only current through the ground rods should be leakage through grounded
power filters and live to ground fault current which has somehow managed to not
cause the circuit protection to operate.
If you have two services, the resulting neutral voltages will not be in phase.
Ok.
If you now tie these two neutrals (grounds) through an unplanned wire, as in
this story, the phase unbalance voltage (the difference between the two
neutrals) will produce a current that's split between that wire, and the
ground around the building, in proportion to the impedance of the two paths.
This is critical to the difference between neutrals and grounds. Neutrals can
routinely and safely carry current but grounds are not supposed to carry
significant currents except for brief periods between the occurrance of a live
to ground fault it's clearance by the operation of circuit protection.
Neutrals must be insulated from everything else including ground except where
they are grounded at the service entry. They particularly must not be
connected to exposed metalwork or signal carrying conductors as a substitute
for a ground connection.
Because grounds are not expected to have significant current flowing in them,
different points on the ground conductors will not be at significantly
different potentials and issues connecting two different grounded pieces
of equipment together with signal carrying conductors should not arise.
The wire impedance is likely to be far lower than that of the ground
(especially in New England), and the phase unbalance current in a large
building might well amount to a lot of amps.
The strap melting in this case is likely to have been due to either
inappropriate use of neutrals as grounds or grounds as neutrals or a live to
ground fault combined with excessively high impedance in the ground connection
causing a major fault to go undetected. Multiple supplies and phase imbalance
can't account for it without very serious flaws also being present.
Regards,
Peter Coghlan.