26 Aug
2015
26 Aug
'15
1:07 p.m.
From: Paul Koning
What happened is that the "grounds" were
offset enough, and with enough
of a current supply, that the ground strap that's supposed to connect
the row of RP06 drives melted.
This sort of thing is a major electric code violation: you can certainly have
multiple services, but all the grounds are required to be connected by
substantial wire; you're not allowed to stick ground rods in at
multiple places and leave it at that.
I'm pretty blown away that the various grounds could be offset by that much,
to produce that kind of current when they were tied together. Wow.
Noel
26 Aug
26 Aug
2:01 p.m.
On Aug 26, 2015, at 4:07 PM, Noel Chiappa <jnc at
mercury.lcs.mit.edu> wrote:
I can't vouch for the truth of the story; I heard it a long time ago from a fairly
reliable source.
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. 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. 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.
Now suppose that you have unbalanced phases, which will generally be the case. That
produces a neutral current, which dissipates through the ground rods. If you have two
services, the resulting neutral voltages will not be in phase.
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. 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.
paul
From: Paul Koning
What happened is that the "grounds"
were offset enough, and with enough
of a current supply, that the ground strap that's supposed to connect
the row of RP06 drives melted.
This sort of thing is a major electric code violation: you can certainly have
multiple services, but all the grounds are required to be connected by
substantial wire; you're not allowed to stick ground rods in at
multiple places and leave it at that.
I'm pretty blown away that the various grounds could be offset by that much,
to produce that kind of current when they were tied together. Wow. 
27 Aug
27 Aug
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.
8:54 a.m.
On 08/27/2015 04:27 AM, Peter Coghlan wrote:
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.
There have been a number of crazy
systems used in the US,
generally referred to as "open delta".
One of them uses two single-phase transformers, and the
common connection between them is grounded.
This, you have two unbalanced 220 V circuits (on different
phases) with a common point. Any 3-phase motor or device is
perfectly happy with this, although one "corner" of the
three-phase triangle is grounded. A benefit is you can use
common split single-phase panels and double-pole breakers
for it. This system is called "corner grounded open delta".
Another system uses one center-tapped residential 120/240 V
transformer and one 240 transformer (or can use two split
transformers as long as the center tap of the 2nd
transformer is not used). The center tap of the split
transformer is grounded. This allows you to derive standard
120/240 V single-phase service AND 240 V 3-phase service
from only 2 transformers. But, it gets really
confusing, as
the 3-phase service is grounded at the center between two of
the 3-phase lines. You have to use 3-pole breakers on this,
as none of the 3-phase lines is at neutral/ground
potential. This system is called "center-grounded open delta".
Then, the typical 3-phase service can be 120/208 V Wye
service, with the center tap of the Wye grounded. This
gives both 120 V for office equipment and 208 for industrial
loads.
Also, there is 240 V Delta service, where loads cannot be
placed between line and neutral. These transformers
generally have a small balance transformer that is just 3
windings with one end all tied together and grounded.
Often, they have a ground fault interrupter connected to the
the balance transformer. If any load draws more than a
small current to ground, the transformer is shut down. But,
you can run unbalanced loads from one line to another.
Welders and big computer gear were often set up this way.
Jon
26 Aug
26 Aug
2:23 p.m.
On 8/26/2015 3:07 PM, Noel Chiappa wrote:
Consider, say, 10 circuits, 20A each sharing a neutrals. Suppose each
neutral as a total resistance of 0.1 ohm. If the neutrals were done
properly, the voltage drop would be 20 x 0.01, or 2 V. Share one
neutral, though, and you are up to a whopping 20V.
And if that happens, guess what: some of the current is going to
happily use the earth ground (or an FE) as the path of least resistance.
From: Paul
Koning
What happened is that the "grounds"
were offset enough, and with enough
of a current supply, that the ground strap that's supposed to connect
the row of RP06 drives melted.
This sort of thing is a major electric code violation: you can certainly have
multiple services, but all the grounds are required to be connected by
substantial wire; you're not allowed to stick ground rods in at
multiple places and leave it at that.
I'm pretty blown away that the various grounds could be offset by that much,
to produce that kind of current when they were tied together. Wow.
Noel
3181
days inactive
3182
days old
4 comments
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participants (5)
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cctalk＠beyondthepale.ie -
cube1＠charter.net -
elson＠pico-systems.com -
jnc＠mercury.lcs.mit.edu -
paulkoning＠comcast.net