--- "R. Measures" <r@somis.org> wrote:
>
(snip
> The SB-220 chassis needed to find its
> way to the
> ground/neutral terminal strip on the breaker box,
> and I saw to it.
The function needed by the SB-220 is ground only, it
does not need neutral, as causes no currents to flow
in that wire under normal operating conditions.
> Sure, if a HV primary winding dead-shorts on the
> 220, there will be c.
> 55Vrms between the chassis and ground for as long as
> it takes to open the
> circuit breaker,
No, there won't. The center tap of the transformer,
when wired for 220V, does not connect anywhere except
to the amplifier's fan. Assuming that you did not add
a connection from the primary center tap to chassis,
then your present use of the third wire is "ground",
not neutral. So this risk is non-existent.
That's a good thing, too. Let's say you HAD tied the
transformer primary center-tap to the chassis
(academic note to the readers: having a direct
connetion from anything on the primary side to chassis
is exactly the condition that the NEC seeks to avoid
by treating ground and neutral separately), and you
DID have that primary short. That 55V shows up on the
amplifier chassis. And it has a lot of current
capability behind it - enough to blow the breaker in
your box, after some time delay (seconds, if the
overload isn't 10X or so). Presumably, your amplifier
chassis is connected to the other equipment in your
shack by way of a nice, fat braid on the coaxial cable
feeding the input to the amplifier. That goes to your
HF rig, whose chassis is interconnected to other
items.
While the bulk of the current will, hopefully, pass
through the #6 wire to the breaker box, according to a
law attributed to Kirchoff, it will be distributed
among the various paths that it finds. I have
repaired equipment with blown ground foils on PC
boards that could only have happened if the
equipment's chassis/board grounding system were used
to conduct substantially more current than it was
designed for. At most risk seem to be low power
accessories.
> >
> >Rich, it sounds as if you are simply fortunate.
> You
> >have a 240VAC outlet whose neutral is not shared
> with
> >other circuits.
>
> ** The neutral in the 240V outlet for the amplifier
> is shared with every
> 120V and every 240V circuit in the house because All
> of the neutrals
> connect to the neutral/ground terminal strip in the
> breaker-box. If I am
> fortunate, it is that I know Ohm's law good enough
> to get by.
I am guilty of incorrectly phrasing my statement. The
neutral wire from your amp's 240VAC outlet carries
current only from that outlet - no other outlets. On
120VAC circuits, the daisy-chaining of wiring from one
outlet to another means that the neutral wire from the
breaker box carries return current for all the
outlets. In that case, if you were to connect the
first outlet's neutral to equipment chassis, any
voltage developed between neutral and ground due to
other equipment on the same circuit, would show up
between that equipment's chassis, and any exposed
infrastructure metal (which, by today's NEC is
required to be electrically connected to ground
through a conductor which carries no normal current).
By the way, you pointed something out which is worth
mentioning. I can describe fault conditions which
would cause hazards depending on the various
conditions we discuss. One can always point out how
unlikely that is - but when discussing safety
preventives, we are ALWAYS discussing low-probability
items. The goal of design for safety isn't to make
something safe in "normal" operation - we presume
normal operation is safe. Rather, the goal of design
for safety is to consider various single-point
failures, and to ensure that safety is not
compromised. Connecting electrical neutral to
chassis does not guarantee that a hazard will exist.
However, it does increase the number of scenarios
under which someone could be injured. Since it is low
cost and low time expense to not tie neutral to
chasiss, it seems prudent to follow that standard.
>
> >Since the third wire goes back to the
> >box, and at the box it is connected to both ground
> and
> >neutral, then it is simply a matter of naming
> whether
> >you call it "neutral" or "ground".
> >
> ** even if it is the "wrong" color.
Yes, that is my point. The color does not define it
as ground. Your usage of it does.
>
> >If, in fact, you have it connected to chassis, and
> if,
> >in fact, you are not using it for return current,
> then
> >whatever name you have chosen to call it, you are
> >functionally using it in the NEC definition of
> >"ground".
>
> ** true, although it is the wrong color. Maybe I
> should pick up some
> red, green, white and black fingernail polish at
> K-Mart? I know black
> fingernail polish sounds weird, but I know it is
> available somewhere
> because I see teenbabe freaks wearing the stuff,
> along with
> color-coordinted black lipstick and purple spiked
> hair.
> >
That would depend on your purpose of coloring the
wire. If adding some color would in some way improve
your own usage, then go for it. The NEC won't
consider it any more conforming, however.
I think the spiked hair comes from the teenbabes using
hair dryers in apartments with 1920s wiring that
wasn't intended to support them.
> >If the Heathkit power cord has a green wire, and
> the
> >"third" wire in your outlet is green,
>
> ** The third wire is bare #12 Cu.
That's acceptable to NEC as a ground, given
appropriate conditions, which I'm not familar with.
As I rewire my own house, I'm using MC cable, which
uses an aluminum sheath surrounding any number of
insulated THHN style wires. Ground is green in this
case.
>
> > and you are not
> >sending return current down it, then you have it
> wired
> >as ground - not neutral.
> >
> >If you are sending return current down that wire,
> AND
> >you have it connected to chassis, then you are
> using
> >it simultaneously for ground and neutral.
> >
> __ Which is what I do on the 240v outlet for the
> tetrodes-with-handles
> mains outlet. // The imbalance current is c.
> 85mA-avg in the
> neutral/ground because I utilize one side of the
> mains-neutral potential
> to power a half-wave rectified, C-filtered 160VDC
> supply for operating
> the high-speed T/R relays. Although there are some
> who would be alarmed
> about the potential-drop in 99' of #6 wire at 85mA,
> I am not one of them.
It is not the normal operating current that is of
concern. It is the current during fault conditions
which is of concern. The reason we concern ourselves
with whether return currents go down the wire is
because that is an indicator of how the wire is used.
That current goes much higher during faults.
>
> >The defining item is what kinds of currents do you
> use
> >the wire to carry. If you use the wire to carry
> >normal return currents, then your usage of the wire
> is
> >as a "neutral". If you use the wire to carry fault
> >currents, then you are using it as a "ground". If
> you
> >are using it to carry both, then you are using it
> as
> >both neutral and ground, which is possible within
> the
> >realm of physics, but there are more fault
> conditions
> >that will result in a hazard, than if you used it
> >differently.
> >
> >So, being a bit repetitive - it is a neutral or
> ground
> >only depending on your usage of the connection.
>
> ** Both, but within reason in my opinion because
> 85mA in 99' feet of #6
> is no biggie. Sure, I could install a 50w isolation
> transformer, but
> would it buy me anything signicficant?
> - So if is it OK according to Code for a neutral
> wire to carry current
> in a 120v outlet, why it is apparently a Code no-no
> for the neutral wire
> in a 3-wire 240V outlet to carry current?
You have incorrectly stated the code. It is NOT a
no-no for the neutral wire in a 3-wire 240V outlet to
carry normal (return) current. However, if you do
have a 3-wire 240V outlet with a neutral, then you do
NOT have a ground in that outlet, and must seek one
elsewhere, to be in conformance with code. I do not
know if a 3-wire 240V outlet with hot-neutral-hot is
even allowed under code today, although it probably
has been in the past.
>
> >The color (green for ground, white for neutral) is
> only
> >the NEC's ruling on what color should represent
> what.
> >A green wire is not ground - a wire that will carry
> >fault current is ground. It might be green (thus
> >meeting NEC code) or it might be white (thus
> violating
> >NEC code) but it is still ground.
>
> ** During a lightning strike, green, black, red,
> black and white carry
> fault current.
Irrelevant to NEC. There are two fault currents of
concern. The first is a short-circuit fault which
does not involve a short to "ground". This is the
point regarding how many fault conditions can be
considered. Here are the possibilities:
1) Per NEC code, "ground" and "neutral" are treated
as separate conductors. This will be true if, inside
your equipment, you have assured that "neutral" is not
connected to the equipment chassis, "ground" does not
connect to the primary wiring of the equipment, and
"ground" is connected to equipment chassis.
When wiring is executed in this manner, a
short-circuit (insulation breakdown) from the
transformer primary to the chassis will cause a
voltage to appear on the chassis, until the breaker
trips. During the time interval from fault to trip,
it doesn't take much of this voltage to cause currents
to flow through the chassis of your various equipment
that could damage the equipment. Other possible
scenarious involve a person standing outside, in
contact with a supposedly grounded portion of your
antenna system. There are a LOT of ground-plane
verticals mounted on poles, without a stiff selection
of ground rods driven into the soil adjacent to them.
I'm sure you do not have one, though, so this is not a
risk to your installation. During the
primary-to-chassis fault, using the voltage-divider
relationship, we would see roughly 60 volts from that
antenna "ground" to the soil. Garden soil is
intentionally kept moist, and few gardeners worry
about wet shoes.
The more common faults, which we discuss frequently on
AMPS, involve components which fail without shorting
primary to chassis.
A short-circuited rectifier diode, for instance will
cause high currents in the hot lead. If the primary
transformers use a combination of hot and neutral,
then there may also be high currents in the neutral,
and this is perfectly legal per NEC code. Let's say
you have a power transformer connected across the 120V
line (for example, for bias and T/R switching) and
somehow the wiring to that transformer shorts together
- but not to chassis. Now, per the voltage-divider
relationship, "neutral" will rise to 60 volts.
However, the chassis will not. You have no risk of
excess currents flowing through other gear, and the
gardener won't notice a thing if she's leaning against
the pole supporting the ground plane antenna.
An arc from HV to chassis will NOT cause currents to
flow down the neutral wire. All of that fault current
flows internally to the amplifier, and it causes a
high current to flow in the transformer primary. That
high transformer primary will flow in the "hot" lead
and possibly the "neutral" lead, but not the "ground"
lead, which is tied to chassis. Chassis remains cold.
Ditto for any other fault in the amplifier which will
cause high currents in the primary side. As long as
"neutral" and "ground" are not tied together inside
the amplifier, the chassis of the amplifier will
remain cold, unless the fault is a short-circuit from
transformer primary to chassis.
2) Now, in this not-to-code scenario, we connect
"ground" and "neutral" together inside the amplifier.
The scenario in which the transformer primary shorts
to chassis poses exactly the same problem as before.
However, we have now created a situation in which ALL
faults which cause high primary currents will cause
the chassis to become energized, to the voltage rise
in the neutral wire.
And that's the main point behind keeping them
separate. With ground and neutral separate, only a
primary-to-chassis fault can energize the chassis.
And, if you've ever looked at the UL standards for
transformer construction, you'll realize that a LOT of
things have to fail in order for this fault to happen
inside the transformer. Therefore, this particular
fault condition, by design, is less likely than almost
any other high-current fault an amplifier can create.
If you tie neutral and ground together, you have
created a condition in which ANY high current event
inside the amplifier can energize the chassis.
>
> ** If the county ever sends an official
> electrical inspector around, I
> will tag along with the four bottles of fingernail
> polish.
>
The inspector could do nothing. Your bare wire meets
the NEC code for a ground wire. Your county inspector
can say nothing about how your amplifiers happen to
use the wire. If I am interpreting you accurately,
your SB-220 is grounded "properly", but your
tetrode-with-handles amplifier isn't. The added risk
is realized if your T/R control transformer has a
primary short circuit, or if the wiring feeding it
suffers a short circuit, or if the rectifier on the
far side shorts, or if somehow HV makes it to the
relay wiring (I can't envision that scenario,
though...). I'm sure you have that line separately
fused inside the amplifier, but during the time
between fault occurrence and the fuse blowing, the
chassis of the amplifier will be energized, and
potentially high currents will flow through other gear
in the shack.
There are many ways to wire 240VAC into an amplifier.
You can be code legal, and you can be safe, and you
can be both. The point behind NEC is that if all
primary wiring follows the NEC "rules", then there are
a lot fewer fault conditions which can cause a hazard.
It's all about improving the statistical
probabilities. We can never make the probabilities
zero, but I sure like the idea of having a system
where there's only one potential fault condition that
can present a hazard, instead of almost every fault
condition that is the subject of debates on AMPs.
73,
Dave W8NF
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