>
>--- "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.
** And if the short is from the primary to the xfmr-case/chassis.
>
>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 same wires furnish power to 4, 120V outlets.
>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.
>
** I do not use a 4-wire system for 240v, 50A outlets. However, a
helicopter tail-rotor could undoubtedly sever #6 wire, so a 4-wire system
could add safety over a 3-wire. Modifying the NEC to require a 20-foot
separation between the routes of the neutral and ground wires would add
even more safety.
>> >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.
** don't county inspectors check color as well as for correct wiring?
>
>>
>> >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.
** I'm getting older, so maybe it would be good to color-code things.
>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.
** Nah, they're Punk and/or Rockabilly berserkos. On Melrose in L.A.,
there's a bunch of clothing stores that sell stuff for 'em that we'd cut
up a use for rags.
>
>> >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.
** #6 wire should not combust carrying a dead short through a 40A
breaker, If it starts a fire, I have a fire hydrant in front of the
house.
>
>>
>> >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.
>
** I purchased a #6, 3-wire electric range cord at the local emporium
for my next ugly amplifier project.
>>
>> >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.
** (such as a neighbor hacksawing your coax at 3am)
>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.
** That's probably because it is becoming common knowledge that
radiation efficiency of a vertical is improved by using elevated
counterpoises instead of ground rods.
>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.
>
** amen
>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.
>
** Except that the ground in the electric-mains box 99-feet away is
probably several times more resistive than my station ground in the
frequently watered flower bed outside the window. .
>>
>> ** 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.
** Neither meets current Code because both have 240v, 3-wire outlets
instead of 4-wire outlets.
>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 can. We live in a helicopter flight corridor and there's a fairly
good reason they call 'em "choppers".
>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.
** Years ago some government agency discovered that people were killing
themselves by falling asleep on sofas while smoking cigarettes -- which
isn't surprising since carbon-monoxide induces sleep and ordinary fabric
can ignite. The safety solution was to mandate the use of fire-proofing
on all upholstery material. However, when this fireproofing material is
exposed to light, it breaks down into dioxin, an agent that is both
carcenogenic and tetratogenic -- i.e., in addition to causing cancer, as
a free added bonus, it also causes DNA glitches in embryos & fetuses.
In other words, smokers are being saved at the expence of moms and babies
that come in contact with newer sofas and the like..
>
later, Dave.
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|