Date: Mon, 26 Oct 2020 08:20:15 +0100 From: Ralf Mardorf <ralf-mardorf@riseup.net> To: freebsd-questions@freebsd.org Subject: Re: What is the "better / best " method to multi-boot different OSes natively WITHOUT VirtualBox(es) ? Message-ID: <20201026082015.24287140@archlinux> In-Reply-To: <79CA7927-080F-4596-96CB-CEFB2322F062@kicp.uchicago.edu> References: <CALMiprbGBaSJQUAA=1HDZAjvsVNK7dqB_5mBb5DKzV16F3hxHg@mail.gmail.com> <20201024111010.5c867e8540a369b826d26703@sohara.org> <20201025065025.6a13dc89@archlinux> <24469.26288.47199.882303@jerusalem.litteratus.org> <20201025135437.35b592a5@archlinux> <c8cbc8c1-2e05-d57f-41de-8e393fa8fc90@holgerdanske.com> <20201025225215.65cfca64@archlinux> <79CA7927-080F-4596-96CB-CEFB2322F062@kicp.uchicago.edu>
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On Sun, 25 Oct 2020 23:02:45 -0500, Valeri Galtsev wrote:
>> On Oct 25, 2020, at 4:52 PM, Ralf Mardorf <ralf-mardorf@riseup.net>
>> wrote:
>> However, galvanic isolation is absolutely safe, unless getting in
>> contact with both conductors, which is impossible in our scenario and
>> even very unlikely when repairing gear. =20
>
>Hm, if galvanic you mean DC isolation (i.e. AC transformer), then I
>disagree. In case of AC stray capacitance (which always exists)
>conducts some of AC line voltage to "DC isolated" part of equipment.
>Only optical isolation is full isolation, but we didn=E2=80=99t learn yet =
how
>to transfer sufficient power using purely optical connection. In
>general, DC isolation, like transformer, is safe, but transformer has
>stray capacitance, and what usually saves the day that low voltage
>part kind of shunts what comes through stray capacitance by not to
>small resistance. There may be less usual situation when highly
>isolated from everything piece of equipment has big capacitive
>coupling to =E2=80=9Cground=E2=80=9D which can be charged gradually throug=
h small
>stray capacitance of transformer (rectifier of one sort or another
>will be in play, call it =E2=80=9Cstray rectifier"). Even though it falls
>under what you call =E2=80=9Cgalvanic isolation=E2=80=9D, it represents bi=
g danger for
>a human, as big capacitor can sustain significant current for long
>enough time to do harm to living being.
>
>My apologies, if I misunderstood you.
>
230 V L=C2=B0----------------=C2=B0 <- Don't get in touch with this conduc=
tor.
50 Hz
N=C2=B0----------------=C2=B0 <- Touching this conductor is ok.
230 V L=C2=B0-----+ || +-----=C2=B0 <- You can touch this conductor ...=20
50 Hz | || |
| || |
N=C2=B0-----+ || +-----=C2=B0 <- ... as well as this conductor, just=
don't
touch both at the same time.
In Germany the isolation transformer usually is an adjustable
transformer, so it's possible to troubleshoot and repair 230 V as well
as 120 V gear. Troubleshooting disconnected gear is hard to do ;).
Indeed, I've forgotten that some circuits contain big capacitors, so
discharging using a resistor is usually done (and sometimes even the
screwdriver is used, not recommended ;).
CRTs and other special gear are something else, far away from a
scenario of a hot computer metal case.
On Sun, 25 Oct 2020 22:34:36 -0400, Kevin P. Neal wrote:
>I've seen that happen. Guitar players hate it when they walk up to a
>mic, start singing, their lip touches the mic and ZAP! They really
>hate that. It looks weird, too. Ever seen a guitar player react like
>he's been punched in the face right as he starts to sing? I have.
On Sun, 25 Oct 2020 18:14:12 -0600, Bob Proulx wrote:
>Ralf Mardorf wrote:
>> >Please explain the risk(s) associate with antistatic wrist straps. =20
>>=20
>> IMO it's a risk for a human being to wear an antistatic wrist strap,
>> when getting in touch with gear that is connected to the mains. =20
>
>ESD wrist straps have a resistor in series to prevent exactly this
>problem from occurring. It's on the order of a 1 M ohm value. This
>protects the wearer in the event they touch live AC mains.
>(Meanwhile... I always preferred the heel straps myself.)
I wouldn't bet on an Alibaba anti-static wrist strap and even not on
branded products, as long as expensive smartphones authorized for
the European market, equipped with original batteries catch fire when
charging.
Audio gear sometimes provides a ground lift switch, then ground isn't
directly connected, but still connected by a capacitor (and sometimes
it's completely cut off, not recommended ;).
Good old MIDI is/was isolated by working opto-couplers. Nowadays USB
MIDI seemingly isn't isolated anymore and for classic MIDI sometimes
there's not enough power to power the opt-couplers or the signal
suffers from bad edge steepness.
However, as a musician and audio engineer I don't trust grounding.
Without doubts grounding is perfect in professional video and audio
studios and probably in a server farm, too. Anywhere else in the real
world grounding often is fishy.
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