Date: Sun, 25 Oct 2020 23:02:45 -0500 From: Valeri Galtsev <galtsev@kicp.uchicago.edu> To: Ralf Mardorf <ralf-mardorf@riseup.net> Cc: freebsd-questions@freebsd.org Subject: Re: What is the "better / best " method to multi-boot different OSes natively WITHOUT VirtualBox(es) ? Message-ID: <79CA7927-080F-4596-96CB-CEFB2322F062@kicp.uchicago.edu> In-Reply-To: <20201025225215.65cfca64@archlinux> 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>
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> On Oct 25, 2020, at 4:52 PM, Ralf Mardorf <ralf-mardorf@riseup.net> = wrote: >=20 > On Sun, 25 Oct 2020 11:39:48 -0700, David Christensen wrote: >> On 2020-10-25 05:54, Ralf Mardorf wrote: >> Please explain how galvanic isolation relates to electrostatic >> discharge mitigation. >=20 > That's a misunderstanding. >=20 >> Please explain the risk(s) associate with antistatic wrist straps. >=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 > IMO if the gear isn't directly connected to the mains, because an > isolating transformer grants galvanic isolation, an antistatic wrist > strap isn't a risk for a human being. >=20 Well, anti-static strap is used when one is working on fully = disconnected from power equipment on the bench which is metal or = slightly conducting mat which is connected to the =E2=80=9Cground=E2=80=9D= which is specially arranged thing I will not go into discussion about. = In this setup there is no danger to person using this strap. Several = people disregard using strap, and single conducting mat. However, they = have set of habits (often from their ham times - radio amateur that is). = These habits will be: putting static sensitive boards on anti-static = bags, and touching bag before picking up the board, then touching metal = chassis before installing board=E2=80=A6 works the same way as strap. > An antistatic wrist strap protects the gear against static charge, but > might be risky for a human regarding potential equalization, assuming > something should be fishy, such as a hot metal case. Once the human > touches the hot metal case a fault current breaker should detect that > something is missing, but galvanic isolation is way more secure than a > breaker that probably is missing that something is missing. >=20 If anything is plugged to AC, there always is stray capacitance, and = through that capacitance the voltage of hot lead can be on some pieces = inside device. Of course, stray capacitance is small, acts as big value = resistor, however =E2=80=9Cdischarging=E2=80=9D even that small resistor = through some high impedance pin of (static sensitive) board can be fatal = for some IC (integrated circuit) on the board. Can be safe but not = always is for a human accidentally touching that spot. Incidentally, I = did myself several times stupid thing: touched 220V (AC) hot lead and = got shock. Neither led to gross consequences, but it could=E2=80=A6 I = was very young then. So, we do our best to work inside equipment when it is fully = disconnected from AC. Pull out all AC cords! > Actually the fault current breaker should already do it's job, when = the > metal case is hot, already before a human does touch it. If it = doesn't, > then touching the metal case might be dangerous with or without > wearing an antistatic wrist strap. >=20 > 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 through 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 = big 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. Valeri > _______________________________________________ > freebsd-questions@freebsd.org mailing list > https://lists.freebsd.org/mailman/listinfo/freebsd-questions > To unsubscribe, send any mail to = "freebsd-questions-unsubscribe@freebsd.org"
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