But it does give you room for error that a nonisolated system doesn't. In a correctly wired system optical isolation isn't necessary. You can not economically protect for every possible fault, but it is wise to do it for probable ones. There was no way we could isolate the LVDT except at greater expense than the damage done, nor in this case should we have thought it necessary. The LVDT was destroyed, the LVDT cable was burned up because it carried a substantial portion of the welding current, and LVDT signal conditioning circuitry was destroyed. In other words the column holding the verifier was bolted to the main machine and not welded to it.Ī welder put his ground wire on the main machine instead of on the part he was welding and did some welding on the post that held the verifier. The physical mounting was not directly attached via a very low resistance joint to the machine. At a Chrysler axle plant on a pinion shim selection machine we had an LVDT on a verifier in a convenient location for the operator. A shorter cable does not necessarily protect you.Ī different type of situation. We were lucky and the damage did not propogate further than the interface chips. A lightning strike close by destroyed all the RS232 chips at both ends of the link. Everything was on the same circuit breaker box. Years ago we had a Calcomp plotter connected to a computer with about 50 ft of cable, no isolation. A steel structure building with a "good" grounding point outside the building, and effective filtering on conductive circuits coming into to the building can be extremely effective as discussed in the above web site. If we can keep lightning current outside our home or building, then we do not have much to worry about, even though the building might rise 100,000 volts above ground potential from a lightning strike. On lightning - a reference that I listed on my NOISE and GROUNDING page - has some very interesting information. In particular the section on "Why is a DIRECT RS232 to RS232 connection risky?" See my discussion on my web page on NOISE and GROUNDING at. Run the experiment and nothing is damaged and data is successfully transferred while the fault voltage is present. Third introduce optical or other non-conductive isolation on all wires between the source and destination. IC buffers would offer no protection and thus be of no value. Interconnect wires burn out and also all sorts of other devices are destroyed. Next from the source make a direct connection, meaning no electrical isolation, from a serial or parallel port to the destination. There is no or negligible current flow from the hot wire and nothing is damaged. Next take a hot 120 wire and connect it to the chassis of the source. The 3 phase 240 is of no importance for this experiment, just the neutral and ground are important. This would be standard procedure at a CNC machine, but more likely the source is 3 phase 240. But neutral must not be connected to the safety ground at the destination. The 3 phase 240 is of no importance for this experiment, just the neutral and ground are important.(end of incorrect paragraph - pminmo in a later post indicated my error)Ĭonnect the chassis of the destination to the 120 V electrical safety ground wire, the safety ground wire is connected to the neutral at the main circuit breaker box. (Badly written paragraph) Connect the chassis of the destination to the 120 V AC neutral and its electrical safety ground wire. (edit 060215-2047 for the purpose of this experiment this paragraph was correct, but for other purposes wrong and could lead someone to incorrectly wire their nuetral) The destination also has its electronic common connected to its chassis. The common of the internal circuitry is connected to the enclosure. Consider a source, possibly a computer, battery powered in a totally enclosed conductive box.
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