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I have recently been troubleshooting a component on a CNC machine. The device is connected via etherCAT to the machines controller. There is also a power cable that goes to the component. Basically the component is one electronic board that controls some gas valves. I have been told that the electronic board can be “short circuited” if I removed the cables and insert them again while the power is on. Is this true-if so-why?

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    \$\begingroup\$ "Short circuit" in this case is an umbrella term for causing circuits to activate out of order or static charge injected before the ground terminals connect. So called "Hot Plug-able" systems have special connector that makes contact with system ground first. A USB connector is like that. \$\endgroup\$ – Sparky256 Apr 8 '18 at 4:21
  • \$\begingroup\$ If the Ethernet cable is a communications channel only, and is not a "Power over Ethernet" (PoE) configuration, the Ethernet jacks on the printed circuit board are typically designed with isolation transformers (for example) that provide galvanic isolation between the wires within the Ethernet cable and the circuitry on the printed circuit board, as well as surge suppression devices. For a "data only" (no power) configuration it seems difficult to imagine that simply disconnecting and reconnecting the Ethernet cable should damage the system. \$\endgroup\$ – Jim Fischer Apr 8 '18 at 4:23
  • \$\begingroup\$ The Com cable is like the one in the link. \$\endgroup\$ – user869218 Apr 8 '18 at 4:38
  • \$\begingroup\$ m.ebay.com/itm/… \$\endgroup\$ – user869218 Apr 8 '18 at 4:39
  • \$\begingroup\$ The power cable is very similar to the etherCAT. \$\endgroup\$ – user869218 Apr 8 '18 at 4:40
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CMOS logic is notorious for SCR latchup if any voltage is applied to signals before power is applied including residual charge that has the capacity to exceed the forward current rating on the two stage 5~10mA ESD protection diode clamps to each rail.

Since long cables have more capacitance (100pF/m) than the human finger tip (100pF in the IEC model of HBM), they can damage CMOS with cable charges with static from dragging on a carpet for example or any other tribolectric generation.

You ought to know how SCR's are used as DC crowbars across the supply, you don't want your logic chips to do this. (e.g. STM, Microchip PIC or ARM chip)

This is an intrinsic SCR Latch-up effect in CMOS logic.

https://en.wikipedia.org/wiki/Latch-up

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