I'm wiring up an electrical box that has several sensors, relays and a PLC. The PLC and relays are powered with a 24Vdc power supply. The sensors, which provide an analog input to the PLC, are powered by a separate 24Vdc power supply.

In wiring all this up, I've inadvertently connected the negative terminals of the two DC power supplied together. The positive terminals are still separate. I've traced all the wires back and cannot find a source of the connection.

Is it bad to leave the negative terminals connected? Should I find the connection and remove it?



1 Answer 1


Typically, you want the grounds (what you are referring to as the negative terminals) connected together anyway. In fact, it can often be dangerous not to.

The connection between the two may be inside the PLC.

  • \$\begingroup\$ Each power supply has its own Earth Ground terminal which I've wired via bare copper wire to the ground terminal from a wall outlet. Do you still suggest I wire the negative terminals of both DC power supplies together? \$\endgroup\$
    – Dan R
    Jun 26, 2019 at 21:09
  • 1
    \$\begingroup\$ Use your multimeter to see if there is an internal connection between the PSU negative and the earth wire (while disconnected from the mains). \$\endgroup\$
    – Transistor
    Jun 26, 2019 at 21:14
  • \$\begingroup\$ I'm using ambiguous terms (which are common here hehe). I didn't mean earth ground, but 'common'. \$\endgroup\$ Jun 26, 2019 at 21:17
  • \$\begingroup\$ I checked the internal connection between the PSU negative and the earth wire. No connection \$\endgroup\$
    – Dan R
    Jun 26, 2019 at 21:40

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