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I am designing a testing box that has as a specification supplying power safely to a product range that requires the following voltages: 24Vdc, 24Vac, 110Vac and 230Vac. I am intending to use transformers controlled by PLC's and a HMI screen.

However, I am stuck with a health and safety issue. Currently one end of the output lead could have live wire when disconnected of the product and therefore hurt the operator.

Is there any component that detects the continuity in a lead, and stops the current automatically when it has been detected that there is no path for the current to flow?

Many thanks in advance, any help would be appreciated.

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    \$\begingroup\$ The simple answer is not to have a bare wire like that. Take a look at how other higher voltage systems manage it - you can't get at the high voltage connection. Use a socket, not a plug. \$\endgroup\$
    – Majenko
    Commented Mar 9, 2015 at 15:40
  • \$\begingroup\$ I would totally agree with not having a wire like that, however the product design apparently cannot be changed and it has been going like this for years, therefore the company would rather find a solution around the testing procedure, other than change its design. Thanks for your input, I use a socket not a plug. \$\endgroup\$
    – MattC
    Commented Mar 10, 2015 at 8:37

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A classic, if complex solution to this, is to superimpose a defined signal on the source test lead. It can be a simple low level square wave, or a digital pattern. Say somewhere around 20kHz to 40kHz. On the other other wire in your continuity tester, you look for the transmitted signal, and only energize the test voltage if it is present. You can do this with a 555 timer chip and a PLL like a 4046.
Just make sure that the supervisory circuit is protected from the high voltage.

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I once had to make a continuity test (part of a machine) that was fed to 120Vac PLC input card. It also had to be ungrounded, because the tested material was grounded. I have used 2 transformers, and the continuity test was done with a low voltage.

I have actually used 120 to 3 V transformers.

I am not sure what exactly is your requirement, maybe it can give you some idea.

enter image description here

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  • \$\begingroup\$ Thanks for your input. It is a good way to carry a continuity test. \$\endgroup\$
    – MattC
    Commented Mar 10, 2015 at 8:38
  • \$\begingroup\$ My challenge is slightly different. In order to test a product I have live wires. However, due to product's design I have to switch the live wires to feed different pins on the pcb. Hence, my issue is the possibility of the operator touching the wire while it is live. One solution is to have a "mechanical enclosure", however, I have been researching whether there is a sort of resistance sensor that detects when the live wire is connected or not. Once it detects that it is not connected it will stop the current. \$\endgroup\$
    – MattC
    Commented Mar 10, 2015 at 8:45
  • \$\begingroup\$ In that case I would recommend GFCI, ground fault detector. If the operator touches it, the current goes through a different path, and it will trip. How big current will the 120VAC draw? It still probably is not allowed. \$\endgroup\$
    – sparky Al
    Commented Mar 10, 2015 at 20:18
  • \$\begingroup\$ If the PCB has a connector with pins, if there are 2 extra pins, jumper them on the PCB, and use them to energize a relay in your box to deliver voltage. the feel voltage for the relay could be 24VDC, you could have different plugs for different PCB's. \$\endgroup\$
    – sparky Al
    Commented Mar 10, 2015 at 20:26

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