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At the time of my plc system inspection i found that my smps input is correctly connected with 230v ac supply but when i check the output + and - voltage with respect to ground i found in both condition the voltage was zero but when i measure in between + and - it was +24 .

And when its output connected to my plc system, it is found that the output voltge + with respect to gound is zero and - with respect to ground is -24 volt, how is this possible amd my system is healthy.

Can you explain it how is this possible??????

Chandu Dhakar
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    Physics has been broken. – Long Pham Jun 03 '18 at 02:44
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    it simply means that you connect power supply to your system with + output to the ground. – Maple Jun 03 '18 at 04:32
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    Have a look at my answer to [Actual electric potential at terminals of a battery](https://electronics.stackexchange.com/questions/355695/actual-electric-potential-at-terminals-of-battery/355706#355706) to see if that helps. – Transistor Jun 03 '18 at 11:51
  • are you asking how it is possible that a negative terminal of a 24V power supply is 24V lower than the positive terminal? .... how do you get to work on PLC systems and yet ask such a question? – jsotola Jun 03 '18 at 19:37

2 Answers2

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Measuring the terminals with respect to ground is meaningless, especially for an isolated supply. If there is a transformer in your supply, and it's isolated, measuring to ground won't mean anything becasue there is a very large resistance between ground and the terminals. In this manner you can make the supply be +24V or -24V depending on how the terminals are connected. You could even take two isolated supplies and make a 48V supply by connecting them in series.

See that pink bar dividing both sides? that isolation is the source of the problem of measuring the voltage across both sides of the SMPS.

enter image description here
Source: http://www.learnabout-electronics.org/PSU/psu34.php

Voltage Spike
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What if you got a 9 volt battery and measured the negative terminal with respect to your house/building earth - what voltage you expect to see? Repeat for the positive terminal and what would you see? In all cases there remains 9 volts across the battery.

Andy aka
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