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I am a novice in power electronics. I don't know when we can call a circuit an extra-low-voltage circuit. for example a circuit in which there's a power supply that converts 240V AC to 24V DC and other components working with 24V DC, is it an extra-low-voltage circuit? the input voltage of the power supply is higher than the extra-low-voltage range but the output of power supply is in the range of extra-low-voltage. Is this circuit considered as an extra-low-voltage one?

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It really comes down to your point of reference. For an individual who works with national grid potentials 100V would appear "extra low". Likewise a high speed digital engineer working with 1.5V would consider 28V to be high voltage

From a legal point of view, at least in Europe, there is the "Low voltage directive" 2014/35/EU

Low voltage is defined as 50Vac --> 1000Vac ( 75Vdc --> 1500Vdc).

Below the 50Vac,75Vdc level is classed as "Extra Low Voltage"

Above 1000Vac, 1500Vdc is classed as "High Voltage"

$$\begin{array}{|c|c|} \hline IEC\ voltage\ range & AC & DC & Defining\ Risk \\\hline High\ Voltage & >1000V_{rms} & > 1500V & electrical\ arcing \\\hline Low\ Voltage & 50-1000V_{rms} & 75 - 1500V & electrical\ shock \\\hline Extra-low\ voltage & <50V_{rms} & < 75V & low\ risk \\\hline \end{array}$$

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  • \$\begingroup\$ Thank you JonRB. so when you call a system an Extra-low-voltage one, to which voltage in your system it refers? to the mains? or the power supply output? which one must be at Extra-Low-Voltage range? \$\endgroup\$ – MINA ASHTIANI Jul 27 '16 at 8:10
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    \$\begingroup\$ As long as both the input and the output are lower than 50Vrms it can be classed as an extra-low voltage piece of equipment. if the input is at 240V and the output < 50Vrms then it is a low-voltage i/p, extra-low voltage output. If the output is variable then the maximum it can goto defines its range (ie a 0-300V output variable supply set at 10V is still a low voltage supply NOT an extra-low voltage) \$\endgroup\$ – JonRB Jul 27 '16 at 8:19

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