What is an extra low voltage circuit?
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"

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