Why tester does not work on DC ?

The tester is consisting of a small lamp, resister and spring contact and screw tip. 

All the arrangements will be connected in series with each other.  When the man touches the 230 Volts Line conductor, the current flow starts from tester tip -> resister -> bulb -> spring contact-> human body -> ground. 

Due to this current flow, the tester lamp starts glowing. 
You should focus on here. where the current flow passes through the ground -> source. 

The electron reaches the source through the ground. 

Therefore, the AC system the source of the neutral should be grounded properly.

In case if you did not ground the neutral terminal, then your tester does not glow when the technician touches his tester in live part. 

Such a system work as an isolation transformer even he touches the live part without tester he does not get shock.

Coming to Direct current source, which has two terminals, considering as positive and negative. 

In this, both terminal has some potential, which means if you have 110 Volts DC supply means, the voltage across positive to ground will be +55 Volts and the voltage across the negative terminal will be -55 volts. 

Hence the DC system negative terminal is not recommended to connect with ground terminal. 

So that, DC negative or positive terminal is not connected with the ground that’s why the technician touches the DC source terminal means, the tester does not glow because of there is not path to flow of current. 

This is the main reason for tester is not glowing on DC source.

Other Fact:
A mains tester works because the potential difference between the live and Earth makes a very small (imperceptible) current flow through the tester and our body to Earth. 

This current is just enough to light the lamp / Neon. 
When you stick it in the Neutral socket, the potential difference (just a volt or two, usually) is not enough to give sufficient current to light the neon.

When the tester light glows, it means electrons flow like,from live terminal > tester > body > floor & walls of building > inside the earth>goes back to the generator, via earth or neutral conductor.

The above description is for AC supply only ( path complete thorough earth or neutral to generator neutral), however, please note the philosophy is not applicable for DC supply where - V is in floating condition (not grounded or earthed) so due to un completion of that circuit tester light will not glow.

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