Why tester does not work on DC

Why tester does not work on DC:

Tester is a device which is used to check the conductor weather it is in live or not in AC low voltage power system. You can see the electrician they always keep it in their packet. Tester is working under principle of leakage current. In this, article we are going to see why tester is not glowing on DC source.

Working principle of Tester:

Tester is consisting of a small lamp, resister and spring contact and screw tip. All the arrangement will be connected in series with each other. Refer the diagram. 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 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 isolation transformer even he touches the live part without tester he does not get shock.

Why Tester do not glow on DC supply:

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.

Key Points on Tester is not glowing on DC source:

  • DC do not have current return path when the man touches the DC terminal. Example. Without battery charger
  • But in Some case i.e battery charger is connected with the battery unit, at that time if the man touches then, he will get shock. At that time the current flow will be DC terminal -> ground -> battery charger source.

Advice to Purchase Tester:

  • Check The voltage rating of the tester
  • Check the turning tips quality

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