Checks for circuit continuity with live supply
Generally, if possible, troubleshooting is done with a disconnected power supply, but in some circumstances, faultfinding is only possible if the circuit is live.
Therefore, the circuit under testing remains connected with the power supply. This uses the circuit power supply itself as a source of energy for testing.
This kind of testing should be done with extreme care following safety precautions.
As shown in Figure 3.6, the integrity of a power supply or continuity of electric path can be checked by using test lamps. Test lamps are connected in between twophases. Thus, as with the dead circuit test, a continuity test can be performed. In addition,a lamp-type visual tester can be used for simple continuity testing. Alternatively, voltage indicators or multimeters can be used for checking voltage and the continuity of the conductors or electrical path.
While checking three-phase voltage, use two lamps connected in series and not a single lamp. Currently, most manufacturers give test voltage details for test points that helps tocheck the integrity of a particular section. Generally, equipments consisting of electronic cards follow this kind of practice.
While checking the voltage at these test points, measuring instruments must be accurate. Therefore, a comparison of voltages at these test points is sufficient to draw conclusions.
Diagnostics for a single-phase motor can be undertaken with this kind of visual indicators. This requires a sound knowledge of circuit and wiring arrangements – depending upon the test done, interpretation varies and so does an accurate fault diagnosis.
It is always advisable to check voltage between line-to-line than line-to-earth, since for the latter, the results may be misleading.
Testing on a live circuit requires extreme caution and should be restricted to LV circuits.
Precautions should be taken to prevent inadvertent contact of the technician with live parts.
The probes and tools must be insulated with minimum exposure of conducting parts. This will minimize inadvertent bridging of two terminals which are at different potentials which can cause a short-circuit and arcing leading to burn injuries to the technician.