Exploring Isolation Transformer Testing

Testing procedures, including type and routine tests, are used to check the specifications and performance of a transformer. Transformers are delivered after passing both type testing and standard inspections by the manufacturer. The basics of isolation transformers and the procedures for testing them on your own are covered in this article.

Isolation Transformers

A stationary device that physically and electrically divides circuits is called an isolation transformer. It has isolated main and secondary windings. It uses a magnetic induction technique to convey electrical energy, ensuring equal voltage and current capabilities. Eliminating voltage spikes in supply lines is its primary function in order to avoid service interruptions and equipment damage. Additionally, it eliminates noise effects and ground loop interference caused by secondary side or equipment grounding.

Maintenance and Testing of Isolation Transformers

A power transformer is an important piece of electrical substation equipment that requires preventative maintenance on a regular basis to maintain peak performance and a long functional life. Transformer features are inspected and tested as part of routine maintenance operations.

Regular preventative maintenance and emergency or breakdown maintenance are the two types of transformer maintenance. The likelihood of emergency maintenance is reduced by routine maintenance. 

The cleanliness, winding temperature, oil temperature, and load hours of a transformer have to be checked daily for maintenance. Oil must be added to the transformer and the tank housing must be checked for oil leaks if the MOG’s oil level is insufficient.

Check the oil level in the oil cap underneath the silica gel breather once a month, and top it off if necessary. Another essential step is a weekly inspection and cleaning of the silica gel breather breathing holes. 

To ensure optimal operation, the cooling system’s oil pumps, air fans, and control circuits must be tested annually. The transformer’s bushings should be cleaned using gentle cotton cloths, and while being cleaned, the bushings should be checked for cracks.

Type of Transformer Testing

Temperature Rise Test
This test determines the transformer’s temperature rise  at full load. This testing of isolation transformers for industries guarantees that the transformer can operate at its rated load without overheating.

Winding Resistance Test
To determine I2R losses and winding temperature, transformer winding resistance testing is necessary. It is carried out as a type and standard test to make sure that the transformers are in good condition and to look for loose connections, damaged conductor strands, excessive contact resistance in tap changers, and high voltage leads and bushings. The current-voltage, bridge, Kelvin bridge, and automatic winding resistance measuring kits are some of the several techniques.

Hipot (High-Potential) Test
An insulation test known as the “dielectric withstand test,” or “hipot test,” determines if a transformer can sustain high or unstable voltage without rupturing. The test checks to see whether any insulation fails while it is running. Other tests for examining the insulation of a transformer include partial discharge, switching impulse, and lightning impulse tests.

Ratio Test
Transformer performance is influenced by the turns or voltage ratio that it has. In order to guarantee the proper functioning of electrical power transformers, the transformer ratio test is an important test. Apply a three-phase 415 V supply to the HV winding while leaving the LV winding open to conduct the test. To ascertain the precise voltage ratio, measure the induced voltages at the HV and LV terminals. Test each tap point separately, then repeat the procedure.

Insulation Resistance Test
The effectiveness of the insulation system in a power transformer must be checked through an insulation resistance test. It is one of the suitable testing of isolation transformers for corporate offices as it is not required to do the test on each phase in three-phase transformers. 

Vector Group Test
For a three-phase transformer to operate in parallel, proper vector grouping requires a vector group test. Select a transformer with the same phase order and phase divergence to ensure parallel operation. Make sure the transformer’s vector group matches the existing system before making a purchase, and run a vector group test to confirm the specifications.

Steps to Test an Isolation Transformers

Due to transitory electromagnetic fields, isolation transformers restrict direct current electrical energy from passing through a transformer. These transformers ensure that the AC voltage is the same at both ends by having identical primary and secondary windings. Testing of isolation transformers for residential homes requires that the primary and secondary voltages are the same and that it does not pass DC electrical energy from the primary to the secondary.

1. Connect one red alligator clip test lead to the positive terminal of the battery, and the other to the primary terminal of the transformer. Transformers are unable to conduct direct current due to sporadic electromagnetic fields. 

2. Connect the other end of the test lead to one of the transformer’s primary terminals.

3. Connect a black alligator clip test lead to the transformer’s final primary terminal as well as the battery’s negative terminal. A multimeter should be turned on and its scale set to “volts DC.” 

4. Put a black probe on one of the secondary terminals of a transformer, and a red probe on the other secondary terminal. 

5. Look for a value of higher than zero volts on the multimeter display, which denotes a bad transformer. If there are no voltages, go to the next step.

6. Remove the test leads with the alligator clips, then connect the red and black leads to the AC power source terminals. 

7. Change the scale of the multimeter to “volts AC.” The multimeter should be turned on with the scale set to “volts DC.” 

8. Check the multimeter display after turning on the AC power supply. Probes should be taken out of the transformer’s secondary terminals. 

9. On the primary and secondary terminals, attach the red and black leads.

 10. Check for identical voltage on the multimeter display. If not, discard the transformer as necessary. 

11. Black lead should be attached to the final primary transformer terminal.


An isolation transformer is a device that delivers electrical energy without making physical contact. In distribution and transmission lines, these transformers are used to step between high and low voltage levels. Prior to making a choice, it is essential to take into account the type, qualities, uses, and benefits of an isolation transformer. For testing of isolation transformers in Hyderabad after purchase, choose experienced professionals.