Hot spotsHot spots from degrading contact inhibits the contacts ability to carry nominal or overload currents and, dependant on the severity of the contact state, this can result in temperature rises.Fig 30: Hot spotsAt the point where a hot spot is detected you may see a much higher rise in temperature than the overall temperature measured, this would increase the resistance and a greater chance of fire: ■Hot spots are the source of high frequency waves (Harmonics). When these waves pile up at a location, they will cause equipment damage by Resonance Phenomenon ■Hot spots are indicators of impending failure of the equipment ■There are sources of electrical energy losses (loose connections) ■Hot spots are the primary cause for a major explosion of electrical equipment ■It is one of the main reasons for failure of current transformer (especially in HV circuits)Calibration in the fieldCalibration of low resistance ohmmeters can be checked in the field by the use of a shunt. Calibration is done using individual current and potential 12-gauge copper leads, to ensure correct current distribution through the shunt and an accurate potential measurement. Be aware that 'test probes' do not provide accurate positioning of the leads to check instrument calibration. They can, however, be used to determine the relative calibration of the instrument.Table 8: Commercially available shuntsResistance ±0.25% ValueCurrent Rating10 Ω1 mA1 Ω10 mA0.10 Ω100 mA0.01 Ω1 A0.0010 Ω10 A0.0001 Ω100 AThese calibration shunts when used with a Certificate of Calibration, traceable to National Standards, help the field service engineer demonstrate to a customer the accuracy of the tests being conducted.AppendicesTesting of transformersRegular tests on transformers can help identify problems that reduce system performance and can lead to unexpected outages. The d.c. resistance of a transformer winding can indicate the internal temperature of the winding, when the resistance at ambient is compared to the hot resistance. The ideal test method is to make resistance readings at one minute intervals as the hot winding is cooling. When this data is charted, the resistance at time zero can be estimated. This test is one of the mandatory tests done when the transformer is manufactured and might also be used in the field if the transformer is accessed while still heated up.The typical test will show excessive overheating in the coils due to fatigue or corrosion of the internal coil and / or the internal connections. Low resistance tests on transformers addresses small, medium, large single, large poly-phase and auto-transformer windings. Tests are done on: ■Dual windings with the test current connected through the windings in opposed polarities ■Wye to wye windings with and without a neutral connection; the leg of the other winding is connected to the potential lead to measure the voltage at the internal connection ■Wye to delta windings; a jumper is used to connect the current from the wye winding to the delta winding (this test mode reduces the test time) ■Delta to delta windings; the test time can be improved by connecting the current jumper to the primary and secondary of the same phase in opposed polaritiesTaps are used to improve voltage regulation and are adjusted daily. Excessive wear and loosening due to vibration can be identified with low resistance measurements. Consecutive tests can be done on secondary tap changers (shorting style of taps). Large transformers have many tap positions and test time will be reduced, as the test current does not have to be shut off between tests. Tests on primary taps (open taps) must be done as individual tests with the test current shut off between tests.The low resistance ohmmeter must have sufficient current capacity to saturate the windings. The time taken to test will depend on the available test current. Large transformers require special attention prior to performing the tests. The insulation between the windings will store energy, similar to the dielectric in a cable, and must be discharged before a test can be done.When three-phase transformers are tested, interaction will occur between the primary and secondary windings. This situation is most evident when transformers with Wye and Delta windings are tested, and can be minimized by connecting the test current to flow through both primary and secondary windings. The net effect is to reduce the mutual coupling between the windings and minimize the flow of circulating current in the delta winding. The recommended test current is between 1 - 10% of the nominal current, but not above 15%. Over 15% will cause heating, as it will affect their resistance value significantly. The lower test currents reduce stress in the magnetic core of the winding, but will increase the test time.Large test currents produce large forces on the core and can cause damage and generate heat, which will affect the resistance value.It is also important that the instrument discharges the transformer when the measurement is completed. If not, lethal voltages can be present at disconnection. Dedicated test instruments with these features integrated are available.Warning: Never use a non-dedicated LRO to measure the winding resistance on a power transformer. Lethal voltages can be present if a winding is not discharged correctly before the test leads are disconnected.Motor bar to bar testsHelical spring point probes are used to measure the value of the bar to bar resistances of the rotor in a d.c. motor (see Fig 31). This test is typically done at the 10 A current level with the typical coil resistance measurements in the 6000 microhm range. These tests identify broken/ loose welds or solder connections between the coils and commutator bars. The resistance measurements should remain consistent. Readings can be higher on a heated motor, due to the temperature of the coils. As the coils cool, the resistance values can drop to some prior reference value recorded at ambient temperature.Fig 31: Bar to bar test on d.c. motor rotorwww.megger.com 2928 A guide to low resistance testing www.megger.com
