Insulation and dielectric
Core insulation resistance testing measures the resistance between a transformer's core and its tank or frame after the core ground is lifted. It shows whether the core is grounded at only the intended point or has picked up a second ground that could cause circulating current and local heating.
Core insulation resistance testing checks the insulation that separates a transformer's magnetic core from its tank, frame and clamping structure. A transformer core is built from thin steel laminations that must be held at a defined electrical potential, so it is intentionally connected to ground at a single point. Everywhere else it is meant to be insulated from the tank and from the clamps. The test opens that single intended connection and uses an insulation tester to measure the resistance from the core to ground. A high reading means the core is isolated everywhere except at the normal ground. A low reading means there is a second path to ground, which may be a bridging piece of debris, a damaged insulating sheet, a displaced part or contamination. The test is done on liquid-filled power and distribution transformers where a core ground connection is brought out, and on large dry-type air-cooled transformers where the core and clamp insulation can be reached. It is a simple, quick measurement that supports the larger set of electrical tests on a transformer, including the winding insulation tests and the excitation current test.
A core that is grounded in two places forms a closed loop with the tank or frame. The alternating magnetic flux of the transformer links that loop and induces a circulating current in it, and the current heats the laminations, the ground connection and the nearby insulation and oil. The heating is local and can be considerable, and it can degrade the insulating liquid and produce combustible gases that show up in dissolved gas analysis long before anything is visible from outside. In the worst case the heating damages the core insulation further, burns the ground strap or leads to a failure. Another reason for the test is the safety of the arrangement. A core that is not grounded at all can develop a floating potential that discharges to the tank. Finding a second ground at acceptance, before the unit is energized, allows it to be traced and removed while the manufacturer or the installer is still available. Finding it in maintenance, when gas analysis or temperature trends point to a hot spot, helps to separate a core fault from other causes of heating.
The core and its clamps are insulated from the tank by thin sheets, coatings and spacers. Insulation tests are based on applying a direct voltage and measuring the small current that flows through that insulation. When the insulation is intact, the current is very small and the resistance is very high. If a metallic particle, a piece of wire, a bent part, a conductive deposit or damaged insulation creates a second path, the resistance falls sharply. The reading is a measure of the path, not of the quality of any one material, and a low value near zero means a solid contact, while a moderate value points to a conductive film or contamination that may be moisture, sludge or metallic dust. Because transformer flux is strong, even a modest second ground can support a substantial circulating current, and that is why this simple measurement is so useful. The direct voltage used is chosen to stress the core insulation modestly and not to damage the thin insulation. Moisture, oil quality and temperature affect the result, which is why conditions are noted. In some cases a transient reading appears because a conductive bridge burns clear when the test voltage is applied.
The transformer is de-energized, isolated from every source and grounded under the owner's lockout and permit procedures, and the windings are grounded for safety. The core ground connection is located. On many liquid-filled units it is brought out through a small bushing or an accessible terminal on the cover or tank, and on others it is reached through a handhole or inspection opening, and on dry-type units it is accessible at the core clamps or frame. The intended core ground strap is disconnected so that the core is isolated, a step that must never be forgotten because a transformer must not be energized with its core ungrounded. An insulation tester set to an appropriate test voltage for the construction is connected between the core terminal and the tank or frame. The voltage is applied for the short period called for by the procedure, the reading is taken, and the circuit is discharged. Where more than one ground has been brought out, for example the core, the frame clamps and the core-to-clamp connection, each is tested separately. After the measurement, the core ground connection is restored, tightened and verified with a continuity check, and the tank, the temperature of the oil or the surroundings and the humidity are noted. The reading is compared with the factory test and with earlier tests of the same unit.
The record holds the insulation resistance measured between the core and the tank or frame, and for each additional core or clamp ground that was tested separately, with the test voltage applied and the duration of the application. It notes the temperature of the oil or the surroundings, the relative humidity, the way the core ground was lifted and the condition of the connection, the instrument and its calibration status, and the confirmation that the ground connection was restored. The condition of the oil level or a reference to related gas analysis results is recorded where relevant. These details help a later reader compare results fairly and confirm that the unit was returned to service with its core grounded.
No single number decides whether the core insulation is acceptable, and this page does not give one. The reading is judged by several observations used together. The first is the absolute level of resistance, read against the published guidance for the equipment type, with ANSI/NETA ATS-2025 for acceptance testing and ANSI/NETA MTS-2023 for maintenance testing, and against the manufacturer's factory test result. A reading near zero indicates a solid second ground, while a very high reading is the expected condition. The second is trending against earlier results on the same transformer, where a decline matters even if the reading is still high. The third is comparison among the several core and clamp insulation paths of the same unit, where one that stands apart suggests where the problem lies. The fourth is correlation with other evidence, including dissolved gas analysis, temperature patterns, excitation current and insulation power factor results. The fifth is the stability of the reading, since a value that drifts or jumps during the test suggests a conductive bridge or moisture. The test is listed as optional in some sections of the standards, so the owner's practice and the manufacturer's recommendations matter. Refer to your own licensed copies of the standards named above for the governing language.
Acceptance and maintenance tests differ in aim. At acceptance the measurement confirms that the core insulation survived shipping, handling and installation, and the result is added to the factory record as a baseline. In maintenance it is made when a transformer shows signs of heating or abnormal gas generation, after an internal inspection or repair, and sometimes on a schedule. The construction changes the access. On liquid-filled transformers the core ground may be brought out to an external terminal, in which case the test is made without opening the tank, or it may be inside the tank where it can only be reached through an opening, and the work is planned accordingly. On large dry-type air-cooled units the core and its clamps are reached directly. Some transformers have separate insulated core sections, or core and frame grounds that are brought out separately, and each path is tested. The test voltage and the evaluation depend on the unit's design and the manufacturer's guidance. In every case the ground must be restored before the unit is energized, and the measurement supplements the winding insulation resistance, power factor, excitation current and gas analysis, without replacing them.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
A core grounded in two places forms a closed loop with the tank, and the transformer's alternating flux induces a circulating current in that loop. The current causes local heating and can degrade the oil and insulation. A single grounding point holds the core at a defined potential without creating a loop.
It means there is an additional path between the core and the tank or frame, such as debris, a displaced part, damaged insulation or contamination. A reading near zero suggests a solid contact. The unit needs further investigation, often with gas analysis and temperature data, and the cause must be found and corrected.
No. The core ground is opened only for the measurement and must be restored and verified before the transformer is energized. A core left ungrounded can develop a floating potential and discharge to the tank. The restoration of the connection should be recorded as part of the test.
The insulation measurement itself requires the core ground to be lifted and a direct voltage to be applied, so it is done with the transformer de-energized and isolated. Other methods, such as measuring the current in the core ground strap, can indicate a second ground on a transformer in service.
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