Technical guide
Detailed information about Primary Tests
primary tests — this guide covers the scope, application method, quality criteria and reporting process for your needs.
What do primary tests verify?
Primary tests verify the condition of the main equipment that carries and transforms electrical energy after installation, commissioning, and maintenance. The method to be applied for the power transformer, current-voltage transformer, circuit breaker, cable, busbar, bushing, and grounding system is selected according to the equipment's construction and the test purpose. In a new facility, manufacturing and installation defects are investigated, while in an operating facility, signs of aging, looseness, pollution, humidity, and mechanical wear are assessed.
Plan-Test does not regard the primary testing service as merely connecting a device and reading a value. Before the test, equipment isolation, nameplate information, the connection group, grounding, and environmental conditions are checked. The device used, its serial number, calibration information, the test connection, and the applied voltage or current are recorded in the report. The measurement result is compared with the standard, the manufacturer's limit, and any historical values and is presented together with a technical interpretation.
Current and voltage transformer tests
Current transformer testing can cover ratio, polarity, winding resistance, excitation or saturation curve, knee point, and insulation measurements. In protection-class CTs, the saturation characteristic that affects relay performance is important; in metering class, ratio and phase accuracy matter. Not leaving the secondary circuit open, using the short-circuit bridges correctly, and checking the grounding point are critical for safety. Differences between phases and nameplate values are evaluated together.
Voltage transformer testing is carried out with ratio, polarity, winding resistance, insulation, and fuse-circuit continuity checks. For capacitive voltage transformers, additional construction-specific assessments may be required. Because ratio and polarity errors directly affect metering, measurement, and protection functions, the primary results are linked to secondary circuit verification. At the end of the test, it is verified with a checklist that the connections have been restored to the operating position and that temporary short circuits or groundings have been removed.
Power transformer tests: TTR, winding resistance, and insulation
Power transformer tests provide valuable information about the transformer's electrical integrity and the consistency between phases. DC winding resistance can reveal differences in connections, tap changer contacts, and winding continuity. The TTR test checks the turns ratio, vector group, and the accuracy of the tap positions. The results are interpreted taking temperature, measurement current, the transformer's magnetic state, and manufacturer tolerances into account.
Insulation resistance and polarization index measurements give an idea about the general condition of the winding-to-ground and inter-winding insulation. Tan delta testing and capacitance measurement help assess the change in losses in the bushing and insulation system. A single test does not reveal all failure types in a transformer; for this reason, oil analysis, frequency response analysis, or other advanced methods are assessed under a separate scope when needed. The measurement combination is determined according to the transformer's history and the suspected failure mechanism.
Circuit breaker tests and contact transition resistance
Circuit breaker tests verify the conductivity and insulation of the main contacts and the timing performance of the mechanism. Contact transition resistance measurement at the micro-ohm level can reveal connection or contact degradation in the poles. Opening-closing times, inter-pole simultaneity, bounce, and motion characteristics are recorded with devices suited to the breaker type. When coil current and mechanism behavior are examined together, clues are obtained about problems arising from friction, adjustment, or power supply.
In SF6, vacuum, or oil circuit breakers, there are manufacturer-specific check items. Gas density, mechanism pressure, spring charging, auxiliary contacts, counters, heaters, and interlocks can be included in the test program. The results are compared not only with the general standard but with the manufacturer limits for the same make and model. The change between pre- and post-maintenance measurements shows the effect of the intervention performed. For critical deviations, correction and re-test before energization are recommended.
Safety and measurement quality in high voltage testing
During the high voltage testing service, the test area is physically bounded, responsible persons are assigned, and it is verified that the equipment is isolated from all energy sources. The absence-of-voltage check and the discharge and grounding steps are explained in the method statement. While the test voltage is applied, unauthorized entry is prevented; after the test, the safe discharge of capacitive loads is awaited. In high-current work such as primary injection, cable cross-section, connection tightness, and heating are monitored separately.
For measurement quality, correct connection and interference management are as important as device calibration. Shielding, grounding, the arrangement of connection cables, surface cleanliness, ambient temperature, and humidity can affect the results. The Plan-Test team determines the expected value range before the test and does not blindly accept inconsistent results. The reliability of the result is increased through repeat measurement, a connection check, or an alternative test method. Raw data is stored so that the assessment in the report remains traceable.
How is a primary test report evaluated?
The primary test report should clearly state the facility, equipment code, make and model, serial number, nameplate information, test date, environmental conditions, and the device used. For each test, the connection, applied value, measurement result, and acceptance criterion are shown. Differences between phases or taps are presented with tables and, where necessary, graphs. Unsatisfactory results are explained together with the possible cause, the recommended verification, and the operational risk.
Plan-Test reports are prepared to serve as a basis for commissioning, maintenance, and failure analysis decisions. Measurements on new equipment form a reference for future predictive maintenance. If a trend is observed in operating equipment, a re-test period, maintenance, or further investigation is recommended. It is important that results are not divorced from context; a value that is within limits but changing rapidly relative to the past may also require follow-up. This approach turns primary tests from a one-off compliance check into technical data that supports the equipment life cycle.
Plan-Test’s quality, safety and reporting approach
Plan-Test does not treat the service scope merely as a list of operations to be performed on site. The facility's operating objective, outage availability, existing documentation, equipment history, and risk level are evaluated together. Before the work begins, responsibilities, access conditions, de-energization and re-energization steps, and reporting expectations are clarified. This is intended to keep teams working from the same plan, prevent critical checks from being skipped, and make efficient use of site time. Because each project is planned according to its own technical conditions, the method, devices, team size, and delivery documents to be used are clearly defined at the proposal stage.
Our quality approach is based on explaining what the result means rather than presenting the measurement result on its own. The calibration status of the test devices used, the ambient and load conditions, the connection arrangement, the applied standard, manufacturer criteria, and any previous measurements are linked together in the report. Nonconformities are classified by severity; findings requiring immediate intervention, issues that can be addressed in planned maintenance, and trends that should be monitored are separated from one another. This method helps the investor, the operations manager, and the site team turn the same technical data into a decision-ready output.
Occupational health and safety is an inseparable part of all services. The site-specific risk assessment, personal protective equipment, lockout-tagout steps, absence-of-voltage verification, and grounding and bonding rules are reviewed before the field work begins. Inspections carried out in an energized facility are based on safe approach distances and operating instructions. Plan-Test's goal is to combine technical verification with a safe working discipline in the same process and deliver sustainable, traceable, and auditable engineering services in 1–380 kV energy facilities.
At the proposal and mobilization stage, clearly defining the scope matters for both the employer and the site team. Plan-Test determines the required activities by reviewing the existing drawings, the equipment list, the facility's operating regime, previous reports, and the employer's expectations. Out-of-scope items, necessary preparatory work, the need for de-energization, operational accompaniment, and third-party responsibilities are put in writing. When a new condition arises on site, no uncontrolled scope change is made before its impact is evaluated in terms of technical, schedule, and resource aspects. In this way, all parties understand throughout the project which activity is being performed and why, which data is expected, and against which criterion acceptance will be made. This clarity reduces repeat visits and misplaced expectations while making the delivered engineering service comparable.
The quality of field work depends on the team's experience and cross-disciplinary communication as much as on the equipment used. Plan-Test assigns tasks according to the test or maintenance steps and opens points requiring primary, secondary, project, and operational knowledge to joint evaluation. Measurement devices are used with accessories suited to the application, their calibration validity is checked, and raw data is stored securely. When a critical result is observed, the connection, device settings, environmental conditions, and the equipment's operating history are reviewed again instead of jumping to a hasty interpretation. Verification with a different method is proposed when necessary. This culture of technical scrutiny helps prevent incidental or erroneous measurements from steering the decision process and helps define the real risk on site more accurately.
Completion of the service does not end with the delivery of the report; the actions in the report must be implementable and traceable. Plan-Test links its recommendations to priority, the responsible discipline, the recommended due date, and the verification method. If the employer wishes, a scope for a technical meeting, post-correction check, or repeat measurement can be established for critical items. When new results are compared with historical data under the same equipment code, the facility's condition trend becomes visible. These records support the preparation of the future maintenance budget, spare parts plan, modernization decisions, and outage calendar. The goal is not to document a single project moment, but to build a corporate technical memory that contributes to safe operation and high availability targets throughout the energy facility's entire life cycle.
In every engagement, the relevant national legislation, the grid operator's requirements, IEC and IEEE standards, and the equipment manufacturer's instructions are considered together. While the general provisions of the standards are adapted to the facility's actual conditions, the contract and approved project requirements are also preserved. Where several criteria give different limits, the acceptance approach used is explained in the report; uncertain points are recorded with the employer as a technical decision. Using unique site codes instead of equipment names in documents strengthens traceability among drawings, test results, photographs, and actions. With this documentation discipline, Plan-Test aims to ensure that the technical evidence needed during an audit, provisional acceptance, warranty discussion, or a future failure investigation can be found quickly and reliably.
For the site program you need, it is sufficient to share the facility type, voltage level, equipment count, location, and target date. After the preliminary review, the technical team clarifies the appropriate scope, preparations, and delivery plan.