Testing

Testing of Electrical Equipment

Testing means examining the object to verify suitability of its performance in particular application. These tests may be quite simple to most complex. Simplest test may be observation by trained human whereas complex test may require precise instruments and setup handled by specially trained group. Testing may also be useful to pin point the defect and hint for the remedy to make it perfect.

In general testing is to observe the effect of specific condition/s on particular parameter/s of the object. The object may be equipment, plant, etc. The condition may be single or more (High Voltage on Wet Insulators) and so also monitoring parameter may be one or more (test of new medicine on human for Effectiveness as well Side Effect) depending upon requirements related to test piece. The basic aim of the testing is to ascertain satisfactory working of the equipment for planned service under likely operating conditions. Appropriate test procedures are developed and standardized for different tests on various equipments and are generally followed.

Testing has role throughout the life of equipment at different stages with concern objectives. Testing starts with designed data of equipment. It is cross verified with alternatives to confirm suitability and/or observations on prototype before regular production. Type Test is meant for this purpose. Residual Life Assessment (RLA) test at end of service life is useful to decide viability for scrap out or refurbish.

Equipment production may be either routine product like that of general purpose small motors or customized production as per requirement of customer like that of generator transformer. Tests requirement differs in some aspects accordingly.

Testing is related to three stages in the life of equipment.

Design and Development

This is factory base activity where equipments are produced. This has three steps as designing, developing and testing. Each step is handled by respective experts.

Design Stage

Designing can be considered as pre birth activity of equipment. Designed parameters are cross checked for different aspects using appropriate methods. In some case design is verified on simulators by virtual testing. Type Test is done on a specimen of new design to verify the design perfection. This test is destructive and hence test piece has to be discarded.

Development stage

Checking starts from quality of inputs to match design consideration. In process interim tests are done on key components to confirm that job so far is in order. This is important because it is a task to locate and correct the defect after assembly.

Testing Stage is last step of this stage where in all finished products has Routine Tests. These are non destructive test and all successfully tested pieces are ready for dispatch in the market. Some basic tests are mandatory as per statutory requirements. Some more tests may require while manufacturer is using mark of quality assurance agencies like ISI, ISO, etc. Some time manufacture voluntarily takes some specific tests. The objective may be to assure performance guarantee and have reputation for their product.

In some case, particularly for customized product, some tests required by purchaser have to be performed in presence of their representative. This is known as Factory Acceptation Tests (FAT). This is as per agreement between supplier and purchaser forming part of purchase order.

Most of the above routine tests are done by testing expert with test facilities at factory. However some tests requiring special setup has to be done at NABL accredited recognized laboratory.

Erection and Commissioning

This is site activity where machine suppose to perform during the life. Erection and commissioning is handled by team of trained experts in the respective field. This stage includes receiving, unloading, inspection, shifting, storing, assembly, fixing, testing, commissioning and trial run of the equipment till handing over to the O&M team. Due care has to be taken in each step to avoid internal or external damage to equipment. It may be very costly affair in term of money and time delay for transport to and back from works for rectification if damaged. Equipment has to be handled very carefully to avoid falling, tilting, knocking rubbing while unloading, shifting, erecting etc. Also care has to be taken in storing according to requirements of equipment to protect it from moisture, heat, fire, dust, insect, theft or loss of parts, etc.

After the assembly and installation at site, commissioning tests are taken before power on. Equipments are already tested in all respect at works before dispatch. But any adversity after testing at works has to be ruled out. Generally after testing at works, the equipment may be lying there till delivery. While dispatching, equipment is shifted and loaded for transportation to the site. There may be vibrations during transport from works to the site, unloading at site, shifting, storing, assembly, etc. During all these activities there may be possibility of loose or open connection, dislocation of internal part, ingress of moisture, damage to insulation etc. Test facilities similar to works or laboratory may not be available at site. So healthiness of equipment is confirmed before power on by site tests. These tests are known as Commissioning Test (or pre commissioning test). Mostly these are indirect tests like checking IR, PI, PD, Tan delta etc. Also checking and removing internal packing of delicate moving parts, confirm CT circuit is closed, perfect earthing and connections, protective devices functionality, all connections on terminal box, etc before power on.

In some case, particularly for customized product, Site Acceptance Tests (SAT) as specified by the purchaser has also to be performed jointly at site in presence of representatives of supplier and purchaser. 

Finally trial run is taken jointly with O and M team as handing over procedure. All drawings, manuals, test results etc are also transferred to O and M team.

Operation and Maintenance

This stage is actual service period of the equipment and is longest period in its life. Here operation and maintenance team is associated with equipment. The objective of the team is to manage for satisfactory service availability of equipment during working hours without breakdown. Team has to monitor operation of the equipment and infer any abnormality. They have to plan and execute maintenance as required during non working hours. Preventive Maintenance is done regularly to avoid sudden break down during working hours and more damage. They may follow Periodic Maintenance also known as Schedule Maintenance which planned based on past experience on similar equipments. Or may resort to Predictive Maintenance called Condition Based Maintenance. In this case, equipment condition is checked periodically and decided for maintenance requirement. These tests are Maintenance Tests or checks. Some scheme are available for continuous on line condition monitoring. It is possible to predict development of fault early stage by Signature analysis. Corrective Maintenance based on component Failure Analysis provides optimum equipment performance is known as Reliability Centered Maintenance.

O and M team has to manage spares and specialist for the maintenance work when so required. Maintenance planning is very crucial job as missing of any aspect may held up progress and delay in completion of the job.

After major maintenance work of equipment, before putting back in service, all the care has to be taken as if re-commissioning it.

Experienced person of the maintenance team can judge some defect by personal observation without aid of any instrument.

Sight

Visual Inspection : It is possible to find out burnt or over heated terminal or cable, open connection, cracks, displacement of parts, rubbing surfaces, dust, water or insects etc.

Sound

Listening Sound : Characteristic sound of dynamic machine or humming sound of static equipment may signify rubbing, knocking, misalignment, loose parts, bent shaft, bearing failure, single phasing etc.

Smell

Sniffing : Type of the smell from the equipment may indicate over heating or burning of paper, cotton, enamel, plastic, rubber, grease, oil etc. So it is possible to guess the fault.

Sense

Touch and Feel : By touching the equipment one can feel the vibration, temperature, dampness, loose parts / nut bolts etc and can evaluate the problem. (NB: Before touching the equipment in service, precautions against electric shock, such as proper earthing, standing on rubber mat, using rubber shoe, etc has to be ensured)

Also measuring Tan delta, IR, PI, PD, signature analyses, etc used to detect the problem 

Generally electrical machines are tested with following Criteria.

Current Criteria

When machine is energized, current suppose to flow through designated circuit. This conducting path for current has to be perfect without breaks / loose connections. The break in the circuit results in no flow of current and nonfunctioning of the equipment. Loose connection cause high resistance at the point. Flow of current in high resistance path will produce more heat at the point. At known locations like contacts have heat dissipation arrangement to limit temperature rise within limit. But more heat at unexpected locations may result in overheating, oxidation and formation of non conducting film causing further increase of resistance. Ultimately it may deform, melt or fuse off.

This path is tested by current flow at various levels and manners. It may be for normal full load operation, normal operating deviations, short time abnormal condition and ultimate level till failure. Last one is destructive test for capacity check known as type test. Most of the above testing may be at various levels of current on higher side. In some case testing for low side is also required.

Voltage Criteria

Current is supposed to flow only through intended path in the equipment known as circuit made of conductor. Any diversion of current is harmful and waste. Therefore insulation is provided throughout this path to block diversion. However some leakage current flows through the insulation that may be either between phases or to the ground via equipment body. Leakage current should be within limit to avoid deterioration / damage to insulating system and therefore insulation resistance suppose to be very high. This leakage current is proportional to applied voltage and therefore insulation system is tested by voltage at various level such as rated voltage, normal deviation, short time high voltage and ultimate high level. In some case testing for low side is also required.

Safety

Whenever electricity divert from conducting path towards the body of the equipment due to failure or weakness of insulation or by mistake, human or animal may face electrical shock if they come in contact with such faulty equipment. Earthing system is provided for protection under such condition. But it may not serve the purpose unless earthing system, protective relaying and switchgear are properly coordinated and healthy. Therefore periodic testing of protective system has to be arranged. Most important is earth pit resistance that is likely to increase with time. Periodic watering is required to maintain moisture. It is   very essential to measure earthling resistance and if remaining high, it has to be reactivated.

Performance Test

Tests done to estimate various losses, efficiency, input output relation at various loading, speed variation, voltage regulation, temperature rise, etc to interpret the performance of the equipment under various operating conditions.

Mechnical

These tests include like vibrations, noise, balancing, alignment, rigidity, etc.

Direct loading on equipment for testing purpose causes undue stress and decline strength to some extent. Therefore to avoid this, non destructive testing using acoustic emission/resonance, ultrasonic, infrared, electromagnetic, radiography, magnetic particle etc are adopted wherever feasible.