Routine Tests of Circuit Breakers

Routine tests are the factory checks run on every circuit breaker before it leaves the works. IEC 62271-100 and IEC 62271-1 group them as follows

1. Power frequency over voltage withstand test
2. Dielectric test on auxiliary circuit and control circuit
3. Measurement of resistance of main circuit or contact resistance test
4. Tightness test or SF6 gas leakage test
5. Design and visual checks
6. Mechanical operation tests.

Each test below confirms one part of that standard routine-test set. Type tests such as lightning-impulse and switching-impulse dielectric tests are not repeated on every unit.

Power Frequency Over Voltage Withstand Test

A Power system can see a temporary power-frequency overvoltage after a load change or a tap-changer error. The power frequency over voltage withstand test checks whether the breaker’s main-circuit insulation can stand that stress. Lightning and switching impulses are covered in type tests. The routine test uses the power-frequency value named for the rated voltage.

A full dielectric type-test series is wider than this check. The dry power-frequency withstand test is the routine dielectric test on the main circuit.

One Minute Dry Power Frequency Voltage Withstand Test

A power-frequency overvoltage on the network usually lasts far less than one minute. The factory test still holds the specified voltage for a stated short duration, commonly one minute in older procedures, to show that the main-circuit insulation can stand that stress without flashover.

The test is performed with the breaker dry. The applied power-frequency voltage is the value given in the relevant IEC table for that rated voltage, with an altitude correction if the test site requires it.

One common arrangement for an SF6 Circuit Breaker is this. The tops of all poles of circuit breakers of the same voltage rating are bonded with a copper conductor and earthed. The bases are earthed. The bottoms of the poles are bonded together.

That bond is then taken to the phase terminal of a single-phase high-voltage cascaded transformer. The High voltage transformer is a cascaded auto transformer whose input can be raised from zero to a few hundred volts so the secondary can reach the required kilovolts. The test voltage is applied at the bottom terminal of the breaker by that cascaded transformer. Raise it slowly to the specified value, hold it for the stated time (often 60 s) and then lower it to zero. Measure leakage current to earth. It must stay inside the allowed limit. A disruptive discharge fails the insulation.

Dielectric test on auxiliary and control circuit

Auxiliary and control circuits can also see a short overvoltage. IEC 62271 requires a power-frequency dielectric test on those circuits. Older factory wording used 2000 V for one minute. Current IEC 62271-1 uses 1 kV or 2 kV for a short duration. Use the voltage and hold time named in the edition on the rating plate. There must be no disruptive discharge.

Measurement of the resistance of main circuit

The resistance of the main circuit is obtained from the DC voltage drop across each pole. Inject a direct current and measure the voltage drop. The injected current is a DC value in the range used for the type test, often from 100 A up to the rated current of the circuit breaker. The measured resistance must not exceed 1.2 times Ru, the resistance recorded before the temperature-rise type test.

Tightness Test

This check is used mainly on gas-insulated switchgear. IEC 62271-1 judges tightness by the annual relative leakage rate of the closed or sealed pressure system, typically 0.5 % or 1 % per year for SF6, and requires the routine result not to exceed the type-test leak. One workshop bagging method wraps each joint in thin polythene for several hours and reads the trapped concentration in ppm. A figure such as 3 ppm in 8 hours is a local workshop limit, not the IEC annual rate.

Visual Checks

The circuit breaker is checked by eye for nameplate language and data, identification marks on auxiliary equipment, paint condition and corrosion on metal surfaces.

Mechanical Operation Test

The circuit breaker must operate cleanly at the maximum and minimum allowed auxiliary and control supply. Close and trip at least five times at the specified maximum control voltage and at least five times at the specified minimum. Repeat the close and open checks at rated control voltage. IEC treats 110% of rated control voltage as the usual upper limit. Closing is checked down to 85% of rated voltage. Opening or trip is checked down to 70% on DC supplies (85% on AC). Times at those limits differ from the rated-voltage times, but every time must stay inside the specified band. For a pneumatic breaker, also operate at least five times at maximum, minimum and rated operating pressure. If the rating plate includes rapid auto-reclose, run at least five open-close cycles and compare the open-to-close interval with that plate. When circuit breakers are shipped as separate units and rebuilt on site, the manufacturer should take part in the commissioning tests to confirm the assembled unit. Record close and open times and the interval between consecutive operations for every required sequence. Record fluid-pressure change during operation where it applies.
A no-load operating cycle can be used to draw the no-load travel curve. That curve must stay inside the envelope of the reference mechanical travel characteristic.

N.B: Record these quantities during the mechanical test of circuit breaker

• Closing time of each pole
• Closing time difference between poles or closing mismatch time
• Opening time of each pole
• Opening time difference between poles or opening mismatch time
• Close-Open time of each pole
• Time difference between two consecutive opening operations (O-C-O)
• Maximum bounce of moving contact during closing operation
• Total bounce of moving contact during closing operation
• Over travel of moving contact
• Contact speed for closing in deg/ms (as transducer is of rotary type)
• Contact speed for opening in deg/ms (as transducer is of rotary type)
• Damping time during opening
• Spring charging time

When sub-assemblies are fitted on site, the mechanical travel characteristic must still match the factory reference at the end of commissioning. The manufacturer should give the exact on-site method. Otherwise the contact-stroke traces cannot be compared. Those traces are taken with a travel transducer or a similar sensor on the circuit breaker mechanism.
Check every connection in the control and auxiliary circuit in the kiosk. Confirm that control and auxiliary switches show the true open and closed positions. Operate every auxiliary device at the specified maximum and minimum control voltage.