Introduction to M2 Class Circuit Breakers
The M2 class of circuit breaker belongs to a special class defined by IEC 62271-100. This means these circuit breakers require minimal maintenance and can operate mechanically for an extended lifespan. If a breaker performs at least 10,000 mechanical operations without any even minor damage, it comes under M2 class.
M1 vs M2 Class – What is the Difference?
Before discussing the M2 class circuit breaker, we need to focus briefly on the M1 class circuit breaker. Since there is M2 class breakers, there must be an M1 class circuit breaker. We generally use M1 class circuit breakers for normal indoor mechanical applications. An M1-class CB can perform 2,000 mechanical switching operations without any major maintenance. On the other hand, M2 class circuit breakers offer much longer endurance.
An M2 class breaker is a special service breaker. These possess an extended mechanical endurance. Generally, these breakers demand no major maintenance below 10,000 mechanical operations. This implies that M2 class circuit breakers require very minimal maintenance. The circuit breakers must pass through a special type of mechanical endurance test.
Applications of M2 Class Circuit Breakers
Engineers prefer M2 circuit breakers for installations that require very frequent switching operations.
Procedure of Mechanical Endurance Test for M2 Class Circuit Breaker
Here we will discuss only M2 class circuit breakers. This is because we do not generally use M1 class circuit breakers for extra high voltage switching purposes in electrical substations.
Setup Requirements Before Testing
Normally, we use M1-class circuit breakers in household applications. Hence, this will not come under the scope of our discussion. For M2, we erect the circuit breaker on its structure. If the breaker is an SF6 circuit breaker, it must contain SF6 gas at normal pressure. We switch on the spring charging motor to supply it with rated voltage. We energize the control circuit of the breaker with rated voltage, either 220V or 110V DC, according to the rating of the control circuit.
Operations at Minimum Control Voltage
As rated, 85% of the rated control voltage is the lower margin of supply control voltage. If the control voltage is 220V, we will apply 85% of 220V or 187V (DC). Sometimes we may also use 70% of 220V. Otherwise, we can go for 85% of control voltage for closing operations and 70% of control voltage for opening operations. We need to perform the closing operations followed by opening operations 500 times at that minimum control voltage level. We must maintain a time gap between each closing and opening operation to restore the circuit breaker to its normal condition and prevent overheating of its mechanical parts.
Operations at Rated Control Voltage
Then we will repeat the same operation another 500 times at the rated supply voltage of the control circuit.
Operations at Maximum Control Voltage
Then we apply the maximum rated control voltage, which is 110% of the supply, either 220V or 110V DC. We will repeat the same operation sequence, meaning a closing followed by an opening operation with a specific time interval of 500 times at 110% of rated maximum control voltage.
At last, we go for 500 close–open (CO) operations with rated control voltage. Here also, we should maintain a time gap between two consecutive close–open (CO) operations. This time gap is also necessary to settle down the heat generated during the mechanical movements of different gears and levers of the circuit breaker.
After that, the breaker must go for 5 close–open operating cycles at rated supply voltage, 5 close–open cycles with minimum supply voltage, and 5 close–open operations with maximum supply voltage. We record the operating characteristics of the breaker during these operating cycles. Although it is not necessary to attach all the contact travel reports for all operations after the test, we should enclose at least one such report with the final test documents.
We should check the contact’s travel characteristics and all other parameters before and after the extended mechanical endurance test. If everything is under limit the test confirms the class M1 of the circuit breaker.
Endurance Test Procedure for M2 Class Circuit Breakers
We repeat the same operating sequences five times to make the total operations 10,000 times to confirm the M2-class of circuit breakers. In that case, 5 close–open operations at minimum, rated, and maximum supply voltage, with recording the operating characteristics, are performed after completion of a total of 10,000 operations.
Post-Test Evaluations and Acceptance Criteria
The standard does not allow any major maintenance during these operations. Although it permits lubrication and mechanical adjustment during the test, it does not allow us to change the breaker’s contacts. If any minor maintenance is done during this sequence of operation, that should be recorded in the test report. We must check the contact travel characteristics and all other parameters before and after the extended mechanical endurance test. If all the parameters are within the specified tolerance after 10,00 operations, the circuit breaker is said to have successfully passed the M2 class confirmation test.