Reliability estimation of moulded case circuit breaker through experimental degradation analysis

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International Journal on Mechanical Engineering and Robotics (IJMER)

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ISSN (Print) : 2321-5747, Volume-4, Issue-1,2016 39

Reliability estimation of moulded case circuit breaker through experimental degradation analysis

1Greegory Mathew, ²Krishna Gaikwad

1Department of Mechanical Engineering, Sardar Patel College of Engineering, Andheri –West, Mumbai -400058

2Department of Mechanical Engineering, Thakur College of Engineering & technology, Kandiwali –East Email: [email protected], [email protected],

Abstract— Life test of circuit breakers on mechanical endurance test show degradation in its component strength due to wear, fatigue, shock loads, etc. Degradation in mechanical strength leads to failure of the component in shear, bending, etc. and hampers the ability of the mechanism to operate the circuit breaker. he paper demonstrates a technique to predict the reliability of a moulded case circuit breaker based on experimental degradation analysis.\The permanent joints(riveting) of the components of the mechanism are replaced by fasteners to facilitate the replacement of failed components. The circuit breaker is tested for life on a test bench. Failed components are replaced by new ones, so that the test can be continued and the life of other components can be estimated. The paper is an applied research paper. The components of the mechanism form a series RBD. Reliability is predicted by computing individual component reliability and using the series law

Keywords: degradation analysis, functional, degradation, reliability

I. INTRODUCTION

Highlight a American National Standards Institute (ANSI) defines a circuit breaker as: “A mechanical switching device, capable of making, carrying and breaking currents under normal circuit conditions. Also capable of making and carrying for a specified time and breaking currents under specified abnormal circuit conditions, such as those of a short circuit.” They are complex engineering systems used for protection of distribution feeder systems, transformers, etc.

A Moulded case circuit breaker (MCCB) is a circuit breaker having a supporting housing of moulded insulating material forming an integral part of the circuit breaker. Present day, Situation shows MCCB in all low voltage (less than 1000V) applications, in residential electrical distribution panel, in the industrial power distribution centre, and in main power feed panels used in large buildings such as offices, hospitals, and shopping centers.

Circuit breaker failures have resulted in catastrophic damages and loss of human lives in the past. Repeated failure leads to annoyance, inconvenience and a lasting customer dissatisfaction that can play havoc with the responsible company's marketplace position. Hence continual assessment of new product reliability and ongoing control of the reliability of everything shipped are critical necessities in today's competitive business arena

The importance of achieving reliability requirements on today’s systems cannot be understated. Reliability is the probability of equipment or processes to function without failure when operated correctly for a given interval of time under stated conditions. Reliability is defined as the ability of a system or component to perform its required functions under stated conditions for a specified period of time (IEEE; 1990). Noncompliance with reliability requirements may result in reduced mission effectiveness and billions of dollars in added operating and support costs over the life cycles of systems.

Every physical component possesses an inherent strength.

During operation, these components are subjected to a certain level of stress. The stress strength interference model is based on the concept that the applied stress should always be lower than the strength of the component for safe working. If the stress is higher than its strength, the components will be subjected to failure.

Stress is a variable which tends to produce a failure of a component or of a device of a material. In general engineering application, the stress may be induced due to mechanical load, environmental conditions, temperature and electric current etc. Strength is the ability of component to accomplish its required function (mission) satisfactorily without failure when subjected to the external loading. Stress and strength follow a statistical distribution and their relationship is represented in a stress strength model.

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II. LITERATURE REVIEW

E J Barbero and T M Damiani, 2003, In further investigations concerning to calculate components reliability based on the SSI model, presents a simple model to predict the time- and environment-dependent degradation of tensile strength of unidirectional E-glass fibre composites, P R.Potdar and SB Rane,2013, presents a method to predict reliability of Moulded Case Circuit Breaker (MCCB) mechanism based on Stress Strength Interference (SSI) with degradation analysis, Nandan Sudarsanam, 2005, presents a method of estimating stress- strength interference reliability under a time dependent degradation analysis. David W. Coit, et. al., 2005 developed a methodology was to correlate field life with observed degradation for electronics modules. This procedure was developed by identifying common deterioration characteristics in field units, modeling observed trends and then developing a model to predict future deterioration trends. Chendong Zhu, et. al., 2003 Developed a robust, time dependent stress methodology for investigating mixed mode reliability degradation in advanced SiGe HBTs. E.W. Kreutzet. al., 2000 Discusses the influence of degradation on performance and life time of high-power diode laser bars. Young Kap Son, , 2011 considers multiple competing failure modes for systems with degrading components in terms of system functionality and system performance with the assumption that system functionality is dependent of system performance. Component degradation explains not only the component aging processes leading to failure in function, but also system performance change over time. Rafik Medjoudj, et. al., 2011 a circuit breaker degrades gradually due to the use, and it is also subject to the shock process resulted from the stress of isolating the fault when a short circuit occurs in the system. Based on failure mechanisms developments, the wear out of the circuit breaker contacts is modeled. Yongming Liu, et. al., 2006 a general methodology for fatigue reliability degradation of railroad wheels is proposed in this paper.

Both fatigue crack initiation and crack propagation life are included in the proposed methodology using previously developed multiaxial fatigue models.

III. DEGRADATION ANALYSIS

The stress-strength interference model is one analytical method used to compute reliability of system or component. M. Baohai, et. al., 1997, it is found to be useful in situations where the reliability of a component or system is defined by the probability that a random variable Y (representing strength) is greater than another random variable X (representing stress). Nandan Sudarsanam, 2005, any state of component where Y falls below X represents the component to be in unacceptable state or to have failed. Once the distribution and

parameters of X and Y are determined, the reliability can be calculated by estimating the probability X<Y, which is computed by equation

Where, f(x) is the probability density function (pdf) of stress (X) and f(y) is the probability density function (pdf) of strength (Y).

Mechanical endurance test show degradation in its component strength due to wear, fatigue, shock loads, etc.

Degradation in mechanical strength leads to failure of the component in shear, bending, etc. and hampers the ability of the mechanism to operate the circuit breaker. There are four degradation models available for reliability analysis.

Those are linear, exponential, power and logarithmic. A power degradation model is typically seen in situations where the level of the mean rate of wear is monotonically decreasing. Based on the work by E J Barbero and T M Damiani, 2003, a power degradation model is represented in equation

where, S(t) is the value of tensile strength at time t and is the initial inert strength. Parameters α & β are constant values and are dependent on the component to be tested and the environmental conditions.

Accuracy of the reliability predicted through degradation models depends on the accuracy of estimation of the model parameters. A better approach is to calculate the reliability values of each component through experimentation or by subjecting the circuit breaker to accelerated life testing and measuring the wear in strength of the components. Hindrance in the ability to perform the stated function is a state of failure for the component.

The various states of the circuit breaker are On, Off, Reset and Trip positions. The circuit breaker consists of 26 components connected serially. In MCCB mechanism, the extension spring induces load on the various components of the mechanism. The intensity of spring force varies with respect to the state of the circuit breaker i.e. with respect to the position of the spring. From analysis of MCCB mechanism, the spring force on the upper link pivot pin is maximum in On state of the circuit breaker.

Hence stress calculation for the pin is done considering On state of the Circuit breaker. Similarly all stress calculations are carried out for other components considering maximum stress. The design is verified through real time testing on a life test setup. The different modes of possible failure for some of the components are listed in the table below.

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Sl. No. Description Shear Bending Buckling Tension Crushing Bearing

1 Upper link Y Y Y

2 Lower link Y Y Y

3 Latch link Y Y Y

4 Latch bracket Y Y

5 Fork Y Y Y

6 Floating pin Y Y Y Y

7 Spring pin Y Y Y Y

8 Trip plate pivot Y Y Y Y

9 Latch link stopper Y Y

10 Lower link pin Y Y Y

11 Drive shaft pin Y Y Y

12 Upper link pin Y Y Y

13 Fork hinge pin Y Y Y

14 Coupler rivet Y Y Y

15 Side plate Y Y Y Y Y

A total of 5 circuit breakers were selected for the study.

The components of the mechanism are connected or held between side plates with the help of permanent riveting.

The circuit breaker is tested on a life cycle test setup. The successful cycles completed by the circuit breaker mechanism indicates the life of the weak link. Testing a new circuit breaker for life will again result in the same component being fatigued. It will not be possible to calculate the life of the other components in the mechanism. To save on the financial constraints, the mechanism rivets are replaced by fasteners. The weak link that fails is replaced by a new component. The new failure will now be in some different component. This will help to calculate the life of the other component.

I. RELIABILITY ANALYSIS

Reliability of the MCCB mechanism depends on the components of the mechanism. Failure of any single component of the mechanism will affect the overall functioning of the mechanism. The components are thus in a series configuration by the RBD logic. The lifetimes for all components of the 5 circuit breaker mechanisms were determined experimentally. Using weibull plot, various distribution parameters were estimated and the reliability of all 26 components are plotted using minitab.

The reliability plot of the spring pin is shown in the figure below. The estimated reliability of some components at

different cycles of operation have been summarized in the table below.

Table : Reliability of various MCCB components at different cycles

Sr.

No

Component Reliability (10000)

Reliability (20000)

Reliability (30000) 1 Floating Pin 1 0.99988 0.98735

2 Spring Pin 1 0.99347 0.88142

3 Drive shaft pin

1 0.99974 0.99782 4 Upper link

1 0.99962 0.99752 5 Trip plate

pivot

1 1 0.99962

6 Latch link stopper

1 0.99974 0.99951 7 Fork hinge

1 0.99954 0.98083 8 Lower link

1 0.99962 0.99898

9 Side plate 1 0.99962 0.99898

10 Coupler rivet

1 1 0.9996

11 Lower link 1 0.99999 0.99998

12 Upper link 0.999539 0.84491 0.56287

13 Latch link 1 0.99969 0.99897

14 Fork 1 0.99969 0.99896

15 Latch bracket

1 0.99969 0.98998

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Rmech (30000) = 0.98735 x 0.88142 x 0.99782 x 0.99752 x 0.99752 x 0.99962 x 0.99962 x 0.99931 x 0.99997 x 0.99958 x 0.99951 x 0.9997 x 0.98083 x 0.98083 x 0.99898 x 0.99898 x 0.9996 x 0.9996 x 0.99998 x 0.99998 x 0.56287 x 0.99897 x 0.99896 x 0.99998 x 0.99998 x 0.99931

Rmech (30000) = 0.4584 Rmech (20000) = 0.8345 Rmech (10000) = 0.9995

0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 1.05

10000 cycles20000 cycles30000 cycles

Reliability

Number of cycles

Reliability of MCCB Mechanism

10000 cycles 20000 cycles

V. CONCLUSION

At the design and development stage of a product, reliability analysis of the complete system is simpler based on experimental degradation analysis. Reliability of the MCCB mechanism at 30000 cycles, 20000 cycles and 10000 cycles are 0.4584, 0.8345, and 0.9995 respectively. From reliability analysis, we can conclude that the mechanism will survive minimum 10000cycles without any failure. Decision of warranty period and maintenance strategy for MCCB can be decided from reliability analysis.

REFERENCES

[1] Ever J Barbero, Thomas M Damiani, Phenomenological prediction of tensile strength of E-glass composites from available aging and stress corrosion data, Journal of reinforced plastics and composites, 2003.

[2] Prathamesh R.Potdar, Santosh B.Rane, Reliability analysis of moulded case circuit breaker mechanism based on stress strength interference with degradation analysis, International journal of engineering science and innovative technology, 2013.

[3] Nandan Sudarsanam, Methods of estimating stress- strength interference reliability under a time dependent degradation analysis, Oklahoma state university, 2005.

[4] David W. Coit, John L. Evans, Nathan T. Vogt and James R. Thompson, A method for correlating field life degradation with reliability prediction for electronic modules, Quality and reliability engineering international, 2005

[5] Chendong Zhu, Qingqing Liang, Ragad Al-Huq, John D Cressler, Alvin Joseph, Jarle Johansen, Tiangbing Chen, Guofu Niu, Greg Freeman, Jae-Sung Rieh and David Ahlgren, An investigation of the damage mechanisms in Impact ionization induced mixed mode reliability stressing of scaled SiGe HBTs, International electronic devices meeting, 2003

[6] E.W. Kreutz, Nicolas Wiedmann, JuKrgen Jandeleit, D. Ho!mann, Peter Loosen, Reinhart Poprawe, Reliability and degradation mechanisms of InGa(Al)As/GaAs DQW high-power diode lasers, Journal of crystal growth, 2000

[7] Young Kap Son, Reliability prediction of engineering systems with competing failure modes due to component degradation, Journal of Mechanical Science and Technology, 2011 [8] Rafik Medjoudj, Rabah Medjoudj and D.Aissani,

Reliability modeling and data analysis of vacuum circuit breaker subject to random shocks, World academy of science, engineering and technology, 2011

[9] Yongming Liu, Liming Liu, Brant Stratman, Sankaran Mahadevan, Multiaxial fatigue reliability analysis of railroad wheels, Reliability engineering and system safety, 2006

[10] M. Baohai, Z. Jingyi, Q. Lihua, The Applications of Fuzzy Reliability in the Hydraulic Cylinder Design, Technical Paper, Beijing University of Aeronautics and Astronautics, Beijing, China, 1997.

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