Circuit breakers or contactors
Author: Ted Olsen
01/01/2005 - Volume II - Issue I
We are often asked to discuss the application considerations that favor the use of medium-voltage breakers, and those that favor the use of medium voltage NEMA Class E2 controllers (fused contactors). This is one of those recurring questions, so this issue of Tech Topics addresses it.
To compare the application of medium-voltage circuit breakers and of fused contactors, we must understand the basic characteristics of each switching technology.
The table below shows the major characteristics of medium-voltage circuit breakers and medium-voltage fused contactors that influence the application. Of course, the entries are generalized, and the information varies by the voltage and current ratings of the equipment. However, the table is valid for an overall understanding.
From the data in the table, we make these observations:
1. Medium-voltage circuit breakers are favored when:
a. Typical loads: transformers, capacitors, larger motors, generators, distribution feeders
b. Ratings required exceed those of contactors (360A or 720A at up to 5kV, 360A at up to 7.2kV)
c. Continuous load current is high (i.e., larger transformers, larger motors)
d. Switching is not very frequent (i.e., weekly or monthly); high endurance (1000's of operations) is satisfactory
e. Process continuity is critical (i.e., no time for fuse replacement)
f. Reduced voltage starting is not needed (RV starting complicates switchgear bus arrangements)
2. Medium-voltage NEMA Class E2 controllers (fused contactors) are favored when:
a. Typical loads: motors, smaller transformers
b. Continuous load current is low or moderate (i.e., smaller motors or transformers)
c. Switching is very frequent (i.e., daily or several times per day); very high endurance (100,000s of operations) is needed
d. Process continuity compatible with fuse replacement time
e. Reduced voltage starting is needed to reduce starting duty (and voltage fluctuation) on system
Historically, circuit breakers have been used for medium-voltage motors in certain industries, especially in utility generating stations. As these stations have aged, and station operation has changed from base-load to peaking service, many of these motor-starting circuit breakers have experienced total operations well in excess of the endurance required by the ANSI standards. As a result, these applications have had higher maintenance costs than if medium-voltage fused contactors had been originally. In contrast, users in the process industries have long favored the use of fused contactors for such applications and have enjoyed long service with lower maintenance costs.
When applied properly, both medium-voltage circuit breakers and medium-voltage contactors should provide decades of reliable service. Applied incorrectly, either can lead to major headaches.