3VA27 molded case circuit breakers & 3WL10 air circuit breakers
Manual, 10/2018, L1V30499596002-01
15
Current-carrying 3WL10 circuit breakers
Air circuit breakers of the 3WL range are designed as current-carrying circuit breakers. Their
place in the power distribution is upstream of molded case circuit breakers and they must
have time-selective current-carrying characteristics. They are equipped with ST releases for
short-time-delayed short-circuit protection. The INST release for instantaneous short-circuit
protection can be disabled in certain circumstances.
The air circuit breakers can carry the full short-circuit current for a certain duration without
the contacts lifting and limiting the short-circuit current. This gives lower-level devices
enough time to disconnect the outgoing feeder with the fault. These are the positive
characteristics of a protection device that is most frequently used near to the incoming
feeder and that, like the 3WL10 air circuit breaker, is time-selective and current-carrying in
the event of a fault.
Current-limiting 3VA27 molded case circuit breakers
"Current limiting" means that the peak value of the prospective impulse short-circuit current i
p
is limited to a smaller let-through current i
D
. Effective current limitation means that the circuit
breakers and busbar trunking systems can be constructed more compactly. In the event of a
short-circuit, the molded case circuit breaker substantially reduces the magnitude of the let-
through currents, i.e. reduces the load reaching downstream equipment (less thermal load,
lower dynamic forces). The let-through energy is also significantly reduced. 3VA molded
case circuit breakers are designed to be current limiting. IEC EN 60947-2 (VDE 0660-101),
section 2.3, page 12, defines current-limiting molded case circuit breakers in the following
way:
"Circuit breaker that, within a specified range of current, prevents the let-through current from
reaching the prospective peak value and which limits the let-through energy (I
2
t) to a value
less than the let-through energy of a half-cycle wave of the symmetrical prospective current."
These are the positive characteristics of a protection device that is most frequently used on
the outgoing side and that, like the 3VA27 molded case circuit breaker, is dynamically
selective and current limiting in the event of a fault.
Advantages of the platform approach
This product platform (ACB/MCCB) provides users with a selection of product variants
depending on their requirements whereas the accessories, electronic trip units, electronics,
connections, and outer dimensions are mainly the same.
This is made possible by the modular design of the contact system inside the product.
The user or plant constructor does still have to perform integration tests, but the physical
design and the system of accessories remain largely intact.
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