Page 126 - L . V S W I T C H G E A R
P. 126
intelligent
Molded Case Circuit Breakers
Molded Case Circuit Breakers
An MCCB provides protecƟon bLJ electromagneƟc device͘ Both these
combining a temperature sensiƟve devices act mechanicallLJ on the trip
device with a current sensiƟve elec- mechanism͘
tromagneƟc device͘ Depending upon the applicaƟon
The tradiƟonal molded-case circuit and required protecƟon, an MCCB
breaker uses electromechanical will use one or a combinaƟon of
(thermal magneƟc) trip units that diīerent trip elements that protect
maLJ be Įdžed or interchangeable͘ against the following condiƟons:
An MCCB provides protecƟon bLJ ͻ Thermal overloads
combining a temperature sensi- ͻ Short circuits
Ɵve device with a current sensiƟve ͻ Ground faults
Thermal Kverload :
/Ŷ ĂŶ ŽǀĞƌůŽĂĚ ĐŽŶĚŝƟŽŶ͕ ƚŚĞƌĞ͛Ɛ Ă ƚĞŵƉĞƌĂƚƵƌĞ ďƵŝůĚƵƉ ďĞƚǁĞĞŶ ƚŚĞ ŝŶƐƵůĂƟŽŶ ĂŶĚ
ĐŽŶĚƵĐƚŽƌ͘ /Ĩ ůĞŌ ƵŶĐŚĞĐŬĞĚ͕ ƚŚĞ ŝŶƐƵůĂƟŽŶ͛Ɛ ůŝĨĞ ǁŝůů ĚƌĂƐƟĐĂůůLJ ƌĞĚƵĐĞ͕ ƵůƟŵĂƚĞůLJ
ƌĞƐƵůƟŶŐ ŝŶ Ă ƐŚŽƌƚ ĐŝƌĐƵŝƚ͘ dŚŝƐ ŚĞĂƚ ŝƐ Ă ĨƵŶĐƟŽŶ ŽĨ ƚŚĞ ƐƋƵĂƌĞ ŽĨ ƚŚĞ ƌŵƐ ĐƵƌƌĞŶƚ
;/ ƐƋƵĂƌĞĚͿ͕ ƚŚĞ ƌĞƐŝƐƚĂŶĐĞ ŝŶ ƚŚĞ ĐŽŶĚƵĐƚŽƌ ;ZͿ͕ ĂŶĚ ƚŚĞ ĂŵŽƵŶƚ ŽĨ ƟŵĞ ƚŚĞ ĐƵƌƌĞŶƚ
ŇŽǁƐ ;ƚͿ͘
Ground Fault CondiƟon:
A ground fault actually is a type of short
ĐŝƌĐƵŝƚ͕ ŽŶůLJ ŝƚ͛Ɛ ƉŚĂƐĞͲƚŽͲŐƌŽƵŶĚ͕ ǁŚŝĐŚ
probably is the most common type of fault
on low-voltage systems (600V or less).
Usually, arcing ground-fault currents are not
large enough to be detected by the stan-
ĚĂƌĚ D ƉƌŽƚĞĐƟǀĞ ĚĞǀŝĐĞ͘ Ƶƚ͕ ŝĨ ůĞŌ
ƵŶĚĞƚĞĐƚĞĚ͕ ƚŚĞLJ ĐĂŶ ŝŶĐƌĞĂƐĞ ƐƵĸĐŝĞŶƚůLJ ƚŽ
ƚƌŝƉ ƚŚĞ ƐƚĂŶĚĂƌĚ ƉƌŽƚĞĐƟǀĞ ĚĞǀŝĐĞ͘ tŚĞŶ
this happens, it usually is too late, and the
ĚĂŵĂŐĞ ŝƐ ĂůƌĞĂĚLJ ĚŽŶĞ͘ Ŷ ĞdžĂŵƉůĞ ŽĨ
ƚŚŝƐ ŝƐ Ă ŵŽƚŽƌ ŚĂǀŝŶŐ ĂŶ ŝŶƚĞƌŶĂů ŝŶƐƵůĂƟŽŶ
ĨĂŝůƵƌĞ͘ tŚŝůĞ ƚŚĞ ĐƵƌƌĞŶƚ ŇŽǁ ŵĂLJ ďĞ ƐŵĂůů͕
it must be detected and eliminated before
ŵĂũŽƌ ŵŽƚŽƌ ĚĂŵĂŐĞ ƚĂŬĞƐ ƉůĂĐĞ͘ WƌŝŽƌ ƚŽ
ƚŚĞ ŝŶƚƌŽĚƵĐƟŽŶ ŽĨ ĞůĞĐƚƌŽŶŝĐ Ɛ͕ ƐĞƉĂƌĂƚĞ
ŐƌŽƵŶĚ ĨĂƵůƚ ƉƌŽƚĞĐƟŽŶ ĚĞǀŝĐĞƐ ǁĞƌĞ ƵƐĞĚ
ƚŽ ƉƌŽǀŝĚĞ ƚŚŝƐ ĂĚĚŝƟŽŶĂů ůĞǀĞů ŽĨ ƉƌŽƚĞĐ-
ƟŽŶ͘ dŽĚĂLJ͛Ɛ ŵŽĚĞƌŶ ĞůĞĐƚƌŽŶŝĐ ŚĂƐ ƚŚĞ
ŐƌŽƵŶĚ ĨĂƵůƚ ƉƌŽƚĞĐƟŽŶ ĂƐ ĂŶ ŝŶƚĞŐƌĂů ƉĂƌƚ ŽĨ
the trip unit.
126
