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Molded Case Circuit Breakers intelligent
Kverload Trip AcƟon:
Short-Circuit
KǀĞƌůŽĂĚ Žƌ ƚŚĞƌŵĂů ƚƌŝƉ ĂĐƟŽŶ ƵƐĞƐ Ă ƉŝĞĐĞ ŽĨ ďŝŵĞƚĂů Trip AcƟon:
heated by the load current. This bimetal is actually
two strips of metal bonded together, with each having
^ŚŽƌƚͲĐŝƌĐƵŝƚ ƚƌŝƉ ĂĐƟŽŶ ƵƐĞƐ ĂŶ ĞůĞĐ-ƚ ŝ
Ă ĚŝīĞƌĞŶƚ ƚŚĞƌŵĂů ƌĂƚĞ ŽĨ ŚĞĂƚ ĞdžƉĂŶƐŝŽŶ͘ dŚĞLJ ĂƌĞ ^Ś ŝƚ ƚ ŝ Ɵ ů
ĨĂĐƚŽƌLJͲĐĂůŝďƌĂƚĞĚ ĂŶĚ ŶŽƚ ĮĞůĚͲĂĚũƵƐƚĂďůĞ͘ ƚƌŽŵĂŐŶĞƚ ŚĂǀŝŶŐ Ă ǁŝŶĚŝŶŐ ƚŚĂƚ͛Ɛ ŝŶ
heat will cause the bimetal to bend. That part of the series with the load current. When a
ďŝŵĞƚĂů ŚĂǀŝŶŐ ƚŚĞ ŐƌĞĂƚĞƌ ƌĂƚĞ ŽĨ ĞdžƉĂŶƐŝŽŶ ŝƐ ŽŶ ƚŚĞ ƐŚŽƌƚ ĐŝƌĐƵŝƚ ŽĐĐƵƌƐ͕ ƚŚĞ ĐƵƌƌĞŶƚ ŇŽǁŝŶŐ
outside of the bend curve. To trip the CB, this bimetal through the circuit conductor causes the
ŵƵƐƚ ĚĞŇĞĐƚ ĞŶŽƵŐŚ ƚŽ ƉŚLJƐŝĐĂůůLJ ƉƵƐŚ ƚŚĞ ƚƌŝƉ ďĂƌ ŵĂŐŶĞƟĐ ĮĞůĚ ƐƚƌĞŶŐƚŚ ŽĨ ƚŚĞ ĞůĞĐƚƌŽ-
and unlatch the contacts. ŵĂŐŶĞƚ ƚŽ ŝŶĐƌĞĂƐĞ ƌĂƉŝĚůLJ ĂŶĚ ĂƩƌĂĐƚ
the armature. When this happens, the
armature rotates the trip bar, causing the
CB to trip.
dŚĞ ŽŶůLJ ƟŵĞ ĚĞůĂLJ ĨĂĐƚŽƌ ŝŶǀŽůǀĞƐ ƚŚĞ
ƟŵĞ ŝƚ ƚĂŬĞƐ ĨŽƌ ƚŚĞ ĐŽŶƚĂĐƚƐ ƚŽ ƉŚLJƐŝ-
ĐĂůůLJ ŽƉĞŶ ĂŶĚ ĞdžƟŶŐƵŝƐŚ ƚŚĞ ĂƌĐ͖ ƚŚŝƐ
usually is less than one cycle.
Short-Circuit CondiƟon: DĂŐŶĞƟĐ ĞůĞŵĞŶƚƐ ĂƌĞ ĞŝƚŚĞƌ ĮdžĞĚ Žƌ
adjustable, depending upon the type of
ĂŶĚ ĨƌĂŵĞ ƐŝnjĞ͘ &Žƌ ĞdžĂŵƉůĞ͕ ŵŽƐƚ
Usually, a short circuit occurs when abnormally high ƚŚĞƌŵĂů ŵĂŐŶĞƟĐ ďƌĞĂŬĞƌƐ ĂďŽǀĞ ƚŚĞ
ĐƵƌƌĞŶƚƐ ŇŽǁ ĂƐ Ă ƌĞƐƵůƚ ŽĨ ƚŚĞ ĨĂŝůƵƌĞ ŽĨ ĂŶ ŝŶƐƵůĂƟŽŶ 150A frame size have adjustable mag-
ƐLJƐƚĞŵ͘ dŚŝƐ ŚŝŐŚ ĐƵƌƌĞŶƚ ŇŽǁ͕ ƚĞƌŵĞĚ ƐŚŽƌƚͲĐŝƌĐƵŝƚ ŶĞƟĐ ƚƌŝƉƐ͘
ĐƵƌƌĞŶƚ͕ ŝƐ ůŝŵŝƚĞĚ ŽŶůLJ ďLJ ƚŚĞ ĐĂƉĂďŝůŝƟĞƐ ŽĨ ƚŚĞ ĚŝƐƚƌŝ-
ďƵƟŽŶ ƐLJƐƚĞŵ͘ dŽ ƐƚŽƉ ƚŚŝƐ ĐƵƌƌĞŶƚ ŇŽǁ ƋƵŝĐŬůLJ ƐŽ ƚŚĂƚ
major damage can be prevented, the short circuit or
instantaneous element of an MCCB is used.
ƚLJƉŝĐĂů ƟŵĞ ĐƵƌƌĞŶƚ ĐƵƌǀĞ ĨŽƌ ĂŶ ŝŶƐƚĂŶƚĂŶĞŽƵƐ ĞůĞ-
ŵĞŶƚ͕ ĂƐ ƐŚŽǁŶ ŝŶ &ŝŐ͘ Ϯ͕ ƐŚŽǁƐ ƚŚĂƚ ŝƚ ǁŝůů ŶŽƚ ŝŶŝƟĂƚĞ
Ă ƚƌŝƉ ƵŶƟů ƚŚĞ ĨĂƵůƚ ĐƵƌƌĞŶƚ ƌĞĂĐŚĞƐ Žƌ ĞdžĐĞĞĚƐ WŽŝŶƚ͘
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