What is the difference between the TM-D and TM-G and MA tripping units for compact NSX MCCB Circuit Breakers
Circuit breakers equipped with thermal-magnetic trip units are used mainly in industrial and commercial electrical distribution applications:
TM-D, for protection of cables on distribution systems supplied by transformers
TM-G, with a low pick-up for generators (lower short-circuit currents than with transformers) and distribution systems with long cable lengths (fault currents limited by the impedance of the cable
For Thermal protection (Ir)
Thermal overload protection based on a bimetal strip providing an inverse time curve I2t, corresponding to a temperature rise limit. Above this limit, the deformation of the strip trips the circuit breaker operating mechanism.
This protection operates according to:
Ir that can be adjusted in amps from 0.7 to 1 times the rating of the trip unit (16 A to 250 A), corresponding to settings from 11 to 250 A for the range of trip units
a non-adjustable time delay, defined to ensure protection of the cables.
For Magnetic tripping (Im)
Short-circuit protection with a fixed or adjustable pick-up Im that initiates
instantaneous tripping if exceeded.
TM-D: fixed pick-up, Im, for 16 to 160 A ratings and adjustable from 5 to 10 x In for
200 and 250 A ratings fixed pick-up for 16 to 63 A ratings
ForMA magnetic trip units
In distribution applications, circuit breakers equipped with MA magnetic-only trip
units are used for:
short-circuit protection of secondary windings of LV/LV transformers with overload protection on the primary side.
Magnetic protection (Im)
Short-circuit protection with an adjustable pick-up Im that initiates instantaneous tripping if exceeded.
Im = In x ... set in amps on an adjustment dial covering the range 6 to 14 x In for 2.5 to 100 A ratings or 9 to 14 In for 150