C Curve circuit breakers are the standard tripping curves.
D Curve circuit breakers are designed to absorbed high starting currents with out the circuit breaker tripping during this start up period. These are used predominately for motor protection.
Some manufacturers refer to these as “Curve 1” circuit breakers

Riken RKB 6ka ranges of Mcb’s have the following characteristics:
“C” Curve – Will trip within 20 milliseconds at 5xIn
“D” Curve – Will trip within 20 milliseconds at 10xIn

It is worth noting that both C & D curves will trip in almost the same time when the current drawn exceeds 25 times the rating of the circuit breaker

This small unit that can easily be fitted to the larger moulded case circuit breakers is used to remotely trip a circuit breaker. This is done in conjunction with remote core earth leakage units or safety monitors in an installation that can switch off the power when necessary.

Auxiliaries are fitted to almost all contactors and can be fitted to circuit breakers. The basic purpose of an auxiliary is to operate in conjunction with the main contacts of the contactor or circuit breaker and can be used for various purposes, for example:

• To give a remote “trip” indication
• To remove power from control circuits
• To give visual indication of the units position (On / Off or Trip)

Most overloads are activated by the movement of bi-metal strips that move according to the rise in temperatures and excess current will cause them to move until the overload trips. It is natural then, that the ambient will have some effect on the operating of not only overloads but thermally activated circuit breakers (Thermal magnetic types). It is for this reason that most overloads have been calibrated to operate in temperatures NOT exceeding 40 degrees Celsius.

A DOL starter is connected across either a single phase and neutral or 3 phases for 380/400Volt motors which can be connected  permanently in either star or delta.
A star/delta starter has 6 leads from the starter to the motor and initially the motor is started in the ‘star” mode and after a preset period of a few seconds it switches the motor into a delta connection which then supplies each coil of the motor with 2 phases. This results in lower current being drawn through the overload. The current drawn is √3 of the current drawn by the motor.

Most motors are designed to take in a safety factor and so they actually operate at a lower current than shown on the name plate. The proper method is when the motor is running correctly one should ascertain the current consumption at that point and set the overload SLIGHTLY higher than the current drawn in under normal working conditions. This will allow the thermal overload to trip within the safety limits of the motor.

The cause of this is that the connections to the motor were incorrect. Check the motor connections for correct phase rotation on each of the motor windings.

This varies with most manufacturers but a good rule of the thumb is that most contactors will drop out at 55/60% of the rated voltage, and will increase again at about 75/100% of the voltage

Discrimination is also referred to as "selectivity" and is the co-ordination between upstream and downstream circuit breakers in such a manner that a fault appearing at a given point in a network is cleared by the upstream circuit breaker closest to the fault and that no other upstream circuit breaker will trip.

Cascading is the use of current limiting capacity circuit breakers upstream of any given reference so as to permit installation of lower rated fault level circuit breakers downstream of that unit and by doing this the lower "ka" fault level downstream circuit breaker costs can be lessened.