Functional units. This topic explains coordination type I and type II in motor feeders, standard readings of a switch gear components, selection factors for ACB and MCCB, microprocessor-based releases, CT, and VT. Type I and Type II coordination in motor feeder standards like IEC 60947-4-1 and IS 13947 specify motor production requirements for the selection of various power components for motor starters or feeders. Recall that MCCB or SFU, where the contactor and overload relay are connected serially prior to connecting the motor power cable. Assume a short circuit at the power cable terminal location at the switch gear. Type I coordination requires that under short-circuit conditions no component of the starter shall cause any danger to persons or installation. The motor feeder may be rendered unsuitable for further service without repair and replacement of parts. Type I coordination is not in practice in the industry for obvious reasons. Type II coordination requires that under short-circuit conditions components used in starter shall not cause any danger to persons or installation. These components after the short-circuit shall be suitable for intended functionality without any repair or replacement of spare parts. However, slight welding of contactors above contacts is considered acceptable if the contacts can be separated and re-used. Also the time current characteristics of the overload relay should not get adversely affected due to fault current. This ensures safety, low downtime, and availability of intended functionality without any replacement of spare parts. From the curve it can be considered that fuse will not act for starting current transients. However, as indicated in the curve, until the crossover point, thermal overload relay will act and any current beyond that point fuse will act. Remember that thermal overload relay is only a sensor and it acts through its auxiliary contact to break the buffer circuit by de-energizing the contactor's coil. It may also be observed that the contactor breaking capacity is more than the current at the crossover point. The table displays information on the standard readings for the components, MCBs MCCBs, SFU, ACB, and busbar for continuous current rating and short-circuit breaking capability. Selection of ACB or MCCB depends on current rating, which is dependent on total connected load, transformer rating, ambient temperature, atmospheric conditions, and altitude. The rating shall be done for ambient temperature, altitude, atmospheric condition, and special application like the capacitor. Selection of ACB or MCCB also depends on short-time withstand capability, breaking, and making capacity depending on current rating, release type or externally CT operated relay type, terminal capacity and arrangement for termination of cables. Ampacity or current carrying capacity of the ACB shall be at least 110 percentage of the estimated load connected to the feeder. In case the ACB or MCCB is used as an incomer to the switchboard it shall be matched with the busbar ampacity rating. Short-circuit withstanding capability and breaking capacity shall be matched to short-circuit withstanding capability of the switch gear busbars. ACBs and MCCBs are provided with auxiliary contacts for indication of its status at a remote location. In order to monitor the status of the switching device on or off and trip condition, tripped or not tripped from a remote location, the MCBs, MCCBs, SFUs, contactors, and ACBs are provided with potential free auxiliary contacts. These contacts can be wired up to the remote location, if located nearby, or they can be communicated through intelligent electronic devices, or IED, example, bay control unit, microprocessor based relay etc. Microprocessor based releases will have additionally auxiliary contacts for indicating the type of faults. MCCBs may be used for outgoing feeders. They can also be used as incomer if the rating is within manufacturing range. The MCCBs are very compact compared to ACB, and are available up to ratings of 1,200 amperes, with short-time rating of 85 kiloamperes per one second. MCCBs limit the through fault current, as the fault is cleared within the first half cycle duration. It has very low let-through energy, and cutoff so that the sizing of outgoing cables can be to match with the let-through energy rather than the switchboard fault level. The operating mechanism is a quick-make, quick-break and trip-free. It is independent of the manual speed of operations. MCCBs have built-in thermal and magnetic releases, or microprocessor based releases, to provide protection against overload and short-circuit. Protection settings may be adjustable depending on make and type. Push to trip facility to verify healthiness of mechanism. MCCBs are less expensive than ACBs and occupy less space as compared to ACBs. MCCBs need to be motorized if the remote operation is preferred. Microprocessor based releases are built-in with ACBs and MCCBs to provide protection against overloads and short-circuit currents. The releases built within the ACBs are more sophisticated than MCCB releases. The releases are self-powered and do not require any external auxiliary power supply for their functioning. Apart from protective functions, the releases may have the following features depending on the make and type; display of RMS currents and individual fault annunciation through LEDs. Self-diagnostic watchdog facility. Different models of releases are available and used in the industry depending on the requirement. Microprocessor based releases are superior to ordinary thermal magnetic over-current releases though both are in use. CTs and VTs are used for a metering and protection functions. They are available in cast resin type, and tape wound versions. Toroidal and rectangular CTs are manufactured to suit various busbar sizes. CTs and VTs are covered in detail in a separate module. Here's a quick recap of what you've learned this far. Type 1 coordination requires that under short circuit conditions, components of the starter shall not cause any danger to persons or installation. However, motor feeder may be rendered unsuitable. Type 2 coordination requires that under short-circuit conditions, components used in starter shall not cause any danger to persons or installation. These components, after the short-circuit, shall be suitable for intended functionality without any repair or replacement of spare parts. Ampacity or current carrying capacity of the ACB shall be at least 110 percentage of the estimated load connected to the feeder. MCCB is very compact compared to ACB and is available up to ratings of 1,200 amperes. MCCBs limit the true fault current as the fault is cleared within the first half cycle duration. It has very low let-through energy. Microprocessor based releases are built-in within ACBs and MCCBs to provide protection against overloads and short-circuit currents. CTs and VTs are used for metering and protection functions.
