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NF50-HRP Datasheet(PDF) 80 Page - Mitsubishi Electric Semiconductor |
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NF50-HRP Datasheet(HTML) 80 Page - Mitsubishi Electric Semiconductor |
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80 / 105 page  79 7. SELECTION In selecting MCCBs for a particular application, in addition to purely electrical aspects of load and distri- bution conductor systems, physical factors such as panelboard configuration, installation environment, ambient-temperature variations, vibration, etc. must also be considered. MCCBs are rated for an ambient of 40 °C, and where panelboard internal temperatures may exceed this, the MCCBs installed should be derated in accor- dance with Table 7.1. 1. Actual load currents may exceed the nominal-val- ues. 2. Load currents may increase with time, due to dete- rioration of load devices (i.e., friction in motors). 3. Source voltage and frequency may vary. 7.1 Motor Branch Circuits The following discussion assumes single motors and cold-start operation. 7.1.1 General Considerations The starting current ( IMS) and time (TMS) for the mo- tor, and its full-load current, dictate the rated current, long-delay trip and instantaneous-trip curves for the MCCB as shown in Fig. 7.2. A safety-margin of up to 50% should be considered for the starting time, to allow for voltage variations and increase in load fric- tion. The instantaneous-trip curve should be at least 1.4 x normal starting current to allow for the effect of the DC component attendant to the low power factor (about 0.3) of the starting current. For -delta start- ing the unphased-switching allowance increases the 1.4 margin to 1.9. For running restarting the unphased- switching allowance increases the factor to 2.4. Rated currents MCCB trip curve Starting current and long-delay trip Inrush and inst. trip Motor starting current TMS IMS Current Fig. 7.2 MCCB and Motor Starting 7.1.2 Motor Breaker Where starting times are relatively short and currents are small, the Mitsubishi M Line motor breakers can be used without the need for a motor starter. 7.2 For Lighting and Heating Branch Cir- cuits In such circuits, switching-surge magnitudes and times are normally not sufficient to cause spurious tripping problems; however, in some cases, such as mercury- arc lamps or other large starting-current equipment, the methods presented in §7.1 above should be con- sidered. In general, branch MCCBs should be selected so that the total of ratings of the connected loads is not more than 80% of the MCCB rating. Fig. 7.1 MCCB Selection Consideration Table 7.1 MCCB Deratings Due to Installation Factors Panelboard max. internal temp. ( °C) 50 55 60 Load allowable, due to panelboard temp. (%) 90 80 70 Supply system Ambient temperature Main,submain or branch use Ambient conditions Wire connection Load current Operation conditions Load Short circuit Installation and connection style Service purpose Regulations ? |
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