Current-carrying equipment, ratings

Deciding the ratings of current-carrying equipment, devices and components... 

To the basic current requirement is applied the derating faetors for various service conditions, as noted in Section 1.5.4.2. The equipment, devices and components may then be cbosen to be as close (nearest higher) to this rating as possible from the available standard ratings. Based on these ratings, the minimum cross-sectional areas of the other current-carrying parts used in the circuit, such as interconnecting links and the cables-are calculated. 

Both ratings, thermal and short-time, will define the cross-sectional area of the current-carrying components. The more severe of the two will then prevail. In the following text we explain the procedure to assign the short-time current rating to equipment, a device or a component. 

The ampacity or rated current carrying capacity of the electrical service entrance conductors that connect the utility company s lines to the plant s service entrance equipment must be a minimum of 125% of the calculated maximum demand for continuous loads plus 100% of the maximum calculated demand for non-continuous loads. Service entrance conductors and equipment with higher ratings or provisions to increase the rating of the service entrance conductors are recommended. 

The consumer s mains equipment must provide protection against overcurrent, that is, a current exceeding the rated value (lET Regulation 430.3). Fuses provide overcurrent protection when situated in the live conductors they must not be connected in the neutral conductor. Circuit-breakers may be used in place of fuses, in which case the circuit-breaker may also provide the means of isolation, although a further means of isolation is usually provided so that maintenance can be carried out on the circuit-breakers themselves. 

Since the electrical supply system carries the full apparent power (VA), a current higher than is theoretically necessary to supply the power demand needs to be supplied. Equipment must therefore be rated to carry the full apparent power plus the losses of the supply system, which are proportional to the square of the current. 

Filtered air may be used to purge a complete room, or it m be confined to a specific area and incorporate the principle of laminar flow, which permits operations to be carried out in a gentle current of sterile air. The direction of the airflow may be horizontal or vertical, depending upon the type of equipment being nsed, the type of operation and the material being handled. It is important that there is no obstruction between the air supply and the exposed product, since this may resnlt in the deflection of microorganisms or particulate matter fiom a non-sterile surface and canse contamination. Airflow gauges are essential to monitor that the correct flow rate is obtained in laminar flow units and in complete suites to ensure that a positive pressure fiom clean to less clean areas is always maintained. 

Analysis of the data from the current conflicts in Afghanistan and Iraq shows that the increased frequency of field tourniquets in treating traumatic limb injuries has contributed to lower mortality due to battlefield injuries. This was accomplished with extremely low rates of tourniquet-induced complications. Contributing to the low rate of complications was the personnel training in the treatment of extremity injuries. The analysis further indicated that all battlefield personnel should carry tourniquets and that they should be carried directly on their person and not stowed in their vehicle, back packs, or other equipment bags. 

The measurement of polarographic currents is carried out by various techniques according to the rate of the reaction involved. Whereas for reactions with a half-time greater than about 15 sec the measurement of mean currents in the classical polarographic arrangement is more useful, for faster reactions it is necessary to use special equipment, as will be discussed separately. 

Overheating of cables and equipment will occur if they become overloaded. Electrical equipment and circuits are normally rated to carry a given safe current which will keep the temperature rise of the conductors in the circuit or appliance within permissible limits and avoid the possibility of fire. These safe currents define the maximum size of the fuse (the fuse rating) required for the appliance. 

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