Short-time ratings

When external secondaiy resistance is used for improved starting charac teristics, short-time-rated resistors are employed. As the motor accelerates, steps of resistance are cut out on a time or current basis to give the desired accelerating torque and current characteristics. 

Curve with short time rating, but output not exceeding C.M.R. - Curve with short time rating, but output more than C.M.R. 

If the cycle has a short-time rating, with a period of energization and one of rest, the motor will obviously cool during the de-energizing period, and depending upon the peak load and tlie rest periods, a comparatively lower output motor can perform duties at higher loads. The equivalent output for the load cycle of Figure 3.16 is... 

Since an autotransformer is in the circuit for only a short period (during the start only), it can be short-time rated. The rating of the autotransformer can be calculated from... 

Since the transformer will be in the circuit for only 15 to 20 seconds, the approximate short-time rating of the transformer can be considered to be 10-15% of its continuous rating. The manufacturer of the auto transformer would be a better judge to suggest the most appropriate rating of the transformer, based on the tapping and starting period of the motor. 

Note The above example is only for a general reference. The CDF of the transformer, for the short-time rating, should be increased writh the starting time and the number of starts per hour. Refer to the transformer manufacturer for a more appropriate selection. 

The resistance units, when used only for starting purpose, are in the circuit for only a short time and are thus short-time rated. However, when they are employed for speed control, braking or plugging operations, in addition to the starting duty, they may be rated for continuous duty. The resistance units are thus classified according to their duty demand, i.e. number of operations per hour (c/h). See Chapter 3 on the duty cycle. 

For motors with a short-time rating, the duration of the test should correspond to the specified short-time rating. At the end of the test the specified temperature rise limits should not exceed. At the beginning of the test, the temperature of the motor should be within 5°C of that of the cooling air. 

Note With the availability of modern circuit breakers with higher short-time ratings of 65 kA/80 kA/100 kA, it is now possible to use even larger transformers up to 2500 kVA, depending upon their other merits. 

Approximate one-second kA short-time rating (rounded ofl ) 61 68 110 127 127 145 216... 

A breaker, usually an MCCB or an MCB on an LT system, can be provided with backup HRC fuses to enhance their short-time rating. This may be done when the available MCCBs or MCBs possess a lower short-time rating than the fault level of the circuit they are required to protect, and make them suitable for the fault level of the circuit. But this is not a preferred practice and is seldom used. As a rule of thumb, the device that is protecting must be suitable to withstand electrically and endure mechanically the system fault current for a duration of one or three seconds, according to the system design. 

The duration, however, is no criterion for a current limiting type protecting device, and a protected equipment, device or component can have a short-time rating commensurate with the tripping characteristics of the protecting interrupter. Accordingly these two types of tripping characteristics are explained below. 

The value of short-time rating (/ J. of the system may now exceed, the thermal rating of some of the equipment, devices and components, i.e. /. >... 

If the short-time lating of the interrupting device is higher than the fatilt level of the system, which is the case with modern interrupting devices, the fault level of the system alone will prevail for the busbars, components and hardwaie. For example, for a system fault level of. SO kA. if the interrupter used is of 65 kA short-time rating, the bus system and all associated components w ill be designed for 50 kA onlv. 

Since will be too low (< 5 ms for a 50 Hz system) 1. will be much less than 7% of 1 in all situations. To assign a short-time rating to the protected devices and components in such cases is therefore of little relevance. As noted above, current is the cause of heat, for which is assigned the thermal duty of a current-carrying device, component or part. Also note the following ... 

Short-time rating and effect of momentary peak or dynamic currents 15/488... 

This is not material in voltage transformers, as neither the voltage measuring instruments nor the protective relays will carry any inrush current during a switching operation or a fault. No short-time rating is thus assigned to such transformers. 

In most applications, a bar primary CT is generally used and a normal CT may be suitable. But for too small ratings, where the use of a wound primary CT is imperative, short-circuit effects must be considered, except the CTs for an LT system, where the fault level for such small ratings may be very low and may not matter (Section 13.4.1(5)). For applications on an HT system, where a wound primary CT is imperative, choice of a CT from standard wound primary CTs may still be possible, meeting the minimum requirements of class of accuracy, VA burden and short-time rating. lEC 60044-1 indicates for measuring and protection CTs the maximum short-time factors (STF) that can be obtained economically for a normal wound primary CT where... 

A 16.6 Skin and proximity effects in a multicore cable 16/547 A 16.7 Short-time rating of cables 16/547 A 16.8 Termination of cables 16/548... 

Short-time rating 5-10 minutes, although the relay will trip much earlier. 

The harmonic content may be measured through harmonic analysers and expressed as a percentage of the fundamental component. The cuiTent and voltage ratings of the reactors will depend upon their application. A series reactor connected permanently in the circuit, for instance, will be rated continuously and for full system voltage, whereas a reactor used in the ground circuit may be short-time rated and rated for the likely maximum ground fault current. 

These are meant to limit the inrush currents occurring during a switching operation of a capacitor. They are connected in series with the capacitors and may be short-time rated for the values of the inrush currents and continuously rated for normal line currents. They are almost the same as the series reactors with fixed reactance. 

These are meant to limit the ground fault current and are used between the neutral of the system and the ground. They are single-phase and may be short-time rated, otherwise they are the same as the current limiting reactors (Figure 27.12). Their ratings can be calculated by... 

As discussed in Section 13.4.1(5). these sections are under the cumulative influence of two pow er sources and may be tested for a higher short-time rating, which would be the algebraic sum of the two fault levels, one of the generator and the other of the generator transformer as noted in Table I 3.8. Also refer to Figures 31,1 and 13.18 for more clarity. 

For the tap-offs, connecting a UAT through the main bus section between the generator and the generator transformer, however, as discussed above, the momentary peak current will depend upon the short-time rating of such tap-offs. The likely ratings are noted in Table 13.8. 

Time rating (shall be continuous or any short-time rating given in 10.36)... 

Short-TImB Rating. The short-time rating defines the load that can be carried for a short and definitely specified time. 

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