Current rating

Another consideration of petroleum assessment analysts is whether, and to what degree, the vast resources of unconventional petroleum in the world can be captured by advances in petroleum production technologies, thereby converting them into conventional sources of petroleum. It is a simple fact that the ia-place resources of petroleum in tar sands, heavy oils, and oil shale can guarantee the future supply of petroleum for hundreds of years at the current rate of consumption, provided they can be produced at competitive costs. 

Bills are due on monthly account with a 2 percent discount for cash. Overdraft and deferred-tax interest are compounded daily at nominal annual interest rates of 15 and 9 percent respectively. Corporation tax, capital gains tax, and personal income tax rates are 50, 40, and 30 percent respectively. The current rate of inflation is at 8 percent per year. The traditional return expected hy investors is 7 percent per year net of all taxes in real terms. 

I Normally the external resistances are connected in star. But for larger motors, calling for high resistance grids, they may also be connected in delta, to reduce their current rating and hence the cost. Insulation is not a limiting factor usually, in case of resistance grids. 

These are unidirectional and uncontrollablet static electronic devices and used as static switches and shown in Figure 6.14. A diode turns ON at the instant it becomes forward biased and OFF when it becomes reverse biased. By connecting them in series parallel combinations, they can be made suitable for any desired voltage and current ratings. Whether it is a transistor scheme or a thyristor scheme, they are used extensively where a forward conduction alone is necessary and the scheme calls for only a simple switching, without any control over the switching operation. They are used extensively in a rectifier circuit to convert a fixed a.c. supply to a fixed d.c. supply. 

The triac may, however, have some limitations in handling frequencies higher than normal. In such cases, they can be simulated by using two SCRs in inverse parallel combinations as illustrated in Figure 6.22(b). Now it is known as a reverse conducting thyristor. An SCR has no frequency limitations at least up to ten times the normal. The required voltage and current ratings are obtained by series-parallel connections of more than one thyristor unit. 

To contain the temperature of the electrical circuits within safe limits for a particular temperature class of the surroundings, the maximum current rating for a minimum size of a conductor is also stipulated in ICC 60079-1 I. The constructional requirements also stipulate the minimum clearances and creepage distances in air between the conducting parts of all the intrinsically safe L lectrical eirctiits. 

Continuous current rating (CMR) to carry the circuit current continuously. 

Rated continuous current rating and permi.ssible temperature rise 13/371... 

Rated short-time current rating or fault level of a system (breaking current for an interrupting device) Duration of fault... 

Rated continuous current ratings and permissible temperature rise... 

The current ratings should generally be selected from series R-10 of lEC 60059 which comprises the following numbers ... 

Rated short-time current rating or fault level of a system... 

These values will vary with the spacings between the conductors and their current ratings. In both cases, the content of loss is low. The resistance r, being very small, is neglected. ... 

LT. sysfews HT sy. items Dielectric strength Continuous current rating... 

Thermal rating or continuous current rating This... 

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. 

Figure I.T29 illustrates a simple distribution system and location of the main buses, devices and components to define the current ratings of all such devices and components under different operating conditions. The idetil ctirrent ratings of these components are given in Table I.TI, for an easy illustration.

Similarly, a contactor is a simple circuit making and breaking device, and is protected by other protective devices used in association with it. It has no role in the decision making for the current rating of other... 

Table 13.15 Current rating and technical data for 1100 V, single-core flexible, PVC insulated copper conductor cables for control and power wiring...

Cross-sectional Equivalent diameter Nominal thickness Nominal overall Maximum resistance Current rating d.c. or... 

The ratings and sizes of main components and cables can be selected from manufacturers catalogues. But cables required for the switchgear internal control and power wirings, being typical of all, are normally identified by their cross-sectional area rather than the current ratings. We have therefore piovided the technical data and current ratings for the most common sizes of such cables for a ready reference in Table 13.15. 

Standard current ratings (based on Renald Series R-10 of ISO 3) - 3/1973... 

Type tests are conducted on the first enclosure of each voltage, current rating and fault level to demonstrate compliance with electrical and constructional design parameters. The tests provide a standard reference for any subsequent enclosure with similar ratings and constructional details. The following tests may be conducted to demonstrate verification of the following ... 

Each feeder is considered at its optimum rating, based on the current rating of the motor or the rating of the power fuses in a. SFIJ or F.SU feeder while the current may be much less in actual operation. 

This test is conducted to verify the suitability of the equipment to withstand a prospective short-circuit current that may develop on a fault. It may also be termed the steady slate symmetrical fault current or the short-time (withstand current) rating of the equipment. When the equipment is an interrupting device, it is referred to as its symmetrical breaking current. 

Circuit whose current is being summed Current rating Main CT ratio VA burden shored by the main CTs... 

Rated short-time current Rated primary current... 

Approx. Current rating capacitance per Direct in In duct phase ground... 

Cunciii ralin . for u o core Current rating for J-. J.5-,, 4-( [Pg.537]

No. of cores Conductor Thickness Thickness Armouring Thickness. Appro.x. Appro.x. Max. d.c. Approx. Approx. Approx. Current rating Short- Normal delivers ... 

C/)n u tor (Alumiiutw ) Thickness Lecul sheath. Armour ins Appro.x. Approx. Nor)na Drum Approx. Maxi Appro.x. Appro.x. Appro.x. Current rating Short-... 

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