Current rating Fault

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

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

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 ... 

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. 

This minimum conductor size will take account of the healing effects only during the fault, irrespective of the current rating of the conductor. The required conductor size may be more than this, depending upon the continuous eurrent it has to carry, as discussed later. 

The above situation may not always be true particularly when the current rating is low, say up to 600 A and the system fault level is still high. In this case much less cross-section of aluminium would be used, and the number of supports and fasteners would also be less. Then the fasteners will also be of smaller cross-sections. In such cases the suitability of fasteners will be more relevant. 

In the case of parallel operation the maximum transformer rating is limited by the fault rating of the switch-gear, while for a single transformer in-feed the limitation is by the full-load current rating of the switchgear. 

Vacuum and SF6 devices were developed at about the same time as competitors. There is little to choose between them since they are both simple mechanisms. Some engineers in the past considered the possibility that the vacnnm conld be lost while the circuit breaker was in its on state was a serions disadvantage. However, the technology has greatly improved and thousands of vacnnm circnit breakers are in service. Vacuum circuit breakers are limited to about 13.8 kV due to insulation difficulties across the open contacts. Current ratings at 13.8 kV are limited to about 3000 A, with corresponding fault making duties up to 100 kA peak. SF6 equipment tends to be preferred to vacuum equipment. 

Enables the ontgoing cable sizes to be significantly smaller due to the reduced fault clearing time provided by the fnses. Cables are sized for rated running current and fault current withstand when a major fanlt occnrs at the load terminals. See Chapter 9. 

The selected protective device should have a current rating which is not less than the full load current of the circuit but which does not exceed the cable current rating. The cable is then fully protected against both overload and short-circuit faults (lET Regulation 435.1). Devices which provide overcurrent protection are ... 

It can be seen that the prospective fault current required to trip an MCB in the required time is a multiple of the current rating of the device. The multiple depends upon the characteristics of the particular devices. Thus ... 

The recloser is another overcurrent device that automatically trips and recloses a preset number of times to clear or isolate faults. The concept of reclosing is derived from the fact that most faults are of the temporary type and can be cleared by de-energizing the circuit for a short period. Reclosers can be set for a number of operation sequences depending on the action desired. These typically include instantaneous trip and reclose operation followed by a sequence of time-delayed trip operations prior to lockout of the recloser. The minimum pick-up for most reclosers is typically set to trip instantaneously at two times the current rating. 

When a fault occurs, the overcurrent device must safely open and interrupt the fault. Overcurrent devices such as fuses, circuit breakers, and self-protected combination controllers, which interrupt fault currents, must have an interrupting rating equal to or greater than the available short-circuit current at their line-side terminals. Control devices, such as motor starters and overload relays, must have a short-circuit current rating equal to or greater than the available short-circuit current. That is to say, they must be able to withstand the fault current for the time it takes the overcurrent device to interrupt the fault. In practice, the actual current which flows during a fault is less than the available short-circuit current, and it is this current which the control device must withstand. 

Another example is the selection and use of suitably insulated cables. Their voltage and current ratings should not be exceeded where this might cause an insulation failure and the consequential exposure of a live conductor that would produce the risk of electric shock and bum injuries to anyone who may touch it. Yet another example is high voltage switchgear, such as circuit breakers. These devices may be required to clear short circuit faults on... 

I In the event of a fault in the d.c. link it will add to the circuit impedance and limit the rate of rise of fault current, since under a transient condition... 

Rated momentary peak value of the fault current (making current for an interrupting device)... 

The rated momentary peak value of the fault current, /y, will relate to the dynamic rating of an equipment. It is also known as the making current of a switching device and defines its capability to make on fault. 

The electrical rating on a fault or short-term high current thermal efllecls, expressed by / c sc. ke., short-circuit current (I q) and its duration Use - I or 3 seconds). 

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. 

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