Through-fault condition

Figure 15.25 A through-fault condition outside the protected zone in a differential scheme...

Fault condition particularly when the LT distribution is fed through a large transformer and the outgoing feeders tre protected by a current limiting device, HRC fuses or breakers. In the event of a fault, on a large... 

Note To avoid unnecessary wear and lear of Ihe machine, ihe blades arc braked when ihe machine is not in operation. In fact the main protection to the iiiachinc is through the brakes only. During an overloading or system fault condition the blades are braked and the machine ceases to generate. The control panel monitors closely all the operating... 

These call for a closer proteciion, w hich is possible through a single point niolor protection relay (MPR). Since a single MPR provides protections against unfavourable operating as well as fault conditions, we discuss this relay separately in Section 12.5. 

But this may not always be true, as it is possible that one or more CTs in the faulty circuit may saturate partially or fully on a severe through-fault and create a short circuit (T 2 = 0) across the magnetizing circuits of all the CTs that are saturated. Refer to Figures 15.26(a) and (b). The CTs resistances, however, will fall across the relay circuit. Assuming that the other sets of CTs in the circuit remain functional, this would cause a severe imbalance and result in a heavy imbalanced current through the relay and an unwanted trip. Under such a condition. 

This relay may be used only under unrestricted fault conditions, with three CTs as shown, tf the scheme is used under a restricted fault condition, with the fourth CT in the neutral, the directional relay will remain immune to any fault occurring outside the zone of the three CTs, as the fault current through the fourth CT will offset the residual current, detected by the three CTs (.Section 2l.6.,f), rendering the whole scheme non-functional. 

The Fig. 4.2.1 shows that when a ground fault occurs, at the end of one of the feeders there is an RLC circuit formed by the resistor (Rll and R12), inductance (Lll and LI 2) and capacitance (C12) of the feeders. Under fault conditions the current flows through these elements, this stream may contain a component of a frequency (1 to 10 kHz) that could generate a series resonant circuit. 

Failure A failure is a permanent interruption of a system s ability to perform a required function. It can only be accommodated by a reconfiguration of the system. Fault A system fault is a deviation of the system structure or the system parameters from the nominal conditions [2]. Appropriate actions may enable to recover from a component fault without replacing the component. The fault may be accommodated through fault tolerant control. 

F. Seven cycles of adding 40 F feedwater at 800 gpm to the steam generator through the downcomer feedwater nozzles during a steam line break. This provides for one steam line break incident with the emergency feedwater cycled a maximum of seven times (faulted condition) ... 

The steps executed after fault detection are termed alarm interpretation which classify the actual fault, its characteristics (occurrence time, fault size, consequences, etc.), and the root cause. Fault characterization and quantification is required to determine the immediate process state and to determine whether the fault can be safely accommodated at that process state. Based on this input, fault accommodation may be performed through reconfiguration when standby devices in healthy condition are available or through fault-tolerant control (FTC) where the... 

Emergency standby power is normally off and does not start (manually or automatically) until the utility AC feed fails. A diesel generator can be started within 10 to 30 s if the system has been maintained properly. Such an interruption, however, is far too long for DP hardware. Most DP systems cannot ride through more than 8 to 22 ms of power interruption. Systems that can successfully ride through short-duration power breaks, as far as energy continuity is concerned, stiU may enter a fault condition because of electrical noise created by the disturbance. 

Guided diagnostics can be used on certain types of hardware to facilitate semi-automated circuit testing. The test connector typically taps into one of the system board input/output ports and, armed with the proper software, the instrument goes through a step-by-step test routine that exercises the computer circuits. When the system encounters a fault condition, it outputs an appropriate message. 

If, for any reason, there was a breakdown of insulation in a part of an electric circuit or in any apparatus such as, say, a hand-held metal-cased electric drill, it is conceivable that current would flow external to this supply circuit, if a path were available. For example, the metalwork of the drill may be in contact with a live internal conductor at the point of insulation breakdown. Or, take the example of someone working at a switch or socket outlet from which the cover had been removed before the electricity supply had been isolated. In both circumstances the person concern could come into contact with a live part, either metalwork of the drill made live because of the internal fault condition or a live terminal exposed by removal of the cover which, if the conditions were right, would allow an electrical current to flow through the body to earth. The former is an example of electrical shock received by indirect contact whilst the latter is an example of electrical shock from direct contact. If the total resistance of the earth fault path were of a sufficiently low value, the current could kill or maim. 

The designers then went one step further - in addition to the normal spring-loaded overpressure safety valves on the top (steam-side) of the steam drums, they also installed a mechanical overpressure safety valve on the underside (the waterside) of the steam drum. This was highly unusual - the overpressure safety valves on steam drums are normally only on the steam-side (the top). The waterside overpressure safety valve meant that, in extrane overpressure fault conditions, high-density water - instead of low-density steam - from the steam drum would be blown through the roof, so the many tonnes of water in the drum available to feed a steam-sodium leak accident could be removed in a few seconds. (By contrast, it would take very much longer to remove all the water if only the steam-side overpressure relief valves were used.)... 

Alternatively, if on close inspection of another damaged socket outlet the electrician realizes it is possible to touch an earth connection through the cracked plate, it will attract a 02 code there is potential danger here because the earth wire may become live under fault conditions. 

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