Protection overcurrent

They do not operate against overloads as these are taken eare of by the overcurrent relay or any other overcurrent protective device. 

Nole In both the above case.s. which are almost similar, so far as the sw itching heats of the stator or the rotor are concertied, the overcurrent protection (noted at serial no. 4) is redundant, as its time constant is much higher (of the order of several minutes) compared to the temperature rise, particularly of the rotor, which is linear and much more rapid under such conditions. Therefore, such protection saves the machine from excessive thermal stresses. 

Overcurrent protection. To provide a thermal replica protection, the relay is set according to motor s heating and cooling (/ - 1) curves supplied by the motor manufacturer. If these curves are not available, they can be established with the help of motor heating and cooling time constants, as in equations (3.2) and (3.4). A brief procedure to establish the motor thermal curves when they are not available is explained in Section 3.6. 

Thus, besides voltage unbalance it can also detect an inter-turn fault, which leads to a current unbalance. A small amount of unbalance is already detected by the thermal element of the overcurrent protection but a severe unbalance, such as during a single phasing, would require quicker protection and hence, this protection. The relay may be set for an of around 3% or so. 

Role of an OCR this is only an overcurrent protection device and does not provide short-circuit protection. 

Overcurrent protection is normally not provided in generator and generator transformers to save the machines from likely outages on momentary overlo.ids. In the event of overloading, the normal practice is to shed some of the loads on the transmission network. 

Residual current operated circuit breakers, without integral overcurrent protection. General rules ... 

Electrical accessories - Circuit breakers for overcurrent protection for hou.seliold and similar installations 8828/1996 BS EN 60898/1991... 

Figure 2-6 Current-boosted 3-terminal regulator without overcurrent protection.

The seeond method of overeurrent proteetion is pulse-to-pulse overcurrent protection. This method guarantees a maximum safe power switeh eurrent. A eurrent sensing element (resistor or eurrent transformer) is plaeed in series with the power switeh(es). It views the instantaneous eurrent flowing through the power switeh and will instantly eutoff the power switeh if a preset instantaneous eurrent limit is exeeeded. This eireuit must be very fast and will proteet the power switeh from all forms of instantaneous overeurrents ineluding eore saturation. It is not a form of eurrent-mode eontrol sinee this proteetion limit is fixed and not infiueneed by external parameters. 

Current mode control has an inherent overcurrent protection. The highspeed current comparator provides pulse-to-pulse current limiting. This form of protection is a constant power form of overload protection (see Section 3.11). This form of protection folds back the current and voltage to maintain a constant power into the load. This may not be optimum for all products, especially where the typical failures slowly increase the failure current. Another form of overload protection can also be placed in the circuit. 

Figure 3-50 Methods of constant current limiting (a) discrete overcurrent limiting (constant-current limiting) (b) precision resistive current-sensing overcurrent protection (constant-current limiting) (c) use of a current transformer to sense ac current.

This converter is intended to function as a bulk power supply for a distributed system. It has only one +28VDC output at 10 A. This is going to be a classic ZVS quasi-resonant half-bridge converter that is, variable frequency, voltagemode controlled with averaging overcurrent protection. It is representative of the designs using the available control ICs on the market today. 

Arcing contacts in Division 2 areas must be installed in explosion-proof enclosures, be immersed in oil, be hermetically sealed, or be non-incendive. High-temperature devices must be installed in explosion-proof enclosures. Fuses must be enclosed in explosion-proof enclosures unless the fuses are preceded by an explosion-proof, hermetically sealed, or oil-immersed switch and the fuses are used for overcurrent protection of instrument circuits not subject to overloading in normal use. 

The power cables to the motor should preferably be buried or otherwise routed away from or protected against fire or other potential source of damages. The feeder circuit should be independent so that plant power can be shut-off without interrupting the pump operation. Conventional feeder overcurrent protection is not permitted by NFPA 20. 

Conductive polymer material used for the positive temperature coefficient resistor (PTC) overcurrent protection device Activated carbon cathode and anode... 

Unfortunately, the protector provided in many OEM computers and power supplies is for overvoltage or overcurrent protection for failing components, and its response time is fa,r too long for the rapidly rising surges and transients entering the system. 

Fuses should not be used to protect secondary voltage feeders. The time current characteristics of fuses above 100 A will not coordinate with the groimd fault pickup currents and time delays of the main overcurrent protection (circuit breaker or fused disconnect switch) ground fault protection. A main load break disconnect switch can be equipped with current-limiting fuses to reduce the available short-circuit current from the utility and should have a three-phase voltage relay for single-phase protection. 

Ground fault protection, including circuit breakers or relays, should be set as low as possible without causing nuisance tripping. The ground fault pickup and time delay for the main overcurrent protection should coordinate with... 

The unrestrained operation of these relays is used as back-up overcurrent protection for downstream relays in case they fail to respond. 

Overcurrent protection is usually provided by a (51) relay, which has separate elements for each phase. The overcurrent curve should be chosen with a margin below the Pt characteristic of the... 

It is not normally necessary to provide overcurrent protection in the bus-section circuit because the presence of overcurrent, not caused by an in-zone fault, would be detected by an outgoing circuit relay. For the busbar to be overloaded the outgoing system must also be overloaded. Introducing an overcurrent relay in the bus-section circuit will add comphcation to the coordination of the incoming and outgoing relays, since a time margin is necessary between each relay. In systems where there are large induction motors the coordination can already be awkward to achieve. 

If there is a large number of motors to restart (or also called reaccelerate) then there will be a large surge of main feeder current if they all start at the same time. This will cause a severe volt-drop at the common busbars, which will cause the run-up times to be extended and the possibility of the overcurrent protection relay at the main feeder circuit breaker to trip. Similarly if there is a group of motor control centres all in the same state of restarting their motors, then their common busbar e.g. main generator switchboard, will experience a significant volt-drop which may not be acceptable. 

Circuit breakers for overcurrent protection for household and similar installations. 

EN 60204-1 requires the use of a control transformer on machines with more than five electromagnetic coils. The transformer must be installed after the mains switch to ensure isolation. Small transformers are not typically suited for control circuits since they are designed for simple resistive loads. Overcurrent protection is required in accordance with standards EN 60742, 60204-1, lEC 76-5. The type and setting of the overcurrent device should also be in accordance with the recommendations of the transformer supplier. The preferred secondary voltages are 24/48/ 115/230 V, 50/60 Hz. Fault-free operation must be verified prior to using lower voltages. 

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