Overcurrent protection circuits

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. 

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

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... 

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. 

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. 

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... 

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. 

All live power conductors, except the earthed neutral, shall employ overcurrent protection selected as low as possible but adequate for the anticipated start-up currents. Control circuits conductors connected directly to supply voltages and circuits feeding control circuit transformers shall be protected. Refer to EN 60204-1 for more information. 

The consumer s mains equipment must provide protection against overcurrent, that is, a current exceeding the rated value (lET Regulation 430.3). Fuses provide overcurrent protection when situated in the live conductors they must not be connected in the neutral conductor. Circuit-breakers may be used in place of fuses, in which case the circuit-breaker may also provide the means of isolation, although a further means of isolation is usually provided so that maintenance can be carried out on the circuit-breakers themselves. 

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 ... 

The overcurrent protection device protecting circuits not exceeding 32 A shall have a disconnection time not exceeding 0.4s (lET Regulation 411.3.2.2). 

Overcurrent, short circuit and earth fault protection... 

In order that an overcurrent protective device can operate successfully, meeting the required disconnection times, of Regulations 411.3.2.2, that is, final circuits not exceeding 32 A shall have a disconnection time not exceeding 0.4 s. To achieve this, the earth fault loop impedance value measured in ohms must be less than those values given in Appendix 2 of the On Site Guide and Tables 41.2 and 41.3 of the lEE Regulations. The value of the earth fault loop impedance may be verified by means of an earth fault loop impedance test as described in Chapter 14 of this book. The formula is ... 

Additional protection by 30 mA RCD is required in addition to overcurrent protection for all socket outlet circuits to be used by ordinary persons and intended for general use. 

Touring caravans must be supplied from a 16A industrial type socket outlet adjacent to the caravan park pitch, similar to that shown in Fig. 14.40. Each socket outlet must be provided with individual overcurrent protection and an individual residual current circuit breaker with a rated tripping current of 30mA (lEE Regulations 708.553.1.12 and 708.553.1.13). The distance between the caravan connector and the site socket outlet must be not more than 20m (Regulation 708.512.3). These requirements are shown in Fig. 14.42. 

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