Times disconnection

It is therefore common practice to use 5 seconds in (9.7) as the disconnection time for motor cables. This choice also corresponds with standardised data given by the manufacturers of fuses and moulded case circuit breakers for their let-through current as calculated from their graphical data. 

For high voltage cables, and low voltage feeder cables between switchboards, the disconnection time can be reduced from 5 seconds to not less than 0.2 seconds. The time of 0.2 seconds is determined from the total clearance time of a circuit breaker protected by a fast acting overcurrent relay. For high voltage motor circuits in which the short-circuit protection is provided... 

IEC60364 Part 4 Chapter 41 makes reference to several important definitions regarding the design of the insulation within low voltage equipment, whether the equipment is portable or fixed, and the necessary disconnection time of the source protective device. These are summarised as follows -... 

The standard recommends two nominal disconnection times 0.4 and 5.0 seconds. The time of... 

Where the distribution circuit feeds a stationary item of equipment, not socket outlets and not portable equipment, the disconnection time may be taken as 5.0 seconds. This apphes to motors. 

The nominal time of 0.4 seconds is intended for circuits supplying socket outlets, regularly moved portable equipment and Class 1 hand-held equipment. For voltages (Vp ) different from 240 Vac, the disconnection time (fdis) of 0.4 seconds becomes approximately related as. 

Note Some oil companies specify a lower disconnection time tdis than 5.0 seconds, e.g., 1.0 second. This significantly increases the disconnection current by a factor of about 3.0 times. This ensures a much lower permissible limit to Zioopf, and thereby making it more necessary to use an earth leakage circuit breaker. Indirectly this reduction in time should be accompanied by ensuring that the earth return impedance Z r (and Zer) is kept very low i.e. as far below... 

Clearly, connection and disconnection times of network components depend on two more factors the number of repair teams assigned to the network maintenance, and the repair policy implemented. As to the first factor, the mean disconnection times of individual components increase as the number of repair teams decreases, and vice versa. If there are fewer than N repair teams, a component s repair may not start immediately after its failure, but the average delay decreases along with the increasing number of repair teams, and is equal to zero if this number reaches N. However, such case should be considered only for theoretical purposes, because in practice the number of repair teams is usually considerably smaller than the number of all components. For example, the mean disconnection times of components computed forN repair teams can be used as lower bounds of these mean times in case of less thanN repair teams. 

CLOSED FORMULAS FOR THE EXPECTED VALUES OF CONNECTION AND DISCONNECTION TIMES... 

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). 

A10mm PVC sheathed mineral insulated (Ml) copper cable is short-circuited when connected to a 400 V supply. The impedance of the short-circuit path is 0.1 Cl. Calculate the maximum permissible disconnection time and show that a 50 A Type B MCB to BS EN 60898 will meet this requirement. 

Disconnection times for various overcurrent devices are given in the form of a logarithmic graph. This means that each successive graduation of the axis represents a 10-times change over the previous graduation. 

In order that an overcurrent protective device can operate successfully it must meet the required disconnection times of lET Regulation 411.3.2.2, that is, final... 

For final circuits less than 32 A the maximum operating time of the protective device is 0.4 s. From Fig 3A2(a) it can be seen that a current of about 90 A will trip the 15 A fuse in 0.4s. The small insert table on the top right of Fig 3A2(a) of the lET Regulations gives the value of the prospective fault current required to operate the device within the various disconnection times given. 

According to lET Regulation 411.3.2.2 all final circuits not exceeding 32A in a building supplied with a 230V TN supply shall have a maximum disconnection time not exceeding ... 

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 ... 

The maximum permitted value given in Table 2A of the On S/feGu/c/e for a 20 A MCB protecting a socket outlet is 2.3 Has shown by Table 12.2. The circuit earth fault loop impedance is less than this value and therefore the protective device will operate within the required disconnection time of 0.4s. 

The object of the test is to ensure that the CPC is correctly connected, is electrically sound and has a total resistance which is low enough to permit the overcurrent protective device to operate within the disconnection time requirements of Regulation 411.4.6, should an earth fault occur. Every protective conductor must be separately tested from the consumer s main protective earthing terminal to verify that it is electrically soimd and correctly connected, including the protective equipotential bonding conductors and supplementary bonding conductors. 

The object of this test is to verify that the impedance of the whole earth fault current loop line to earth is low enough to allow the overcurrent protective device to operate within the disconnection time requirements of Regulations 411.3.2.2 and 411.4.6 and 7, should a fault occur. 

Metal reinforced concrete floors could perhaps be made part of the equipotential zone by a bonding connection to the reinforcement. It should be effective if the metal mesh is electrically continuous. Where this is impracticable and there is a substantial shock risk - in wet kitchens or laundry rooms, for example - it is suggested that the location be regarded as outside the equipotential zone (see section 471-08-03) and the disconnection times reduced to those in Table 41 A. Alternatively, an insulating floor covering should be provided. 

Regulation 551-04-04 addresses protection against indirect contact for static inverters, typically used for uninterruptable power supplies in installations where continuity of supply is crucial. Where the disconnection times of section 413-02 cannot be achieved, supplementary bonding must be used to minimise the risk of a shock between exposed metalwork. A warning is provided in Regulation 551-04-05 about the possible deleterious effects on the operation of protective devices, such as circuit breakers, of direct current generated by the static inverter or filters. 

The increased risks arising from the harsh environment are recognised by reductions in the required disconnection times for TN systems, set out in Table 604A. For example, for 230 V systems, the normal 0.4 s disconnection time is reduced to 0.2 s. It may not be possible to achieve these disconnection times because of earth loop impedance restrictions, in which case RCD protection will normally need to be specified. Note also that the 50 V touch voltage value used in Section 413 is reduced to 25 V. 

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