Electrical Protection System Design and Operation

Transcription

1 ELEC9713 Industrial and Commercial Power Systems Electrical Protection System Design and Operation 1. Function of Electrical Protection Systems The three primary aims of overcurrent electrical protection are: Personnel injury protection Equipment damage protection Coordination and proper discrimination of operation In this section we will consider only the latter two aspects. Personnel protection, or electrical safety, will be considered later, together with earthing of power systems. Power circuit protection covers a wide range of measures that are used to protect equipment against damage from abnormal fault events that may occur in the system. The protection is used to prevent damage to electrical circuits and components caused by any of the following effects: Current overload (overload current up to about 6pu) Short circuit current effects (SC currents to 20pu) Earth fault current (and possible arcing effects) Excessive thermal heating (by overload or limitation of thermal dissipation) Voltage excursions outside the specified operation limits (high and low) ELEC9713: Industrial and Commercial Power Systems p. 1

2 Unbalanced 3-phase currents and voltages Frequency variations (usually under-frequency operation) Loss of synchronism of AC synchronous motors The protection design should be able to sense very quickly the existence of any of the above faults when they occur and to then effect a disconnection of the electrical supply if the fault severity warrants this. In addition, if the system has any separate components with different protection zones, the operation should be able to discriminate so that only the protection closest to the fault operates first. In this part of the course we are interested only in Overcurrent Protection, which covers overload, short circuits and earth faults from the above list, although protection against overcurrents will necessarily provide some degree of protection also against over-temperature effects. The overcurrent protection design should have the following qualities: Reliability: should be dependable and secure Fast action, if necessary, with speed of isolation dependent on fault magnitude Selectivity (discrimination) in its isolation of circuitry In general, it is normal to have both primary and backup protection levels in the event of failure of the primary protection. Protection is also designed in Protection Zones, where each protection item covers a specified ELEC9713: Industrial and Commercial Power Systems p. 2

3 section. In this way all items can be covered by the protection. It also follows that some duplication may be involved and this is often necessary to achieve good reliability. For overcurrent protection we require the following functions to be performed: Current monitoring This is normally done by a current transformer (CT): a very low impedance transformer which acts as a current source to a protection timing relay (the CT burden). Timing of operation This requires a specific current-time (I-t) operating characteristic this may be a protection relay or a microprocessor controller. It provides the tripping signal to a circuit breaker after a suitable preprogrammed time delay, i.e. short for high currents and longer for low currents. A fuse has a rudimentary timing system as part of its 2 pre-arcing Itcharacteristic. A circuit interrupter This is either a fuse or a circuit breaker which is actuated by the relay after some appropriate time. In some smaller power circuits which use small circuit breakers (called miniature circuit breakers [MCBs] and some larger capacity moulded case circuit breakers ELEC9713: Industrial and Commercial Power Systems p. 3

4 [MCCBs]) the current sensor and the timer may be incorporated within the breaker housing. For example bimetal strips are often used. They have a similar type of I 2 t characteristics to the fuse. In large power supply systems the current sensor, the timing relay and the interrupter will be separate items. 2. Zones of Protection For most efficient protection operation, we need to divide the power system into zones of protection, with each zone served by its own protection device. Zones may (and should) have some overlap for greater reliability. Zones of protection may include, for example: Transformers These need high-speed differential protection with 6 CTs, three on each side to monitor input and output. Motors/generators Need high speed differential protection, again with 6 CTs. Overhead lines These need high speed pilot wire protection with 6 CTs. The pilot wires are relatively long cable connections from the ends of the line to the ELEC9713: Industrial and Commercial Power Systems p. 4

5 protection relays in a substation. The pilot wires may be telephone type connections. Feeders and final circuits Overcurrent protection with a CT in each phase and earth fault protection with residual current devices (RCDs). 3. Primary and Backup Protection This may be required in some situations where additional security is required, such as in computer installations and in various process industries etc. where reliability of operation is paramount. In general there will be some back-up protection available in most systems due to their radial configuration nature and thus the presence of a number of series-arranged protective elements which see the same fault current. 4. Protection Relays Protection is achieved by the use of protection relays which act together with specific sensors (such as current transformers) for the particular parameters that are included in the protection scheme. The sensor will then generate an output which is compared by the relay to some predetermined upper limit which is the allowable normal operating limit. When this limit is exceeded, the relay will then operate to flag the problem or to trip a ELEC9713: Industrial and Commercial Power Systems p. 5

6 circuit breaker to isolate the faulty zone of the system. In the latter case, the isolation is normally done after some appropriate time delay which is programmed into the protection scheme (the time delay may be effectively zero in some cases where the fault is severe designated Instantaneous tripping). In some cases it may be necessary to prevent operation in some situations where overcurrents normally occur: for example motor starting. Thus the protection relay/sensor combination has both a sensing function and a time delay function incorporated when it is used to automatically protect the electrical system. The protection relay cannot operate alone, it needs to have input from sensors such as CTs. 4.1 Classification of relay types The whole range of protection relays can be classified into various groups as follows (the groups are not exclusive and relays may be included in more than one group): Function Auxiliary Protection Monitoring Control Type Electromagnetic Electronic/Computer-based Non-electric (e.g. thermal, pressure, mechanical etc.) ELEC9713: Industrial and Commercial Power Systems p. 6

7 Parameters sensed Current Voltage Frequency Power Temperature Pressure Velocity In this case, we will be concerned only with current sensing relays which are of the electromagnetic, electronic and microprocessor computer-based types. In the international numbering identification scheme for protection relays, the different types of common protection relays are designated numerically as follows: 49 = thermal relay 50 = instantaneous overcurrent relay 51 = time-delay overcurrent relay 64 = earth fault protection relay 67 = directional overcurrent relay 79 = reclosing relay Current sensing relays are always used in conjunction with current transformers (CTs) which are used as low impedance current sources to activate the associated relay. The accuracy, design and choice of CTs is important. In particular, the linearity and frequency response of current transformers need attention. ELEC9713: Industrial and Commercial Power Systems p. 7

8 5. Overcurrent Protection by Current Transformers and Relays For large scale systems and circuits where there are many radial lines with a number of fault current protective devices located in series, the time discrimination of the operation to a through-fault becomes of great importance in achieving optimum performance with minimal disruption. Whereas HRC fuses, contactors and moulded case circuit breakers can achieve some rudimentary discrimination in circuits at low voltage (415/240V), large high power circuits and high voltage circuits require much greater flexibility in operating times. As well as needing this greater flexibility in the I-t characteristics, it also becomes physically difficult to incorporate current sensing elements in the CB housing for high voltage and high current levels. For this reason, at the high voltage and high current levels, fault protection is achieved by use of a separate current transformer, as the fault current sensor, feeding a separate protection relay which is then used to trip the circuit breaker after a time determined by the relay operating characteristics. The protection relay has a well-defined current-time characteristic, so that a specific current into the relay will cause the relay to trip and thus operate the breaker at a well-defined time after the fault initiation. The ELEC9713: Industrial and Commercial Power Systems p. 8

9 relationship between current and time of operation can be very different, depending on the relay type. However, in general, there is the usual inverse I-t relationship, with high fault currents causing operation in the shortest times. Protection relays have facilities to change their operating times and tripping currents over a wide range of values and this is then used to grade the operating time of series circuit protection to achieve proper discrimination of operation for the particular application. Old style protection relays relied on induction disc electromechanical type relays. However modern protection relay practice uses electronic timing units with microprocessor control. These allow for a much greater flexibility in the choice of current-time characteristics and allow easier design to accommodate normal transient high current events such as transformer inrush current and motor starting currents. In large building electrical systems, protection relays are used for: Feeder circuit protection Electrical plant protection Motor protection Until recently the relays for applications 1 and 2 would have been induction disc relays as mentioned above and those for application 3 would have been thermallyactivated relays. Both of these relay types are electromechanical in operation, requiring some physical movement of elements, generated by the electrodynamic ELEC9713: Industrial and Commercial Power Systems p. 9

10 force of the current, to close the contacts. For high voltage (eg 11kV) applications and for very high capacity low voltage installations, thermal relays would not normally be used. Modern protection devices for these purposes are now almost all solid state or static relay devices. These provide timing purely by electronic means, with no time delay caused by mechanical movement. This electronic timing allows a much greater variation of I-t characteristic and with microprocessor control the programming of almost any complicated I-t characteristic is possible. This makes discrimination much more reliable and much easier to achieve. 5.1 Current Time characteristics of protection In order to be compatible with the very many electromechanical relays still in service, the electronic relays have their base I-t curves identical to those of the old style relays. However the flexibility and ease of variation of these curves with modern relays is then much greater in the static relay types. The Standard Curves used for protection are: 1 Inverse IDMT (Inverse definite minimum time) 2 Very inverse IDMT 3 Extremely inverse IDMT 4 Instantaneous operation 5 Long time earth fault 6 Definite time operation relay (eg 2, 4, 8 seconds etc) ELEC9713: Industrial and Commercial Power Systems p. 10

11 Illustrations of these are shown below: ELEC9713: Industrial and Commercial Power Systems p. 11

12 5.2 Unit protection While good discrimination in operation of serial fault protective devices can be done to achieve minimal disruption, the problem may occur that devices furthest ELEC9713: Industrial and Commercial Power Systems p. 12

13 away from a remote fault location will have to have, in effect, a delayed, operating time to allow the nearer protectors time to operate. Then, in the event of a fault at their terminals, for example, their response time will be delayed. This may lead to problems with overheating, for example. Thus, while it is necessary to delay operation for through fault currents, if a fault occurs within an item such as a transformer, the protection should also be able to operate very quickly in response to such an internal fault. This can be achieved by the use of unit protection, which provides protection only for faults within that unit. Unit protection is achieved by the use of two current transformers (CTs) as sensors. One is located at the input and one at the output and the response of each CT unit is compared. Any difference in the current (differential current) response will mean a leakage of current in the item and will indicate a fault within the item of equipment. The current difference can be used to operate an instantaneous trip relay. For important items of equipment such fast operation will be the primary protection means and slow overcurrent protection will be the back-up means. 5.3 Current Transformer requirements The basic current sensing element in use with all relays is the current transformer and thus its measurement accuracy must be very high. ELEC9713: Industrial and Commercial Power Systems p. 13

14 There are some substantial problems in achieving this for high fault currents. The problems lie in the fact that current transformers are (generally) magnetic core devices and thus have non-linear characteristics if the currents are high and cover a wide dynamic range. This can affect the accuracy and the response to higher frequency currents. For this reason, normal metering CTs, used to measure demand and energy use, cannot be used as protection CTs because protection CTs require a quite different range of current measurement. The design of protection CTs in particular is a major problem Current Transformer Accuracy Protection CTs have a dynamic range of about 1 25 per unit of rated current over which good (though not necessarily high) accuracy must be maintained. Metering CTs have a dynamic monitoring range of only about per unit over which high accuracy must be maintained. CT accuracy is determined by the core exciting current and this should thus be minimised. The magnetic circuit should be designed so that the CT is not driven into saturation by high fault currents, which may be up to 20 times rated current. This is not a problem for metering CTs which look at only normal load conditions where levels are only up to about 150% of rated current at most. High permeability low loss cores are used for CTs to minimise losses and magnetising currents. ELEC9713: Industrial and Commercial Power Systems p. 14

15 5.3.2 Differential use The use of two CTs for unit protection systems, where the outputs are compared and subtracted, relies on the two CTs being identical in their characteristics. This is not always the case even for the same design. Such differences may be a limitation of the sensitivity of the differential protection scheme. In the case of transformer unit protection schemes, where the input and output currents are quite different, because of the transformer ratio, the CT design and choice is thus of paramount importance. 5.4 Protection Relays For time discrimination purposes, there are three standard time-current characteristics used for overcurrent protection relays, these being variants of IDMT (Inverse Definite Minimum Time) characteristics: Standard Very Inverse Extremely Inverse t t t = 3 t = log10 M 3s at M = = t = 1.6s at M = 10 ( log ) 2 10 M 0.6 = t = 0.6s at M = 10 log M ( ) 3 10 ELEC9713: Industrial and Commercial Power Systems p. 15

16 where M is the Plug Setting Multiplier (PSM) or the ratio of the actual fault current to the relay s pre-set current trip level. Each relay has adjustable time and current multipliers incorporated which allow wide variation of the relay I t characteristic curve, as is shown below in the typical I-t curves for relays. [As noted earlier there are also definite time protection relays which provide a single, constant, operating time above a specified tripping current level and a sensitive earth fault relay which detects earth faults in an analogous way to earth leakage safety switches (RCDs) and has a more sensitive trip current setting level]. Operating Time, t Time variation by TMS Operating current variation by PS Current (PSM) The Plug Setting (PS) parameter of the relay (also often called the Tap Setting ) is used to vary the nominal relay operating current. ELEC9713: Industrial and Commercial Power Systems p. 16

17 The Time Multiplier Setting (TMS) parameter of the relay (also called the Time Dial Setting ) is used to vary the operating response time Plug Setting (PS) The plug setting allows tappings of the relay coil (or the equivalent in electronic relays) to give seven discrete settings in terms of the rated trip current. These are: 50%, 75%, 100%, 125%, 150%, 175% and 200% of the rated relay operating (tripping) current. Thus, for a standard relay operating current of 5 amps (usually relays are rated at either 1 or 5 amps tripping current), the tripping (or pick up ) current can be varied between 2.5 amps and 10 amps for that relay. This is used to adapt the operating current of the relay to the required operating conditions of the circuit or the load protected by the relay. The above settings are those for standard overcurrent relays. Earth fault relays are also widely used and these have more sensitive tripping currents: 20%, 30%, 40%, 50%, 60%, 70% and 80% of the rated tripping current Time Multiplier Setting (TMS) ELEC9713: Industrial and Commercial Power Systems p. 17

18 Each relay has a continuously variable TMS to change the operating time, at a specified or chosen plug setting, over certain specified limits. The range of variation is of the standard setting. For example, a TMS of 0.2 at a PS of M = 10 will give an operating time of 0.2 x 3 = 0.6 seconds. [The operating time from the I-t curve at M=10 is 3 seconds]. Using the Plug setting and the Time multiplier setting, it is thus possible to get wide variations in the I-t curves of relays to allow proper time discrimination of different protection operation, even when identical relay types are used in serial (radial) circuit connections. 6. Determination of Relay Settings The choice of the plug setting required is determined by the normal operational requirements of the particular application, taking into consideration the current transformer ratio, the allowance for brief overload periods of transformers, for example, and other common transient overload events such as motor starting. In the idealised operation of the relay, pick-up will occur at a plug setting multiplier (PSM) of just in excess of 1.0. However, in practice, there may be some inaccuracies and/or mechanical delays in the relay operation which will make the pick-up level rather higher than 1.0. To ensure that operation will always occur when a fault is present, a ELEC9713: Industrial and Commercial Power Systems p. 18

19 minimum PSM of the fault current of 1.3 is often chosen as the practical minimum basic fault operation level, where operation can be relied on to take place. Similarly, a PSM value which is just less than 1.0 may not always guarantee non-operation of the relay: nuisance or non-fault generated operation may occur at a PSM of about 0.95 in some cases. Thus a value of PSM which is less than about 0.8 is generally used in the normal operational phase design to ensure non-operation of the relay under normal operational conditions. Thus, a PSM between about 0.8 and 1.3, under normal system operation, should be avoided. Basic time/current characteristics of IDMT relay. ELEC9713: Industrial and Commercial Power Systems p. 19

20 Time grading between relays in series. Because of the potential variations in timing between identical relays due to mechanical effects, it has been standard to allow about sec minimum delay between the designed operation of two relays in series when designing for discrimination. Such delays are able to reduced with microprocessor based timing. 6.1 An example of a discrimination calculation A 3-phase 20 MVA transformer supplies an 11kV busbar through an 11kV circuit breaker as shown below. It is required to determine the settings for the transformer overcurrent protection relay so as to give proper time discrimination with the feeder sub-circuit relay, with settings as shown, when there is a fault current of 5,000 ELEC9713: Industrial and Commercial Power Systems p. 20

21 amps as shown. The ratio of the current transformer feeding the relay is 1000/5 amps. The relays are both standard IDMT. Transformer IDMT relay 11kV busbar Transformer 33/11kV 20MVA current transformer [1000/5] circuit breaker CT [400/5] Feeder IDMT relay PS=125% TMS=0.3 feeders Feeder relay: The fault current of 5000 amps through the feeder relay CT gives 5000 x 5/400 = 62.5 amps to the relay. The rated relay current is 5 amps. With the specified PS = 125%, the pick-up current = 1.25 x 5 = 6.25 A. Thus, the Plug Setting Multiplier = 62.5/6.25 = 10 ELEC9713: Industrial and Commercial Power Systems p. 21

22 Hence, using the relay curves, the operating time from the standard IDMT curve gives a tripping time of 3 seconds for a PSM = 10 at TMS = 1.0. With a time multiplier of 0.3, this gives an actual operating time of 0.3 x 3 = 0.9 sec. For proper discrimination, the transformer relay must operate after the feeder relay at 5000 amps. It is usual to add an extra time margin of about seconds to allow for various uncertainties in the operation time. Thus, we need the transformer protection relay to operate after = 1.4 seconds at 5000 amps fault current. Transformer relay setting determination: We first have to determine the transformer relay Plug Setting using the normal load conditions. For a 3-phase 20 MVA transformer, the rated current level at the 11kV secondary side is I = 1050 A We allow a permissible short term overload of 1.3 per unit to occur with the transformer without tripping of the relay, so this requires that a current of 1.3 x 1050 = 1365 A should not operate the relay. Thus, at this current the PSM should be less than 1.0, say 0.9 to ensure non-operation at normal loads up to 1.3 per unit. ELEC9713: Industrial and Commercial Power Systems p. 22

23 At 1365 A, the actual relay current from the CT is 1365 x 5/1000 = 6.8 A. For a rated relay operating current of 5 amps, this gives a multiple of 6.8/5 = 1.36 times relay operating rating. The nearest plug setting to this is 150%, so we try that first to see if the PSM is satisfactory. This gives the PSM = 6.8/1.5x5 = This is close enough to 0.9 and should ensure nonoperation, so we chose PS = 150%. Having determined the PS from the normal operation, we now use this to determine the operating time at the specified fault current level. For a 5000 amp fault, the relay current is 5000 x 5/1000 = 25 A. Thus, the fault current PSM of the transformer relay is 25/1.5x5 = From the IDMT curve at PSM = 3.33, the relay operating time = 5.7 seconds at a TMS of 1.0. We need an operating time of 1.4 seconds for proper discrimination with the feeder relay operation to give the minimum operating time commensurate with the feeder relay operation. ELEC9713: Industrial and Commercial Power Systems p. 23

24 Thus, the TMS of transformer relay is obtained from 1.4/5.7 = (say 0.25). Thus, the required settings of the transformer relay for proper discrimination are: PS: 150% TMS: An example of a discrimination calculation Supply A CT 800:1 X B CT 500:1 X C CT 250:1 X D Relay A Relay B Relay C 125A fuse 100A fuse Total load from B = 220A Fault level at B = 8000A Total load from C = 200A Fault level at C = 7000A 80A fuse Total load at D is 180A. Includes a 15kW motor. I S = 6I FL Fault level at D = 6000A. Radial system fed from A At D C B Maximum loads 180A 200A 220A Fault levels 6000A 7000A 8000A ELEC9713: Industrial and Commercial Power Systems p. 24

25 The motor at D is 15kW. Its full-load current is 28A and the starting current is 6 times the full-load current during the starting period of 5 seconds. The fuses shown are the largest rating at each busbar (i.e. the longest operating times). We want to find appropriate Plug Settings and Time Multiplier Settings for R A, R B, and R C (which are IDMT) to achieve proper discrimination. 1. Relay R C at C Peak steady-state load = 180A. This is with motor at full load. During motor starting: I = = 320A ( ) Must choose P.S. to ensure that R C does not operate in either situation. i.e. want PSM < 1 for both For the steady state, want PSM 0.9 Rating of R C is 1A (or 250A on busbar) 180 want PS PS=100% or 1.0 is thus OK For starting, can relax the requirements to: PSM =1.28 too high!: try PS = 1.25 ELEC9713: Industrial and Commercial Power Systems p. 25

26 Hence: =1.02 OK so use PS = 1.25 Rated I = 1.25A or = 312.5A I Fault = 6000A at D PSM = = 19.2 during fault From IDMT characteristics: T op = 2.32 seconds at PSM = 19.2 We can assume the 80A fuse at D will operate in less than 0.3 sec. Thus we need R C to operate after 0.3 sec. 0.3 TMS = = At PS = 1.25, TMS = 0.13, we get discrimination with the fuses at D. However, we can get faults between D and C and R C has to handle them also. Maximum I F for R C will occur at the CT at C, where I F 7000A. This will determine the minimum operating time of R C for discrimination with R B PSM = = T op = 2.22s With TMS = 0.13 t = 0.29s op ELEC9713: Industrial and Commercial Power Systems p. 26

27 2. Relay R B at B C.T. ratio = 500:1 Steady-state load = = 380A During starting = = 520A Thus: Start: 380 = 0.76 so PS = 100% is OK = 1.04 OK PS = 100% is OK 500 Thus relay setting of 1A or 500A 7000 I F = 7000A PSM = = PSM = 14 t op = 2.6s For the actual minimum operating time, we use the minimum operating time of R C plus a margin of (say) 0.4 seconds to allow for variations i.e. ( ) top R B = = 0.69s 0.69 TMS = = Have to check that this will discriminate with fuses at C (the 100A). Use I-t curves. Maximum I F handled by R B will be 8000A: 8000 PSM = = 16 t op = 0.67s 500 Use this to discriminate with R A. ELEC9713: Industrial and Commercial Power Systems p. 27

28 3. Relay R A at A C.T. ratio = 800:1 Steady-state load = = 600A Start load = = 740A Thus steady-state: Start: = OK = 0.75 PS = 100% is OK Use PS = 100% I F = 8000A 8000 PSM = = 10 top = 3.0s 800 We require: = 1.07s 1.07 TMS = = Also have to check discrimination with fuses at B: i.e. 125A. Thus: Relay CT Ratio Plug Setting R C 250:1 125% R B 500:1 100% R A 800:1 100% Time Mult. Setting ELEC9713: Industrial and Commercial Power Systems p. 28

29 Time-current curves with discrimination. ELEC9713: Industrial and Commercial Power Systems p. 29

30 Appendices 1. Coordination between protection device and cable: Refer to the Wiring Rules and see also AS/NZS :2004 Electrical accessories circuit breakers for overcurrent protection for household and similar installations. Three types of MCBs: Type B have magnetic trip settings from 3 to 5 times rated current (with mean tripping current of 4 times rated current). Used when load is constant and not subject to high inrush current, eg. resistive loads. Type C have magnetic trip settings 5 to 10 times rated current. Suitable for general purpose, most common. Type D have magnetic trip settings 10 to 20 (or even 50) times rated current. Used mostly for highly inductive loads, eg. motors. Coordination between the protection device and the protected cable is achieved if: IB IN I Z and I2 1.45I Z where: I B = current for which the circuit is designed I N = nominal current rating of the protective device I Z = continuous current-carrying capacity of cable I 2 = current ensuring effective operation of protection device ELEC9713: Industrial and Commercial Power Systems p. 30

31 For MCBs: I2 = 1.45IN 1.45 IZ IN = I Z For HRC fuses: I2 = 1.6IN 1.45 IZ IN = 0.9I Z Conventional tripping current = current value that causes CB to trip within the conventional time. Conventional tripping current of CB is 1.45 times its rated current. Conventional non-tripping current of CB is 1.13 times its rated current. Conventional time is 1h for CB with rated current <64A or 2h for above. Preferred values of rated current: 6, 8, 10, 13, 16, 20, 25 32, 40, 50, 63, 80, 100, and 125A Standard values of rated short-circuit capacity: 1500A, 3000A, 4500A, 6000A, 10kA Type B: if 3 I N, opening time not less than 0.1s. If 5 I N, trip in less than 0.1s. For safety protection purpose, AS3000 states that maximum disconnection times must not exceed: 0.4 s for final sub-circuits that supply socket outlets (<64A), or hand-held class 1 equipment, or portable equipment for manual movement during use. 5 s for other circuits including sub-mains and final sub-circuits supplying fixed or stationery equipment. Example: Single circuit connected to 15A socket outlet. Cable is 2.5mm 2 Cu conductor, 2 core & earth V75 TPS, nominal ambient temperature not exceeding 40 o C. ELEC9713: Industrial and Commercial Power Systems p. 31

32 Option 1: Install cable on a surface partially surrounded by thermal installation Table 9 of AS I Z = 18A Nominal commercial current rating of protection device I N = 16A (a) Coordination requirement: IB( 15A) IN ( 16A) IZ ( 18A) O.K (b) Overload protection requirement: I2 1.45I Z = = 26.1A For MCB: IN = IZ = 18A For HRC fuse: IN = 0.9IZ = 16.2A Thus, both coordination and protection requirements are met by using either 16A MCB or 16A HRC fuse. Option 2: Install cable on a surface in air. Table 9 of AS I Z = 26A Available nominal commercial current rating of protection device I N = 16A; 20A; 25A (a) Coordination requirement: IB( 15A) IN ( 16, 20 or 25A) IZ ( 26A) O.K (b) Overload protection requirement: I2 1.45I Z = = 37.7A For MCB: IN = IZ = 26A For HRC fuse: IN = 0.9IZ = 23.4A Thus, both coordination and protection requirements are met by using either 25A MCB or 20A HRC fuse. ELEC9713: Industrial and Commercial Power Systems p. 32

33 2. Residual current devices (RCD): Also known as earth leakage circuit-breakers (ELCBs). These are devices which interrupt the current in a circuit whenever a predetermined level of current (usually 30mA) to earth is detected. Their purpose is to help prevent electrocution accidents for the cases when the current flows through the body to the general ground mass earth. Supply - Single Phase N L Supply - Three Phase N L1 L2 L3 N L Load Ring core of magnetic material N Load L1 L2 L3 Under healthy conditions the currents in the conductors passing through the toroidal transformer are balanced, there is no flux induced in the core and no voltage induced in the secondary coil of the transformer. Should some current flow to earth through a fault and thus return to the supply transformer without returning through the appropriate primary of the core-balance device, then the vector sum of the currents through the primaries will be equal to the earth fault current. A magnetic flux will ELEC9713: Industrial and Commercial Power Systems p. 33

34 then be generated in the toroidal core and a voltage induced in the transformer secondary. This voltage is used to trip the circuit breaker. For safety protection purpose, AS3000 states that RCDs with rated current of 30mA and tripping time of 0.3s are required for: all circuits supplying lighting and socket outlets in domestic installations socket outlet circuits in residential sections of other electrical installations. Note there are certain situations where RCD is not used as it can cause nuisance tripping, e.g. stoves, water heaters. 3. Operating time of over-current relays: The IEC and ANSI/IEEE Standards define the operating time of overcurrent relays by the following formula: kβ t = + L α ( I I ) 1 S where k is the time multiplier setting (TMS); (I/I S ) is the plug setting multipler (PSM). ELEC9713: Industrial and Commercial Power Systems p. 34

35 Description Standard α β L Standard Inverse Very Inverse Extremely Inverse Long-time Inverse Moderately Inverse Very Inverse Extremely Inverse Inverse Short-time Inverse IEC IEC IEC UK IEEE IEEE IEEE CO8 CO ELEC9713: Industrial and Commercial Power Systems p. 35