Ct Selection Guide 1c2268
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Current transformers: function, characteristics, guide to selection Function Current transformers (that from now on will be identified as C.T.) are born to reduce the value of the currents circulating inside an electric plant; these can go from some amperes to thousands of amperes or can have standard values (for instance 1 or 5 A.), suitable to feed test and measuring equipments. A transformer can be imagined as a device containing two separate circuits: a primary circuit where the plant current flows and a secondary circuit where it flows the output current proportional to primary current. They are usually used with meters (ammeters, KWH-meters, transducers, etc.) or with control devices such as earth leakage relays, load control, recorders, protection relays. Transformers treated in this document can be only used in alternating current circuits; for measurements in direct current circuits we have to adopt other solutions such as shunts or Hall-effect transformers. The main functions of an instrument transformer are: -
to allow going back with precision to the primary current value by measuring the current inside secondary circuit;
-
to galvanically insulate the secondary current from the primary current separating, in this way, a world composed of very high voltages and currents, and therefore potentially dangerous, from the world in which the s are working, where the highest security level must be assured (both for men and for machines);
-
to allow the use of quite easy and cheap meters to measure quantities with very high values (thousands of amperes can be measured by a C.T. and a moving-iron ammeter with an approximate error of 2%).
Characteristics Luckily, for instrument transformers there is a standard (IEC 44-1) which exactly fixes the use and electrical characteristics; according to this standard, our products are classified as transformers to measure the current in low-voltage applications. Low-voltage applications mean plants where primary circuit voltage doesn’t exceed 0.72kV or, in specific cases, 1.2kV. To these use voltages correspond insulation voltages respectively of 3kV and 6kV. 99% of our production us designed to work at 0.72/3kV. The most common symbols used to represent a current transformer are the following:
Description Two-winding transformer Note – Voltage instantaneous polarities can be shown as in Form 2 of the symbol. Example: Two-winding transformer represented with the voltage instantaneous polarity indicators. Two instantaneous currents which enter from the winding ends marked by the points, produce fluxes which get added. The symbols highlight the presence of two terminals for the primary circuit and two terminals for the secondary circuit. In low-voltage C.T.’s very often primary terminals don’t physically exist since the primary circuit is composed of a bar ing through a hole in the C.T. itself. The symbols emphasize also the physical separation between the two circuits as a guarantee for the insulation; the energy transfer between primary and secondary circuit occurs for magnetic coupling. Once defined the voltage range for our C.T.’s, let’s define also the current range: our models cover primary currents from some amperes to 6000 A. with a wide range of models which can meet the various mechanical and mounting requirements; for secondary winding, standard currents of 1 and 5 A. are usually used. Before entering a detailed discussion about the characteristics that identify a C.T., it is necessary to give some definitions: -
rated primary current: primary current value to which the C.T. performances are referred;
-
rated secondary current: secondary current value upon which the C.T. performances are based;
-
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rated transformation ratio: ratio between primary and secondary rated current;
-
ratio error: error introduced in the measurement for which the real transformation ratio doesn’t correspond to the rated transformation ratio;
-
phase error: secondary current is not perfectly in phase with primary current, as we might expect from an ideal transformer;
-
accuracy class: a C.T. classification which defines the highest permitted errors (both ratio error and phase error) under certain test conditions;
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http://www.jensenelectric.dk/docs/ime/current_transformers_manua...
-
burden or load: it is the impedance of the circuit fed by secondary winding, expressed in ohms and power factor;
-
rated power: it is the value of apparent power provided by the C.T. when it feeds the rated burden;
-
thermal current: highest primary current that a C.T. can permanently stand with secondary winding connected at the rated burden.
To clarify the ideas, our range of C.T.’s concentrates on accuracy classes 0.5 – 1 and 3 with rated powers (or burdens as they are usually called) between 2 VA. of a TAIBB with 100 A. and 100 VA. of the biggest current transformers in our range (series TAU). It is important to note that high burdens are obtained with high primary currents and that, vice versa, there are physical and cost limits when you want to obtain high burdens with low currents. This peculiarity sets some limits to the manufacturing of ing bar C.T.’s when currents are low and compels the customer to choose a wound-primary current transformer (our series TAQ) in which the primary circuit is actually linked up with two terminals. Upon the basis of these definitions, it is possible to bring in the C.T. accuracy classes. The accuracy classes express the ratio and phase error limits measured in fixed test conditions. The classes established by the standard are the following: 0.1-0.2S-0.2-0.5S-0.5-1-3-5 the three values in bold figures cover 98% of our production. Let’s see directly their meaning through the tables established by the standard:
Tab.12 Limits of current error and phase displacement for measuring current transformers for special application
Tab.13 limits of currents error for measuring current transformers (classes 3 and 5)
Limits of phase displacement are not specified for class 3 and class 5 For classes 0.2-0,5 and 1 the ratio and phase error must not be higher, at the various test currents, than the highest errors shown in the table when the load on secondary winding has a whatsoever value between 25% and 100% of rated load. For instance, let’s suppose to carry out a test for class 0.5 on a TAI 400/5; the general catalogue states that a TAI 400/5 must supply 8 VA in class 0.5. We arrange a 8 VA load, then we feed the C.T. with a current of 400 A (100%) and we with a suitable test system that ratio error is lower than 0.5 as well as phase error is lower than 30 minutes. The same check is extended to the other current values established in the table. The transformer is ed if no error exceed the fixed limits. The last of above mentioned definitions is worth an explanation: the thermal current standard value for our C.T.’s is 120%
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of rated current; this means that they can continuously work with an overload of 120%. Owing to thermal dissipation, this is not possible for models with higher currents; in this case the highest load coincides with the rated one. In this context it is also necessary to hint at the so-called C.T.’s suitable for overloading which are designed to stand permanent overloads of 200% or 400% of rated current; this kind of C.T.’s is present in our catalogue and they are devoted to feed kWh meters for tariff use (ENEL, municipal concerns). Until now we have spoken about instrument C.T.’s, but it exists another important C.T.’s family designed to feed protection devices (network-phasing, earth, single-pole relays, etc.), which have the peculiarity to work with a current much higher than the rated one, assuring errors within determinate limits. They are defined according to the following parameters: -
accuracy class: it expresses the measuring accuracy to the itted limit current: there are 2 accuracy classes, 5P and 10P where P stands for protection
-
accuracy limit factor: it expresses the current limit value to which is required the above mentioned accuracy: standard values are 5-10-15-20-30
The following table shows the highest itted errors at the rated current (second and third column) and at the limit current (last column) for 5P and 10P CT’s:
Tab.14 Limits of error for protective current transformers
As marking example, two transformers marked 5P10 and 10P20 meet the conditions in the columns 2 and 3 at the rated current and the conditions in the last column 10 times and 20 times the rated current respectively.
A guide to the choice Whoever skims through a transformer catalogue, faced with plenty of models, shapes, sizes and mounting solutions cannot help wondering: but is all that really necessary? The answer is yes: to this variety of offer corresponds an as big variety of mounting options, requirements and habits. Luckily, most of the work to choose the model is made by the end-; anyway, since we are suppliers, we should always be able to judge the choices and suggest possible alternatives when this is necessary. A C.T. is chosen according to its mechanical and electrical characteristics. Let’s start from the mechanical ones. The first mechanical characteristic to take into consideration is the size of the ing bar or the cable diameter: this is a first factor for choosing a C.T., but not the only one. The next step is to that C.T. external dimensions meet the bonds imposed by the bar system (for ing cable C.T.’s this bond is usually less binding). In particular it is necessary to take into consideration C.T. width, which must meet the distance between the bars, in its turns determined by the distance between the main switch poles. Many of our C.T.’s have been designed keeping into consideration this datum, obtained from the catalogues of the most widespread switches. A third mechanical element to take into is the mounting system for which we have three different options: on (through plastic or metallic feet) on DIN rail or direct mounting on primary bars. The last element is the protection and anti-intrusion degree obtained through terminal covers, which can be sealable in case of tariff applications. A series of our C.T.’s (series TAI) has these three options built in the transformers; for all the other series, these are supplied in separate plastic bags with mounting by the end-. C.T. housings are made with V0-type self-extinguishing thermoplastic material. Electrical characteristics: for a correct choice you have to consider the following ones: -
secondary rated current required rated burden required accuracy class
Primary and secondary currents are defined by the end- on the basis of the plant data; as far as the other two characteristics are concerned, although they are also tied to the use, it is convenient to make some considerations which allow to make clear some choice criteria. As we have already seen, burden is essentially the power that a transformer must be able to supply in rated conditions and it is opportunely expressed in VA. In order to establish the burden that a C.T. must supply in a certain application, let’s consider the following diagram:
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(please note as various meters connected with a C.T. must be placed in series among them, contrary to the meters connected with voltage transformers which must be placed in parallel among them. Besides, please note that for a correct phase relationship between primary and secondary winding, terminals are identified with some letters: to P1 corresponds s1 and to P2 corresponds s2. To the secondary winding of this C.T. are connected two meters; in this case C.T. shall be able not only to supply the necessary power to feed them but also to supply the power dissipated by the connecting cables: called Pf the power dissipated by the cables and P1 and P2 the one for the two meters we have: P = Pf+P1+P2 (VA) This is the lowest burden that the C.T. must have in order to correctly feed the meters; but this is not enough because to consider it right for the purpose it is necessary to define required burden and accuracy class. In its turns this is correlated with the accuracy class of the fed meter: the more accurate is the meter the more accurate must be the connected C.T. in order to guarantee that the measuring chain meets the required whole accuracy. To clear the ideas, we list here-follow some possible pairs C.T.’s/meters: -
class 3 C.T.’s connected with: maximum demand ammeters RQT and sometimes moving iron meters RQ
-
class 1 C.T.’s connected with: ammeters RQ and self-supplied transducers
-
class 0,5 C.T.’s connected with: power or current transducers, kWh-meters, multifunction meters
-
class 0.2 C.T.’s connected with: kWh-meters and multifunction meters with class 1 energy counting
You will surely that when we spoke about C.T.’s accuracy class we mentioned two kinds of errors: ratio error and phase error. Well, if it is obvious the reason why we have to reduce the ratio error to a minimum (an ammeter connected with a C.T. does nothing but measure C.T. secondary current and if it is wrong, consequently also the measurement is wrong), it is much less evident the importance to limit phase error. Phase error becomes important when we measure powers: in fact, active power formula P=V*I* cosphi shows that power is tied to the phase angle between voltage and current (cosphi); therefore it is essential, for an accurate measurement, to reduce the phase error introduced by C.T.’s to a minimum. Power transducers as well as energy kWh-meters require C.T.’s with very high phase accuracy. Always to clear the ideas, let’s return to the burdens and give some tables which report the typical burdens of some meters manufactured by our company as well as typical power losses on the connection cables.
Power dissipated by connection cables Section mm² 1 1.5 2 2.5 4 6 10
1 0.677 0.5 0.4 0.25 0.167 0.1
2 1.333 1 0.8 0.5 0.333 0.2
Connection cable length (m) 3 4 5 2 2.667 3.333 1.5 2 2.5 1.2 1.6 2 0.75 1 1.25 0.5 0.667 0.833 0.3 0.4 0.5
7 4.667 3.5 2.8 1.75 1.167 0.7
10 6.667 5 4 2.5 1.667 1
Power dissipated by the meters
Analogue meters – Circuit loadings FAMILY RQE-IE RQE AQE D4E NP2E – RP2E NP3E – RP3E ET70AV
4 of 5
TYPE 48 / 72 / 96 144 48 / 72 / 96 / 144 DIN 4 module
RANGE 5A 5A 5A 5A 5A 5A 5A
Circuit loading VA 1.1 1.7 0.7 1.1 1.1 1.1 1.1
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ES90 PA96E RQTN RQTEN
PROFILE 72 / 96 72 / 96
5A 5A 5A – 15 minutes 5A – 15 minutes
1.8 0.6 2.8 3.8
Measuring transducers – Circuit loadings FAMILY D8W – D8RW D4I D4I4
Input 5A 5A 5A
Input 230 – (400)V
Circuit loading VA 0.5 1.5 – 2.5 0.5
Multifunction meters – Circuit loadings FAMILY Multifunction meter with C.T. Multifunction meter with shunt
Input 5A 5A
Input 230 – (400)V 230 – (400)V
Circuit loading VA 0.5 0.7
KWh-meters – Circuit loadings FAMILY KWh-meters with C.T. KWh-meters with shunt D4EWMS
Input 5A 5A 5 – 16A
Input 230 – (400)V 230 – (400)V 115 – 230V
Circuit loading VA 0.5 0.7 8
With these tables we are now able to correctly evaluate a C.T.: let’s suppose that we want to measure a current of 400A by a C.T. with secondary 5A which has to feed a maximum demand ammeter with burden P1=2.8VA and a Nemo 96 with burden P2=0.5VA, that the distance between the meters and the transformer is 3 meters and that we use a cable with section 1 mm² (Pf= 3VA). Which C.T. have we to choose? The whole required burden is the sum of the three: P = Pf+P1+P3 = 3+2.8+0.5 = 6.3VA Since we have to feed also a very accurate meter such as Nemo 96 which requires a low ratio and phase error, it is convenient to choose a C.T. with accuracy class 0.5; in conclusion, we need a C.T. with ratio 400/5, secondary winding 5A, accuracy class 0.5 and burden of at least 6.3VA. By looking up our catalogues you can identify as possible candidates TAI300, TAI310, TAI400; the model is completely defined by the bonds imposed by the mechanical requirements (sizes and opening for bar and cable ing).
Tlf. (+45) 4353 0222 - Fax (+45) 4353 0797 - Mail jensenelectric.dk Gisselfeldvej 12 - DK-2665 Vallensbæk Strand
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http://www.jensenelectric.dk/docs/ime/current_transformers_manua...
Current transformers: function, characteristics, guide to selection Function Current transformers (that from now on will be identified as C.T.) are born to reduce the value of the currents circulating inside an electric plant; these can go from some amperes to thousands of amperes or can have standard values (for instance 1 or 5 A.), suitable to feed test and measuring equipments. A transformer can be imagined as a device containing two separate circuits: a primary circuit where the plant current flows and a secondary circuit where it flows the output current proportional to primary current. They are usually used with meters (ammeters, KWH-meters, transducers, etc.) or with control devices such as earth leakage relays, load control, recorders, protection relays. Transformers treated in this document can be only used in alternating current circuits; for measurements in direct current circuits we have to adopt other solutions such as shunts or Hall-effect transformers. The main functions of an instrument transformer are: -
to allow going back with precision to the primary current value by measuring the current inside secondary circuit;
-
to galvanically insulate the secondary current from the primary current separating, in this way, a world composed of very high voltages and currents, and therefore potentially dangerous, from the world in which the s are working, where the highest security level must be assured (both for men and for machines);
-
to allow the use of quite easy and cheap meters to measure quantities with very high values (thousands of amperes can be measured by a C.T. and a moving-iron ammeter with an approximate error of 2%).
Characteristics Luckily, for instrument transformers there is a standard (IEC 44-1) which exactly fixes the use and electrical characteristics; according to this standard, our products are classified as transformers to measure the current in low-voltage applications. Low-voltage applications mean plants where primary circuit voltage doesn’t exceed 0.72kV or, in specific cases, 1.2kV. To these use voltages correspond insulation voltages respectively of 3kV and 6kV. 99% of our production us designed to work at 0.72/3kV. The most common symbols used to represent a current transformer are the following:
Description Two-winding transformer Note – Voltage instantaneous polarities can be shown as in Form 2 of the symbol. Example: Two-winding transformer represented with the voltage instantaneous polarity indicators. Two instantaneous currents which enter from the winding ends marked by the points, produce fluxes which get added. The symbols highlight the presence of two terminals for the primary circuit and two terminals for the secondary circuit. In low-voltage C.T.’s very often primary terminals don’t physically exist since the primary circuit is composed of a bar ing through a hole in the C.T. itself. The symbols emphasize also the physical separation between the two circuits as a guarantee for the insulation; the energy transfer between primary and secondary circuit occurs for magnetic coupling. Once defined the voltage range for our C.T.’s, let’s define also the current range: our models cover primary currents from some amperes to 6000 A. with a wide range of models which can meet the various mechanical and mounting requirements; for secondary winding, standard currents of 1 and 5 A. are usually used. Before entering a detailed discussion about the characteristics that identify a C.T., it is necessary to give some definitions: -
rated primary current: primary current value to which the C.T. performances are referred;
-
rated secondary current: secondary current value upon which the C.T. performances are based;
-
1 of 5
rated transformation ratio: ratio between primary and secondary rated current;
-
ratio error: error introduced in the measurement for which the real transformation ratio doesn’t correspond to the rated transformation ratio;
-
phase error: secondary current is not perfectly in phase with primary current, as we might expect from an ideal transformer;
-
accuracy class: a C.T. classification which defines the highest permitted errors (both ratio error and phase error) under certain test conditions;
05/04/2012 01:42 PM
Generelt om traformere
http://www.jensenelectric.dk/docs/ime/current_transformers_manua...
-
burden or load: it is the impedance of the circuit fed by secondary winding, expressed in ohms and power factor;
-
rated power: it is the value of apparent power provided by the C.T. when it feeds the rated burden;
-
thermal current: highest primary current that a C.T. can permanently stand with secondary winding connected at the rated burden.
To clarify the ideas, our range of C.T.’s concentrates on accuracy classes 0.5 – 1 and 3 with rated powers (or burdens as they are usually called) between 2 VA. of a TAIBB with 100 A. and 100 VA. of the biggest current transformers in our range (series TAU). It is important to note that high burdens are obtained with high primary currents and that, vice versa, there are physical and cost limits when you want to obtain high burdens with low currents. This peculiarity sets some limits to the manufacturing of ing bar C.T.’s when currents are low and compels the customer to choose a wound-primary current transformer (our series TAQ) in which the primary circuit is actually linked up with two terminals. Upon the basis of these definitions, it is possible to bring in the C.T. accuracy classes. The accuracy classes express the ratio and phase error limits measured in fixed test conditions. The classes established by the standard are the following: 0.1-0.2S-0.2-0.5S-0.5-1-3-5 the three values in bold figures cover 98% of our production. Let’s see directly their meaning through the tables established by the standard:
Tab.12 Limits of current error and phase displacement for measuring current transformers for special application
Tab.13 limits of currents error for measuring current transformers (classes 3 and 5)
Limits of phase displacement are not specified for class 3 and class 5 For classes 0.2-0,5 and 1 the ratio and phase error must not be higher, at the various test currents, than the highest errors shown in the table when the load on secondary winding has a whatsoever value between 25% and 100% of rated load. For instance, let’s suppose to carry out a test for class 0.5 on a TAI 400/5; the general catalogue states that a TAI 400/5 must supply 8 VA in class 0.5. We arrange a 8 VA load, then we feed the C.T. with a current of 400 A (100%) and we with a suitable test system that ratio error is lower than 0.5 as well as phase error is lower than 30 minutes. The same check is extended to the other current values established in the table. The transformer is ed if no error exceed the fixed limits. The last of above mentioned definitions is worth an explanation: the thermal current standard value for our C.T.’s is 120%
2 of 5
05/04/2012 01:42 PM
Generelt om traformere
http://www.jensenelectric.dk/docs/ime/current_transformers_manua...
of rated current; this means that they can continuously work with an overload of 120%. Owing to thermal dissipation, this is not possible for models with higher currents; in this case the highest load coincides with the rated one. In this context it is also necessary to hint at the so-called C.T.’s suitable for overloading which are designed to stand permanent overloads of 200% or 400% of rated current; this kind of C.T.’s is present in our catalogue and they are devoted to feed kWh meters for tariff use (ENEL, municipal concerns). Until now we have spoken about instrument C.T.’s, but it exists another important C.T.’s family designed to feed protection devices (network-phasing, earth, single-pole relays, etc.), which have the peculiarity to work with a current much higher than the rated one, assuring errors within determinate limits. They are defined according to the following parameters: -
accuracy class: it expresses the measuring accuracy to the itted limit current: there are 2 accuracy classes, 5P and 10P where P stands for protection
-
accuracy limit factor: it expresses the current limit value to which is required the above mentioned accuracy: standard values are 5-10-15-20-30
The following table shows the highest itted errors at the rated current (second and third column) and at the limit current (last column) for 5P and 10P CT’s:
Tab.14 Limits of error for protective current transformers
As marking example, two transformers marked 5P10 and 10P20 meet the conditions in the columns 2 and 3 at the rated current and the conditions in the last column 10 times and 20 times the rated current respectively.
A guide to the choice Whoever skims through a transformer catalogue, faced with plenty of models, shapes, sizes and mounting solutions cannot help wondering: but is all that really necessary? The answer is yes: to this variety of offer corresponds an as big variety of mounting options, requirements and habits. Luckily, most of the work to choose the model is made by the end-; anyway, since we are suppliers, we should always be able to judge the choices and suggest possible alternatives when this is necessary. A C.T. is chosen according to its mechanical and electrical characteristics. Let’s start from the mechanical ones. The first mechanical characteristic to take into consideration is the size of the ing bar or the cable diameter: this is a first factor for choosing a C.T., but not the only one. The next step is to that C.T. external dimensions meet the bonds imposed by the bar system (for ing cable C.T.’s this bond is usually less binding). In particular it is necessary to take into consideration C.T. width, which must meet the distance between the bars, in its turns determined by the distance between the main switch poles. Many of our C.T.’s have been designed keeping into consideration this datum, obtained from the catalogues of the most widespread switches. A third mechanical element to take into is the mounting system for which we have three different options: on (through plastic or metallic feet) on DIN rail or direct mounting on primary bars. The last element is the protection and anti-intrusion degree obtained through terminal covers, which can be sealable in case of tariff applications. A series of our C.T.’s (series TAI) has these three options built in the transformers; for all the other series, these are supplied in separate plastic bags with mounting by the end-. C.T. housings are made with V0-type self-extinguishing thermoplastic material. Electrical characteristics: for a correct choice you have to consider the following ones: -
secondary rated current required rated burden required accuracy class
Primary and secondary currents are defined by the end- on the basis of the plant data; as far as the other two characteristics are concerned, although they are also tied to the use, it is convenient to make some considerations which allow to make clear some choice criteria. As we have already seen, burden is essentially the power that a transformer must be able to supply in rated conditions and it is opportunely expressed in VA. In order to establish the burden that a C.T. must supply in a certain application, let’s consider the following diagram:
3 of 5
05/04/2012 01:42 PM
Generelt om traformere
http://www.jensenelectric.dk/docs/ime/current_transformers_manua...
(please note as various meters connected with a C.T. must be placed in series among them, contrary to the meters connected with voltage transformers which must be placed in parallel among them. Besides, please note that for a correct phase relationship between primary and secondary winding, terminals are identified with some letters: to P1 corresponds s1 and to P2 corresponds s2. To the secondary winding of this C.T. are connected two meters; in this case C.T. shall be able not only to supply the necessary power to feed them but also to supply the power dissipated by the connecting cables: called Pf the power dissipated by the cables and P1 and P2 the one for the two meters we have: P = Pf+P1+P2 (VA) This is the lowest burden that the C.T. must have in order to correctly feed the meters; but this is not enough because to consider it right for the purpose it is necessary to define required burden and accuracy class. In its turns this is correlated with the accuracy class of the fed meter: the more accurate is the meter the more accurate must be the connected C.T. in order to guarantee that the measuring chain meets the required whole accuracy. To clear the ideas, we list here-follow some possible pairs C.T.’s/meters: -
class 3 C.T.’s connected with: maximum demand ammeters RQT and sometimes moving iron meters RQ
-
class 1 C.T.’s connected with: ammeters RQ and self-supplied transducers
-
class 0,5 C.T.’s connected with: power or current transducers, kWh-meters, multifunction meters
-
class 0.2 C.T.’s connected with: kWh-meters and multifunction meters with class 1 energy counting
You will surely that when we spoke about C.T.’s accuracy class we mentioned two kinds of errors: ratio error and phase error. Well, if it is obvious the reason why we have to reduce the ratio error to a minimum (an ammeter connected with a C.T. does nothing but measure C.T. secondary current and if it is wrong, consequently also the measurement is wrong), it is much less evident the importance to limit phase error. Phase error becomes important when we measure powers: in fact, active power formula P=V*I* cosphi shows that power is tied to the phase angle between voltage and current (cosphi); therefore it is essential, for an accurate measurement, to reduce the phase error introduced by C.T.’s to a minimum. Power transducers as well as energy kWh-meters require C.T.’s with very high phase accuracy. Always to clear the ideas, let’s return to the burdens and give some tables which report the typical burdens of some meters manufactured by our company as well as typical power losses on the connection cables.
Power dissipated by connection cables Section mm² 1 1.5 2 2.5 4 6 10
1 0.677 0.5 0.4 0.25 0.167 0.1
2 1.333 1 0.8 0.5 0.333 0.2
Connection cable length (m) 3 4 5 2 2.667 3.333 1.5 2 2.5 1.2 1.6 2 0.75 1 1.25 0.5 0.667 0.833 0.3 0.4 0.5
7 4.667 3.5 2.8 1.75 1.167 0.7
10 6.667 5 4 2.5 1.667 1
Power dissipated by the meters
Analogue meters – Circuit loadings FAMILY RQE-IE RQE AQE D4E NP2E – RP2E NP3E – RP3E ET70AV
4 of 5
TYPE 48 / 72 / 96 144 48 / 72 / 96 / 144 DIN 4 module
RANGE 5A 5A 5A 5A 5A 5A 5A
Circuit loading VA 1.1 1.7 0.7 1.1 1.1 1.1 1.1
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ES90 PA96E RQTN RQTEN
PROFILE 72 / 96 72 / 96
5A 5A 5A – 15 minutes 5A – 15 minutes
1.8 0.6 2.8 3.8
Measuring transducers – Circuit loadings FAMILY D8W – D8RW D4I D4I4
Input 5A 5A 5A
Input 230 – (400)V
Circuit loading VA 0.5 1.5 – 2.5 0.5
Multifunction meters – Circuit loadings FAMILY Multifunction meter with C.T. Multifunction meter with shunt
Input 5A 5A
Input 230 – (400)V 230 – (400)V
Circuit loading VA 0.5 0.7
KWh-meters – Circuit loadings FAMILY KWh-meters with C.T. KWh-meters with shunt D4EWMS
Input 5A 5A 5 – 16A
Input 230 – (400)V 230 – (400)V 115 – 230V
Circuit loading VA 0.5 0.7 8
With these tables we are now able to correctly evaluate a C.T.: let’s suppose that we want to measure a current of 400A by a C.T. with secondary 5A which has to feed a maximum demand ammeter with burden P1=2.8VA and a Nemo 96 with burden P2=0.5VA, that the distance between the meters and the transformer is 3 meters and that we use a cable with section 1 mm² (Pf= 3VA). Which C.T. have we to choose? The whole required burden is the sum of the three: P = Pf+P1+P3 = 3+2.8+0.5 = 6.3VA Since we have to feed also a very accurate meter such as Nemo 96 which requires a low ratio and phase error, it is convenient to choose a C.T. with accuracy class 0.5; in conclusion, we need a C.T. with ratio 400/5, secondary winding 5A, accuracy class 0.5 and burden of at least 6.3VA. By looking up our catalogues you can identify as possible candidates TAI300, TAI310, TAI400; the model is completely defined by the bonds imposed by the mechanical requirements (sizes and opening for bar and cable ing).
Tlf. (+45) 4353 0222 - Fax (+45) 4353 0797 - Mail jensenelectric.dk Gisselfeldvej 12 - DK-2665 Vallensbæk Strand
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05/04/2012 01:42 PM
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