Okružní 31, 638 00 Brno  
legmet@cmi.cz  
 +420,545,555,414  
 
CZECH 
METROLOGY 
INSTITUTE  
Executive summary for the EC (not part of this legislation)  
'Voltage instrument transformers' may be placed on the market and put into operation in the Czech Republic 
as specified measuring instruments  in accordance with Act  No 505/1990 on metrology, as amended. Specified 
measuring instruments are, pursuant to this  act, measuring instruments whose the type is specified in the 
generic list of specified measuring instruments (Decree No  345/2002) and that are also intended (by the 
manufacturer/importer) for measurement with significance for protection of the public int erest in the areas of 
consumer protection, obligations, sanctions, fees, tariffs and taxes, health protection, environmental 
protection,  occupational safety or other public interests protected by special legislation . This is therefore a 
purpose similar to the one defining specified products – non-automatic measuring instruments and instruments 
pursuant to Directives  2014/31/EU and 2014/32/EU. The requirements of this legislation do not apply to 
measuring instruments not placed on the market in the Czech Rep ublic for the above purposes, defined by 
Act No 505/1990 on metrology.  
The purpose of this notified legislation is to lay down metrological and technical requirements for these 
specified measuring instruments. This legislation also stipulates tests for typ e approval and verification of 
specified measuring instruments of this type.  
 (End of executive summary.)  
 
 
P U B L I C   D E C R E E  
 
As the authority with substantive and territorial jurisdiction in the matter of defining metrological and technical 
requi rements for specified measuring instruments and stipulating test methods for type approval and 
verification of specified measuring instruments pursuant to §  14(1) of Act No  505/1990 on metrology, as 
amended, and in accordance with the provisions of §  172 e t seq. of Act No  500/2004, the Administrative Code, 
as amended (hereinafter the ‘CAP’), the Czech Metrology Institute (CMI) commenced ex officio proceedings 
pursuant to §  46 of the Administrative Code and, based on supporting documents, issues the followin g: 
 
 
 
I. 
D R A F T  M E A S URE O F  A  G E N ERAL  N A TUR E  
number: 0111 -OOP -C089 -22 
 
laying down the metrological and technical requirements for specified measuring instruments, 
including testing methods for verification of the following specified measuring instruments:  
'voltage  instrument transformers'  
 
 Measure of a General Nature No 0111 -OOP -C089 -22 
 
2 
 1  Basic definitions  
For the purposes of this Measure of a General Nature, terms and definitions according to VIM and VIML1 and 
the terms and definitions below apply.  
1.1 
voltage instrument transformer (hereinafter 'instrument transformer' or 'VIT')  
a voltage transformer that helps transmit an information signal to measuring instruments, electricity meters, 
etc. 
1.2 
primary winding  
the winding to which the voltage to be transformed is applied  
1.3 
secondary winding  
the winding to which voltage circuits of measuring instruments, electricity meters, breakers or similar devices 
are connected  
1.4 
nominal primary voltage   
the value of the primary voltage indicated on the instrument transformer plate and on which its operation is 
based. The standard values of nominal voltage of a single -phase transformer connected between the phase 
conductor of the three -phase system and the ground, or between zero point and ground, are given by the 
product of the nominal system voltage multiplied by a factor of 1/ 3 
1.5 
nominal secondary voltage  
the value of the secondary voltage indicated on the instrument transformer plate and on which its operation is 
based. For single -phase instrument transformers connected between phase and ground in three -phase systems 
where the nominal value of the primary voltage is divided by 3, the secondary voltage value is divided by 
3, so the resulting transformation ratio remains the same.  
1.6 
voltage increase factor   
a factor whose product with the rated primary voltage gives the highest voltage at which the instrument 
transformer must meet the warming requirements for the prescribed time and accuracy requirements  
1.7 
voltage error (ratio error)  
error caused by the instrument transformer due to the fact that the actual voltage ratio is not equal to the 
nomina l ratio. This voltage error, expressed as a percentage, is given by the formula:  
                                                        
()
pp sn
UUUUK 100−=  
 (1) 
 
1  TNI 01 0115 International Vocabulary of Metrology – Basic and General Concepts and Associated Terms (VIM) and 
International Vocabulary of Terms in Legal Metrology (VIML) are part of the technical harmonisation compendium 
‘Terminology in the Area of Metrology’, which is publicly accessible at www.unmz.cz.  Measure of a General Nature No 0111 -OOP -C089 -22 
 
3 
 where  Kn..........   is the nominal ratio,  
 Up   ........   is the actual primary voltage , 
 Us  .........   is the actual secondary voltage, corresponding to Up under measurement conditions.  
1.8 
phase angle error   
the phase difference between primary and secondary voltage phasors; the orientation of the primary and 
secondary voltage phasors is chosen in such a way that in an ideal instrument transformer the phase angle error 
is zero. The phase angle error is considered positive if the secondary current phasor precedes the primary 
current phasor. It is usually expressed in arc minutes or centirad ians. 
NOTE: This definition is correct only for the alternating sinusoidal voltage.  
1.9 
two-pole insulated instrument transformer  
an instrument transformer for which all parts of the primary winding, including terminals, are insulated from 
ground to an in sulation level corresponding to the nominal insulation level  
1.10 
single -pole insulated instrument transformer   
a single -phase instrument transformer that has one end of its primary winding directly grounded or a three -
phase transformer designed to connect  the primary winding to a star with a directly grounded zero point  
1.11 
real ratio   
the ratio of the actual primary voltage to the actual secondary voltage of the instrument transformer  
1.12 
nominal ratio  
the ratio of nominal primary voltage to nominal secondary voltage  
1.13 
impedance  
the impedance of the secondary circuit, expressed in ohms, at a given power factor (inductive or capacitive). 
The impedance is usually expressed as the apparent power in VA consumed at the stipulated power factor a nd 
at nominal secondary current.  
1.14 
nominal impedance  
value of the load from which the prescribed accuracy requirements are derived  
1.15 
nominal load  
the load value in volt -amperes at a given power factor that the instrument transformer transmits to the 
secondary circuit at nominal secondary voltage and connected nominal impedance  
1.16 
rated load  
the value of the apparent power at the nominal voltage that can be taken from the secondary winding without 
exceeding permitted warming. The ra ted load must be specified in units of ‘VA’.  
Its default values are: (25; 50; 100)  VA and their decimal multiples, these extreme load values being Measure of a General Nature No 0111 -OOP -C089 -22 
 
4 
 determined at the rated secondary voltage and at a power factor of 1.  
NOTES:  
1. At the rated load, the limi ts of voltage errors and phase angle errors for all secondary windings may be exceeded.  
2. If there is more than one secondary winding, the rated load for each winding must be indicated separately.  
1.17 
accuracy class  
a designation assigned to an instrument transformer whose voltage error and phase angle error do not exceed 
permissible values under prescribed operating conditions  
1.18 
maximum voltage for an instrument transformer  
the maximum effective value of the combined AC voltage between phases for which the instrument 
transformer is designed with regard to its insulation  
 
 
1.19 
nominal insulation level  
the combination of voltage values that characterises the insulation of the  instrument transformer in terms of its 
ability to withstand electrical stress  
1.20 
exposed environment  
an environment in which the instrument transformer is exposed to atmospheric overvoltage  
NOTE: Such a  voltage instrument transformer  is usually conne cted to an external line either directly or by a short 
cable.  
1.21 
unexposed environment  
an environment in which the instrument transformer is not exposed to atmospheric overvoltage  
NOTE: Such a  voltage instrument transformer  is usually connected with  a cable grid.  
1.22 
nominal frequency   
the value of the frequency from which the requirements of this legislation are derived  
1.23 
auxiliary winding   
the winding of a single -phase instrument transformer that is intended, in an assembly of three single -phase 
instrument transformers for connection to an open triangle, for:  
a) creation of the sum voltage in fault states;  
b) suppression of oscillations during r esonance phenomena.  Measure of a General Nature No 0111 -OOP -C089 -22 
 
5 
 2  Metrological requirements  
2.1 Normal operating conditions  
2.1.1 Ambient air temperature  
Measuring transformers are classified in three categories as shown in Table 1.  
Table 1 — Temperature category  
Category  Lowest temperature  
(°C) Highest temperature  
(°C) 
-5/+40  -5 +40 
-25/+40  -25 +40 
-40/+40  -40 +40 
NOTE: Storage and transport conditions must also be considered when selecting the temperature category . 
2.1.2   Other operating conditions for an indoor instrument transformer  
The other operating conditions to be taken into account are:  
a)  the effect of sunlight; may be neglected;  
b)  the influence of ambient air, if it is heavily polluted by dust, smoke, corros ive gases, steam or salt;  
c)  normal humidity conditions are as follows:  
   the mean relative humidity value measured over a period of 24 hours does not exceed 95  %;  
   the average water vapour pressure over 24 hours does not exceed 2.2 kPa;  
   the average relative humidity over the course of one month does not exceed 90%;  
   the average water vapour pressure over a month does not exceed 1.8 kPa.  
2.1.3   Other operat ing conditions for outdoor instrument transformers   
Other operating conditions that are considered are as follows:  
 a) if the average ambient air temperature measured over a period of 24 hours does not exceed 35 ° C, it 
can be neglected;  
 b) solar radiati on exceeding 1000 W/m 2 (high noon on a clear day) must be taken into account;  
 b) the ambient air, if it is heavily contaminated by dust, smoke, corrosive gases, vapours or salts, must be 
taken into account;  
 d) wind pressure not exceeding 700 Pa (this v alue corresponds to a wind speed of 34 m/s) is not taken into 
account;  
 e) the presence of condensation or precipitation must be taken into account.  
2.2   Nominal parameter values for instrument transformers  
2.2.1   General requirements for nominal data  
The nominal data of the voltage instrument transformers, including their accessories, must be selected 
according to:  
  a) the maximum voltage for the instrument transformer ( Um);  
  b) the prescribed insulati on levels;  
  c)  the nominal frequency ( fR);  
  d) the nominal load;   Measure of a General Nature No 0111 -OOP -C089 -22 
 
6 
   e) the nominal accuracy class;  
  f) the nominal voltage.  
The nominal data shall be applied under normal reference atmospheric conditions (temperature (20  °C), 
pressure (101.3 kPa) a nd humidity (11  g/m 3)). 
2.2.2   Maximum voltage for an instrument transformer  
The maximum voltage values for the instrument transformer must be selected from Table 2.  
The maximum voltage for the instrument transformer is selected as the nearest value of Um that is equal to or 
greater than the maximum voltage of the system where it is installed.  
For an instrument transformer installed under normal insulation conditions, Um must be at least equal to Usys.  
For a measuring transformer installed outside the normal isolation environment range, U  m may be selected 
greater than the nearest Um value equal to or greater than Usys according to special needs.  
NOTE: By way of example, the selection of a Um value gre ater than the nearest normalized value Um equal to or greater 
than Usys when the voltage measuring transformer is to be installed at an altitude higher than 1000  m in order to 
compensate for the drop in the withstand voltage.  
2.2.3   Prescribed insulation  levels  
The choice of insulation level of the primary winding terminals of inductive instrument transformers must be 
derived from the highest voltage Um for the instrument transformer according to Table 2.  Measure of a General Nature No 0111 -OOP -C089 -22 
 
7 
 Table 2 - Prescribed primary terminal insulation levels for instrument transformers  
Maximum 
instrument 
transformer 
voltage  Um  
(effective value)  
(kV) Nominal test 
voltage at 50  Hz 
(effective value)  
(kV) Rated 
atmospheric 
pulse test voltage 
(peak value)  
(kV) Rated withstand 
voltage at 
switching pulse 
(peak value)  
(kV) 
0.72 3 ---  
1.2 6 ---  
3.6 10 20  
40  
7.2 20 40  
60  
12 28 60  
75  
17.5 38 75  
95  
24 50 95  
125  
36 70 145  
170  
52 95 250  
72.5 140 325  
100 185 450  
123 185 450  
230 550  
145 230 550  
275 650  
170 275 650  
325 750  
245 395 950  
460 1,050   
300 395 950 750 
460 1,050  850 
362 460 1,050  850 
510 1,175  950 
420 570 1,300  950 
630 1,425  1,050  
550 630 1,425  1,050  
680 1,550  1,175  
800 880 1,950  1,425  
975 2,100  1,550  
 
2.2.4   Additional requirements for primary winding insulation  
2.2.4.1   Requirements for grounded primary winding terminal insulation  
The primary winding terminal to be grounded and insulated from the casing and frame must pass a test using Measure of a General Nature No 0111 -OOP -C089 -22 
 
8 
 a short -term test voltage at a frequency of 50  Hz with an effective value of 3  kV for 1 minute applied between 
this terminal and the frame.  
2.2.4.2 P artial discharges  
Partial discharge requirements are applicable to instrument transformers with Um greater than or equal to 
7.2 kV. Partial discharge levels must not exceed the limits set out in Table 3.  
Table 3 - Test voltages for measuring partial disch arges and permissible levels  
Grid ground type  Primary winding 
connection  Test voltage for 
partial discharge 
measurement 
(effective value)  
(kV)  Permissible partial discharge levels (pC) 
Type of insulation:  
liquid or gas  solid  
Grounded centre 
network (ground 
connection factor 
 1.4) Phase -ground  Um 10 50 
1.2Um/3 5 20 
Phase -phase  1.2Um 5 20 
It network  or 
network with 
inefficiently 
grounded centre 
(ground link factor 
> 1.4) Phase -ground  1.2Um 10 50 
1.2Um/3 5 20 
Phase -phase  1.2Um 5 20 
NOTES:  
1.  If the network midpoint is not defined, values apply to an IT network or network with an inefficiently 
grounded midpoint.  
2.  The maximum permitted levels of partial discharges are also valid for frequencies other than the nominal 
frequency.  
2.2.5  Insulation requirements between winding sections  
For interconnected terminals from each section, the test voltage of the industrial frequency between sections 
3 kV must be specified.  
2.2.6   Requirements for the isolation of secondary windings  
The specified test voltage at 50  Hz for secondary insulation is 3  kV. 
2.2.7   Values of nominal frequencies  
The nominal frequencies are 16  2/3 Hz, 50  Hz. 
2.2.8   Nominal load values  
The nominal values of apparent load power at power factor 1 expressed in VA  are: 
(1; 2.5; 5; 10) VA (Load Range I).  
The nominal apparent power values at an inductive power factor 0.8 expressed in VA are:  
(10; 25; 50; 100) VA (Load range II).  Measure of a General Nature No 0111 -OOP -C089 -22 
 
9 
 NOTE: For a given instrument transformer that has one of the nominal load values associated with the accuracy class, 
the provision of nominal non -standard loads associated with another accuracy class is not ruled out.  
2.2.9  Accuracy classes and permissib le errors of instrument transformers  
2.2.9.1   Determination of the accuracy class  
For instrument transformers, the accuracy class is determined by the maximum permissible voltage error 
expressed as a percentage at the rated voltage prescribed for the re levant accuracy class.  
2.2.9.2   Accuracy classes  
The accuracy classes for single -phase inductive instrument transformers are:  
0.1; 0.2; 0.5; 1.0.  
2.2.9.3  Permissible current errors and angle errors for instrument transformers  
The voltage error and the error of the angle at rated frequency shall not exceed the values given in Table 4   at 
any voltage between 80  % and 120  % of the nominal voltage and with a load in the range of:  
a) any values between 0 VA and 100  % of the nominal load value for instrument transformers with a load 
of (1; 2.5; 5; 10) VA at Power factor 1 (Load range I);  
b) between 25  % and 100  % of the nominal load for instrument transformers with a load of (10; 25; 50; 
100) VA at power factor 0.8 (Load range II).  
Errors in instrum ent transformers shall be determined at the terminals of the instrument transformer and shall 
include the effect of fuses, resistors and integral parts of the instrument transformer.  
For instrument transformers with taps on secondary windings, the error re quirements apply to the highest 
transformation ratio, unless otherwise specified.  
Table 4  – Permissible voltage errors and phase angle errors of instrument transformers  
Class  
of accuracy  Voltage error  
U (%) Phase angle error U 
Minutes  Centiradians  
0.1 0.1 5 0.15 
0.2 0.2 10 0.3 
0.5 0.5 20 0.6 
1.0 1.0 40 1.2 
2.2.10 Nominal voltage values of instrument transformers  
2.2.10.1 Nominal primary voltage values  
The nominal values of the primary voltages of three -phase instrument transformers and single -phase 
instrument transformers for use in single -phase systems or for phase -to-phase measurements in three -phase 
systems must correspond to the nominal voltages set out in Tables 5 to 8. The nominal value of the primary 
voltages of t he single -phase voltage transformer connected between one phase of the three -phase system and 
ground or between the system zero point and the country must be 1/ 3 of one of the values of rated voltages.  
NOTE: The property of voltage instrument transformer s is based on the nominal primary voltage, whereas the nominal 
insulation level is based on the highest voltage for the instrument transformer.  
 
 Measure of a General Nature No 0111 -OOP -C089 -22 
 
10 
 Table 5 — Three -phase systems with a rated AC voltage exceeding 1  kV and not exceeding 35  kV and 
related equipment a 
Highest voltage for equipment (kV) Nominal system voltage (kV)  
3.6 3.3 3 
7.2 6.6 6 
12 11 10 
(17.5)  – (15) 
24 22 20 
36 33 30 
40.5 – 35 
NOTE:  Above are two series (columns) of nominal network voltage.  
a  These networks are generally three -wire unless specified otherwise.  
These values are phase -to-phase voltages.  
Values in brackets are not considered preferential.  
 
 
Table 6 — AC three -phase systems with a nominal voltage above 1  kV and not exceeding 35  kV and 
related equipmenta used in the Czech Republic  
Highest voltage for 
equipment (kV)  Nominal system voltage (kV)  
3.6 3 
7.2 6 
12 10 
25 22 
38.5 35 
a  These networks are generally three -wire unless specified otherwise.  
These values are phase -to-phase voltages.  
 
 
 Measure of a General Nature No 0111 -OOP -C089 -22 
 
11 
 Table 7 — Three -phase systems with a rated AC voltage exceeding 35  kV and not exceeding 230  kV 
and related equipmenta 
Highest voltage for equipment (kV) Nominal system voltage (kV) 
(52) (45) – 
72.5 66 69 
100 90 – 
123 110 115 
145 132 138 
(170)  (150)  (154)  
245 220 230 
a Values in brackets are not considered preferential. These values are phase -to-phase voltages.  
NOTE 1: Above are two series (columns) of nominal network voltage.  
NOTE 2: Of the following groups, only one value may be used for the highest voltage for the device:  
Group 1: 123  kV or 145  kV; 
Group 2: 245  kV (see Table 7) or 300  kV or 362  kV (see Table 8)  
 
 
Table 8 — Highest voltage for three -phase AC system equipment with a rated voltage above 245  kVa 
Highest voltage for equipment (kV)  
(300)  
362 
420 
550b 
800c 
1,100  
1,200  
a Values in brackets are not considered preferential. Values are phase -to-phase voltages.  
b 525 kV is also used.  
c 765 kV is also used.  
NOTE: Only one value from the following groups can be used as the highest voltage for 
equipment:  
Group 1 245  kV (see Table 7) or 300  kV or 362  kV (see Table 8);  
Group 2 362  kV or 420  kV; 
Group 3 420  kV or 550  kV; 
Group 4 1100  kV or 1200  kV. Measure of a General Nature No 0111 -OOP -C089 -22 
 
12 
  
2.2.10.2  Values of rated secondary voltages  
The nominal value of the secondary voltage must be selected according to practice at the place of application 
of the instrument transformer. The values of these voltages shall be considered as values for single -phase 
instrument transformers that are connected in single -phase systems or between phases in three -phase systems, 
and for three -phase instrument transformers. The nominal values of the secondary voltage are  100 V and 110 
V. 
For single -phase instrument tr ansformers that are designed for phase -to-ground connection in a three -phase 
system where the rated primary voltage is the selected value divided by 3, the rated secondary voltage must 
be one of the above values divided by 3. This will ensure the nominal  ratio of the instrument transformer.  
2.2.10.3  Nominal values of the voltage increase factor  
The voltage increase factor is determined by the highest operating voltage, which depends on the system and 
the grounding conditions of the primary winding of the instrument transformer.  
The values of the voltage increase factor relevant to the different grounding conditions are given in Table 9 
together with the permissible duration of the maximum operating voltage (i.e. nominal duration).  
Table 9 — Nominal values of voltage increase factors  
Voltage 
increase factor  Permissible 
duration  Primary winding connection method and grounding 
conditions  
1.2 Permanent  Between phases in all systems.   
Between the zero point of the instrument transformer and the 
ground in all systems.  
1.2 Permanent  Between phase and ground in a system with an effectively 
grounded centre.  
 1.5 30 s 
1.2 Permanent  Between phase and ground in a compensated network with 
automatic disconnection of the ground connection.  1.9 30 s 
1.2 Permanent  Between phase and country in an IT network without automatic 
disconnect of the ground connection or in a compensated 
network without automatic disconnect.  1.9 8 h 
3  Technical requirements  
3.1  General requirements for warming of instrument transformers  
The warming of windings, magnetic circuits or any other components of instrument transformers must not 
exceed the relevant values given in Table 10 if operated under specified conditions. Th ese values shall be 
based on the operating conditions referred to in Article 2.1.1.  
The warming of the winding is limited by the lowest class of insulation, either of the winding itself or the 
medium surrounding it.  
If the instrument transformers are in en closures, attention must be paid to the temperature surrounding the 
coolant inside the housing.  
If the specified ambient temperatures are higher than those referred to in Article 2.1.1, the permissible 
temperature increase according to Table 10 must be red uced by a value equal to the higher ambient 
temperature.  Measure of a General Nature No 0111 -OOP -C089 -22 
 
13 
  
Table 10 — Temperature limits for different parts, materials and dielectrics of the instrument 
transformer  
Instrument transformer parts  Greatest permissible 
warming (K)  
Oil voltage instrument transformer:    
— oil in upper layer   50 
— oil in upper layer, hermetically sealed   55 
— centre of winding   60 
— centre of winding, hermetically sealed   65 
— other metal parts in contact with oil  as for windings  
Instrument transformer with solid or gas insulation  — 
windings in contact with insulation materials according to the 
following temperature classes:   
Y   45 
A   60 
E   75 
B   85 
F   110 
H   135 
− other metal parts in contact with the above material classes  
classes  as for windings  
Connections, screwed or equivalent  
− Bare copper, bare copper alloy or bare aluminium alloy  as for windings  
• in air  50 
• in SF6 75 
• in oil  60 
− Silver plated or nickel plated   
• in air  75 
• in SF6 75 
• in oil  60 
− Tin plated   
• in air  65 
• in SF6 65 
• in oil  60 
3.2  Requirements for external insulation  
For inductive instrument transformers for outdoor use with a ceramic insulator sensitive to contamination, the 
required isolation distances measured over the surface of the insulator are shown in Table 11.  
 
 Measure of a General Nature No 0111 -OOP -C089 -22 
 
14 
  
Table 11 — Insulation surface distances  
Degree  of 
contamination  Minimum nominal surface distance mm/kV 1) Ratio = surface distance 
divided by clearance  
I low 16 
 3.5 II medium  20 
III high 25 
 4.0 
IV very high  31 
1) The ratio of the surface distance between phase and ground to the effective value of the phase -to-phase 
voltage of the highest voltage for the equipment.  
NOTES:  
1.  It is found that the quality of the surface insulation is greatly influenced by the shape of the insulator.  
2.  For very lightly contaminated surfaces, a nominal surface distance of less than 16  mm/kV may be used 
based on operating experience. The general ly accepted lower limit is 12  mm/kV.  
3.  In cases of exceptional contamination, the nominal surface distance of 31  mm/kV may not be sufficient. 
Depending on operating experience and/or the results of laboratory tests, a higher nominal surface 
distance valu e must be used.  
3.3   Short -circuit resistance  
An instrument transformer must be designed and manufactured to withstand an external short circuit lasting 
1 second without damage due to thermal and mechanical influences.  
3.4   Software  
If an instrument transformer includes software that is essential for its metrological characteristics, it must be 
identifiable by the manufacturer as a separate numerical version that is in conformity with the approved 
measuring instrument type. It must be possible to iden tify software during the operation of the measuring 
instrument in a simple manner. The installed software must be secured against accidental or unauthorised 
external interference. If the software needs to be reinstalled (as measuring instrument maintenance ), new 
verification of the measuring instrument’s metrological properties must be conducted.  
4  Instrument markings  
4.1  In general  
This marking applies to single -phase instrument transformers as well as to single -phase instrument 
transformers assembled into one unit and connected as a three -phase instrument transformer or to a three -
phase instrument transformer having a common core fo r three phases.  
4.2  Terminal markings  
Priority marking of terminals of inductive instrument transformers shall be in accordance with Figures 1 to 
10. The capital letters ‘A’, ‘B’, ‘C’ and ‘N’ refer to the terminals of the primary winding, the lower -case letters 
‘a’, ‘b’, and ‘n’ indicate the corresponding terminals of the secondary winding. The letters ‘A’, ‘B’ and ‘C’ 
refer to fully insulated terminals, and the letter ‘N’ indicates the end of the winding to be grounded and the Measure of a General Nature No 0111 -OOP -C089 -22 
 
15 
 insulation of which is les s than that of the other terminals. The letters 'da' and 'dn' mark the terminals of the 
auxiliary secondary winding.  
4.3  Polarity indications  
Terminals marked with the same lower -case and capital letters must have the same polarity at the same time.  
NOTE: The plate may contain information relating to several load combinations and accuracy classes that the instrument 
transformer complies with.   
 
 
  
 
 
 
  
 
 
 
 
 
 
Figure 1 — Single -phase two -pole Figure 2 — Single -phase single -pole 
insulated transformer with one 
secondary winding  insulated transformer with reduced 
insulation and one secondary winding  
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 3 — Three -phase transformer with one secondary winding  
 
 
 
Measure of a General Nature No 0111 -OOP -C089 -22 
 
16 
  
Figure 4 — Single phase transformer  Figure 5 — Three -phase transformer  
with two secondary windings  with two secondary windings  
 
 
Figure 6 — Single phase transformer  Figure 7 — Three -phase transformer  
with one secondary winding with a tap  with one secondary winding and tap  
 
 
or 
or 
or 
or 
or Measure of a General Nature No 0111 -OOP -C089 -22 
 
17 
  
 
 
Figure 8 — Single phase transformer with two secondary windings and taps  
 
 
Figure 9 — Single phase transformer with one auxiliary winding  
 
 
or 
or 
or Measure of a General Nature No 0111 -OOP -C089 -22 
 
18 
  
 
Figure 10 — Three -phase transformer with one auxiliary winding  
4.4 Information on the nameplate  
All voltage instrument transformers must have a nameplate containing at least the following information:  
a) the manufacturer’s name or brand name;  
b) year of manufacture, serial number and type designation;  
c) nominal frequency;  
d) maximum operating voltage for equ ipment;  
e) the prescribed level of insulation;  
f) temperature category;  
g) weight in kg (if ≥  25); 
h) class of mechanical requirements (for Um ≥ 72 kV);  
i) nominal primary and secondary voltages (e.g. 66/0.11 kV);  
j) nominal load plus corresponding accuracy class (e.g.  50 VA, class 1.0);  
NOTE: If the instrument transformer has two separate windings, the load of each secondary winding in VA must be 
indicated for the corresponding accuracy classes and nominal voltage of each winding.  
k) the voltage increase factor and the corresponding duration of the overvoltage;  
l) where applicable, the software version may also be shown on a display;  
NOTE: Points (d) and (e) can be combined into a single fraction (for example,  72.5/140/325 kV).   
All information must be indelibly marked directly on the voltage instrument transformer or on a plate firmly 
attached to the instrument transformer. If severa l thermal insulation categories are used, the temperature 
category that limits the warming of the winding must be provided. For gas -insulated instrument transformers, 
the highest operating voltage of the instrument transformer must be indicated on the type  plate as the nominal 
Measure of a General Nature No 0111 -OOP -C089 -22 
 
19 
 voltage for the instrument transformer. For instrument transformers in load range I. this information must be 
provided immediately before the nominal load information (e.g. 0 VA — 10 VA class 0.2). The type plate may 
contain a combina tion of several loads and precision classes that the instrument transformer meets.  
5  Measuring instrument type approval  
The type -approval process for a voltage instrument transformer includes the following tests and activities:  
5.1   Warming test  
For th is test, the instrument transformer shall be stipulated to have reached a steady -state temperature if the 
temperature increase per hour does not exceed 1 K. The ambient temperature must be between +  10 °C and 
+ 30 °C. If the instrument transformer has more  than one secondary winding, the test must be carried out on 
each secondary winding. Auxiliary or protection windings must be loaded between 0 VA and 100  % of the 
nominal load value of this winding. During this test, the instrument transformer must be conf igured as it will 
be configured in operation. The warming must be measured using the electrical resistance increment method. 
The warming of parts other than windings can be measured using thermometers or thermocouples.  
The voltage to be applied to the instrument transformer during this test must comply with the following points:  
a) All instrument transformers irrespective of the voltage increase factor and duration (see Table 9) shall 
be tested with 1.2 times the nominal primary voltage. If the value of th e full load of the secondary 
winding is determined, the instrument transformer must be tested at the nominal primary voltage, at a 
load corresponding to the value of the full load of this secondary winding and at a power factor of 1. 
Any other secondary wi nding must be unloaded. If a full load value is specified for more than one 
secondary winding, the instrument transformer must be tested sequentially for each winding, each of 
which shall be loaded with the corresponding full load value at a power factor o f 1. The test must 
continue until the temperature of the instrument transformer reaches a steady state.  
b) Instrument transformers with a voltage increase coefficient of 1.5 for a duration of 30 s or 1.9 for 30 
seconds shall be tested at their respective volt age factor with a duration of 30 seconds as soon as 
temperature of the instrument transformer reaches a steady state at 1.2 times the nominal voltage. The 
temperature increase must not exceed the value given in Table 10 by more than 10 K.  
NOTE: A relative ly short duration of 30 seconds for overvoltage does not represent a measurable increase in 
temperature after measurement at nominal voltage. As a result, the negative effect caused by overvoltage on the 
instrument transformer can be best assessed indirect ly from observed deficiencies during dielectric type tests.  
c) Instrument transformers having a voltage increase factor of 1.9 for a duration of 8 hours shall be tested 
at 1.9 times the nominal voltage for 8 hours immediately after the instrument transformer' s temperature 
reaches a steady state at 1.2 times the nominal voltage. The temperature increase must not exceed the 
values given in Table 10 by more than 10 K.  
If one of the secondary windings is auxiliary or protective, the test shall be carried out in ac cordance with the 
above test procedure, starting with the test referred to in (a), at 1.2 times the nominal primary voltage and then 
immediately followed by the test according to 5.1(c).  
In the pre -test preparatory process with 1.2 times the nominal primar y voltage, the auxiliary or protective 
winding is unloaded. During the test with 1.9 times the rated primary voltage for 8 hours, the auxiliary or 
locking windings must be loaded with a load corresponding to the standard value of the full load (see 1.18), 
while the other windings are loaded with nominal load value.  
NOTE: Voltage measurement must be carried out on the primary winding, as the value of the secondary voltage may be 
significantly less than the nominal value of the secondary voltage multiplied by  the voltage increase factor.  Measure of a General Nature No 0111 -OOP -C089 -22 
 
20 
 5.2  Short -circuit resistance test  
During this test, the initial temperature of the instrument transformer must be between +  10 °C and +  30 °C. 
The instrument transformer must be powered from the primary side, the secondary windings shall be short -
circuited. The short -circuit duration is 1 second.  
NOTE: This requirement shall also apply where fuses are an integral part of the instrument transformer.  
During the short circuit, the effective value of the supplied voltage at the primary terminals must not be lower 
than the nominal voltage.  
NOTE: For inductive instrument transformers, the test may be performed by supplying power from the secondary side, 
with the terminals of the primary winding short -circuited.  
The instrument transformer has passed this test if, after cooling to ambient temper ature, it meets the following 
requirements:  
a) is not visibly damaged;  
b) its errors do not differ from those recorded before the tests by more than half of the limit errors in its 
accuracy class;  
c) complies with the dielectric tests referred to in Article 5.7 with test voltages reduced to 90  % of the 
prescribed values;  
d) inspection of insulation at the surface of both primary and secondary windings shows no significant 
damage (e.g. carbonisation).  
A check pursuant to (d) is not required if the current density in the winding does not exceed 160 A/mm 2  for 
a copper winding whose conductivity is not less than 97  % of the prescribed value. The current density shall 
be determined by measuring the symmet rical effective value of the short -circuit current at the secondary 
winding (divided by the nominal transmission in the case of primary windings).  
5.3   Primary winding impulse voltage test  
The test voltage must be supplied between each primary voltage m ains terminal and the ground. Grounded 
primary winding terminals or untested terminals in the case of a two -pole insulated instrument transformer, at 
least one terminal of each secondary winding, frame, casing (if any) and core (if it should be grounded) m ust 
be grounded during the test. The impulse test generally consists of the introduction of reference and test 
impulses. The reference impulse voltage must have an amplitude between 50  % and 75  % of the nominal test 
voltage. The peak value and the shape of  the impulse voltage must be recorded during the test. Proof of failure 
is a change in the shape of the pulse voltage waveform when comparing the reference and test impulse 
waveforms. For fault detection, a recording of current(s) in the ground circuit or voltage transferred to the 
secondary winding may be used, but only as additional detection to record the voltage waveform.  
NOTE: Grounding can be done through a suitable shunt or current recording device.  
5.3.1  Atmospheric impulses test  
The test voltage must have an appropriate value pursuant to Table 2  according to the highest voltage for the 
device and the specified insulation level.  
5.3.1.1   Winding with Um < 300 kV   
The test must be performed using both positive and negative polarity. At leas t 15 consecutive pulses of the 
same polarity must be applied, without correction for atmospheric conditions. The instrument transformer shall 
be considered to have passed impulse tests for each polarity if the following conditions are met:  
 each series ( 'plus' and 'minus') has at least 15 impulses;  
  non-renewable insulation is not breached, which is confirmed by five consecutive withstand impulses, 
following each arc;  Measure of a General Nature No 0111 -OOP -C089 -22 
 
21 
  the instrument transformer has passed even if there have been no more than 2 arcs in  each series (this 
procedure leads to a maximum of 25 pulses in series), no evidence of insulation breach (e.g., a deviation 
of the waveform of the recorded quantity during production unit tests serving as verification tests) may 
be found.  
If arcs have occ urred and evidence cannot be provided during the test that they occurred in self -restoring 
insulation, the instrument transformer must be removed and inspected after the dielectric tests have been 
completed. If damage to non -renewable insulation is observe d, the instrument transformer failed the test.  
5.3.1.2  Winding with Um  300 kV  
The test must be performed using both positive and negative polarity. Apply 3 consecutive impulses of each 
polarity, uncorrected for atmospheric conditions. The instrument transformer has passed the test if:  
  no arcs occurred;  
  there was no other evi dence of insulation failure (for example, there were no changes in the waveform 
shape of the recorded pulsed quantities).  
5.3.2   Switching impulse test  
The test voltage must have the appropriate value in Table 2   according to the highest voltage for equ ipment 
and according to the specified insulation level. The test must be performed using positive polarity. Fifteen 
consecutive impulses, corrected for atmospheric conditions, are applied to the instrument transformer. For 
outdoor instrument transformers, this test must be carried out in rain (see Article 5.4).  
NOTE: In order to avoid over -saturation of the core, it is permissible to adjust the magnetic state of the core in an 
appropriate manner between the individual impulses.  
The instrument transformer has passed the test if:  
 non-self-restoring internal insulation was not breached;  
 no arc occurred along non -self-restoring external insulation;  
  the instrument transformer has passed this test even if no more than 2 arcs occurred; this is confirmed 
by 5 consecutive impulses after each arc applied after the last impulse in the series;  
  no other evidence of insulation breach was found (for example no changes in shape occurred during 
the course of recorded values).  
If arcs have occurred and  proof cannot be provided during the test that they occurred in self -restoring 
insulation, the instrument transformer must be disassembled and inspected after the dielectric tests have been 
completed. If damage to non -self-restoring insulation is observed,  the instrument transformer must be 
considered to have failed the test.  
NOTE: Impulses during which there are arcs to the walls or ceilings of the laboratory shall not be considered.  
5.4   Rain tests of outdoor transformers  
For instrument transformers wi th Um < 300 kV the test must be carried out with AC voltage at 50  Hz and the 
test voltage value according to Table 2, depending on the highest voltage for the instrument transformer 
corrected for atmospheric conditions.  
For instrument transformers with Um ≥ 300 kV, the test must be performed by positive polarity switching 
pulses with the test voltage value as specified in Table 2, depending on the highest voltage for the instrument 
transformer and the nominal insulation level.  Measure of a General Nature No 0111 -OOP -C089 -22 
 
22 
 5.5   Electromagnetic Compatib ility Test (EMC) — measurement of radio 
influence voltage (RIV)  
Radio influence voltage requirements apply to instrument transformers with Um ≥ 123  kV, designed to be 
installed in air insulated substations.  
Because radio influence voltage may be affected by fibres or dust on insulators, it is permitted to wipe the 
insulator with a clean cloth prior to the start of measurement.  
The following pr ocedure must be followed:  
An instrument transformer with accessories must be dry and clean and at approximately the same temperature 
as the laboratory where the test is being performed.  
The test shall be carried out under the following atmospheric conditio ns: 
Ambient temperature: (10 to 30) °C  
Atmospheric pressure: (0.870·105 to 1.070·105) Pa 
Relative air humidity: (45 to 75) %  
The test leads and their ends must not be a source of radio influence voltage.  
In order to  avoid interfering discharges from the primary terminals, they must be appropriately shielded by 
imitation service shielding. It is recommended to use a piece of pipe with spherical ends.  
 
 
Figure 11 - Electromagnetic compatibility test arrangement  
Legend  
T  test transformer  
C& a  tested item  
Z  filter 
B  termina l with corona formation protection  
M  measuring set with input resistance Rm Measure of a General Nature No 0111 -OOP -C089 -22 
 
23 
  
 
The test voltage is applied between one of the terminals of the primary winding of the tested item (C a) and the 
ground. The frame, cover (if any), core (if it is intended to be grounded) and one terminal of each secondary 
winding are grounded.  
The measuring circuit must be tuned in the 0.5 MHz to 2 MHz band with measurement frequency recording. 
The result s are expressed in microvolts.  
Impedance between test conductor and ground, (Z S + (R  1 +  R2 //  RM)) in Figure 11, must be 300  Ω ± 40 Ω 
with a phase angle not exceeding 20° at the measuring frequency.  
A capacitor C S can also be used instead of the filter  Z S; sufficient capacity is 1  000 pF.  
The filter Z must have a large impedance at the measuring frequency in order to separate the source of the 
industrial frequency from the measuring circuit. An appropriate value of this impedance at the measuring 
frequency can be found between 10,000  Ω and 20,000  Ω.  
The background noise level of background radio interference (RF interference caused by an external field and 
a high -voltage transformer) must be at least 6 dB (preferably 10 dB) below the given level of ra dio interference.  
Prior to the measurement itself, a voltage of 1.5 × Um/√3 is applied and maintained for 30 seconds. The voltage 
is then reduced to 1.1 × Um/√3 for approximately 10 seconds, and  is maintained at this value for 30 seconds 
prior to the actual RF interference measurement.  
The instrument transformer shall be considered to comply with this test if the interfering RF voltage level does 
not exceed 2  500 μV at 1.1 Um/√3.  
5.6  Accuracy te sts  
Type -approval tests to verify the requirements of 2.2.9.3 in the case of class 0.1 to 1 instrument transformers 
shall be carried out for 80  %, 100  % and 120 % of nominal voltage at nominal frequency   
  for load range I at 0  % and 100  % nominal load at load factor cos β = 1; 
  for load range II and at 25  % and 100  % at load factor cos β= 0.8.  
5.7   Primary winding test at industrial AC frequency  
During the induced voltage test, the test voltage frequency may be higher than the nominal frequency to p revent 
the magnetic core saturation. Test duration must be 60 seconds.  
If the test voltage frequency is more than twice the rated frequency, the test time may be reduced to less than 
1 minute according to the relationship:  
Trvání  zkoušky  (s)=dvojnásobek  jmen ovitého  kmitočtu
zkušební  kmitočet×60 s 
Trvání zkoušky (s)  Duration of the test(s)  
dvojnásobek jmenovitého kmitočtu  double the nominal frequency  
zkušební kmitočet  test frequency  
 Measure of a General Nature No 0111 -OOP -C089 -22 
 
24 
 5.7.1   Winding with Um < 300 kV   
The test voltage for a winding with Um < 300 kV must have the appropriate value given in Table 2 according 
to the highest voltage for equipment. If there is a large difference between the applicable highest voltage for 
equipment ( Um) and the nominal primar y voltage, the induced voltage must be limited to five times the nominal 
primary voltage.  
5.7.1.1  Two -pole insulated instrument transformers  
Two-pole insulated instrument transformers must be subjected to the following tests:  
a)  Applied voltage test  
The test voltage is applied between the ground and between all interconnected primary winding 
terminals. The frame, case (if present), core (if it is to be grounded) and all secondary winding terminals 
must be interconnected and grounded.  
b)  Induced voltage test 
At the manufacturer’s discretion, the test may be carried out either by supplying the secondary winding 
with a voltage of such magnitude as to induce the prescribed test voltage in the primary winding or by 
supplying the primary winding directly to th e prescribed test voltage.  
The test voltage must be measured in each case on the side of the primary winding. The frame, the case 
(if present), the core (if it is to be grounded) and one terminal of each secondary winding and one 
terminal of the primary wi nding must be interconnected and grounded.  
The test may be carried out at the test voltage applied to each terminal of the primary winding for half 
the time, but not less than 15 seconds for each terminal.  
5.7.1.2   Single -pole insulated instrument transfo rmers  
Single -pole insulated instrument transformers must be subjected to the following tests:  
a)  Applied voltage test   
The test voltage must have the corresponding values according to 2.2.4.1 and must be applied between 
the terminal of the primary winding to be grounded and the ground.  
The frame, case (if present), core (if it is to be grounded) and all secondary winding terminals must be 
interconnected and grounded.  
b)  Induced voltage test  
The test must be carried out according to 5.7.1.1. The pri mary winding terminal that will be grounded 
during operation must be grounded during the test.  
5.7.2   Winding with Um ≥ 300 kV   
The instrument transformer must be subjected to the following tests:  
a)  Applied voltage test   
The test voltage must have the c orresponding value according to 2.2.4.1   and the test must be performed 
according to 5.7.1.2.  
b)  Induced voltage test  
The test voltage must have the corresponding value according to Table 2, depending on the nominal 
value of the atmospheric pulse test vol tage. The test must be carried out according to 5.7.1.2.  Measure of a General Nature No 0111 -OOP -C089 -22 
 
25 
 5.8   Measurement of partial discharges  
5.8.1   Test circuit and measuring equipment  
The measuring equipment must measure the apparent charge Q expressed in picocoulombs (pC). It must be 
calibrated after being connected to the test circuit. Broadband equipment must have a minimum bandwidth of 
100 kHz with an upper frequency not greater tha n 1.2 MHz. Narrowband equipment must have its resonant 
frequency within 0.15 MHz to 2 MHz. The 0.5  MHz to 2  MHz range is recommended but, where practicable, 
measurements shall be made at the frequency ensuring the highest sensitivity. Sensitivity must allo w detection 
of partial discharges with an amplitude of 5 pC.  
NOTES:  
1.  Interference must be significantly lower than the indicated sensitivity. Known impulses caused by external 
interference are not considered.  
2.  Bridge connection can be used to suppress external interference.  
3.  If equipment allowing electronic interference suppression is used, it must be proven by means of changes to its 
parameters that it allows the detection of repeated impulses.  
5.8.1.1  Te st procedure for single -pole insulated instrument transformers  
After pre -test stress according to Methods A or B, the test is carried out at the voltage specified in Table 3 and 
partial discharge measurement is carried out for 30 seconds.  
During this time  the measured partial discharge level must not exceed the values given in Table 3.  
Method A:  The voltage level is obtained by reducing the voltage after the induced voltage test without 
interruption.  
Method  B: Measurements of partial discharges is carried out after the induced voltage test. Voltage is 
increased to 80 % of the test voltage during the induced voltage test, kept at this level for at least 60 seconds 
and reduced without interruption to the voltage for the measurement of partial discharges.  
Unle ss otherwise specified, which method is chosen is at the manufacturer's discretion. The test method chosen 
must be specified in the test report.  
5.8.1.2  Test procedure for two -pole insulated instrument transformers  
The test circuit for two -pole insulated  instrument transformers must be the same as for single insulated 
instrument transformers, but the test must be carried out twice by applying the test voltage at each terminal, 
the second terminal being connected to the low voltage terminal, frame and hous ing (if any).  
5.9   AC voltage test of secondary winding and between winding parts  
A 3 kV test voltage must be applied for 60 seconds between the shorted terminals of each winding section or 
each secondary winding and ground.  
5.10   Software test  
5.10 So ftware test  
Software tests are carried out on the basis of submitted documentation and functional checks using Welmec 
7.2. 
5.10.1 Documentation  
It shall be assessed whether the documentation contains the following information, including that referred to 
in subsequent paragraphs:  Measure of a General Nature No 0111 -OOP -C089 -22 
 
26 
 - a description of the software in terms of its function and instrument function;  
- a description of the accuracy of the computational algorithms;  
- a description of the user interface, menus and dialogs;  
- unambiguous identification of the software;  
- a description of the hardware components;  
- operating and user manual for electricity meter software.  
5.10.2 Identification  
Documentation check: an assessment  is performed whether the identification creation algorithm is described 
in the documentation and whether it is part of the dynamic part that is generated while running.  
Functional check: a check is performed that the identification conforms to the manufac turer’s information and 
whether it is possible to invoke the display of identification while the instrument is running. If the device has 
multiple modes in which identification can be displayed, all are tested.  
5.10.3 Functionality  
Documentation check: the  documentation is assessed to determine whether it includes a basic description of 
the instrument’s function and, where applicable, a description of the computational algorithms and data flow.  
Functional check: the instrument is checked to determine wheth er it works in accordance with the 
documentation. A test is performed using the black box method and comparing inputs and outputs with 
simulated or independently read inputs and outputs. This test may be replaced by further type -approval testing.  
5.10.4 In fluence through the user and communication interface  
Documentation check: a check is performed to determine whether the documentation contains:  
- a description of the implementation of the user and communication interface;  
- a description of the physical cons truction (where applicable, physical interface security);  
- a complete list of all commands of the user and communication interface with sufficient description 
and assignment to functions or data operations;  
- a declaration that the list of all commands is com plete.  
Functional check: randomly selected user interface commands (e.g., menu items on the display) are tested. The 
manufacturer must supply all necessary accessories to make it possible to test the selected communication 
instructions in laboratory condit ions. Response to requirements outside the manufacturer’s specifications is 
tested: other commands, other ranges of values, interruption of communication, replacement of the instrument 
during communication.  
5.10.5 Protection against Changes  
Documentation c heck: it is assessed whether the documentation includes a description of protection against 
accidental and intentional changes and how appropriate the design of this protection is.  
Functional check: the device’s response to power outages and outages of com munication means (if used) is 
tested. A test is performed to determine whether any user interface dialogs modifying the data in the instrument 
are implemented in such a way that the user is required to confirm choices.  
5.10.6 Protection of transmitted data (if required)  
Documentation check: the documentation is assessed to see whether it contains:  
- information needed to reconstruct the transmitted data;  
- a description of the protection against accidental and deliberate changes during transmission;  
- a descriptio n of the communication interfaces and communication protocols;  
- proof of authenticity of transmitted data;  
- a description of the detection of erroneous data generated during the transmission;  
- a description of the protection in the event of delay or transmission through communication interfaces.  Measure of a General Nature No 0111 -OOP -C089 -22 
 
27 
 Functional check: data transfer is tested with regard to its possible outages — the response to communication 
interruptions and the response to damaged  data. A test is performed to see whether data blocks contain all the 
data needed to identify them. If a custom protocol is created for communication, its implementation is tested. 
If a standard protocol is used (a routine protocol using standard libraries ), its correct use is checked with respect 
to the data flow in the program.  
5.10.7 Protection of stored data (if required)  
Documentation check: the documentation is assessed to see whether it contains:  
- a list of items to be stored;  
- a description of the protection against accidental and deliberate changes to stored data;  
- proof of the authenticity of the stored data;  
- a description of the display of the stored data;  
- a description of the operation for writi ng stored data;  
- description of the capacity and management for data storage.  
Functional check: the system’s response to power failure is tested with a view to retaining relevant data. It is 
tested whether memory is physically protected against substitution  or reset by the user. The display of stored 
data is tested.  
5.10.8 Software separation  (if required)  
Documentation check: the documentation is assessed to see whether it contains:  
- a list of items that are part of the part of software affecting measuremen t results (legally relevant 
software — LRSW);  
- a description of how the indication of legally relevant information is protected from confusion with 
information  
- generated by the part of the software that does not affect the measurement results (legally not relevant 
software — LNRSW);  
- a description of the protective interface and its implementation.  
Functional check a test is performed whether the display of data generated from LRSW is sufficiently 
distinguishable from the data on the display generated from L NRSW.  
6  Initial verification  
Before initial verification, the number of the type -approval certificate must be indicated on the instrument 
transformer plate.  
When verifying voltage instrument transformers, the following tests are carried out:  
a)  technical inspection and the check that terminals are marked correctly;  
b)  determination of voltage and angle errors of the instrument transformer.   
Furthermore, the conformity of the software version with the version specified in the type approval certif icate 
is checked.  
6.1   Necessary aids  
The following test equipment is used for the verification of instrument transformers:  
6.1.1   Measuring set -up  
The measuring set -up for testing in the verification of an instrument transformer consists of a referenc e 
measuring device, a reference voltage transformer or a reference voltage divider and voltage transformer load, Measure of a General Nature No 0111 -OOP -C089 -22 
 
28 
 auxiliary measuring instruments for measuring voltage, frequency, distortion and supply circuits with 
regulation.  
6.1.2  Instrument transformer  loads  
Loads are used with cos β = 0.8 or cos β = 1, specified for the given frequency according to 2.2.7, allowing 
with the corresponding error  the configuration  of the nominal value given by the combination of the load 
itself and the impedance of the measuring device. The instrument transformer loads are given by the values in 
2.2.8.  
6.1.3   Power supplies  
Power supplies are used with harmonic waveform voltage control over the required range with appropriate 
adjustment accuracy, stability and distorti on of less than 5  %. 
6.1.4   Distortion meter  
A distortion meter is used to monitor compliance with the condition of Article 6.2 for monitoring the shape of 
the voltage curve.  
6.2   Conditions during the tests  
Instrument transformers are tested at the nominal frequency with a maximum deviation of 1 %, at a current 
harmonic whose distortion factor does not exceed 5 %, an ambient temperature of +15 ° C to +25 ° C, a relative 
air humidity of not more than 75 % and an external magnetic field up to 1 mT.  
6.3   Description of tests  
6.3.1   Physical damage  
Instrument transformers are checked for physical damage, that nameplate information is complete, correct and 
legible, and that terminals are properly connected.  
6.3.2   Correct marking of terminals  
When  checking the correctness of the marking of the terminals of the instrument transformer, the tested and 
reference transformer are connected in the measuring set -up in such a way that the primary voltage is connected 
to the corresponding terminals.  
This cor responds to the parallel connection of terminal 'U' of the reference with terminal 'A' and terminal 'V' 
with terminal 'B' or 'N' of the instrument transformer being tested. The secondary terminals of both 
transformers are connected in parallel to the ident ically marked terminals of the measuring device. If errors 
can be measured by the measuring device when the primary current is set to 10  % I1N, the terminals are marked 
correctly. Otherwise, the instrument transformer is excluded from further testing.  
6.4  Determination of voltage and phase angle errors of instrument transformers  
When determining an instrument transformer's voltage and angle  errors, the measuring device and the 
instrument transformer load must be connected by separate cables to the seconda ry terminals of the tested 
transformer.  
For an instrument transformer with several secondary windings, its voltage and angle errors are determined 
when the winding that is not being tested:  
a) remains open (zero load);  
b)  is loaded with the nominal load ZN. Measure of a General Nature No 0111 -OOP -C089 -22 
 
29 
 Current and angle errors are read on the set -up's measuring device, and if needed corrected for errors in the 
reference and the measuring equipment. The arrangement for determining the errors of the instrument 
transformer by means of a reference instrum ent transformer is in Figure  12, the use of a reference capacitive 
divider is in Figure  13. If the reference transformer errors are known, the transformer under measurement is 
corrected according to the relationship  
UX = UN + UM , UX = UN + UM,  (2) 
where UX and UX are the errors of the transformer under measurement (%;  ' ), UN a UN are the errors of the 
reference being used (%;  ' )  
 
and UM a UM are system data for error evaluation (%;  ' ). 
 
Figure 12 — Arrangement to determine errors using a reference instrument transformer  
vX 
TX 
n 
a 
A 
N 
TN 
vN 
uN 
UN 
VN 
 VOLTAGE LOAD  
REFERENCE  
VIT 
TESTED  
VIT 
vX 
SYSTEM FOR VERIFICATION OF 
THE VIT, e.g. TETTEX 2767  
U1 
U2X 
 U2N 
uN 
 vN 
 uX 
 vX 
VOLTAGE 
SOURCE  
vX 
 uX Measure of a General Nature No 0111 -OOP -C089 -22 
 
30 
  
Figure 13 — Arrangement to determine errors using a capacitive divider  
Voltage and phase angle errors of the instrument transformer are detected in the range between 80  % and 
120 % of the nominal voltage value U 1N, where they must comply with the values in Table 4. For verification, 
it is recommended to check errors at 80  %, 100  % and 120  % of the nominal voltage value U 1N. For nominal 
loads (1; 2.5;  5; 10) VA and power factor 1 (load range I) are verified at a load of 0 VA and 100  % of the 
nominal load. For nominal loads (10;  25; 50;  100) VA and with an inductive power factor of 0.8 (load range 
II) the verification is carried out at a load of (25 and 100) % of the nominal load. Error magnitudes must not 
be greater than those shown in Table 4.  
Maximum permissible   voltage and angle  errors must not be exceeded for an instrument transformer with 
attached accessories if they are an integral part of the transformer.  
The setting of the primary voltage at each measured point must be guaranteed with a maximum permissible 
error of ±  0.5 % of the measured value. During the verification of the instrument transformer load impedance 
errors in must not be greater than ±  3 % of the required value. Errors in the real and imaginary components of 
a general type impedance must not exceed 3  % of the nominal values.  
6.5   Determining voltage and phase angle  errors in combined instrument 
transformers   
6.5.1  For combined current and v oltage instrument transformers, depending on design the current transformer 
must comply with the conditions specified in the regulation for current transformers and voltage transformers 
with those specified in Table 4,  or corresponding to requirements for the current part.  
6.5.2  For combined instrument transformers, the following interactions must be determined:  
R'N 
 uN 
        ELECTRONIC  
       VOLTAGE DIVIDER  
             
 vN       
A 
 
 
 
N 
a 
 
 
n 
 
 Z 
C1 
NX 
vN 
ND 
EVALUATION CIRCUITS  
KEYBOARD DISPLAY  
I 
uN 
 vX 
SYSTEM FOR VERIFICATION OF THE  
VIT, e.g. TETTEX 2767  
TX 
U2X 
I2X 
RN 
TD 
U2N 
uX 
I2N 
NN 
Ur 
U1 
C2 
U1N 
RX Measure of a General Nature No 0111 -OOP -C089 -22 
 
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 a)  the influence of the current instrument transformer on the voltage instrument transformer;  
b)  the influence of the voltage instrument transformer on the current instrument transformer.  
6.5.3 The influence of the tested current instrument transformer on the tested voltage instrument transformer 
is determined by detecting  voltage and phase angle  errors of the tested voltage instrument tra nsformer:  
a)  when the primary terminals of the current instrument transformer are disconnected;  
b) at a primary current of 120  % of  I1N (or at the extended range current) and at 25  % and 100  % of the 
nominal secondary load ZN. 
6.5.4 The influence of the teste d voltage instrument transformer on the tested current instrument transformer 
is determined by detecting the current and phase angle  errors of the tested current instrument transformer:  
a)  when the primary terminals of the voltage instrument transformer are d isconnected;  
b) at a primary voltage of 120 % U1N and at 25 % and 100 % of the nominal secondary load.  
6.5.5 Detected current, voltage and angle  errors of the operating combined instrument transformer during 
interaction must comply with the conditions given in 2.2.10.3 and Table 4 for separate error detection.  
7  Subsequent verification  
Follow -up verification is carried out in the same way as the  initial verification in chapter  6.  
The verification of the measuring instruments are subject to the metrological requirements applicable at the 
time they were put into circulation.  
8  Measuring instrument check  
When checking measuring instruments pursuant to § 11a of the Metrology Act at the request of an entity that 
could be affected by its incorrect measurement, Chapter 6 is followed.  
9  Notified standards  
For the purposes of specifying the metrological and t echnical requirements for measuring instruments and 
specifying the testing methods for their type approval and verification arising from this General Measure, the 
CMI shall notify Czech technical standards, other technical standards or technical documents of international 
or foreign organisations, or other technical documents containing more detailed technical requirements 
(hereinafter ‘notified standards’). The CMI shall publish a list of these notified standards attached to the 
relevant measures, together  with the general measure, in a manner accessible to the public (at www.cmi.cz ). 
Compliance with notified standards or parts thereof is considered, to the extent and under the conditions 
stipulated by a general measure, to  be compliance with the requirements stipulated by this measure to which 
these standards or parts thereof apply.  
Compliance with a notified standard is one of the ways to demonstrate compliance. These requirements may 
also be met by using another technical  solution guaranteeing an equivalent or higher level of protection of 
legitimate interests.  Measure of a General Nature No 0111 -OOP -C089 -22 
 
32 
 II. 
G R O U N D S  
Pursuant to § 14(1)(j) of the Metrology Act, the CMI has issued this Measure of a General Nature toward the 
implementation of §  6(2), §  9(1) and  (9), and  § 11a(3) of the Metrology Act, laying down metrological and 
technical requirements for specified measuring instruments and tests for type approval and verification of 
specified measuring instruments - 'voltage instrument transformers'.  
Decree No  4.1.4 specifying measuring instruments for mandatory verification and measuring instruments 
subject to type approval, as amended, classifies the measuring instruments under Item  4.1.4 in the annex 
entitled ‘List of specified measuring instruments’ as meas uring instruments subject to type approval and 
mandatory verification.  
This legislation (Measure of a General Nature) will be notified in accordance with Directive (EU)  2015/1535 
of the European Parliament and of the Council of 9  September  2015 laying dow n a procedure for the provision 
of information in the field of technical regulations and of rules on information society services.  
III. 
I N S T R U C T I O N S  
In accordance with §  172(1) CAP, in conjunction with §  39(1) CAP, the CMI has stipulated a time limit for 
comments of 30  days from the date of posting the draft on the official notice board. Comments submitted after 
this deadline will not be considered.  
The persons concerned are hereby invited to comment on this draft Measure of a General Nature. Wi th regard 
to the provisions of §  172(4) CAP, the comments shall be submitted in writing.  
In accordance with §  174(l) CAP in conjunction with §  37(l) CAP, it must be clear who is making the 
comments, which measure of a general nature they concern, how it co ntradicts legislation or how the measure 
of a general nature is inaccurate, and they must be signed by the person making them.  
The supporting documents for this draft measure of a general nature may be consulted at the Czech 
Metrological Institute, Depart ment of Legal Metrology, Okružní 31, 638 00 Brno, upon appointment by 
telephone.  
This draft Measure of a General Nature will be posted for a period of 15  days.  
 
  
       Director General