METREL A1472 User manual

Leakage current clamp
A 1472, A1579
User manual
Version 1.2, Code no. 20 752 271

2
Distributor:
Manufacturer:
METREL d.d.
Ljubljanska cesta 77
1354 Horjul
Slovenia
web site: http://www.metrel.si
e-mail: [e-mail]
Version note
1.1: manual is valid also for A 1579
© 2014, 2019 METREL
Mark on your equipment certifies that this equipment meets the
requirements of the EU (European Union) concerning safety and
electromagnetic compatibility regulations.
No part of this publication may be reproduced or utilized in any form or by
any means without permission in writing from METREL.

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1Warnings...........................................................................................4
2Description of leakage current clamp .............................................5
2.1 Meaning of symbols....................................................................6
3Maintenance......................................................................................7
3.1 Inspection ...................................................................................7
3.2 Cleaning......................................................................................7
3.3 Service and calibration................................................................7
4Leakage current clamp operation....................................................8
4.1 Measuring of leakage currents....................................................8
4.1.1 Origins of leakage currents......................................................8
4.1.2 Leakage currents in normal operation.....................................9
4.1.3 Leakage currents in fault conditions......................................10
4.1.4 Problems caused by leakage currents...................................11
4.2 Measuring leakage currents with current clamps.......................13
4.2.1 Direct method........................................................................13
4.2.2 Differential method................................................................14
4.3 Testing procedures...................................................................15
4.3.1 Finding sources of excessive leakage current.......................15
4.3.2 Insulation resistance testing..................................................17
4.4 Influencing quantities ................................................................18
4.4.1 Influence of nearby external magnetic field ...........................18
4.4.2 Influence of load current in differential leakage current test...18
4.4.3 Influence of external electric field..........................................19
4.4.4 How to recognize accuracy problems ?.................................19
4.4.5 How to minimize the influencing quantities............................19
4.4.6 Consideration of influencing quantities..................................20
4.5 Other important current clamp parameters................................21
4.5.1 High measuring range...........................................................21
4.5.2 Clamp diameter.....................................................................21
4.5.3 Protection category...............................................................21
5Technical specifications ................................................................22

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1 Warnings
To ensure a high level of operator's safety during using of the leakage
current clamp the following warnings have to be considered:
Do not use the current clamp if any damage is noticed!
Do not leave open signal output terminals of the current clamp
with current output (A 1472, A 1579) during measurement to avoid
damage and electric shock on secondary side. Never enclose
conductor in the jaw unless the clamp is connected to measuring
instrument!
Consider all generally known precautions in order to avoid risk of
electric shock while dealing with electric installations and
equipment!
Do not extend hands over protection barrier during measurement
to prevent of electric shock!

!
Symbol on the current clamp indicates possible hazardous live
conditions if the required safety measures are ignored.
Only a competent, authorized person is allowed to carry out
service intervention!
If the leakage current clamp is used in a manner not specified in
this User manual, the provided protection can be impaired!
The leakage current clamp may only be used within the operating
ranges as specified in this User manual.

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2 Description of leakage current clamp
The A 1472 is a 1000/1 ratio leakage current clamp for measuring
alternating currents in the range from 0.5 mA to 10 A.
The A1472 current clamp is specially designed to be used in combination
with METREL DeltaPAT testers.
Refer to chapter 5. Technical specification for list of all measuring
instruments suitable to work with the A1472 current clamp.
Main clamp parts:
1. Current sensor
2. Protection barriers
3. Conductor opening
4. Signal output terminals
1
2
3
4
A 1472
2.1 Difference to A 1579
The leakage current clamps differs only in applied safety banana
colours (4), those are red and black for the A 1579 instead green and
brown for the A 1472.
A 1579
Warning:
Do not leave open signal output terminals of the current
clamp during measurement to avoid damage and electric
shock on secondary side, never enclose conductor in the
jaw unless the clamp is connected to measuring
instrument!

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2.3 Meaning of symbols
Symbol on the current clamp indicates that it is
possible to use the current clamp on non-insulated
conductors.
!
Symbol on the current clamp indicates possible
hazardous live conditions if the required safety
measures are ignored.
Symbol indicates that the current clamp can be
applicable in presence of external magnetic fields of
up to 100 A/m.
> 100 A/m

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3 Maintenance
3.1 Inspection
To maintain operator safety and ensure reliability of the current clamp it is
good practice to inspect it on a regular basis. Check that the enclosure and
connections are undamaged.
The jaw surface must be clean. Pollution on jaw surfaces reduces the
current clamp sensitivity.
3.2 Cleaning
Use a soft cloth moistened with soapy water or alcohol to clean non-
metallic surface of the current clamps and leave them to dry totally before
using it.
Notes!
Do not use liquids based on petrol or hydrocarbons!
Do not spill cleaning liquid over the current clamps!
To clean jaw cut surfaces use slightly oiled soft cloth.
3.3 Service and calibration
It is essential that your clamp is regularly calibrated in order to guarantee
the technical specification listed in this User manual. We recommend 2-
year calibration interval. Metrel encloses an original calibration certificate
with every new instrument and current clamp.
For recalibration and repairs under or out of warranty time please contact
your distributor for further information.

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4 Leakage current clamp operation
4.1 Measuring of leakage currents
4.1.1 Origins of leakage currents
Leakage currents are currents driven by active conductors of a distribution
system or electrical equipment to earth and/or protective conductors.
Leakage currents flow wherever a conductive path (resistance and/or
capacitance) is applied between active and earthed conductors/protected
parts. Fig.1 shows leakage currents that usually flow in electrical
installations in normal operation.
Fig. 1 Leakage currents in electrical installations in normal operation
The relation between leakage current and (insulation) resistance between
line and earth is defined by the Ohm’s law.
 
)(/
_ kRUmAIinsnomresleak
Eq. 1
Ileak_res.................resistive leakage current in mA
Unom...................nominal line voltage, e.g. 230 V
Rins....................resistance in kΩ
The relation between leakage current and capacitance between line and
earth is shown in Equation 2.
   
nFCfUmAInomcapleak  6
_1028.6
Eq. 2
Ileak_cap.................capacitive leakage current in mA
Unom...................nominal line voltage, e.g. 230 V
f.........................mains frequency
C(nF).................capacitance in nF
Capacitive and resistive leakage currents are forming the overall (total)
leakage current. For continuous leakage currents of main frequency,
Equation 3 can be used.
 
2_ 2_capleakresleakleakage IImAI
Eq. 3
Ileakage.................overall leakage current
Shapes of leakage currents are usually much more complicated. That will
be described later in this article.

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4.1.2 Leakage currents in normal operation
Typical leakage currents in normal operation are:
Leakage currents caused by input filters in electrical equipment
connected between live and earth (filters, monitors). The capacitors
in input filters have a capacitance of ca. 10 nF to 100 nF (see Fig.1).
Leakage currents caused by conductors’ insulation resistance and
their capacitances to earth. Long power cables have a lower
insulation resistance and higher capacitances. The capacitance
between a cable and earth is typically about 150 pF/m (sse Fig. 1).
Leakage currents produced by frequency converters, switch mode
power supplies, different rectifiers and similar equipment. The
shape and frequency of this leakage currents can vary from pure
d.c, pulsed d.c. components up to very high frequency.
In general, leakage currents in normal operation increase with the size of
the electrical installation and the number of connected loads.
Table 1 shows the values of leakage currents for some typical
capacitances and resistances.
Capacitance
Capacitive leakage current
@ 230V/ 50Hz
Note
1 nF
0.072 mA
Typical values of
capacitors in input
cables
10nF
0.72 mA
100nF
7.2 mA
1uF
72 mA
Capacitance of long
power cables
Resistance
Resistive leakage current
@ 230V/ 50Hz
10 kΩ
23 mA
500 kΩ
0.46 mA
1.00 MΩ
0.23 mA
Typical limits for
insulation resistance
2.00 MΩ
0.115 mA
Table 1: Leakage current / capacitance relations
In Table 2 typical values of leakage currents for different types of electrical
equipment are shown.
Equipment
Leakage current
Stationary PC system
1 to 3
Photocopy machine
0.5 to 1.5
Floor heating
1mA / kW
Input EMC filter
1 mA
Printer/ fax
up to 1 mA
Photocopy machine
up tp 1.5 mA
Laundry machine, drying machine
up to 2 mA
Cookers, heaters
1mA / kW
Installed power cable 100 m
1 mA

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4.1.3 Leakage currents in fault conditions
Faults in electrical installations and equipment can cause additional
continuous or
short-lasting leakage currents. Typical faults that can cause increased
leakage currents are:
Deterioration of insulation (because of pollution, moisture,
corrosion). This is causing a gradual increase of the resistive
leakage current,
Faults in electronic equipment.
If the neutral and PE wiring are connected together anywhere in the
installation this can result in an improper current distribution
through the neutral and PE conductors.
Typical faults that usually results in a high fault current that will immediately
trip the protective devices are:
Misswiring like changing the N and PE conductors.
Breakdown of insulation between active parts and earth anywhere
in installation or equipment
Short-lasting leakage currents are often a result of:
A sudden change in the voltage or current between active parts
and PE. This can result in a short leakage currents from several us
up to 100ms . Lightning and switching on/ off large loads can cause
such changes.
Fig.2: Fault leakage currents in electrical installations

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4.1.4 Problems caused by leakage currents
Too high leakage currents can cause very critical problems in electrical
installations. The most often problems are nuisance tripping of protective
devices and safety problems due to increased fault, earth and touch
voltages.
Nuisance tripping of protective devices
Nuisance tripping occurs if the sum of leakage currents exceeds the
tripping level of the residual current protective device.
In circuits with a lot of installed equipment the sum of leakage currents in
normal operation can be close to the trip out limit. In this case even a small
added leakage current (caused by switching on/off loads or connection of a
new equipment) can trigger the nuisance tripping.
It must be considered that leakage currents can vary in time and the
tripping could be only occasional.
A solution is to add RCD(s) and divide the circuits in a way that the leakage
currents in normal operation stay below 20% of the nominal value of each
RCD. For further information refer to METREL handbook Guide for testing
and verification of low voltage installations.
Problems because of non- typical shape of leakage current
Leakage currents can have different non-sinusoidal shapes. They are
caused by nonlinearity of loads, nonlinearity of insulation materials and
presence of harmonic voltages and currents in the power systems:
high frequency leakage current
d.c. pulsed and smooth d.c. leakage current
arcing leakage current
harmonic leakage currents
transient and other leakage currents of short duration
The amount of the mentioned leakage currents in power systems is
increasing because more and more electronic controlled devices are
connected to the power system.
It must be considered that standard protection devices and measuring
equipment is not suited for monitoring and measuring of the mentioned
currents.
Impact on protection devices
Atypical shapes of leakage current can blind standard RCDs. For instance
general type RCD’s are not sensitive to leakage currents with pulsed DC
components or smooth DC current. High frequency leakage currents
caused are not dangerous for human safety but can result in wrong
operation of the RCD. It could happen that a dangerous fault leakage occur
at the same time and is not detected by the RCD.
Impact on measuring equipment
It is preferred to use at least TRMS measuring equipment for measuring
leakage currents. For more demanding applications instruments that
include harmonic analysis, time diagrams and DC sensitive measuring
equipment must be used.

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On Fig.3 two non-sinusoidal leakage currents are shown.
Fig.3: Examples of atypical leakage current
Impaired safety due to fault, earth, touch voltages
Leakage currents that are flowing into earth and PE conductors cause a
voltage drop on exposed conductive parts. In case of inadequate protective
measures this voltage can increase above a safe level and presents a
danger for the people if it would be touched.
Furthermore if the protection devices are not respond in due time the
released energies on faulty spots can result in mechanical damage,
overheating or even fire.
For more information regarding safety aspects of fault currents refer to
METRELs handbook Guide for testing and verification of low voltage
installations.

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4.2 Measuring leakage currents with current clamps
The simplest way of measuring leakage current is the measurement with
leakage current clamps. The measuring principle is shown on Figure 4 and
in Equations 3,4.
Fig.4: Measuring principle of current clamp –direct and differential method
LPEout R
N
IU  1
Eq.3
LNLout R
N
IIU  1 )(
Eq.4
Uout......................output voltage of current sensor
IPE, IL, IN...............measured currents
RB........................load (burden) resistor
N.........................number of turns of current clamp
The main advantage of the measurement with current clamp is that
disconnection of conductors is not needed –it is a non-contact measuring
method.
In general two methods are used for the leakage currents measurement.
4.2.1 Direct method
The current through one (embraced) conductor is measured in the direct
method.
Fig.5: Leakage current measurement –direct method

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4.2.2 Differential method
The sum of currents through two or more active (embraced) conductors is
measured in the differential method. If no current is leaking into PE
conductors/ earth the sum of currents through active conductors must be
exactly zero regardless of the load currents. If a leakage current flows it is
of the same size as the measured sum between active conductors
measured with the clamp.
Note
Note that the direction of currents in the conductors must be considered if
the differential method is used.
Fig.6: Leakage current measurement –differential method

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4.3 Testing procedures
4.3.1 Finding sources of excessive leakage current
The described testing procedure in this chapter is applicable for
troubleshooting in the installation and electrical equipment in case of
nuisance tripping of protective devices.
1. Embrace all active conductors (L and N, L1 and L2 and L3, L1 and
L2 and L3 and N) at the origin of installation.
The advantage of using the differential method is that all leakage
currents downstream the point of measurement will be captured.
The same leakage as “seen” by the RCD is measured.
Alternatively, if the main protective conductor at the origin of
installation.is accessible the direct measurement can be used.
If measuring currents through PE conductors it is possible that a
part of the leakage currents will not be measured as they are flowing
through different parallel paths. Refer to METRELs handbook Guide
for testing and verification of low voltage installations for more
information about parallel paths.
2. All suspicious loads should be connected. If the leakage current lies
inside expected limits try to run the suspicious loads in different
operation modes and find out if any of them is problematic. Use
MAX, PEAK, LOGGING functions on the measuring equipment if
available.
3. If the leakage current is too high try to isolate the fault downstream
of the installation. This can be done by disconnection of individual
parts of the installation as shown on Fig. 7 - sub distribution boards,
fuses, switches, appliances etc. When the faulty part is
disconnected the leakage current value will drop or fall to zero.
4. Sometimes it is not allowed to disconnect the mains voltage. In this
case additional measurements (direct or differential) should be
carried out downstream of the installation’s input as shown on Fig.
8. By doing this the measured leakage currents will be limited to a
smaller part of the installation.

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By using any of the procedures described above the faulty point can be
determined very precisely.
Fig. 7: Finding source of excessive leakage current –by disconnection
Fig. 8: Finding source of excessive leakage current –measuring
downstream the installation

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4.3.2 Insulation resistance testing
The standard high voltage insulation resistance test can sometimes not be
carried out –if the installation is not allowed to be disconnect for a longer
time or sensitive equipment is connected.
In this case the leakage current measurement can conditionally be used as
an alternative to the insulation resistance test.
However the results cannot be directly compared because the leakage
currents are flowing through resistance and capacitances between line and
earth. Degradation of isolation causes only a drop of the resistance and not
of the capacitance.
The applicability of leakage current measurement as an alternative to the
insulation resistance test depends on following factors:
The ratio between capacitive and resistive leakage currents in the
installation should be known. If the resistive part is completely
blinded by the capacitive leakage currents, the measurement will
give no indication about the insulation. If both portions are
comparable and the number of loads connected to the mains is not
changing in time at least trends can be determined.
Old measuring results should be available. This will enable
observing the trends.
The capacitive and resistive currents are not simply summed
therefore the interpretation of results should be made by a skilled
person.
Note:
Before using this alternative test check if it is allowed in your country.

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4.4 Influencing quantities
Unfortunately measurements with leakage current clamp are subjected to
different influencing quantities that have a big impact on their performance.
High uncertainty and non-repeatability of readings often lead to wrong
interpretations. Three most important influencing quantities are described in
this document.
4.4.1 Influence of nearby external magnetic field
Current carrying conductors, transformers and chokes in electrical
equipment are producing magnetic fields that are coupling into the sensor
and cables of the current clamp and are disturbing the results. The coupling
should be as small as possible. It strongly depends on the current sensor
quality. Protective measures are proper magnetic shielding, symmetry of
the winding, twisting of all connection cables, high permeability of the
sensor core etc.
4.4.2 Influence of load current in differential leakage
current test
This influencing quantity is similar than the influence of nearby magnetic
fields. During a differential leakage test the sum of currents flowing through
the clamp is measured. Lets assume that the load current through the line
conductor is 10 A and the returning current through the neutral conductor
is 9.999 mA. This means that a leakage current of 1mA is obtained on base
of the measured difference of: (10000 mA –9999 mA).
Any small anomaly in the current sensor will result in an asymmetry and
consequentially in a large measuring error. Protective measures are
symmetry of the winding, high permeability of clamp core etc)
Fig. 9: Influence of external magnetic fields

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4.4.3 Influence of external electric field
The electric field caused by voltage on measured conductors or any other
conductors/ parts in vicinity can interact with the current clamp through
capacitive coupling. This can happen through the sensor, housing, test
cables, measuring instrument etc.. The capacitive coupling factor should be
as small as possible and depends on electric shielding and common mode
rejection of the measuring stage,
Fig. 10: Influence of external electric fields
4.4.4 How to recognize accuracy problems ?
If following problems are noted then it is likely that external fields are
influencing the results:
The size of load currents is higher than several Amperes (especially
critical for the differential measurement).
The result depends on the position of the clamps (in regard to the
measured conductor(s), measured electrical equipment, nearby
current carrying conductors, switchboards etc). If the results
strongly fluctuate in regard to the current clamp position it is unlikely
to get an usable result.
The reading is not zero although no current is flowing through the
clamp.
4.4.5 How to minimize the influencing quantities
Do not use current clamp without information about immunity
against external fields.
The line and neutral conductors should be placed as close as
possible and in the middle of the clamps. If possible they should be
twisted to increase the symmetry (for differential measuring method)
–see Fig. 11.
The protective conductor should be placed in the middle of the
clamps (for the direct measurement).
Avoid placing the current clamp near to other line conductors and/
or current carrying conductors. If using a clamp sensor connected to
a measuring instrument the placement of the cable is important too.
Placing the current clamp/ cables close to grounded surfaces
usually lowers the coupling and improves the immunity.
The current clamp / test cables should be placed as far as possible
from the tested object –see Fig.12.

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Fig. 11: Minimizing influencing quantities, correct placement of conductors
Fig. 12: Minimizing influencing quantities, avoiding external magnetic and
electric fields
4.4.6 Consideration of influencing quantities
Only high quality current clamp can accurately measure in range of mA in
presence of external fields and high load currents. The given resolution or
basic accuracy are not telling a lot about the real quality of the current
clamp. The immunity of the current clamp against influencing quantities is
much more representative.
The IEC (International Electrotechnical Commission) is preparing a new
international standard IEC 61557-13 Hand-held and hand-manipulated
current clamps and sensors for measurement of leakage currents in
electrical distribution systems that will cover the aspect of influencing
quantities and will be launched in 2010. One of its main objectives is to
define rules how to consider the immunity of influencing quantities in scope
of the accuracy of leakage current clamps. This will enable the user to
make a relevant comparison when deciding between different current
clamp models.