Megger DELTA 4000 Reference Manual

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Reference Manual
Applications Guide

DELTA 4000

12 kV Insulation Diagnostic System

ZM-AH02E

Advanced Test Equipment Rentals

www.atecorp.com 800-404-ATEC (2832)

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SWEDEN
Megger Sweden AB

Eldarvägen 4
Box 2970
SE-187 29 TÄBY
Sweden

T [phone]
F [phone]
[e-mail]
www.megger.comEDEN

UNITED STATES
Megger

2621 Van Buren Avenue
Norristown, PA 19403

USA

T [phone]
F [phone]
[e-mail]
www.megger.com

ZM-AH02E

3

NOTICE OF COPYRIGHT & PROPRIETARY RIGHTS
© 2010, Megger Sweden AB. All rights reserved.
The contents of this manual are the property of Megger Sweden

AB. No part of this work may be reproduced or transmitted in
any form or by any means, except as permitted in written license
agreement with Megger Sweden AB. Megger Sweden AB has made
every reasonable attempt to ensure the completeness and accuracy
of this document. However, the information contained in this
manual is subject to change without notice, and does not represent
a commitment on the part of Megger Sweden AB. Any attached
hardware schematics and technical descriptions, or software listings
that disclose source code, are for informational purposes only.
Reproduction in whole or in part to create working hardware or
software for other than Megger Sweden AB products is strictly
prohibited, except as permitted by written license agreement with
Megger Sweden AB.

TRADEMARK NOTICES
Megger® and Programma® are trademarks registered in the U.S.

and other countries. All other brand and product names mentioned
in this document are trademarks or registered trademarks of their
respective companies.

Megger Sweden AB is certified according to ISO 9001 and 14001.

Reference Manual
Applications Guide

DELTA 4000

12 kV Insulation Diagnostic System

4 DELTA 4000 ZM-AH02E

Contents

1 Introduction ................................................. 6

General .................................................................6

Principle of operation .............................................6

Current, capacitance and dissipation factor

relationship ............................................................ 7

Connections for UST/GST Configurations ............... 8

2 Interpretation of measurements ............. 10

Significance of capacitance and

dissipation factor .................................................10

Dissipation factor (Power factor) of typical

apparatus insulation ............................................11

Permittivity and % DF of typical insulating

materials ..............................................................11

Significance of temperature ................................. 12

Significance of humidity .......................................13

Surface leakage ...................................................13

Electrostatic interference ...................................... 14

Negative dissipation factor ................................... 14

Connected bus work, cables etc ................................ 14

3 Testing power system components ......... 16

Transformers ........................................................ 16

Introduction .............................................................. 16

Definitions ................................................................ 16

Two-winding transformers......................................... 16

Three-winding transformers ...................................... 18

Autotransformers ...................................................... 19

Transformer excitation current tests ........................... 19

Shunt reactors ........................................................... 21

Potential transformers ............................................... 21

Current transformers ................................................. 21

Voltage regulators ..................................................... 21

Dry-type transformers ............................................... 22

Bushings .............................................................. 22

Introduction .............................................................. 22

Definitions ................................................................ 22

Bushing troubles ....................................................... 24

Bushing tests ............................................................. 24

Inverted tap to center conductor test C1 (UST) .......... 26

Power and dissipation factor & capacitance test C2 ... 26

Hot collar test ........................................................... 27

Spare bushing tests ................................................... 27

Circuit breakers .................................................... 28

Introduction .............................................................. 28

Oil circuit-breakers .................................................... 28

Air-blast circuit-breakers ............................................ 30

SF

6 Circuit-breakers ................................................... 31

Vacuum circuit breakers ............................................ 32

Air magnetic circuit-breakers ..................................... 33

Oil circuit reclosers .................................................... 33

Rotating machines ............................................... 33

Cables .................................................................35

Surge (lightning) arresters .................................... 35

Introduction .............................................................. 35

Test connections ........................................................ 36

Test procedure .......................................................... 36

Test results ................................................................ 36

Liquids ................................................................. 37

Test procedure .......................................................... 37

Miscellaneous assemblies and components .......... 37

High-Voltage turns-ratio measurements ...............38

Test procedure .......................................................... 38

Temperature considerations ....................................... 38

Index ............................................................. 40

References ........................................................... 42

Appendix A

Temperature correction tables .................... 44

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6 DELTA 4000 ZM-AH02E

1 INTRODUCTION

1

Introduction

Principle of operation

Most physical test objects can be accurately represented as
a two or three-terminal network. An example of a two­terminal capacitor is an apparatus bushing without any test
tap. The center conductor is one terminal and the mount-

ing ange (ground) is the second terminal. An example of

a three-terminal capacitor is an apparatus bushing which
has a power factor or capacitance tap. The center conduc­tor is one terminal, the tap is the second terminal, and the

mounting ange (ground) is the third terminal.

It is possible to have a complex insulation system that
has four or more terminals. A direct measurement of any
capacitance component in a complex system can be made
with this test set since it has the capability for measuring
both ungrounded and grounded specimens.

Figure 1 shows a simplied measuring circuit diagram of

the DELTA 4000 test set measuring a two-winding trans­former

in UST test mode. The test voltage is connected to the HV
terminal and the current is measured at the LV terminal.
Voltage and current are accurately measured in amplitude
and phase and CHL capacitance, dissipation factor, power
loss etc are calculated and displayed.

Figure 1: UST-R test setup for a 2-w transformer

Dissipation factor measurements can generally be per-

formed with two different congurations, UST (Unground­ed Specimen Test) where the ground act as natural guard
or GST (Grounded Specimen Test) with or without guard.

Figure 2 shows a guarded UST measurement. The current

owing through CHL is measured but the current paths

through CH and CL is guarded/grounded and not mea­sured. Figure 3 shows a guarded GST measurement where
the CH current to ground is measured but the current
through CHL is guarded and measured.

General

The intention of this reference-application manual is to
guide the operator in the appropriate method of making
capacitance and dissipation factor/power factor measure­ments on power apparatus and to assist in the interpretation
of test results obtained. It is not a complete step-by-step
procedure for performing tests.

Before performing any test with this apparatus, read the
user manual and observe all safety precautions indicated.

ZM-AH02E DELTA 4000

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1 INTRODUCTION

Figure 2: UST connection for measuring CHL in a two-wind­ing transformer

Figure 3: GST connection for measuring CH in a two-wind­ing transformer

Current, capacitance and
dissipation factor relationship

In an ideal insulation system connected to an alternating
voltage source, the capacitance current Ic and the voltage
are in perfect quadrature with the current leading. In ad­dition to the capacitance current, there appears in practice
a loss current Ir in phase with the voltage as shown in
Figure 5.

The current taken by an ideal insulation (no losses, I

r

= 0)

is a pure capacitive current leading the voltage by 90° (q =
90°). In practice, no insulation is perfect but has a certain
amount of loss and the total current I leads the voltage by
a phase angle q (q< 90°). It is more convenient to use the
dielectric-loss angle d, where d = (90° - q). For low power
factor insulation Ic and I are substantially of the same mag­nitude since the loss component Ir is very small.

The power factor is dened as:

Power factor= cos Θ = sin δ =

Ir

I

and the dissipation factor is dened as:

Dissipation factor = cot Θ = tan δ =

Ir
Ic

PF =

DF

•1+DF

2

DF =

PF

•1 – PF

2

The DELTA 4000 is able to display either dissipation factor
or power factor based on user’s choice.

Figure 5: Vector diagram insulation system

In cases where angle d is very small, sin d practically equals
tan d. For example, at power factor values less than 10
percent the difference will be less than 0.5 percent of read­ing while for power factor values less than 20 percent the
difference will be less than 2 percent of reading.

The value of I

c

will be within 99.5 percent of the value I

for power factor (sin d) values up to 10 percent and within

98 percent for power factor values up to 20 percent.

8 DELTA 4000 ZM-AH02E

1 INTRODUCTION

Connections for UST/GST
Configurations

DELTA 4000 supports two basic groups of operation, GST
and UST mode. GST stands for grounded specimen test
while UST stands for ungrounded specimen test. Within
the two groups the test set can be operated in seven test
modes as summarized in Table 1.1. Measurements are
always made between the high-voltage lead and the lead/
connection in the measure column.

Table 1.1
DELTA 4000 test modes and internal measure­ment connections

UST: Ungrounded specimen testing

Tes t mo de Measure Ground Guard

UST- R Red Blue –
UST- B Blue Red –
UST- RB Red and Blue – –

GST: Grounded specimen testing

Tes t mo de Measure Ground Guard

GST- GND Ground Red and Blue –
GSTg-R Ground Blue Red
GSTg-B Ground Red Blue
GSTg-RB Ground – Red and Blue

In UST test mode, Ground and Guard are internally
connected. Internally the Red and Blue leads are either

connected to be measured or connected to Ground (and
Guard).

In GST test modes the current returning from Ground is
measured. Internally the Red and Blue leads are either con­nected to Ground or Guard to be included in or excluded
from the measurement.

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1 INTRODUCTION

10 DELTA 4000 ZM-AH02E

2 INTERPRETATION OF MEASUREMENTS

in operation with consequent deterioration. Some increase

of capacitance (increase in charging current) may also be

observed above the extinction voltage because of the short
circuiting of numerous voids by the ionization process.

An increase of dissipation factor accompanied by in severe
cases possible increase of capacitance usually indicates
excessive moisture in the insulation. Increase of dissipation
factor alone may be caused by thermal deterioration or by
contamination other than water.

Unless bushing and pothead surfaces, terminal boards, etc.,
are clean and dry, measured quantities may not necessar­ily apply to the volume of the insulation under test. Any
leakage over terminal surfaces may add to the losses of the
insulation itself and may, if excessive, give a false indication
of its condition.

2

Interpretation of

measurements

Significance of capacitance and
dissipation factor

A large percentage of electrical apparatus failures are due
to a deteriorated condition of the insulation. Many of these
failures can be anticipated by regular application of simple
tests and with timely maintenance indicated by the tests.
An insulation system or apparatus should not be con­demned until it has been completely isolated, cleaned, or
serviced and measurements compensated for temperature.
The correct interpretation of capacitance and dissipation
factor tests generally requires knowledge of the apparatus
construction and the characteristics of the particular types
of insulation used.

Changes in the normal capacitance of an insulation material
indicate such abnormal conditions as the presence of a
moisture layer, short circuits, or open circuits in the capaci­tance network. Dissipation factor measurements indicate
the following conditions in the insulation of a wide range

of electrical apparatus:

▪

Chemical deterioration due to time and temperature,
including certain cases of acute deterioration caused by
localized overheating.

▪

Contamination by water, carbon deposits, bad oil, dirt
and other chemicals.

▪

Severe leakage through cracks and over surfaces.

▪

Ionization.

The interpretation of measurements is usually based on
experience, recommendations of the manufacturer of the

equipment being tested, and by observing these differences:

▪

Between measurements on the same unit after successive
intervals of time.

▪

Between measurements on duplicate units or a similar
part of one unit, tested under the same conditions around
the same time, e.g., several identical transformers or one
winding of a three-phase transformer tested separately.

▪

Between measurements made at different test voltages
on one part of a unit; an increase in slope (tip-up) of a
dissipation factor versus voltage curve at a given voltage is
an indication of ionization commencing at that voltage.

An increase of dissipation factor above a typical value may
indicate conditions such as those given in the previous para­graph, any of which may be general or localized in charac-

ter. If the dissipation factor varies signicantly with voltage

down to some voltage below which it is substantially con­stant, then ionization is indicated. If this extinction voltage
is below the operating level, then ionization may progress

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2 INTERPRETATION OF MEASUREMENTS

Dissipation factor (Power
factor) of typical apparatus
insulation

Values of insulation dissipation factor for various appara­tus are shown in Table 2.1. These values may be useful in
roughly indicating the range to be found in practice. Please
note that the higher values are not to be regarded as “OK”
but instead examples of “to be investigated/at risk” data.

Table 2.1
DF (PF) of typical apparatus insulation

Type apparatus % DF (PF) at 20°C

Oil-filled transformer: New, high­voltage (115 kV and up)

0.25 to 1.0

15 years old, high-voltage 0.25--

Low-voltage, distribution type 0.30-­Oil circuit breakers 0.5 to 2.0
Oil-paper cables, “solid” (up to 27.6

kV) new condition

0.5 to 1.5

Oil-paper cables, high-voltage oil-filled
or pressurized

0.2 to 0.5

Rotating machine stator windings, 2.3
to 13.8 kV

2.0 to 8.0

Capacitors (discharge resistor out of
circuit)

0.2 to 0.5

Bushings: Solid or dry 3.0 to 10.0

Compound-filled, up to 15 kV 5.0 to 10.0

Compound-filled, 15 to 46 kV 2.0 to 5.0

Oil-filled, below 110 kV 1.5 to 4.0

Oil-filled, above 110 kV and con-

denser type

0.25--

In IEEE 62-1995, typical values for dissipation/power fac­tor are given as in Table 2.2.

Table 2.2
IEEE 62-1995 power factor values

Typical power factor values @ 20°C

“New” “Old” Warning/alert limit

Power trans­formers, oil
insulated

0.2-0.4% 0.3-0.5% > 0.5%

Bushings

0.2-0.3% 0.3-0.5% > 0.5%

IEEE 62-1995 states; “The power factors recorded for
routine overall tests on older apparatus provide information
regarding the general condition of the ground and inter­winding insulation of transformers and reactors. While the
power factors for older transformers will also be <0.5%

(20°C), power factors between 0.5% and 1.0% (20°C) may
be acceptable; however, power factors >1.0% (20°C) should

be investigated.”

Permittivity and % DF of
typical insulating materials

Typical values of permittivity (dielectric constant) k and

50/60 Hz dissipation factor of a few kinds of insulating

materials (also water and ice) are given in Table 2.3.

Table 2.3
Permittivity and dissipation factor of typical
insulating materials

Material k % DF (PF) at 20°C

Acetal resin (Delrin*) 3.7 0.5
Air 1.0 0.0
Askarels 4.2 0.4
Kraft paper, dry 2.2 0.6
Oil, transformer 2.2 0.02
Polyamide (Nomex*) 2.5 1.0
Polyester film (Mylar*) 3.0 0.3
Polyethylene 2.3 0.02-0.05
Polyamide film (Kapton*) 3.5 0.3
Polypropylene 2.2 0.05
Porcelain 7.0 2.0
Rubber 3.6 4.0
Silicone liquid 2.7 0.01
Varnished cambric, dry 4.4 1.0
Water** 80 100
Ice** 88 1.0 (0°C)
* Dupont registered trademark.
** Tests for moisture should not be made at freezing tempera-

tures because of the 100 to 1 ratio difference of % dissipation
factor between water and ice.

12 DELTA 4000 ZM-AH02E

2 INTERPRETATION OF MEASUREMENTS

Any sudden changes in ambient temperature will increase
the measurement error since the temperature of the appara­tus will lag the ambient temperature.

Dissipation factor-temperature characteristics, as well as
dissipation factor measurements at a given temperature,
may change with deterioration or damage of insulation.
This suggests that any such change in temperature char­acteristics may be helpful in assessing deteriorated condi-

tions. As an example, bushings have typically a rather at

temperature correction with only slightly elevated values
at high temperatures. Generally a bushing showing highly
increased dissipation factor at elevated temperature should
be considered “at risk”.

Be careful making measurements below the freezing point
of water. A crack in an insulator, for example, is easily

detected if it contains a conducting lm of water. When

the water freezes, it becomes non-conducting, and the
defect may not be revealed by the measurement, because ice
has a volumetric resistivity approximately 100 times higher
than that of water. Moisture in oil, or in oil-impregnated
solids, has been found to be detectable in dissipation factor
measurements at temperatures far below freezing, with no
discontinuity in the measurements at the freezing point.

Insulating surfaces exposed to ambient weather conditions
may also be affected by temperature. The surface tempera­ture of the insulation specimen should be above and never
below the ambient temperature to avoid the effects of
condensation on the exposed insulating surfaces.

Significance of temperature

Most insulation measurements have to be interpreted based
on the temperature of the specimen. The dielectric losses
of most insulation increase with temperature; however, e.g.
dry oil-impregnated paper and polyethylene of good quality
exhibit decrease of dielectric losses when temperature is

raised moderately, e.g. from 20°C to 30°C. It is also known

that the effect of temperature depends on the aging status
of the insulation. In many cases, insulations have failed due
to the cumulative effect of temperature, i.e., a rise in tem­perature causes a rise in dielectric loss which in turn causes

a further rise in temperature, etc (thermal runaway).

It is important to determine the dissipation factor-tempera­ture characteristics of the insulation under test. Otherwise,
all tests of the same specimen should be made, as nearly as
practicable, at the same temperature.

To compare the dissipation factor value of tests made on
the same or similar type apparatus at different temperatures,
it is necessary to convert the value to a reference tempera­ture base, usually 20°C (68°F). Examples of standard tables
of multipliers for use in converting dissipation factors at
test temperatures to dissipation factors at 20°C are found in
the Appendix A of this document.

In reality, temperature correction for a specic compo­nent is always individual and pending age/condition.
DELTA 4000 has a unique and patented feature for estimat­ing the individual temperature correction (ITC). By measur­ing dissipation factor over frequency and using mathemati­cal formulas and models of insulation characteristics, the
correct temperature correction can be determined from

5 to 50°C measurement temperature to 20°C reference

temperature. The input data for the calculation is dissipa­tion factor measured from 1 to 500 Hz and the method
is principally based on Arrhenius’ law, describing how the
insulation properties are changing over temperature.

κ = κ

0

·exp(-Wa/kT)

With activation energy W

a

and Boltzmann constant k

The test temperature for apparatus such as spare bushings,

insulators, air or gas lled circuit breakers, and lightning

arresters is normally assumed to be the same as the ambient
temperature. For oil-lled circuit breakers and transform­ers the test temperature is assumed to be the same as the
top oil temperature or winding temperature. For installed
bushings where the lower end is immersed in oil the test
temperature lies somewhere between the oil and air tem­perature.

In practice, the test temperature is assumed to be the same
as the ambient temperature for bushings installed in oil-

lled circuit breakers and also for oil-lled transformers

that have been out of service for approximately 12 hours.
In transformers removed from service just prior to test,
the temperature of the oil normally exceeds the ambient
temperature. The bushing test temperature for this case can
be assumed to be the midpoint between the oil and ambient
temperatures.

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2 INTERPRETATION OF MEASUREMENTS

Significance of humidity

The exposed surface of bushings may, under adverse rela­tive humidity conditions, acquire a deposit of surface mois-

ture which can have a signicant effect on surface losses

and consequently on the results of a dissipation factor test.
This is particularly true if the porcelain surface of a bush-

ing is at a temperature below ambient temperature (below
dew point), because moisture will probably condense on the

porcelain surface. Serious measurement errors may result
even at a relative humidity below 50 percent when moisture
condenses on a porcelain surface already contaminated with
industrial chemical deposits.

It is important to note that an invisible thin surface lm of

moisture forms and dissipates rapidly on materials such as
glazed porcelain which have negligible volume absorption.
Equilibrium after a sudden wide change in relative humid­ity is usually attained within a matter of minutes. This,

however, excludes thicker lms which result from rain, fog,

or dew point condensation.

Surface leakage errors can be minimized if dissipation
factor measurements are made under conditions where the
weather is clear and sunny and where the relative humid­ity does not exceed 80 percent. In general, best results are
obtained if measurements are made during late morning
through mid afternoon. Consideration should be given to
the probability of moisture being deposited by rain or fog
on equipment just prior to making any measurements.

Surface leakage

Any leakage over the insulation surfaces of the specimen
will be added to the losses in the volume insulation and may
give a false impression as to the condition of the specimen.
Even a bushing with a voltage rating much greater than the
test voltage may be contaminated enough to cause a signi­cant error. Surfaces of potheads, bushings, and insulators
should be clean and dry when making a measurement.

It should be noted that a straight line plot of surface resis­tivity against relative humidity for an uncontaminated por­celain bushing surface results in a decrease of one decade
in resistivity for a nominal 15 percent increase in relative
humidity and vice versa.

On bushings provided with a power factor or capacitance
tap, the effect of leakage current over the surface of a
porcelain bushing may be eliminated from the measurement
by testing the bushing by the ungrounded specimen test

(UST).

When testing bushings without a test tap under high
humidity conditions, numerous companies have reported
that the effects of surface leakage can be substantially
minimized by cleaning and drying the porcelain surface and
applying a very thin coat of Dow Corning #4 insulating
grease (or equal) to the entire porcelain surface. When mak­ing a hot collar test, the grease is generally only applied to
the porcelain surface on which the hot collar band is to be
located and to that of one petticoat above and one below
the hot collar band.

When testing potheads, bushings (without test tap), and

insulators under unfavorable weather conditions, the dis­sipation factor reading may, at times, appear to be unstable
and may vary slightly over a very short period of time. The
variation is caused by such factors as the amount of surface
exposure to sun or shade, variations in wind velocity, and
gradual changes in ambient temperature and relative humid­ity. Similar bushings may have appreciably different dissipa­tion factor values for the case where one bushing is located
in the sun while the other is in the shade. A test made on
the same bushing may have a different dissipation factor
value between a morning and an afternoon reading. Due
consideration must be given to variations in readings when
tests are made under unfavorable weather conditions.

14 DELTA 4000 ZM-AH02E

2 INTERPRETATION OF MEASUREMENTS

Negative dissipation factor

Creep currents inside an insulation system or more com­monly on surfaces; create change of potential distribution
that may give increased or decreased dissipation factor, and
in some cases also negative dissipation factor. This condi­tion is most likely to arise when making UST and GST
measurements on specimens who have a capacitance value
of a few hundred picofarad or less. Equipment such as
bushings, circuit breakers, and low loss surge arresters fall
into this category.

The error is usually accentuated if tests are made under
unfavorable weather conditions, especially a high relative
humidity which increases surface leakage.

There appears to be no clear-cut way of knowing whether

an error is signicant or what remedies should be taken to

overcome an error. A frequency sweep may give additional
information. The best advice is to avoid making measure­ments on equipment in locations where negative dissipation
factors are known to present a problem when unfavorable
weather conditions exist, especially high relative humidity.
Make sure the surface of porcelain bushings are clean and
dry to minimize the effects of surface leakage. Make sure all
items such as wooden ladders or nylon ropes are removed
from the equipment to be tested and are brought out of

any electrostatic interference elds that could inuence a

measurement.

Connected bus work, cables etc

A complete disconnected component is preferred when
performing dissipation factor measurements. All connected
bus work, cables, disconnect switches etc may add signi­cant capacitance and losses in GST measurements where
they are in parallel with the desired insulation measure­ment. For this reason, many test engineers will ask that the
equipment under test be totally isolated from connected
apparatus.

UST data is principally possible to measure without fully
disconnecting the test object. The capacitance from the
connected parts results only in a current to ground that is
not measured in UST test mode.

Electrostatic interference

When tests are conducted in energized substations, the

readings may be inuenced by electrostatic interference

currents resulting from the capacitive coupling between
energized lines and bus work to the test specimen. Other

sources for interference may be corona discharges (espe­cially at high humidity) and is some cases DC uctuations
in the grounding system. Trouble from magnetic elds

encountered in high-voltage substations is very unlikely.

To counter the effects of severe electrostatic interference
on the measurement, it may be necessary to disconnect the
specimen from disconnect switches and bus work. Experi­ence in making measurements will establish the particular
equipment locations where it is necessary to break the
connections. The related disconnect switches, leads and
bus work, if not energized, should be solidly grounded to
minimize electrostatic coupling to the test set.

The measurement difculty which is encountered when

testing in the presence of interference depends not only

upon the severity of the interference eld but also on the

capacitance and dissipation factor of the specimen. Unfa­vorable weather conditions such as high relative humidity,
fog, overcast sky, and high wind velocity will increase the

severity and variability of the interference eld. The lower

the specimen capacitance and its dissipation factor, the

greater the difculty is to perform accurate measurements.

It is also possible that a negative dissipation factor reading
may be obtained so it is necessary to observe the polarity

sign for each reading. Specically, it has been found that
some difculty may be expected when measuring capaci-

tance by the GST test method in high interference switch­yards when the capacitance value is less than 100 pF. This

difculty may be minimized considerably by:

▪

Using the maximum voltage of the test set if possible.

▪

Disconnecting and grounding as much bus work as
possible from the specimen terminals.

▪

Making measurements on a day when the weather
is sunny and clear, the relative humidity is less than
80 percent, the wind velocity is low, and the surface
temperature of exposed insulation is above the ambient
temperature.

Tests made by the UST method are less susceptible to in­terference pickup than are tests made by the GST method.
In the UST test method, the capacitive coupled pickup

current in the high-voltage circuit ows directly to ground

after having passed through the high-voltage winding of
the power supply transformer. In the GST test method the
same pickup current, after passing through the high-voltage
transformer winding, must pass through one of the bridge
transformer-ratio measuring arms before reaching ground.

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2 INTERPRETATION OF MEASUREMENTS