Performing Live–Circuit

Installation Tests with a

Fluke 1650 series Tester Application Note
on an IT-system

Performing the Tests

Voltage measurements
The tester can be used as a volt-
meter with voltage and frequency
displayed at the same time by
setting the rotary knob to V.
All voltage measurements are
performed between the L (red)
and the PE (green) inputs. It is not
To measure voltage between phase
necessary to press the Test key.
and earth, connect the red lead
to the L (red) input, the blue lead
To use the test leads, connect the
to the N (blue) input and the green
red lead to the L (red) input and the
to the PE (green) input and insert
green lead to the PE (green) input.
the plug into the outlet. Reverse the
plug in the outlet to measure the
voltage between the other phase
and earth. In an IT-system the
nominal voltage between each of
the phases and earth is 130 V.
This voltage may vary due to the
variation of capacitive loading of
To compensate for lead resistance,
each phase to earth or due to an
short all leads and press Zero. To
existing fault condition.
use the test leads, connect the red
lead to the L (red) input and the blue
lead to the N (blue) input. To use the
mains test cord, connect the red lead
to the L (red) input, the blue lead to
the N (blue) input and the green to
the PE (green) input and insert the
plug into the outlet. Press the Test
key to initiate a test.

Loop impedance measurements

Voltage measurements can also be The 1650 series tester can perform
performed with the mains test cord. two types of loop impedance
To measure voltage phase to phase, measurements on an IT-system,
connect the red lead to the L (red) phase to phase or phase to earth.
input and the blue lead to the PE
(green) input then insert the plug To measure phase to phase, set the
into the outlet. In an IT-system the rotary knob to ZI and select L-N with
phase to phase voltage is around the F1 key.
230 V.
 Select the starting phase of the test
(0º or 180º) with the F4 key. Press the
Test key to initiate the test.

To measure trip current, set the rotary

knob to I∆N. Select the RCD’s rated
current (10-1000mA) with the F1 key.
Select the RCD type (AC, A, AC & S or
A & S) with the F3 key. Select the
starting phase of the test (0º or 180º)
with the F4 key. Press the Test key to
initiate the test.

Either measurement can be performed

at the panel or at a socket. To
measure at the panel, connect the
The impedance being measured by a The remote probe may be used for
leads as shown. With all loads on this
phase to earth test depends on the either loop impedance test. The test
circuit disconnected, the results are
condition of the IT-system. It should key on the probe performs the same
true measurements of the parametric
be a very high impedance on a function as the test key on the
performance of the RCD.
healthy system. Low impedance instrument. This allows tests to be
values may be caused by a shorted initiated while holding the probes.
over voltage protection device at the
supply transformer, loads connected RCD measurements
to the system or an existing first fault All RCD’s have a test button. When
condition. pressing this button a current is
This is not a common test as the state generated through the RCD’s internal
of the system must be known before current coil, and the RCD should trip.
you can determine the significance of This test does not verify whether the
the measured value. RCD suitably protects the installation To measure at a socket, use the
it is in and it does not verify the adapter between the N and the PE
To perform a phase to earth parametric performance of the RCD. inputs. To connect the mains test cord,
measurement, set the rotary knob to The 1650 series tester can measure connect the red lead to the
ZI and select L-N with the F1 key. the parametric performance of the L (red) input and the green to the
To compensate for lead resistance, RCD and determine if it functions adapter and insert the plug into
short all leads and press Zero. Using correctly in the installation. the outlet. Press the Test key to
the test leads, connect the red lead to initiate a test. Reverse the plug in
the L (red) input and the blue lead to There are two types of measurements the outlet to test the other phase.
the N (blue) input. On the circuit to be performed by the 1650 series testers:
tested, connect between phase and trip time and trip current. The trip
earth. Using the mains test cord, time test (∆T) forces the selected
connect the red lead to the L (red) current and measures the time to trip.
input and the green to the the N The trip current test (I∆N) forces
(blue) input, leave the blue lead currents from 50% to 110% of the
unconnected and insert the plug into selected current to determine the
the outlet. Press the Test key to current which causes the RCD to trip.
initiate a test. If the voltage between Both tests also display the maximum
phase and earth is less than 100 V, resulting fault voltage during the test.
testing is disabled. Reverse the plug
If the result is the same in both cases,
in the outlet to test the other phase. To measure trip time, set the rotary
the measurements are not influenced
These tests will trip RCDs. knob to ∆T. Select the RCD’s rated
by leakage currents in the circuit.
current (10-1000 mA) with the F1 key.
If the two results are different,
Select the current multiplier (x1/2, x1,
the cause might be existing leakage
x5) with the F2 key. Select the RCD
currents in the circuit. The correct
type (AC, A, AC & S or A & S) with
value is approximately the average
the F3 key.
of the two measurements.
The Basic Theory of an When the load is resistive, and the
IT-system voltage has the shape of a sine
wave, the current can be described
Voltage as a vector that rotates around a
AC sine waves can be described as plane and draws its instantaneous
in Figure 1. We can think of a value along a time axis (as
voltage vector, Û. previously described for the voltage).
This is rotating around in a plane. The peak value of the current is the
In a 50 Hz distribution system the peak value of the voltage divided by
rotation time is 20 ms. The vector the resistance in the circuit.
rotates 360° or 2π radians, which is The same is valid for the rms value. Figure 1 – Voltage
one period. The next period it For a sine wave current the ratio
repeats itself and we get a periodic between peak and rms value is
signal. We can find the always the square root of 2.
instantaneous voltage with an
instrument that measures the Ieff = Î ⇒ Î = √2 ⇒ Î = 1.414
√2 Ieff Ieff
voltage at each time. This
If you have an instrument that can
instantaneous value is often
measure both peak and rms values
described with small letter (u= the
you can verify that the ratio is 1.414.
instantaneous voltage value). When
If the ratio differs from 1.414 you can
the voltage vector has rotated once
conclude that the current or voltage
it draws a momentary value along
you are measuring is not a pure sine
the time axis. We get a periodic
wave.
sine wave. Figure 2 – Current resistive loads

In Figure 2 you can see that the

A handheld instrument with a
current vector has the same direction
numeric display will only measure
as the voltage vector.
the voltage rms value. This value is
The current is a sine wave and it has
used for calculating effect, Ueff- is
its peak value and zero crossing
the voltage rms value. The ratio
point at the same time as the
between the voltage peak and rms
voltage. The angle between the
value will always be the square
current and the voltage is zero.
root of 2 for a voltage sine wave.
When the load is inductive,
Ueff = Û see Figure 3, the current value is
dependent on the voltage value
√2
and the impedance in the circuit.
Current linear loads
From the figure you can see that
Current will flow in a closed circuit Figure 3 – Current inductive loads
the current has an angle of 90°
where there is a voltage source that
after the voltage.
can generate a current. The current
is the result of the shape of the
When the load is capacitive,
voltage and the kind of load. There
see Figure 4, the current value is
are three linear loads; resistors,
also dependent on the voltage and
capacitors and coils.
the impedance in the circuit. In this
When these loads are connected to a
situation the current will have and
linear voltage, the current will also
angle of 90° before the voltage.
be linear.

Figure 4 – Current capacitive loads

A three phase voltage system can If the loads are unequal the loads
be described as in Figure 5. Three zero point will be displaced related
voltage vectors with an angle of to the voltage source zero point.
120° between them are rotating See Figure 7. The voltage between
around a plane and draw their the load's zero point and the three
instantaneous values along the time phases will be unequal due to this
axis. The phase voltage is measured displacement.
between the zero point of the
voltage source and the phase. The In Figure 8 the three phase voltage
line voltage measured between the source is loaded with three equal
phases is 1.732 times larger than capacitances in a star connection.
the measured phase voltage. The current in all three phases will
Figure 5 – Voltage three phases
be equal and have an angle of 90°
Generally in IT-systems U1=130 V before it’s respectively voltage.
and U1-2= 230 V and in TN systems The star point of the load will have
U1= 230 V and U1-2= 400 V. the same potential as the star point
of the voltage source. The different
•
U1-2= U1 √3 √3=1.732 phase voltages can be measured
between the zero point of the
voltage source and each phase,
In Figure 6 the voltage source is or between the zero point of the
loaded with three similar resistors load and each phase. The measured
in a star connection. The current in value between the zero points and
the three phase conductors is equal each phase will be equal.
and has the same angle as the
voltage. The center point in the star
Figure 6 – Voltage and current, three phases Ohmic resistance
connection of the load will have the
same potential as the center point
of the star connection of the voltage
supply. The different phase voltages
can be measured between the zero
point of the voltage source and
each phase, or between the zero
point of the load and each phase.
The measured value between the
zero points and each phase will be
equal. If you need to measure the
phase voltage you can use three
similar resistances connected in a
star as in Figure 6. Then you get
Figure 7 – Displacement of the load’s zero point
access to a zero-point and can
measure the phase voltage. This
method can also be used to
measure phase power.

Figure 8 – Voltage and current, three phases Capacitive load

If the loads are unequal the load’s For IT-systems, when measuring
zero point will be displaced relative the voltage between phase and
to the voltage source zero point. earth we get a voltage quite similar
See Figure 9. The voltage between to the phase voltage (across the
the load’s zero point and the three phase to the common point of the
phases will be unequal due to this transformer). If we get a resistive
displacement. earth-fault in one of the phases,
the earth potential will move along a
Earth-fault semi-circle around that phase vector.
In an IT-system the zero point of the Where the earth potential will be
transformer is not connected to depends on the size of the resistance
earth. Earth is connected to the and the capacitive coupling to earth. Figure 9 – Displacement of the loads zero point

system through the capacitive In Figure 11 a resistive fault from

coupling each phase has to earth. phase 1 to earth exists. The earth
Normally the common point of the potential has moved along the
transformer is connected to earth semi-circle. As the resistance is
through an over voltage protection reduced, the earth potential moves
unit called a disneyter. See Figure further away form the zero point of
10. This will start conducting when the transformer, along the semi-
the voltage between the zero point circle. We can measure a voltage
and earth increases more than a that is 242 V between earth and
certain value, otherwise it works as one of the other phases in this
an insulator. situation. Finally as the resistance
decreases to zero, we have a short
When measuring voltage between between earth and phase 1 and
each phase and earth in the we will not measure any voltage Figure 10 – IT system
IT-system we are actually measuring between them. Between earth and
the voltage over this coupling the other phases we measure
capacitance. If we are measuring the line voltage of 230 V.
equal voltage between each phase
and earth we can make two In reality, a fault may be caused by
different conclusions: a device which is both inductive
1. There is no earth fault in the and resistive, for example, a current
transformer circuit. The capacitive limiter in a lighting fixture. This will
coupling between each phase and have the effect of moving the
earth is equal for the three phases. voltage of the common point of the
transformer along the lines of
2. If the disneyter is defective it will Figure 12. Then the voltage
make a connection between the between earth and the different
common point of the transformer phases can be more than 400 V.
Figure 11 – Voltage between phase and earth. Resistive earth-fault
and earth (this then becomes a TT
system). In this case we will be
measuring the phase voltage from
the transformer and it will be
equal for all three phases.

Figure 12 – Voltage between phase and earth. Resistance and coil

earth-fault
When there is a short between More developed systems will have
phase and earth in an IT-system larger capacitive coupling, and the
the current is defined by the total leakage current will be larger.
resistance in the fault loop and the
capacitive coupling the system has If the capacitive coupling is the
to earth. The current will follow a same between each phase and
path as shown in Figure 13. The earth, earth will have the same
current will change dependent of voltage potential as the zero point
the capacitive coupling the healthy of the transformer.
phases have to earth. Normally this The current through each
current is quite low and it will not capacitance will have an angle of
trip overload protection units. 90° before its respective voltage. Figure 13 – Fault-current between phase and earth
There will also be equal voltage
When there is a short between two between each phase and earth. If
phases in an IT-system, the current the capacitive coupling between
is limited by the impedance in the each phase and earth is different,
fault loop, the external impedance the voltage between each phase
Zexternal, plus the impedance in the and earth will be different.
conductors, Zinternal. See Figure 14.
Zexternal is the impedance of The magnitude of the leakage
transformers and the wiring system current is dependent on the
upstream of the origin of the capacitive coupling each phase has
installation. Zinternal is the to earth. For example, in a standard
impedance in and wiring system house with an IT-system, this
downstream the origin of the leakage current can be 10 mA.
installation. This current consists of two Figure 14 – PSC in an IT system
We can use an installation tester to capacitive components with an
verify the PSC current and loop- angle of 90° before its respective
impedance between the phases in phase current. The angle between
an IT-system. The installation tester the two components is 120°.
simulates a fault and calculates the See Figure 15.
impedance and PSC. To get an
indication on the Ik2p min you can RCDs in IT-systems
measure at the end of the circuit NEK400:2002 pinpoints a
and multiply the result with 0.76. requirement to disconnect earth-
To get an indication on the three faults in final circuits connected to a
phase value, Ik3pmaks, you can public network transformer.
measure at the point of origin The practical solution is often the
and multiply the result with 1.15. use of RCD’s before the fault.
The reason the results are indications
only is that these factors are based Before a RCD is tested, any load Figure 15 – Leakage current in an installation
on ideal situations regarding downstream from the RCD must be
voltage and temperature. disconnected because all loads
have a leakage current to earth.
Capacitive leakage currents This leakage current will be added
In all mains systems we have to the test current generated by the
capacitive coupling between phases test instrument and influence the
and earth. This capacitive coupling measurements. This is due to the
can be composed of conductor fact that most installation testers
insulation or capacitive loads. with a RCD function work as shown
The result is that the system is in Figure 16. The instrument adds a
loaded with a capacitive leakage resistive load between one phase
current. This current is present even and earth.
if there are no other loads
connected. Figure 16 – Testing a RCD
The current forced by the tester is
in phase with the phase voltage
connected to earth via the resistors
added from the instrument. It is the
total of tester's generated current
and any leakage currents that trip
the RCD.

The result can depend on which

phase the tester connects to earth.
On one of the phase the tripping
current may be less than the true Figure 17
characteristic of the RCD and on the
other it may be greater. The correct
value is approximately the average
of the two measured values.

It may be possible that the RCD

does not trip. There might be a
problem with the RCD, but this is
seldom the case. More likely the
cause is that the RCD is placed
incorrectly. If, for example,
a three phase RCD is placed too
close to the transformer it will not
trip. See Figure 17.

In an IT-system the earth-fault

current is determined by the
capacitive coupling each phase has
to earth. NEK 400 recommends that
the first earth-fault current in a
transformer circuit is calculated to
be 2 mA per kVA transformer size.
Sometimes the capacitive coupling
is so small that an earth-fault
current never reaches the value that
is needed to trip the RCD. This often
happens in underdeveloped
systems.

Sometimes the RCD will trip

without any downstream earth-
faults. The reason might be that
the capacitive leakage current is so
large that it makes the RCD trip.

As already discussed, all loads have

a natural leakage to earth.
This current can be measured with
a leakage current clamp.
Fluke. Keeping your world
up and running.

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Printed in the Netherlands.
02/2004 Pub-ID 10700-eng Rev. 01