TECHNICAL

OVERVIE W

Considerations for
Instrument Grounding
Many people have heard of the term "grounding", but few fully understand its meaning and
importance. Sometimes, even experienced electricians do not treat grounding as a serious
issue. The impact of an incorrect or absent grounding ranges from noise interference.
resonance or humming during the use of electrical equipment to the worst case where
electricity leakage through the chassis causes personal injury or damage to instrument
components. Grounding, therefore, is a very practical issue that should be dealt with properly.
For those who operate electrical equipment frequently, a complete understanding of grounding
theories and applications is necessary in order to become a best-in-class technician.

In the eighteenth century, Benjamin Franklin performed the famous kite experiment to observe
how lightning in the sky was conducted to the earth. This experiment led to the invention of
lightning rods to avoid lightning strikes. From then on, people began to realize that the vast
ground under our feet is a huge electrical conductor. It may not be the best conductor, but it
is certainly a good one. It is so enormous in size that it can sustain a tremendous amount of
current. That is why the voltage level of the ground is set to be zero. Safety regulations require
that all metal parts which do not carry electricity should be kept at zero or the earth voltage
level.

There are several reasons for grounding. Some are for safety purposes, and some are for
maintaining circuit stability. The following are some examples:

–– Power system grounding: As you can see in Figure 1. this design is to prevent the secondary
side from being damaged by the high voltage on the primary side, as the current will be
conducted to the ground through the Grounding Wire to protect human lives.
–– Instrument grounding: By connecting the equipment or chassis to the ground, operators
can be protected from electric shocks if there is electricity leakage.
–– Signal grounding: A zero voltage reference or a loop-back path is provided for all integrated
signals to ensure proper operation or accurate measurements.
–– Shielded grounding: This is used to prevent static electricity from being accumulated.
Ground isolation or conduction can help to reduce noises and electro-magnetic
interference. Examples include shielding rooms, cables, wirelines, guarded terminals of
instruments, transformers and filters.

Transformer
Primary Secondary

High voltage Low voltage

Grounding wire

Earth ground
Grounding electrode

Figure 1.

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Types of Instrument Grounding
Figure 2 shows a commonly used instrument grounding on inputs and AC power. In this case. the
input signal ground is connected to the power ground and when you are making a measurement,
it is important to make sure that the input signal ground is not short-circuited directly to
any point where there is a voltage difference to the earth ground. This is very common when
measuring commercially available low-cost circuits. To reduce costs, these circuits usually do not
use power isolated transformers. Instead, the AC power is directly connected to the circuit. As a
result. a loop is formed between the circuit itself and the earth ground, and a voltage difference
occurs. If the AC power happens to be plugged in the reverse way, or a considerable voltage
difference exists between the neutral line and the earth ground, the combined factors could lead
to very unpredictable results. Therefore, caution must be exercised before the input is connected
for measurement.

To avoid the problem described above, some instruments provide floating inputs as shown in
Figure 3. Each of the inputs is floating from the earth ground. Ideally, as long as the voltage
difference between these two inputs is within an acceptable range, the inputs can be connected
to any voltage point.

Figure 4(a) and 4(b) show some common instruments for output devices. For DC or low frequency
generators, the design shown in 4(a) is usually adopted, while for high frequency (RF) generators,
the model in 4(b) is used. Therefore, special attention is required to avoid the problem described
in item (1) when performing a high frequency measurement. Otherwise, the voltage difference
and conflict may cause damage to the input and output of instrument circuits.

Instrument case Instrument case

Power Power
AC AC
transformer + transformer
plug plug
Sensing Sensing
electronics Input electronics
Input and
and
display display
−
Signal
ground Shield
ground

Figure 2. Figure 3.

Instrument case Instrument case

Power AC Power AC
+ transformer plug transformer plug
Output
Output Output Generating
conditioning
electronics
electronics
−
Case
ground
Shield
ground
(a) (b)

Figure 4.

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Guidelines for Instrument Grounding
While there are some guidelines for grounding, there is no hard and fast rule. In practice, it is
difficult to follow all the guidelines. Rather, it is dependent on the environment and applications.
For instance, before the measurement is performed, evaluations should be made to decide
whether the floating method should be used. The following are some general guidelines:

–– When dealing with different kinds of grounding such as shielded grounding, power
grounding and signal grounding, make sure that they are guided to their own paths to avoid
interaction.
–– Keep the impedance of the ground line low and the path short.
–– Avoid multiple ground loops, which may disturb current flows.
–– Isolate the heavily loaded ground current loop from the small signal loop.

Let’s take a look at the example in Figure 5. For a well-designed power supply equipment. the
voltage ripple between output A and B should be very small. However, if for convenience sake,
point C instead of point D is used as the signal’s ground reference, a significant pulse voltage
will occur and show up on the output circuit even when the resistance between point C and D is
small. This is because the transient current in charging or discharging the capacitors is usually
very high. As a result, as soon as the power is on, damage can be caused to the connected
circuits or instruments.

A
Voltage
regulator

Figure 5.

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Common Causes of Poor Grounding
The AC power (such as 110 V) socket does not provide the ground line (green). as shown in
Figure 6. The earth pole on the plug is usually the longest so that it can be connected to the
ground first when the AG power cord is plugged into the socket.

The actual impedance to the ground is too high which does not comply with electrical
regulations (please refer to the appropriate regulations in your country). Take the example of
the third level regulation of grounding. When the AG voltage to the ground is above 300 V. the
impedance to the ground must be less than 10 Ω. If the AG voltage to the ground is under 150
V. the impedance to the ground should be within 100 Ω.

The neutral line is mistakenly used as the ground line and these two lines are short-circuited
together on the socket. Under normal conditions, the voltage difference between the neutral
line and the ground line on the socket must be within 1.0 V, but this should not be accomplished
by short-circuiting these two lines together.

Swap the hot line and the neutral line arbitrarily. Take Figure 7 as an example. Equipment A
sends out signals while equipment B receives signals. Suppose the AG power sockets to which
Equipment A and B are connected do not provide the ground line. and the hot line and neutral
line are swapped on one of the equipment. Since equipment A and B both have noise filters
installed. a 110 V AG loop is therefore formed accidentally even when the power switch of A and
B is not connected.

Equipment A

N
L

Earth

5-15R

Each pole in the figure represents: Equipment B

L: Line, hot or active conductor
N: Neutral or identified conductor
G: Ground or earth conductor

Figure 6. Figure 7.

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Summary
When installing the equipment in a building, make sure to have an electrician check on the
impedance to the ground and the grounding device to see if they comply with electrical
regulations. 8 AWG wireline should be used as a minimum wire type for instrument grounding.

Use the three-pole AC power socket for the instrument. Make sure the polarity of the hot line
and the neutral line is correct (see Figure 6). The voltage difference between the neutral and
the ground lines should be less than 1 V. At the socket end. the impedance between the neutral
and the ground lines should be lower than 1 Ω.

Find out the appropriate way to do measurements. i.e. whether the instrument's input/output
terminals should be grounded or be floating.

Check the stability of the AC power (ex. +5% to –10% within 120 V) and whether there are
unpredictable impulses, which may cause the measurement to fail or even damage the
instrument. Generally, the transient voltage fluctuation should not exceed ± 15% within 120 V
and the voltage should be restored to 120 V within 0.5 second. The total harmonic component
should be less than 5%.

Verify the grounding of the equipment or device under test (OUT). If voltage differences exist
among equipment, connecting them together may cause conflicting situations. The sudden
pulses generated when the equipment is powered on may also damage vulnerable modules. If
this happens, the links between the equipment and devices under test should be disconnected
before the AC power is turned on. Each equipment and OUT should be reconnected only after
all equipment and devices have stabilized. In so doing, the possibilities of damage can be
minimized. However, this is not the way to eradicate the problem completely. The best solution
is to identify the root causes and fix them.

Reduce and remove unwanted static, interference and noise through proper grounding.

Keysight Test and Measurement Service Centers have been providing comprehensive and
precise repair and calibration services to customers for many years. Our pursuit of quality and
technical innovation enables us to offer better services to our customers. By sharing this article
with you, we hope to help you improve the accuracy of measurements and the life span of your
instruments.

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Appendix 1: AC Power Control Panel
In most countries, electric power companies will provide building occupants AC power
electricity with a Y connection configuration (Figure 8), which offers three live/hot lines (a), (b),
(c) and one neutral line that is colored white. This configuration is called a 3-phase 4-wire AC
power system. The voltage between either one of the live lines and the neutral line is about 108
to 126 V, while the voltage between the two live lines is about 187 to 218 V. The neutral line
inside the AC power transformer is connected to earth, which — in the event of an AC power
transformer failure — bypasses a potentially damaged current into the ground, thus preventing
any damage to the occupant’s equipment. This neutral line also performs as a reference point
for balancing three phase voltages.

Builders should provide one independent grounding line (G) colored green, with a resistance of
about 1 Ω, connected to the AC power control panel. It is not the responsibility of the electric
power company to provide this. In many cases, either the builder or the occupant may overlook
this step, resulting in a missing grounding wire, or the neutral line being mis-used as the ground
line.

The neutral line (N) should not be used as the ground line. The key concern is that the dynamic
loading current. which flows through the neutral line, creates a voltage drop between it (N)
and the earth. If the loading current is high, then the voltage drop will be significant. However,
in normal circumstances there is no loading current in the ground line and the voltage drop
between (G) and the earth is zero. So when we connect the instrument chassis to the ground
line, we can obtain an extremely stable and safe reference plane. Moreover, if electricity
leakage or ESD occurs, the current will flow to the earth, thereby protecting the instrument.
Usually, the voltage drop between (N) and (G) should be within 1 volt AC; it is impossible to
achieve zero volt. For precision instrument measurements, if the ground line is left open or is
connected to the neutral line, this means we have failed to set up an independent grounding
for the instrument. Hence, an ideal grounding reference level required for safe and stable
measurement cannot be achieved. In the worst case, when the voltage difference between
the instrument chassis and the ground is too high, it may damage the other instrument or the
device under test in the same measurement system.

Neutral line (N): Dynamic current flows

(N) (G)
through it. If wire resistance
Mains supply Power panel is high, there will be a
110 V/ 220 V significant voltage drop with
respect to the earth. (i.e.
(a) Live/hot line there is voltage between N
and G)
(b) Live/hot line
Ground line (G): Normally there is no loading
current and voltage drop
is zero with respect to
(N) Neutral line the earth. Ground line is
connected to the instrument
chassis, which protects the
(c) Live/hot line operator and offers an ESD
bypath.
(G) Ground line

(a) (b) (c)

Figure 8.

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Appendix 2: Guide on Testing Power
This is to remind you once again. Please check the AC alternating current (use 120 V as an example) and grounding condition prior to
the use of an instrument. If the following basic requirements are not met, please do not connect it to the power source. Otherwise, the
instrument is extremely vulnerable to damages. This may even lead to dangers to human body.

Step 1
Make sure there are 3 holes in the outlet.
that is, holes for the Live wire, the Neutral
wire and the Ground wire. Do not put on
the plug if the outlet is not with 3 holes. N
(As on Figure 9) L

L: Live Wire
G N: Neutral wire
G: Ground wire
(Front view of the wall socket)

Figure 9.

Step 2
Make sure the polarity of the 3 wire outlet
(Figure 9) is correct: can be tested with
–– Polarity Tester on Figure 10 to find the
polarity via the red, white and yellow
lamps or
–– Multi-meter and refer to the test in
Step 3
(a) (b)

Figure 10.

Step 3 108 ~ 126 V

Use a multi-meter to test the followings:
The voltage between the Live wire and
the Neutral wire should be in the range
of 108 to 126 V. The Voltage surge should
not exceed 120 V ± 15%. The voltage
between the Neutral wire and the Ground
wire should be less than 1.0 V. At the same <1.0 V
Holes facing the power source lead
time. the change should not be more than
1.0 V no matter whether the instrument is
Figure 11.
on or not. (As on Figure 11)

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Appendix 3: Serious Damage comes from incorrect polarity and grounding of AC
3-Wire

Question 1
Why is instrument grounding so important?

Question 2
Why is the polarity and grounding of AC power source so important?

Question 3
What is the threat of AC power source on the Device-under-test to the instrument?

Figure 12.
If we only use the Line and the Neutral Conductor of AC 120 Volts but do not really connect the
Ground Wire, voltage will exist between the Ground Wire (and the chassis) and the ground! This
is because there is still stray capacitance between the Neutral conductor and the Ground Wire
of the power cable or the primary winding of the transformer. Or, there is filtering capacitor
for suppressing noise. As Vg1 or Vg2 on the diagram, it may be as high as 20 V to 60 V! If the
Live and the Neutral wires are reversely connected in one of the instrument Vg1 ≠ Vg2. Simply
imagine what will happen if two objects with different electric potential are put together.

When Vg1 and Vg2 exist, the signal (to the ground) output of Instrument 1 is no longer smaller
than v1. lnstead, it is (vl+Vgl). This signal is strong enough to seriously damage the input circuit
of Instrument 2. The signal will not only circulate between the output ends of Instrument 1 and
input ends of Instrument 2. Ig will flow through the floating ground wire. If the circulation is too
strong, permanent damage will be caused! Therefore, only perfect grounding can solve this
problem. Vg1 = Vg2 = Vg3 = 0 and makes Ig = O.

Problem: Problem:
Reversed live and Noise, harmonic wave, spikes
neutral connection and voltage fluctuation

Instrument 1 Instrument 2
Neutral line Hot line
HI + + HI
120 V AC V1 120 V AC
Hot line Neutral line
Lo — — Lo

GND line
GND GND

Vg1. lg Vg2.

Problem:
Problem: Over-ranged ground
No grounding resistance and poor grounding

Figure 12.

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Figure 13.
When the Device-under-test is an active component or circuit it may sometimes rely on 120 V
AC (but not battery) as a source of power. This device usually uses only 2 wires (Live wire and
Neutral wire) and is not connected to the ground due to costing and commercial use factors.
There may not be even any isolating transformer in the circuit. Therefore, there may be an
unexpected Vg3 voltage. This usually happens in tests using LNB and CATV Amplifier. Without
sufficient care, Ig will cause damages.

120 V AC

Instrument 2
signal receiver
Hot line
+ HI
Device-under-test 120 V AC
Neutral line
— Lo

GND line
GND

Vg3. Vg2.
lg

Figure 13.

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Figure 14.
If the instrument chassis is not grounded. When there is an AC voltage through it (the operator
sometimes may not feel the electricity when he touches its chassis). How can the instrument be
damaged under such conditions?

The facts are:

If the chassis of both instruments (1) and (2) are grounded correctly, both instruments will not
have any voltage difference with respect to the earth.

Because the instrument is using a 3-pronged power source, the ground line inside the power
cord is located in parallel between the live/hot line and the neutral line, and this will create a
stray capacitance between these 3 wire lines. So if the neutral line, instead of the ground line,
is connected to the earth, a voltage of about 30 V to 60 V will be coupled to the ground line
with respect to the earth. This voltage will appear on the instrument chassis and may injure the
operator or damage any DUT and other connected instruments.

When any DUT or other instruments connected to the instrument are not grounded, this
unexpected 30 V to 60 V voltage will find another circuit loop to release the power. It will
generate a current surge and will damage any sensitive DUT or instruments in the circuit loop.
However, if the chassis (ground line) of both instruments are connected beforehand to form an
equal potential, the destructive current loop will not result between instruments (1) and (2).
This explains why operators do not always find damage to their instruments, even though they
do not take instrument grounding precautions. The key principle is that we should connect LI to
L2 before connecting HI to H2 for setting up a good reference plane.

(H1) Device-under-test (H2)

Instrument (1) Instrument (2)
(v signal)

Assuming that
instrument (1)
chasis is not (L1) (L2)
grounded properly,
then 30 V to 60 V When taking measurements, if
AC may exist on 11 and L2 are open while HI and
the chasis H2 are connected, then an
unexpected voltage of 30 V to
(30 to 60 V AC) 60 V AC will flow into the loop
indicated here and may create
damage. However, if both
instruments are well grounded,
there will not be 30 V to 60 V
AC. Only a V signal will exist and
no damage source will appear.

Figure 14.

Device
H1 under test H2

Instrument 1 Instrument 2
signal source signal analyzer

C
L2 L1

Figure 15.

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 Check items Expected specification Tools Check results
AC power quality
1 3-wire polarity AC power outlet, N-G voltage less –– DMM
N L than 1 VAC (but N & G cannot be –– PGT-602
G shorted on the outlet), N-L voltage –– Receptacle Tester1933
120 VAC. –– GAM-2A
2 Voltage and impedance Should be less than 1 VAC and 1 Ω. Grounding wire –– PGR75
between neutral and ground specification 8 AWG, independent ground wiring. –– Fluke 434
line (under full load) –– PGT-602
3 Stable voltage, single phase 120 V or 208 V +5%, –10% –– Fluke 434 Phase A to neutral:
120 volt (i.e. 108 V to 126 V, 187 V to 218 V) –– RPM 901 Phase B to neutral:
Phase C to neutral:
4 Frequency 60 cycle (Hz) ± 0.5 cycle –– Fluke 434
–– RPM 901
5 Surge and sag voltage Less than 120 V ± 15%, can endure for max. 0.5 –– Fluke 434
Sec (30 cycle) –– RPM 901
6 Impulse, transient voltage If transient voltage > 100 VAC (up to 200 us), –– Fluke 434
equipment may damaged. Normally, ETE can stand –– RPM 901
for 150% to 200% VAC with max. 0.2 second
7 Total harmonic Less than 5% –– Fluke 434
–– RPM 901
8 Power factor 0.8 ~ 0.9 –– Fluke 434
–– RPM 901
9 3-phase unbalance ratio Less than 2.5% –– Fluke 434
–– RPM 901
10 3-phase load unbalance ratio Each phase max. 5% ~ 20% –– Fluke 434
–– RPM 901
Environment
11 Temperature 20 ºC –25 ºC best with 23 ºC
12 Humility 40% ~ 60% best with 50% Testo 615
13 Air dust The particle quantity should be less than 45,000
pieces within each cubic fee for those particle size
> O.5 Micron
14 Vibration Less than 0.5 g
15 EMI Less than 0.5 V/M,
Frequency range: 14 kHz ~ 1 GHz
ESD process
16 ESD floor, desk, chair, mat, Floor resistance > 150 KΩ, < 20,000 MΩ. Refer to
cloth, hat, shoe, wrap, bag, specific product spec, the surface resistance are
transit box, etc. between 103 to 1010 Ω, please refer to product
data sheet.
17 ESD fan By necessary
18 ESD charge measurement For specific product, it may be able to just endure –– ACL300
several volts ESD only –– ME-2B2A

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This information is subject to change without notice. © Keysight Technologies, 2014 - 2019, Published in USA , February 22, 2019, 5989-9200EN