Fluke 481, 1621, 1630 Service Guide

Principles, testing methods and applications

EARTH GROUNDING
RESISTANCE

EARTH GROUNDING
RESISTANCE

EARTH GROUNDING
RESISTANCE

EARTH GROUNDING
RESISTANCE

DIAGNOSE intermittent electrical problems
AVOID unnecessary downtime
LEARN earth ground safety principles

Why ground, why test?

Why ground?

Poor grounding not only contributes to
unnecessary downtime, but a lack of good
grounding is also dangerous and increases
the risk of equipment failure.

Without an effective grounding system,
we could be exposed to the risk of electric
shock, not to mention instrumentation errors,
harmonic distortion issues, power factor
problems and a host of possible intermittent
dilemmas. If fault currents have no path to
the ground through a properly designed and
maintained grounding system, they will find
unintended paths that could include people.
The following organizations have recom­mendations and/or standards for grounding
to ensure safety:

• OSHA (Occupational Safety Health

Administration)

• NFPA (National Fire Protection Association)

• ANSI/ISA (American National Standards

Institute and Instrument Society of America)

• TIA (Telecommunications Industry

Association)

• IEC (International Electrotechnical

Commission)

• CENELEC (European Committee for

Electrotechnical Standardization)

• IEEE (Institute of Electrical and Electronics

Engineers)

However, good grounding isn’t only for
safety; it is also used to prevent damage to
industrial plants and equipment. A good
grounding system will improve the reliability
of equipment and reduce the likelihood of
damage due to lightning or fault currents.
Billions are lost each year in the workplace
due to electrical fires. This does not account
for related litigation costs and loss of per­sonal and corporate productivity.

Why test
grounding systems?

Over time, corrosive soils with high mois­ture content, high salt content, and high
temperatures can degrade ground rods and
their connections. So although the ground
system, when initially installed, had low earth
ground resistance values, the resistance of the
grounding system can increase if the ground
rods are eaten away.

Grounding testers, like the Fluke 1623-2 and
1625-2, are indispensable troubleshooting tools
to help you maintain uptime. With frustrating,
intermittent electrical problems, the problem
could be related to poor grounding or poor
power quality.

That is why it is highly recommended that all
grounds and ground connections are checked
at least annually as a part of your normal Pre­dictive Maintenance plan. During these periodic
checks, if an increase in resistance of more
than 20 % is measured, the technician should
investigate the source of the problem, and
make the correction to lower the resistance, by
replacing or adding ground rods to the ground
system.

What is a ground
and what does it do?

The NEC, National Electrical Code, Article 100
defines a ground as: “a conducting connection,
whether intentional or accidental between an
electrical circuit or equipment and the earth, or
to some conducting body that serves in place of
the earth.” When talking about grounding, it is
actually two different subjects: earth grounding
and equipment grounding. Earth grounding is an
intentional connection from a circuit conductor,
usually the neutral, to a ground electrode placed
in the earth. Equipment grounding ensures that
operating equipment within a structure is prop­erly grounded. These two grounding systems are
required to be kept separate except for a con­nection between the two systems. This prevents
differences in voltage potential from a possible
flashover from lightning strikes. The purpose of a
ground besides the protection of people, plants
and equipment is to provide a safe path for the
dissipation of fault currents, lightning strikes,
static discharges, EMI and RFI signals and inter­ference.

2

What is a good ground
resistance value?

There is a good deal of confusion as to what
constitutes a good ground and what the ground
resistance value needs to be. Ideally a ground
should be of zero ohms resistance.

There is not one standard ground resistance
threshold that is recognized by all agencies.
However, the NFPA and IEEE have recom­mended a ground resistance value of 5.0 ohms
or less.

The NEC has stated to “Make sure that system
impedance to ground is less than 25 ohms
specified in NEC 250.56. In facilities with sensi­tive equipment it should be 5.0 ohms or less.”

The Telecommunications industry has
often used 5.0 ohms or less as their value for
grounding and bonding.

The goal in ground resistance is to achieve
the lowest ground resistance value possible
that makes sense economically and physically.

Table of
contents

2

Why ground?

Why test?

4

Why test? Corrosive soils.

Grounding basics

6

Methods of earth

ground testing

Why ground? Lightning strikes.

Use the Fluke 1625-2 to determine the
health of your earth ground systems.

12

Measuring

ground resistance

3

Grounding basics

Components of a
ground electrode

• Ground conductor

• Connection between the ground conductor

and the ground electrode

• Ground electrode

Locations of resistances

(a) The ground electrode and its connection

The resistance of the ground electrode and

its connection is generally very low. Ground
rods are generally made of highly conduc­tive/low resistance material such as steel
or copper.

(b) The contact resistance of the surrounding

earth to the electrode

The National Institute of Standards (a gov-

ernmental agency within the US Dept. of
Commerce) has shown this resistance to be
almost negligible provided that the ground
electrode is free of paint, grease, etc. and
that the ground electrode is in firm contact
with the earth.

(c) The resistance of the surrounding

body of earth

The ground electrode is surrounded by earth

which conceptually is made up of concen­tric shells all having the same thickness.
Those shells closest to the ground electrode
have the smallest amount of area resulting
in the greatest degree of resistance. Each
subsequent shell incorporates a greater area
resulting in lower resistance. This finally
reaches a point where the additional shells
offer little resistance to the ground surround­ing the ground electrode.

What affects the
grounding resistance?

First, the NEC code (1987, 250-83-3) requires a
minimum ground electrode length of 2.5 meters
(8.0 feet) to be in contact with soil. But, there are
four variables that affect the ground resistance of
a ground system:

1. Length/depth of the ground electrode

2. Diameter of the ground electrode

3. Number of ground electrodes

4. Ground system design

Length/depth of the ground electrode

One very effective way of lowering ground
resistance is to drive ground electrodes deeper.
Soil is not consistent in its resistivity and can
be highly unpredictable. It is critical when
installing the ground electrode that it is below
the frost line. This is done so that the resistance
to ground will not be greatly influenced by the
freezing of the surrounding soil.

Generally, by doubling the length of the
ground electrode you can reduce the resistance
level by an additional 40 %. There are occa­sions where it is physically impossible to drive
ground rods deeper—areas that are composed of
rock, granite, etc. In these instances, alternative
methods including grounding cement are viable.

Diameter of the ground electrode

Increasing the diameter of the ground electrode
has very little effect in lowering the resistance.
For example, you could double the diameter of a
ground electrode and your resistance would only
decrease by 10 %.

So based on this information, we should focus
on ways to reduce the ground resistance when
installing grounding systems.

4

Number of ground electrodes

Another way to lower ground resistance is to use
multiple ground electrodes. In this design, more
than one electrode is driven into the ground and
connected in parallel to lower the resistance. For
additional electrodes to be effective, the spacing
of additional rods need to be at least equal to
the depth of the driven rod. Without proper
spacing of the ground electrodes, their spheres
of influence will intersect and the resistance will
not be lowered.

To assist you in installing a ground rod that
will meet your specific resistance requirements,
you can use the table of ground resistances,
below. Remember, this is to only be used as a
rule of thumb, because soil is in layers and is
rarely homogenous. The resistance values
will vary greatly.

Ground system design

Simple grounding systems consist of a single
ground electrode driven into the ground. The use
of a single ground electrode is the most common
form of grounding and can be found outside your
home or place of business. Complex grounding
systems consist of multiple ground rods, con­nected, mesh or grid networks, ground plates,
and ground loops. These systems are typically
installed at power generating substations, cen­tral offices, and cell tower sites.

Complex networks dramatically increase the
amount of contact with the surrounding earth
and lower ground resistances.

Each ground electrode has its own
‘sphere of influence’.

Ground
systems

Single ground electrode

Soil

Type
of soil

Very moist soil,
swamplike

Farming soil, loamy
and clay soils

Sandy clay soil 150 50 25 15 60 30 15

Moist sandy soil 300 66 33 20 80 40 20

Concrete 1:5 400 - - - 160 80 40

Moist gravel 500 16 0 80 48 200 100 50

Dry sandy soil 1000 330 165 100 400 200 100

Dry gravel 1000 330 16 5 100 400 200 100

Stoney soil 30,000 1000 500 300 120 0 600 300

Rock 10

resistivity

R

E

ΩM 3 6 10 5 10 20

30 10 5 3 12 6 3

100 33 17 10 40 20 10

7

Ground electrode depth

- - - - - -

Earthing resistance

(meters)

Earthing strip

(meters)

Multiple ground electrodes
connected

Mesh network

Ground plate

5