IMS-SYSTEM-EARTHING-201204-GGv1
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Earthing Requirements for IMS Equipment
October 29, 2014
Document Number
IMS-SYSTEM-EARTHING-201204-GGv1
October 29, 2014
IMS-SYSTEM-EARTHING-201204-GGv1
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Contents
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List of Figures
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2
Earthing system using 2 grouted earth rods, and the metal casing
of the sensor borehole. . . . . . . . . . . . . . . . . . . . . . . .
6
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List of Tables
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Change record . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
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Introduction
While most electronic circuits will function without being earthed, for safety and
performance reasons proper earthing is essential, especially when dealing with
sensitive electronic equipment. Experience shows that improper earthing practices
can lead to latent problems which are difficult and costly to diagnose. These
problems can degrade or even damage the installed equipment.
Requirements
Here follows a list of guidelines which should be adhered to during site preparation for, and installation of, IMS equipment (failure to comply with any of these
requirements could void your equipment warranty):
• A professionally installed earthing system, providing a low impedance path
to earth (less than 50 Ohms), complying with all applicable codes and regulations should be provided. This is usually the green/yellow wire provided
by the utility, but do not assume simply because such a wire is present that
it complies - have this verified by a professional.
• All electrical connections (a.k.a. bonding) must be by compression connections, mechanical fittings, welding, bolting, or riveting. Twisting of conductors is not acceptable.
• All earthing conductors provided at the AC input and/or negative conductors
at the DC input of any IMS (or 3rd party) equipment must be bonded to the
earthing system.
• If any equipment is to be run off battery power, the negative terminal of the
battery should be connected to the earthing system, using a suitably sized
conductor.
• Ensure all external surge protectors are properly earthed, and that all 3rd
party equipment is installed according to manufacturer’s recommendations.
• All metal equipment parts such as enclosures, racks, raceways and conduits, equipment grounding conductors, and all grounding electrodes must
be bonded together into a continuous electrically conductive system.
An excerpt from IEEE Standard 1100-1999 "IEEE Recommended Practice for
Powering and Grounding Electronic Equipment", section 8.5:
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"Proper grounding techniques are necessary for safety, equipment operation, and performance reasons. The integrity of the facility grounding, and thus the integrity of proper equipment operation, depends on
proper bonding of the grounding electrode systems, proper system
grounding of service equipment and separately derived sources, and
proper equipment grounding for power-related frequencies as well as
higher frequencies. Recommended practice is for all grounding design and installation be compliant to all applicable codes and standards. Refer to the NEC and IEEE Std 142-1991 for proper safety
grounding techniques. Recommended practice is to utilize solidly
grounded ac supply systems and install insulated (non-bare) equipment grounding conductors in circuits supplying electronic load equipment... All grounding electrodes used for grounding of the power system, grounding of communications systems, and grounding of lightning protection systems shall be effectively and permanently bonded
to each other as required by the NEC and NFPA 780-1997."
Designing a Local Earthing System
In certain cases, e.g. for remote applications, or where the electrical distribution system does not provide an adequate earthing system, it may be necessary to
install a local earthing system for each seismic station. In such case, IMS recommends using one or more of the methods discussed below for realising the local
earthing system. Note: these are recommendations only, and do not supersede
those existing requirements imposed by law or otherwise; in fact a professionally
installed system is preferred. It is the customer’s responsibility to ensure that any
earthing system conforms to all applicable codes and regulations.
• If a concrete foundation is going to be laid, to be used as a mounting base
for the equipment enclosure, then it is recommended that a grid of steel reenforcing rods be included at the base of the foundation, to form part of
the earthing system. The rods should be at least 0.5 inches in diameter, and
the foundation must be in direct contact with the earth (no plastic moisture
barriers). A suitable conductor should be electrically bonded to the grid
and left protruding from the foundation for easy connection to the equipment earth. Additionally, an earth rod should be installed near the concrete
base, and electrically bonded with the grid by means of the aforementioned
conductor.
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Figure 1: Earthing system using 2 grouted earth rods, and the metal casing of the
sensor borehole.
Figure 2:
• If the sensor is installed in a borehole within a couple of meters of the equipment enclosure, and the borehole has a metal casing, then the casing can be
used as an effective earth electrode provided that the casing makes good
contact with the surrounding rock, and that the casing extends at least 2
meters below the collar. An additional earth rod is also recommended.
• If neither a conductive foundation or borehole casing is available, then 2
earth rods should be used.
Figure 1 shows a diagram of an earthing system consisting of 2 earth rod electrodes and a metal borehole casing. Figure 2 shows how the metal re-enforcing of
a foundation can be used together with an earth rod electrode.
Note on Earth Rod Electrodes
Earth rod electrodes are usually constructed from copper, copper-clad steel, or
electrically conductive galvanized rods. The material used for the earth electrode
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is important, as it should be electrically conductive and resistant to corrosion.
Aluminum should not be used as it is prone to corrosion when buried in soil.
Earth rods should be at least 20 mm diameter, and as long as practically possible
(up to 5 meters, no less than 1.5 meters), and can be driven into the ground or
cemented into a drilled hole where ground conditions do not allow driving the
rod. Cementing the rod into the hole is preferred.
Two earth electrodes, when connected electrically in parallel, will improve the
earthing system’s resistance to earth by up to 60% compared with a single electrode system. Take note though, that this is only true if the electrodes are placed
at least 2 times their buried depth apart, one being placed near the equipment to
be earthed.
October 29, 2014
IMS-SYSTEM-EARTHING-201204-GGv1
Date
2012/04/02
2014/10/29
Author
GG
GG
Change Control Record
Description
Original release
Updated document format
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Revision
0
1
Table 1: Change record
October 29, 2014