ERICO Practical Guide to Electrical Grounding - Manual (Page 80)

Practical Guide to Electrical Grounding
GROUND CURRENT INTERFERENCE
WITH CATHODE RAY TUBE (CRT)
BASED EQUIPMENT
Low frequency magnetic fields such as those associated
with the power system’s fundamental of 60 Hz and
harmonics from it, will sometimes be seen to interfere with
the normal deflection of the electron beam being used to
paint the image on the CRT’s screen. This magnetic field
interference is seen by the equipment’s operator as a wavy
or rippling display that is often very disconcerting to the
operator. (See Fig. 4-1)
One way magnetic fields of the type involved in this kind of
interference are created in grounding conductors is by any
continuous or nearly so, flow of current in externally
attached supplementary equipment grounding conductors,
grounding electrode conductors, structural steel members,
piping, ducting, cable trays, wireways, etc. Stray ground
currents in any of these items can produce the same effects
on the CRT’s screen.
Fortunately, the effects of these interfering magnetic fields
falls off exponentially with distance between the source of
the field and the equipment that is being affected. Also, the
orientation of the CRT to the lines of force of the magnetic
field affects the severity of the problem. Therefore,
increased spacing and reorientation of equipment is often
the first and a successful step, in the resolution of the
problem.
Another practical approach to reducing the effects of
magnetic fields on a CRT is to increase the number and
location of any grounding/bonding connections between
grounded items, including the one involved in the
interference. For instance, more bonding between cold
water piping, building steel, and grounding electrode
conductors often solves the problem. (See Fig. 4-2)
The foregoing procedure generally works since it breaks up
the currents from one conductor into several smaller ones.
In example, since the magnetic field surrounding a
conductor is proportional to the current’s amplitude, the
process of providing multiple paths for a current reduces
the current in any one conductor and therefore the stray
magnetic field being emitted from it. The best approach of
all however, is to find out how the unwanted current is
getting into the conductor and to fix the problem in
accordance with NEC requirements such as per Section
250-21, Objectionable Current On Grounding Conductors.
GROUND LOOPS
A formal definition of a ground loop that is very general is
provided in IEEE Std. 100-1991, IEEE Dictionary as
follows: . . . a ground loop is “formed when two or more
points in an electrical system that are nominally at ground
potential are connected by a conducting path such that
either or both points are not at the same potential.” While
this is a good general purpose definition, it is not
sufficiently specific for use when dealing with signal level
circuits and grounding connections. Therefore, a more
specific and useful definition as provided in this document
is as follows:
72
Structural
Steel
NEC 250-81 (b)
Water Supply (Street Side)
Ring Ground, NEC 250-81 (d) Rod/Pipe Electrode
NEC 250-83 (c)
Water
Meter
Bonding Jumper
NEC 250-80 (a)
Grounding Electrode
Conductor, NEC 250-94
Metal Underground
Water Pipe, NEC 250-81 (a)
(Must Be Supplimented)
To AC Service Entrance
Grounded Conductor (Neutral)
Water Supply (House Side)
Magnetic Flux Lines
Magnetic Flux Follows The
Easiest Magnetic Path From
One Pole To The Other
Magnetic Shield
Material
No Magnetic Field
Source Of Magnetic Field
Magnetic Field Shielding
Fig. 4-1
Typical Electrodes
Fig. 4-2
Grounding Book 4/14/99 10/5/99 6:02 PM Page 72 (Black plate)