The
concept of electrical bonding developed when we required connecting two or more
electrical conductors together. Electrical bonding is simply joining of two
conductors together. These may be two wires, a wire, and a pipe, or two pieces of equipment. Electrical Bonding
ensures that these two conductors which are bonded will be at the same
electrical potential. That means no current flow can take place between two
bonded bodies because they have the same potential.
What is Electrical Bonding?
Electrical bonding is a process in which
parts of an electrical circuit assembly or sub-modules within a system are
connected electrically by their joints or by any low-resistance bonding media
(Jumpers). Electrical
bonding is the process of connecting metallic objects that may be exposed to
electrical faults or induced voltages to the grounding conductors. This ensures
that in the event of a fault the current will have a low resistance path to
take to trip the over-current devices as quickly as possible, as well as
providing a path for static electricity and induced voltages to drain out.
In other
words, Electrical bonding is simply the act of joining two electrical conductors together. Bonding has to be done by connecting all the metal parts that are not supposed to be carrying current during
normal operations to bringing them to the same electrical potential. It ensures
that these two bonded things will be at the same electrical potential. That
means we would not get electricity building up between two different pieces of equipment.
The main purpose of electrical bonding is to
make a homogenous structure with respect to the flow of radio frequency (RF)
current, so that it would experience minimum barrier as it crosses one surface
to the other without developing electrical potential at the crossover point. Bonding
to electrical earth is used extensively to ensure that all conductors (person,
surface and product) are at the same electrical potential. When all conductors are at the same potential no discharge can occur.
Electrical bonding requirements
The main function of electrical bonding
is to create a homogeneous structure to allow smooth flow of RF current. So that
it will experience minimum resistance to the flow of current. It does not
create any potential at the joint or crossover point.
In order to ensure Electromagnetically
Compatibility (EMC) specifications the various components of a module or all
the modules of a system are connected to a common chassis or common reference
ground via low impedance electrical path that should provide near-zero impedance at all frequencies. This kind
of bonding provided to meet EMC, specifications are known as EMC bond and the process is known as EMC bonding.
Whenever there is the potential
for conductive metal parts which has to become energized, then it must be electrically
bonded to the ground. This includes conduction for wires and cables, raceways
and cable trays, and service equipment enclosures and junction boxes. Bonding
is also required for non-electrical equipment that is similar to our electrical
systems like ventilation ducts, water and gas piping, or stairs and handrails.
This is especially important in the areas where occupants will be able to come
in direct contact with the metal parts or areas where there is the potential
for explosive gases or dust to exist.
Good bonding is required for mounting
line-filter modules on the chassis that serves as a drain for EMI currents, or
for connector shells to equipment enclosures to ensure shielding integrity of
cable shields that are terminated on these connectors, or for ensuring
shielding integrity over seams and joints to avoid leakage of RF energy.
The main purpose of electrical bonding is to prevent voltage difference between two
parts being joined.
Why Electrical bonding is important?
One of the important uses of
electrical bonding is to reduce touch potential, especially in case of long
runs of conductive cable tray. Whenever voltage is applied to a conductor, it can
be a wire or anything else metallic body, there will be some residual voltage
drop across the length of the conductor based on its size, material, and
length. Even highly conductive copper and aluminum has some resistance. Then
due to this resistance based on the distance from the nearest grounding connection, the small potential difference can occur. If there is a difference between a cable
tray and a nearby staircase, then someone who touches both the metal objects at
once, they might experience an electrical shock. More frequently electrical
bonding connections will reduce the potential between nearby metallic objects
and it decreases the chance of unintentional electrical shocks from static or
induced voltages.
On large industrial sites,
potentially flammable or explosive fluids or gasses are available inadequate
amount. Even a small static electrical spark can be disastrous. Hence care must
be taken to ensure that electrical bonding connections are secure and frequent
enough to minimize the risk of fire or explosion.
Residential swimming pools and
fountains can also be susceptible to touch potential differences, especially
when water meets metal surfaces. Metal parts of pools, hot tubs, and fountains
must be bonded to earth to reduce the risk of electrical shock anyone who might
come in contact with these surfaces, especially when the humid environment
increases the chances of providing a path to ground through a person’s body.
Types of Electrical bonding
To
perform bonding at DC or low power
frequencies (i.e. 50 Hz or 60 Hz), a simple but durable and permanent low
resistance joint can be adequate. To accomplish this kind of low DC resistance bond between
these two metal walls, holes can be drilled in the adjoining parallel walls and
the two cabinets can be bolted together with the star washers at the points of
contact between the walls.
These
bonds are created by using bolts and star washers at the point of contacts
between the two walls. But such a bond is not suitable at high RF or Microwave
frequencies due to its high resistance and inductive reactance.
Electrical bond involves dissimilar metals where non-linear
junctions can develop at the bonds, leading to the generation of harmonics and
spurious, especially under the strong RF fields, which causing interference.
To
reduce the above problems, a radical approach is required if the bond is to work at high
frequencies. The most effective method is to divide the bonding process into two categories as mechanical
and electrical bonding.
Mechanical
bonding ensures that an appropriate robust strength is being applied to the
parts which have to be joined. Whereas Electrical bonding ensures that a low
impedance path between parts to be joined which make them electrically robust.
Electrical
Bodings are classified into two types as Direct
bonding and Indirect bonding.
1. Direct Bonding:
In this type of electrical bonding, a specific portion of surface areas of
members to be joined are placed in direct contact by permanent or
semi-permanent bonds. Permanent bonding is carried out by using welding,
soldering or brasing. Whereas Semi-permanent bonding is carried out using
bolted connections. For a satisfactory result of direct bonding, bolt/screw will
serve as faster and pressure of about 90 to 100 Kg/Cm2 need to be maintained.
2. Indirect Bonding:
This type of bonding is used in such
applications where metal to metal contacts are not reliable to be used such as
where parts are frequently removed, parts which use dissimilar metal types,
parts which are exposed to corrosion or parts which will have relative motion
such as hinges. Here in this type of electrical bonding, other than the primary
bond with the help of mechanical joint, indirect bonding is being employed with
the help of straps or jumper wires.
Jumpers are used for
low-frequency or Power frequency (50 or 60 Hz) scenarios and can be shorted or
stranded conductors, generally round in cross-section. These jumpers exhibit
self-inductance and residual capacitance in the electrical circuit at high
frequencies due to the skin effect phenomena. In such cases, sheet-metal straps
(called bond-straps) or special flat braids are used. For sheet-metal straps to
work effectively, its width-to-thickness ratio should be maintained ten or
more, while the length-to-width ratio should not exceed five.
Electrical Bonding of Earth wire
In our
typical electrical and electronics system, a protective earth wire (green or
green-yellow wire) is used for safety purpose to connect with the external
earth.
As
shown in below Fig., this is usually done by providing a bonding terminal essentially
a bolt screwed to the position and held in place by a captive nut or a stud
welded to the enclosure cabinet. While fixing the earth wire, any insulating
surface finish (paint or powder coating) is removed from the contact area.
The contact area of the cabinet should be slightly larger than
the surface area of the lug. A shake-proof or spiky washer is first put into
place to ensure a good lifetime of bonding. Then after over this, the lug is
placed. Another spiky washer is then placed over the lug after which the nut is
tightened. If the bonding is likely to be exposed to moisture or corrosive
environment, a coating of paint or grease is applied over the bond to make it
free from corrosion.
Electrical bonding behavior at RF
At
direct current (DC) or power frequency (50 or 60 Hz) the electrical bonding
resistance should be less than one milli-ohm. The figure below represents the behavior
of electrical bonding at Radio Frequencies (RF). Here due to parasitic effects, an electrical bond is not purely resistive at RF. In addition to resistance there
is self-inductance in series and residual capacitance in parallel of the
bonding circuit.
 |
| Electrical bonding behavior at RF |
This circuit
acts as a parallel resonance circuit. So the circuit provides very high
impedance at higher frequencies. Due to skin effect at high frequency, there
becomes a flow of current via the outer peripheral of the conductor. Hence due to this
bond offers very high resistance.
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