Electrical Bonding

The concepts 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 equipments. Electrical bonding ensures that these two conductors that are bonded together will be at the same electrical potential. That means no current flow will takes place between two bonded bodies because they have same potential.

What is Electrical Bonding?

Electrical bonding is a process in which parts of an electrical assembly or sub-modules within a system are connected electrically by their joints or by any low resistance bonding media (Jumpers). Electrical bonding is a process of connecting metallic objects that may be exposed to electrical faults or induced voltages to the grounding conductors. This ensures that in the events of 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 voltage 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 bounded 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 homogeneous 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 use extensively to ensure that all conductors (Person, surface and product) are at the same electrical potential. When all the conductors are at the same potential then no discharge will 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 doesn’t create any potential at the joint or crossover point.

In order to ensure electromagnetically compatibility (EMC) specification the various components of a module or all the modules of system are connected  to a common chassis or common reference ground via low impedance electrical path that should provide non-zero impedance at all frequencies. This kind of bonding provided to meat 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 equipments that is similar to our electrical system like ventilation ducts, water and gas piping, or stairs and handrails. This is especially important in the areas where occupant will be able to come in direct contact with the metal parts or areas where there is a potential for explosive gas or ducts to exit.

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 the touch potential, especially in case of long runs of conductive cable tray. Whenever voltage is applied to the conductor, it can be a wire or anything its metallic body, there will be some residual voltage drop across the length of the conductor based on its size, material and length. There is some resistance even in highly conductive copper and aluminum. 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 connection will reduce the potential between metallic objects and it decreases the chances of unintentional electrical shock from static or induced voltages.

On large electrical sites, potentially flammable or explosive fluids or gases are available in adequate amount. Even a small static spark can be dangerous. 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 can also be susceptible to touch potential differences, especially when water meats metal surfaces. Metal parts of pools, hot tubes, 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 the ground through a person’s body.

Types of Electrical bonding

To perform bonding at DC or low power frequencies (i.e. 50 or 60 Hz), a simple but durable permanent low resistance joint can be adequate. To accomplish this kind of low resistances, a bond between these two metal walls, a hole can be drilled in the adjoining parallel walls and the two cabinets can be bolted together with the star washers at the point of contact between the walls.

These bonds are created by using bolts and star washers at the point of contact 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 b0nd involves dissimilar metals where non-linear junctions can developed at the bond, leading to the generation of harmonics, especially under the strong RF fields that causing interference.

To reduce the above problem 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 bonding 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 that make them electrically robust.

Electrical bondings 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 member to be joined are placed in direct contact by permanent or semi-permanent bonds. Permanent bonding is carried out by using welding, soldering, or brazing. Whereas semi-permanent bonding is carried out by 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 needed 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 that are 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 circuits at high frequencies due to skin effect phenomena. In such case a sheet-metal strap (called bond strap) or special flat braids are used. For sheet-metal strap to work effectively, its width to thickness ratio should be maintained at 10 or more, while the length to width ratio should not exceeds 5.

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 earth.
As shown in the below fig. this is usually done by providing a bonding terminal essentially a bolt-screwed to the position and held in place by 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 the corrosion.

Electrical bonding behavior at RF

At direct current (DC) or power frequency the electrical bonding resistance should be less than one milli-ohm. The below figure represents the behavior of electrical bonding at radio frequency (RF). Here due to parasitic effect, an electrical bond is not purely resistive ar RF. In addition to the resistance there is a self-inductance in series and a 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 higher frequency, there become a flow of current via the outer surface of the conductor. Hence due to this, bond offer very high resistance in the circuit.

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