Electromagnetic Compatibility: Bonding (Part 4 of 5)

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Bonding of internal protective earth wires. Earth conductors of modules housed in non-metallic enclosures, installed inside a cabinet, have to be connected to local ground reference or mounting plates or chassis. In such cases, a captive nut-and-screw arrangement is useful as shown in Fig. 24. The captive nut is welded on the other side of the mounting plate, and the earth conductor with a lug is fixed by a screw that slides through this nut.

Bonding of protective wire
Fig. 24: Bonding of protective wire

Bonding in electronic cabinets. In any electronic cabinet, direct metal-to-metal contacts/bonds give best RF performance since the contact is over the entire surface rather than a single point. For this reason it is imperative to have local RF reference in the form of a metal plate or sheet, and not a single bolt or stub, as shown in Fig. 25.

Electrical bonding in electronic cabinet
Fig. 25: Bonding in electronic cabinet

The metal plate exhibits low inductance and, hence, provides low impedance at RF frequencies. All modules having a metal enclosure such as filters, switched-mode power supplies and drives are directly installed on this mounting pate, allowing contact over the entire mating surface.

The plate can be connected to either the protective earth coming from the utility or to a dedicated facility earth, the latter being more useful in severe environment where protective earth may already be polluted. If protective earth has to be extended to the modules via, say, a dual-in-line (DIN) rail-mounted terminal block, then it can be done by a separate wire to comply with requirements of electrical safety standards.

Earthing can be extended to hinged doors by using bond straps (braided or sheet metal). For more effective earthing of metal doors, EMI gaskets can be installed over the mating surface at the periphery.

Bonding screened cables with connectors. Screened cables are many a time used to bring signals from the external world to modules inside a cabinet. When these enter the cabinet, either through a connector or otherwise, their shields need to be connected to the local RF reference (which can be the mounting plate or metal enclosure of the module, or even the PCB reference plane) through a low-RF impedance bond.

When shielded connectors with metal or metal covers such as D-type connectors are used, cable shields need to be bonded to the connector body. The simplest way is to use a semi-circular or saddle clamp. As shown in Fig. 26 (a), the shield is exposed in such a way that the exposed surface area is just a shade greater than the clamp surface area.

Care should be taken that curvature of the clamp matches that of the cable to ensure total contact over the entire mating surface. The clamp, when tightened, exerts a small pressure over the shield. The clamp is then fixed on the connector shell at two points. Since the saddle clamp is semi-circular, it may not provide full 360° and, consequently, the connector can be used in situations where shielding-effectiveness requirements are not that high.

The best way to have a true 360° contact as well as to provide good strain relief is to use a circular metallic connector, where the shield is in contact with the connector outer shell over a full circle and a separate provision is made for strain relief. Such connectors give the best RF performance.

As shown in Fig. 26 (b), the outer shell comes in two parts: one that clamps the cable jacket to provide the necessary strain relief and the other that clamps the exposed shield and connects it to RF reference or the module enclosure. The shield is connected to the metal body of the connector via a special EMI gasket called IRIS compression ring, which is a part of the connector back-shell and encircles the cable at the shield.

Electrical bonding of shield to connector
Fig. 26: Bonding of shield to connector

Advantage of using IRIS is that the shield only has to be trimmed and slipped into the connector without disturbing the over-shield braid structure.

Bonding screened cables that have no connectors. In cases where shielded cables do not employ connectors, it is imperative that the shield be bonded to the mounting plate (RF reference) at a point closest to the module or even to the module body itself (which is, in turn, bonded to the mounting plate). There are various methods suggested by manufacturers for bonding the shield to RF reference. Those methods where the exposed shield is bonded to the reference by a separate wire or where shield itself is twisted and bonded to the reference (called pigtails) are generally not recommended since their RF performance is dismal.

Pigtails are typically worse by more than 35dB for frequencies above a few megahertz than other methods. A better method is to employ P-clips, which clamp around the exposed shield and bond to mounting plate (or module body) at a single point. Saddle clamps provide an even better method where the shield is bonded to the clamp, which is, in turn, bonded to the mounting plate at two points.

Conclusion

In the previous and current parts of the article, we have seen various methods of grounding and bonding. These methods are not only important themselves but also contribute in a big way in increasing the efficiency of other design methods to ensure EMC like filtering and shielding.

There is no denying the fact that these measures have to be taken into consideration before going for fixes like filtering and shielding as these are design measures and do not cost much, if taken care of before freezing equipment design. These can further reduce cost of filters and shielding measures by reducing their performance requirements and thus contribute in reducing the overall cost of the equipment.
To be continued…


Chetan Kathalay is working as scientist in Electronics Test and Development Centre, Pune. He is BE in electronics from Nagpur University

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