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Product category: Connectors, Terminals, Busbars and Slip Rings
News Release from: Harting | Subject: Connectors
Edited by the Engineeringtalk Editorial Team on 03 July 2000

Better EMC protection with connectors

This article by David Franklin illustrates the basics and the importance of shielding effectiveness in industrial connectors.

Electromechanical components are not included in the EU legislation concerning EMC, although many of the end-users of these components - i.e the equipment manufacturers - need CE-EMC certification for their applications Electromagnetic compatibility (EMC) is defined as the 'ability of an electrical unit to function satisfactorily in its electromagnetic environment without influencing this environment - to which other units may belong - in an undue way' (from the DIN VDE 0870 specification)

Thus an electrical installation is compatible when, as a source of interference it produces tolerable emissions and as a receiver, shows a tolerable immunity against interference.

To meet these criteria, the equipment must have sufficient shielding capacity.

The shielding efficiency is affected by a number of factors, including the connector's hood and housing, its cable gland, and the overall cable installation.

Ideally, screened components should incorporate continuous and homogeneous shielding.

In practice, however, there will always be weak points in pre-assembled cables.

The critical points for screening in an industrial connector are the contact points between bulkhead-mounted housings and switch cabinet panels, iris springs and cable screens, cable glands and hoods, and the gaps between housings and hoods.

At any of these interfaces, where the homogeneous shielding is interrupted, interference power may enter the system.

To ensure proper operation, the degree of interference immunity (i.e the shielding effectiveness) has to be high enough for the transmitted signals inside the connector not to be interfered with by the surrounding electromagnetic fields.

In addition, the active interference potential of the connector has to be so small as not to interfere with other installations or components in its environment.

The term 'electromagnetic interference' includes all electromagnetic phenomena that have an impact on the performance of a component, a device or an installation.

It includes electromagnetic impulses, drop-ins or a spontaneous increase in the propagation mode.

These electrical signals may overlay and interfere with the usable signal.

Electromagnetic interference consists of the electrical field component E as well as the magnetic field component H.

The electrical field is generated by a potential difference, and the magnetic field is generated by an alternating current in an electrical conductor.

The whole spectrum of frequencies from low frequency (a few hertz) up to high frequency (gigahertz) is potentially a source of electromagnetic interference.

Sources of interference can be divided into 'functional' and 'non-functional, which basically refer to intentional and non-intentional transmission.

Functional sources include communications transmitters, generators for industrial or medical applications, mobile phones, radar sources, manufacturing processes and microwave ovens.

Non-functional sources include automobile ignition, fluorescent lamps, welding equipment, contacts in connectors, relays and contactor coils, electrostatic discharges, static convertors, switching actions in high-voltage networks, devices with clock frequency generators and any equipment producing abrupt changes in voltage and current.

Sources of electromagnetic energy are generally classified according to the frequency range of their emitted spectrum.

Interference in the low-frequency spectrum occurs mainly as conducted interference (via cables etc.) and falls within the frequency range 0-30 MHz.

High energy levels can lead to functional disturbances and even the destruction of equipment.

Interference in the high-frequency spectrum occurs mainly in the radiated state, over a frequency range from 30 MHz up to the gigahertz range.

Energy levels are generally low, but they can still cause functional disturbances in equipment.

_ It is possible to differentiate between four types of coupling: Galvanic coupling: In connectors, galvanic coupling occurs mainly in the earth circuit.

In pre-assembled cables all earth wires are normally connected via common electrical conductors to the earth potential and consequently to the protective earth.

The electrical conductor might be the switch cabinet panel.

This is the way in which interference may be transmitted to the connector.

Inductive coupling: This occurs when an alternating current flows over the housing, the cable gland and the cable screening, and generates a magnetic field.

This magnetic field is variable, and induces an interference voltage.

Capacitive coupling: Between cable conductors as well as between the conductors and the connector housing, there exists a permanent capacitance.

Since there are two conductive elements with a variable potential difference, an electrical current may be induced via the insulation medium (air), leading to interference problems.

Radiated coupling: Where there are high transmission frequencies and 'long' cable runs occur, electromagnetic waves may be emitted from the conductor (antenna principle).

A suitable receiving medium can 'catch' these electromagnetic waves and thus generate an interference voltage or current on top of the transmitted signals.

However, the energy level is normally quite low.

The coupling parameters can be measured by means of the line-injection (parallel-wire) method, according to the VG 95214 specification.

This is a standardised measuring method that determines the correct, reproducible, and thus comparable, results of screened components with respect to the transfer impedance.

The test set-up is designed as a 3-wire system, where the internal receiver line, the screening and the transmitting line injection wire each represent one conductor.

The shielding efficiency of a housing can be represented by the values of shielding effectiveness (as) and the transfer impedance (ZT).

The shielding effectiveness is defined as the ratio of the power radiated within the component to the maximum resulting interference power outside the component in the environment.

It is specified as a logarithmic ratio, and is referred to as the shielding effectiveness ratio (specified in the VG 95214 standard).

The transfer impedance is the ratio of the induced interference voltage between the outer conductor and housings to the interference current applied to the screen.

The transfer impedance is a component screening parameter (specified in the VG 95214 standard).

A range of connector hoods and housings that have been designed especially for EMC applications.

These EMC hoods and housings have high screening values because of two design features: first, the well-developed labyrinth structure; and, secondly, the extensive overlapping contact between hood and housing.

As a result, these hoods and housings achieve a screening attenuation of 60dB at a frequency of 10 MHz, compared to a value of 40dB for the standard industrial connector products.

If the contact between the cable screening and connector hood is realised by an EMC cable gland, there is a higher screening attenuation (by approximately 6-15 dB) than if the connection is to a PE contact.

This is because the iris spring allows a complete 360deg contact, and the low transfer resistance allows optimal flow of unwanted surface currents (on cable braids, for example).

If, on the other hand, the connection of the cable shielding is to a PE contact, the shielding effectiveness is reduced.

This is because the shield braid has to be opened, which weakens its effectiveness.

The result is a higher impact on the signal integrity.

There is a main precondition for a so-called 'optimal system': all the individual components should have the same shielding effectiveness.

With such a system, over-engineering is avoided, and total system costs are kept to a minimum.

The shielding effectiveness of a pre-assembled cable is only as good as the shielding effectiveness of the 'weakest' component.

The ideal situation is where the characteristic of the cable approaches that of the EMC connector, so that both components are equivalent in terms of shielding effectiveness.

This results in a system of high quality, provided that the connection of the screening between cable shield and housing shield is optimal.

One source of interference that is very common in practice is the discharge from an electrostatically charged person touching an electronic component.

Such an electrostatic discharge (ESD) impulse also forms the basis of a test on the shielding effectiveness of connectors.

The results of these tests show that EMC connectors with a high shielding effectiveness can reduce the interference voltage over significant transmission distances.

In practice, using an EMC connector in combination with a cable with a lower screening quality does not necessarily mean an improvement in the total shielding effectiveness.

Factors that have an important influence on the shielding effectiveness include the cable gland, the installation configuration, and corrosion effects in the gland and in the screw thread.

All these factors can lead to a reduction in the shielding effectiveness of the entire system.

Once again, it is important to remember that the weakest link in the chain can undermine the shielding effectiveness of the entire system.

Frequency converters are a well-known source of electromagnetic interference in both conducted and radiated forms.

Manufacturers of frequency converters often advise that wiring continuity should not be interrupted by installing connectors.

However, it is exactly this feature that the customers generally require.

There is an attitude that connectors have a negative influence on the electromagnetic compatibility of frequency converters because of insufficient shielding effectiveness.

This has led to tests being carried out with the aim of quantifying the electromagnetic compatibility of connectors.

The basis of the test set-up rests on the fact that, in a frequency-converter application, the short rise times of the rectangular signals are a cause of high-frequency disturbance signals.

The radio interference transfer was measured according to the European Standard EN 55022/EN 55011.

The whole system is tested with a view to finding out whether the interference transfer (in conducted or radiated form) is influenced in any way by the connectors.

In fact, the comparison shows that the radio interference transfer of the whole system is not influenced, if the EMC connector is assembled to ensure optimum electromagnetic compatibility.

The EMC housings feature a chromatised, corrosion-resistant surface.

Using these parts in combination with an EMC cable gland and an iris spring ensures very good low contact-resistance values.

This ensures a low resistance connection of the cable braid of the motor wiring to the reference ground.

The 'worst-case scenario' for using a connector in a frequency converter application occurs when a standard connector with a painted insulating surface is installed.

If the screening current cannot flow off the housing unhindered (i.e because of corrosion or inadequate construction), the surface currents are interrupted.

Furthermore, a screening connection via a PE contact in the connector has disadvantages.

It is important to remember that an insulating mounting surface and metal screws that isolate the reference ground lead to a very high impedance connection of the connector to the reference ground.

However, even in this 'worst case', the limit for industrial applications is only slightly crossed in the range from 45 MHz to 65 MHz.

In an aggressive industrial environment, corrosion can occur.

In the worst case, these corrosive layers can lead to electrical insulation of the connector mounting screws.

Hence, the connection of the shielding braid of the shielded motor wire to the earth potential has high contact resistance.

In order to ensure that the internal and external electro-magnetic compatibility is maintained, it is advisable to use specially designed EMC connectors in frequency converter applications.

These EMC connectors guarantee a high-integrity, highly conductive connection to ground (the protective earth), and the connection of the shielding braid of the motor wires to the reference ground is achieved with very low impedance.

The corrosion-resistant surface minimises any problems caused by corrosive effects.

Assembly faults (with respect to EMC) are minimised.

As a result, the screened motor wire of a frequency convertor can be established via a Harting EMC connector without influencing the conducted or radiated interference transfer in a negative way.

This article illustrates the basics and the importance of shielding effectiveness in industrial connectors.

It is important to realise that the shielding effectiveness is a function of the design of the overall system.

EMC cable glands give a considerable increase in shielding effectiveness values, even when used with standard housings.

In addition, because of their higher shielding effectiveness values (up to 78 dB at 1 MHz), EMC hoods and housings allow further optimisation of the total system.

The efficacy of this approach has been evaluated in Harting's EMC test laboratory, which is accredited to DIN EN 45001.

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