Control panels: Solutions to protect equipment from electromagnetic interference
In severe industrial environments, when rigor is required, the reliability of the automation depends on the level of disturbance.
The electrical panel must be installed to limit electromagnetic interference as much as possible while respecting EMC (electromagnetic compatibility) rules.
Schneider Electric offers adapted solutions to make the implementation more reliable.
Presentation
Industrial workshops are places where the concentration of electromagnetic disturbances is often high:
- In the metallurgical industry, the electrical power required generates very high magnetic fields near electrolytic cells and induction furnaces.
- The workshops for manufacturing PVC or rubber parts use high-frequency welding processes to make the assemblies.
The propagation of strong magnetic fields and high frequency waves is not easily controlled.
It creates local pollution in the midst of which the control and command equipment must be able to operate.
Multiple means exist to ensure the ElectroMagnetic Compatibility of the process (EMC: ability to operate in a disturbed environment).
To obtain optimal performance, strict rules apply at all levels:
- building earth network,
- communication cables, sensor cables,
- control and command boards.
Control panels
Electromagnetic disturbances are present everywhere, right down to the heart of control panels. Their effects are difficult to predict.
Electromagnetic disturbances are potential sources of malfunction for all electronic equipment:
- regulators, measuring devices, processing analog signals,
- PLCs, communication interfaces, processing digital signals.
These disturbances are difficult to detect because they are fleeting and only appear under certain conditions.
Compliance with design rules is therefore essential to avoid these problems.
Disturbances conducted by sensor cables
Disturbances of different frequencies are superimposed on the original signal. It thus becomes difficult to “understand” by the equipment that receives it, and its treatment becomes difficult if not impossible.
Examples of configuration:
• the cable runs alongside another highly disturbing cable (speed variator / motor link for example),
• the cable is not shielded,
• the shielding is not correctly connected (eg: circulation of “stray” currents caused by the earthing of the 2 ends of a screen, in particular in the case of a TNC network (neutral to earth and earth to neutral).
Disturbances radiated by equipment
Treatment equipment is disturbed by a beam of high frequency waves: it stops suddenly, resets itself for no apparent reason, or gives abnormal results.
However, the input signals are correct.
This type of situation can occur when the controller is grounded incorrectly:
wire too fine, connection too resistive (paint present at the connection point).
The controller enclosure or an electrical panel with too many “windows” can also be the cause.
Some equipment (converters, current choppers, etc.) incorporate oscillators whose voltage, frequency or signal shape makes them capable of emitting disturbances over varying distances, inside or outside the control panel.
The enclosure of the equipment and its connection to the ground of the switchboard are essential to limit radiation.
Design of switchboards according to EMC rules
The electromagnetic compatibility (EMC) of a switchboard represents its ability to operate in a disturbed environment while limiting its own disturbing emissions.
The search for overall performance will go through:
• the reduction of disturbances at the source which could also be external to the switchboard,
• protection of information exchanged with the process throughout its progress including in the switchboard,
• preservation of the input panel from radiated and conducted disturbances.
Role of the earth network: safety and equipotentiality
The regulations relating to the safety of persons impose the equipotentiality of the metallic masses of all the equipment of a building.
Power and IT equipment are therefore all linked to the building’s single earth network.
Thanks to its interconnected meshes, this network also shields pollution by high frequency (HF) waves.
The connection points distributed symmetrically around the building balance the impedance of the earth link.
However, even on installations in perfect condition, a current flow of 50 Hz is observed on certain earth conductors (stray current).
It can reach up to several amps under a few millivolts if the conductor is long enough.
This current can disturb the low-current analog connections (0-10 V sensor lines, etc.) if they are wired without precautions. The digital links are little affected.
Low current signal protection
Use shielded cables
Analog sensor signals and data streams are sensitive to interference.
Shielded cables are used to route them. These cables are also used to create less disturbing drive – motor links.
The shielding consists of:
• a braid, an effective barrier for frequencies up to a few Megahertz,
• a strip, theoretically effective beyond these frequencies but easily degradable during handling,
• a strap and a braid for mechanically robust broad spectrum protection.
Connect the shield to ground
This connection allows earth disturbances to be removed.
The choice of connecting one or both ends to earth favors protection against low or high frequencies (HF):
• on one side, 50 Hz stray currents cannot flow, medium HF protection,
• on both sides, possible presence of 50 Hz current but the barrier against higher frequencies is reinforced.
Filtering
Reduce the conduction of disturbances
Some equipment generates disturbances on their upstream or downstream links: variable speed drives, frequency converters, switching power supplies, etc.
The most appropriate treatment is to place a filter on the disturbed line, as close as possible to the polluting equipment.
The characteristics of the appropriate filter are given by the manufacturers as a function of the voltage, the current in the line and the frequency of the disturbances to be reduced.
Filters
They integrate passive components (inductors, capacitors).
Their metal case is fixed to the earth plate, at the bottom of the switchboard.
The contact surface must be free of paint and insulation, and be maximum.
Filter ferrite
Crossed by the polluted wire, the ferrite ring or tube constitutes an effective filter against high frequencies, it is often used to reduce the disturbances of low level cables.
Several passes of the wire in loops around the ferrite ring increase the attenuation (while preventing the ring from slipping if the wire is thin).
Characteristics of an EMC compliant switchboard
Disturbance mitigation characteristics
If an electrical panel without a hole stops radio waves by a “Faraday cage” effect, the magnetic radiation is attenuated by the sheets depending on the nature of the material.
The greatest attenuation is obtained with Aluzinc sheets.
It is important to know the required attenuation for the two effects (electric field and magnetic field) in order to choose the appropriate table properly.
AluZinc / steel attenuation comparison
Installation of equipment in a switchboard
Importance of implantation
The careless juxtaposition of high current and low current equipment, the routing of cables of different types in the same trunking predispose to severe malfunctions.
Taking into account the rules described below from the design stage will avoid tedious troubleshooting, the installation of filters a posteriori, or even reworking of installation and cabling.
To separate
Specializing the switchgear by power class is the most effective measure to obtain an excellent “EMC” result.
In addition, the separate routing of disturbing and sensitive cables guarantees minimal coupling.
A metal duct ensures the equipotentiality of the switchgear and efficient conduction of LF and HF disturbances.
Partition
The partitioning of the table into two zones:
• power,
• low level,
is an alternative.
A metal partition will further improve EMC by confining each area.
For delicate situations
In general, contactors should be kept away from electronic devices.
It is recommended to equip contactors and relays with anti-interference filters.
Highly disturbing equipment (variable speed drive, frequency converter, etc.) will have less radiation in the switchgear if it is “encapsulated” in a small metal box that is electromagnetically sealed and unpainted.
The box will be carefully connected to the base plate (ground plane).
Reference texts and standards
The devices: radiation and radiation resistance
European directive
CEM 2014/30 / EU
It relates to all electrical and electronic equipment placed on the market or in service, excluding those subject to a specific directive (eg: medical devices).
These must be designed so as not to generate electromagnetic disturbances liable to disturb the operation of other devices.
• They must also be able to function properly within the framework of the intended use (environment, mode of supply, etc.).
The directive is based on IEC international standards:
• for the industrial environment,
IEC 61000-6-2 (immunity),
IEC 61000-6-4 (emission);
• for residential, commercial and light industrial environments,
IEC 61000-6-1 (immunity),
IEC 61000-6-3 (emission).
Facilities: main rules
International
IEC 61000-5-2: Electromagnetic compatibility (EMC) Part 5: Installation and mitigation guide.
Section 2: Grounding and Wiring.
IEC 61000-5-6: Electromagnetic compatibility (EMC) Part 5: Installation and mitigation guides
Section 6: Mitigation of external electromagnetic influences.
IEC 60364-4-44: Electrical installations of buildings Part 4-44: Protection against voltage disturbances and electromagnetic disturbances.
European
EN 50174-2: Information technology – Installation of cabling Part 2: Planning and practices
installation inside buildings.
EN 50310: Application of equipotential bonding and earthing in rooms with Information Technology equipment.
French
Guide UTE C 15-900: Installation in buildings of power networks and communication networks.
NF C90-480: Application of equipotential bonding and earthing in rooms with Information Technology equipment.
NF C 90-480-2: Information technologies – Installation of cabling.
Assembly – wiring of switchboards
Table assembly
Installation of boxes receiving polluted cables
Positioning of filters, power supplies
These components are installed in the board.
When it is assumed that their connection to the outside will be via a polluted cable, then the boxes should be arranged so that only a minimum length of cable can enter.
Thus, the radiation of HF disturbances will be reduced.
External cable routing – entry into the switchboard
External cable routing – entry into the switchboard
Keep high current cables away from low current cables:
• separate ducts if the cables are unshielded D = 5 cm,
• single trunking possible if the low current cables are shielded but with maximum spacing.
Reinforcement of the shielding efficiency of very disturbing cables
The radiation of a shielded drive / motor link cable will be further reduced if it runs in a closed metal duct or, better, in a metal conduit. Chute and tube are earthed at each end.
In the example below, shielded communication signals (*) whose shielding is connected on both sides are less sensitive than analog signals.
The least sensitive signals are placed on the same side as the most disturbing signals and vice versa.
(*) Classification of signals conveyed: from standards IEC 61000-5-2 and IEC 60364-4-44
Organization of cable entries
Group the cables by type of current:
• strong currents: power supply, PEN, etc. actuators,
• low currents (<100 mA): analog communication. Create specific entries in the table, one for each type of current. Screened cable entry
The use of metal 360 ° tightening cable glands ideally preserves EMC.
The cable shielding is connected to the ground of the switchboard over its entire perimeter without being interrupted.
It extends all the way through the internal cable to the terminal block, filter or variable speed drive where it is again connected to ground.
Grounding joints enclosing the shielding at the point of entry are an alternative to cable glands.
Cable circulation in the switchboard
To avoid :
Capacitive effects
Two cables side by side constitute the armatures of a capacitor.
The high-frequency components present in one (transient overvoltages, pulses, for example) pass through the other cable through the stray capacitance.
Inductive loops
An alternating current (50 Hz, harmonics), or impulse (lightning…) circulating in a switchboard forming a loop creates an inductive coil.
All the electrical equipment in this loop will be crossed by a current that is an image of the original current.
Its energy may be significant if the coil is formed by a power cable.
Local electromagnetic disturbances
The variable electromagnetic fields generated when switching a contactor coil or when opening the power contacts interfere with neighboring conductors by coupling.
The effect is enhanced if the disturbing and / or disturbed conductors form turns.
Example: control boards
Grounding of shields
Connecting the ends of the shields
The shields are stopped as close as possible to the equipment to which the cables are connected.
The connection is made with care, enclosing the shielding with a clamp which will be:
• attached to a mass strip or
• fixed on the switchboard earth plate.
To avoid corrosion problems, avoid using galvanized steel flanges on shielding made of tinned copper, as well as aluminum.
Stainless steel is the recommended material.
Connection of “standby” cables
The wires not connected to a potential constitute antennas picking up and radiating high frequency disturbances. This phenomenon will be canceled by connecting them to the nearest ground.
Grounding of metal trunking
Selection guides for “CEM” products
Enclosures reinforced for EMC: Spacial range for disturbed environments
Spacial S3HF, SFHF and SMHF enclosures are used to attenuate electromagnetic disturbances in industrial environments.
These envelopes perform two functions:
1) Provide armor by acting as a Faraday cage.
2) Allow effective protection of sensitive equipment thanks to galvanized sheet frames, in compliance with the implementation rules.
Construction
• Spacial S3HF: these enclosures are made of a single Aluzinc 150 sheet of metal, folded and welded.
• Spacial SMHF: these cabinets are designed from a sheet metal belt made in one piece. On it is added a welded and reinforced bottom. The sheet is made of 55% aluminum as the surface coating to ensure good reflection of electromagnetic waves. A special metal-sheathed seal ensures the necessary electrical continuity at the level of the panels, doors and associations while ensuring the tightness of the whole.
• Spacial SFHF: these cells are designed around a framework made up of closed galvanized steel profiles. This framework receives doors, panels and a roof made of Aluzinc 150 sheet metal.
Application areas
These enclosures are particularly suitable for sensitive equipment:
• programmable controllers,
• electronic circuits and boards, etc .;
They protect against the main disturbing elements: variable speed drives, motors, transformers / rectifiers, power cables, etc.
Fans and accessories for reinforced EMC
