3.12 The synergy of filters and shielding
If a supply filter allows a very high frequency (say 300MHz, harmonics from digital processing) to pass through to the product’s mains lead, the radiation of these frequencies from the lead will compromise the product's radiated emissions.
So another secret of cost-effective EMC is that the correct way to view filtering and shielding is as a synergy, with each one complementing the other. Incorrect filter design or mounting technique can easily compromise radiated emissions and immunity. Likewise, inadequate shielding can easily compromise conducted emissions and immunity.
Some manufacturers only make their filters to work over the frequency range of the conducted emissions tests (up to 30MHz), to keep costs low. Unfortunately such filters often compromise the shielding integrity of shielded enclosures and cause problems with radiated electromagnetic disturbances.
3.13 Filter construction, mounting, and cabling
Filters are easily compromised by RF coupling from the conductors on their unfiltered side to the conductors on their filtered side. Many engineers have been very surprised by the ease with which high frequencies will “leak around” a filter, given half a chance.
Where an external cable to be filtered enters a shielded enclosure or room, the filter should be fixed into the metal wall at the point of cable entry and RF bonded (metal-to-metal) to the metalwork of the wall. For better performance at high frequencies, and to avoid compromising enclosure shielding, a conductive gasket or spring fingers may be fitted to create a low-impedance electrical bond between the filter’s metal body and the wall along the entire circumference of the filter’s cut-out.
An IEC inlet filter installed in a shielded enclosure can only give a good account of itself at frequencies above a few tens of MHz if its body has a seamless construction and its body is RF bonded to the shielding metalwork, as shown in Figure 3G.
Through-bulkhead filters are best, but may be too expensive for some applications, such as mains currents above 10A (the maximum rating of the IEC 320 style mains connector). For higher powers most commercially available mains filters are just rectangular units with screw-terminal connections. Figure 3G shows how to mount such filters using the "dirty-box" method, which encloses the filter in its own box within the main shielded enclosure (the “clean box”) to help achieve good high-frequency performance.
The filter input and output cables in the dirty box must be very short and far away from each other, but even so, very high frequencies may still couple between them and soft-ferrite cylinders may be needed on either (or both) cables.
Filters which are not through-bulkhead types but must have the highest performance often deal with cable coupling by enclosing their input and output terminals inside their metal enclosure (a “dirty box”) and bringing their cables out through standard circular galvanised conduit fittings. Running the cables in conduit effectively shields the unfiltered from the filtered cables, and the filter functions effectively up to the highest frequencies. This is the method used by most of the supply filters intended for EMC test chamber applications. Input and output cables may be screened instead of being run in conduit for the same effect.
3.14 Surge protection devices (SPDs)
3.14.1 Types of SPD
Surge arrestors are variable resistance devices, whose resistance is a function of the applied voltage. They are designed so that they provide a clamping effect when the voltage across them exceeds a certain level, rather like a zener diode.
There are four basic types of SPD:
· Gas discharge tube (GDT), essentially just a spark gap, slow but very high power
· Metal-oxide varistor (MOV), fast and available in a wide range of energy ratings
· Avalanche devices, semiconductors with a zener type action, very fast but not very high power
· SCR devices, another type of semiconductor device, slow but will handle high currents
Figure 3H sketches the voltage/time curves of these four types of SPD when exposed to the leading edge of a typical surge test waveform. It shows that GDT and SCR devices are slow to start suppressing. They have to reach a trigger voltage before they begin to conduct, and during this time they let through surge voltages which may be potentially damaging. They also have a foldback characteristic, which means that once they are triggered and are carrying current, the voltage across them drops to well below the voltage they were previously quite happy to block. Consequently, careful design is needed to make sure that when they are connected to a source of DC current they do not remain in conduction for ever.
MOV and Avalanche devices act like zener diodes, with a knee voltage where they start to draw current. As their current increases their clamping voltage rises slowly.
The wired-in SPDs usually used for mains suppression may be unsuitable for use on signal or data lines, because of too much leakage, or too much capacitance, or impedance mismatch, or a variety of other problems. Special SPDs are available fitted with connectors intended to plug directly into a wide variety of digital and analogue signal cables and be mounted to earthed metalwork, like through-bulkhead filters. Radio antenna and high-speed data lines use matched transmission-line type SPDs, which use connectors such as BNCs.
3.14.2 Are SPDs needed on data lines?
SPDs are often advertised for use on data lines, but this is only really necessary on long cables which remain within a building, where there is a poor earth structure between the equipments at each end such that lighting surges can cause a high voltage to appear between them. Where a product’s cables exit a building (or connect to an external antenna) they should always have surge protection fitted.
For short interconnections (for example between keyboards or printers and PCs) the problems of earth voltage surge differences and fast transient bursts does not exist, and all that is left for an SPD to act on are electro-static discharge (ESD) transients. We shall see in Part 6 that ESD is best dealt with by using plastic to prevent discharges altogether, or else using good earth-bonding of metalwork.
3.14.3 SPDs and data integrity
Surge protection of analogue or digital data is usually not complete in itself. Even though the surges do not cause damage, they will have given a false value or bit. Where no memory or program is involved, as in simple analogue indicating instruments, a momentary glitch in the reading may be acceptable (depending on function), but for some analogue signals and all digital data (such as control signals) a momentarily incorrect signal can alter the stored data or operational mode, and this is usually unacceptable. Very slow data may be able to use filtering to reduce the “spike” to below detection thresholds.
Where glitched data is not acceptable and yet the only protection from surges is the use of SPDs, there needs to be some way of identifying and recovering from the incorrect data. Communication protocols are the usual answer, briefly mentioned Part 1.4.7. There are a number of these, ranging from simple to exotic, all with various overheads and penalties, and it is by far the best to buy the chips which implement protocols proven to be robust in real-life applications (e.g. CAN) rather than imagine that you can create a protocol of your own that will be good enough, even for the most cost-sensitive high-volume applications.
3.14.4 Ratings of SPDs
SPD ratings should really be chosen in conjunction with the design of a building and its lightning protection network. BS6651 Appendix C deals with this issue, and specifies the SPD ratings for equipment fitted in different parts of a building.
This standard, or others which deal with the lightning protection of electronic equipment (e.g. IEEE C62.41–1991) should be used where the generic or product EMC standards are lacking in surge requirements, or where their surge requirements are incomplete (for example, EN50082-1:1992 and EN 50082-2:1995 have no surge requirements at all, whereas EN 50082-1:1997 only applies surge requirements to mains and DC power ports).
BS6651 Appendix C specifies the following SPD ratings for the mains supply for equipment located more than 20 metres from the building’s incoming mains connection (the most benign location, known as Category A):
· Low risk premises: 2kV, 167 Amps
· Medium risk premises: 4kV, 333 Amps
· High risk premises: 6kV, 500 Amps
Buildings not large enough to have a Category A, which includes most residences, are specified as Category B for internally connected equipment:
· Low risk premises: 2kV, 1,000 Amps
· Medium risk premises: 4kV, 2,000Amps
· High risk premises: 6kV, 3,000 Amps
External telecommunication and other signal/data cables (no matter how far they travel within a building), and the mains supplies to equipment mounted outside a building, are known as Category C:
· Low risk premises: 6kV, 3,000 Amps
· Medium risk premises: 10kV, 5,000Amps
· High risk premises: 20kV, 10,000 Amps
When a product is adequately protected against lightning surges, it is generally protected well enough against common surges generated by other means, such as switchgear. Where an application is known or suspected of suffering high levels of surges not of lightning origin, it is generally enough to protect the product to the next higher level of surge than is called for by the lightning protection standards.
Some superconducting magnet or power generation applications, or Nuclear Electromagnetic Pulse (NEMP) involve extreme or special types of surges, and are not addressed here.
3.14.5 Fusing of SPDs
All SPDs fail eventually, and since the majority of products use metal-oxide-varistor types (whose failure mode is to leak increasingly and finally to go short-circuit) in their mains inputs, they may need to be fused to prevent fire or shock hazards.
If the fuse is in the SPD circuit only, when it opens during the surge event that kills the SPD the protected equipment may be exposed to the remaining parts of the surge and damaged. Afterwards, even if the protected equipment is undamaged, it has lost its surge protection and so is very exposed to the next surge that comes along.
If the fuse is in series with the line that also goes to the protected equipment, the opening of the fuse due to SPD failure will disconnect the line to the equipment, which may not be acceptable in critical applications.
There is no easy answer to the problem of SPD fusing, but either of the above methods are generally acceptable providing the SPD is adequately rated for a goodly number of the maximum surges expected to be experienced. Once again, the lighting protection standards come to our aid, with risk assessments based on geography and application which allow the number and magnitude of surges caused by lightning to be assessed so that long-term reliability is likely.
3.14.6 Assembly of SPDs
SPDs, like filters, can be used to suppress unwanted DM or CM signals. In the jargon common to the surge community, DM surges are usually called line-to-line (or symmetrical), and CM surges become line-to-ground (or asymmetrical).
SPDs are always used in shunt mode, to “short out” the surge voltages. Also like filters, SPDs only function as expected when assembled correctly. One of the most important issues is to avoid lead inductance.
When surge currents flow in lead inductance they create voltages which increase the “let-through” voltage. As is shown in Figure 3J, it is best to track (or wire) the incoming power (or signal) directly to the terminals of its SPD, and then connect the protected circuitry to the SPD terminals too.
Figure 3J also shows that the use of SPDs from phase to earth is strongly discouraged, if not prohibited, for portable or pluggable equipment because of the unpredictable earth leakages of SPDs as they wear out, and the resulting safety problems. So it is best to use other means to deal with line-to-earth (common-mode) surges.
However, SPDs between supply phases and ground may be acceptable for permanently-wired equipment, especially if it has duplicated protective earth conductors, so that if one is faulty the other prevents electric shock from any SPD earth leakage. SPDs are commonly connected phase to ground in building installations, to help protect the equipment in the building from lightning surges (e.g. see BS6651 Appendix C), but in such instances the building’s earth structure (its common bonding network) should have a high degree of redundancy and not depend upon any single conductor for safety.
There are other types of surge limiters that are used in series with the circuit to be protected. Different types are used to prevent emissions of surge currents at switch-on, or to protect a circuit from current surges in its supply. These are not discussed here.
3.14.7 The problems of earth lift
Applying SPDs between phase and protective earth in a fixed product which is permanently wired may be acceptable under the relevant safety standards, given adequate protective earthing. But this can create new problems due to the inductance of the protective earth conductor. With ordinary protective conductor wire having an inductance of around 1mH/metre, and lightning surge currents of around 1kA with rise times of around 1ms – trying to suppress a power surge with an SPD connected to a length of earth wire does not actually suppress the beginning of the surge. Instead, it forces the voltage on the earth wire (and the chassis of the product) to follow the initial few microseconds of the surge voltage on the phase conductor. In the avionics world (at least) this phenomenon is called “earth lift”, which sums it up nicely.
This protects the mains input itself, but where there are signal cables connected to the product the surge voltage now present on the earthed chassis exposes the signal cables to the early part of the mains surge, which could damage their associated circuitry (even if opto-coupled, as most opto-couplers are only rated for 500V). So when SPDs are fitted to mains inputs to protect from line-to-ground surges, all signal cables may need surge protection too.
The same problem arises when an external telephone line or LAN is protected by SPDs connected to an equipment’s protective earth or chassis. This can allow surges on signal cables to damage the products’ power supply, necessitating the use of SPDs on the mains too (or else achieving the necessary protection by adequate line-to-ground voltage isolation).
Computer and telecommunication cabinets often deal with “earth-lift” surges by bonding their metal cabinets directly to the common-bonding network of their building with one or more heavy-duty cables each no more than 500mm long. Where they have “system blocks” of several cabinets passing signals between them, modern good practice is to construct a local earth mesh to reduce the inductance of the earth between the cabinets. This adds to the cost of the installation but removes the need for heavy-duty SPDs on all internal communication I/Os when SPDs are fitted on incoming mains and external LAN or telephone lines. Clearly, these earthing and bonding techniques are not appropriate for domestic or portable products.
More details on using SPDs within products to protect circuits may be found in Part 6. Although this is concerned with ESD, many of the circuit comments apply equally to using SPDs for surge protection. |
