EMC vs. Safety/Health

A discussion of EMC protection would not be complete without mentioning the potential conflicts that may exist between EMC protection measures and health/safety protection measures. Safety of electrical equipment is not often compromised by EMC protection measures, but there are a few areas where this can happen if no thought is given to the system design. Particular issues are:

Effects of filters and suppressors:

++ earth leakage currents

++ residual voltage on capacitors

++ fusing of filters and surge protection devices (SPDs)

Effects of screened enclosures:

++ cree page and clearance distances between conducting parts

++ over-temperature and ventilation.

Filter problems

Ground/earth leakage

It’s usual for mains filters to include capacitors between each phase and the safety ground/earth. As discussed about filtering, these capacitors suppress common mode noise emitted by apparatus and also reduce the amplitude of incoming transients.

Since there is an impressed voltage between the live conductor and ground/earth of 230V at 50Hz (in Europe), a current, I, will flow through the live-connected capacitor, and under certain fault conditions through all capacitors, given by I = 230 x (2/1;.50 x C) amps

This translates to 80 x C mA if C is in microfarads and a 10% increase is allowed for the effect of voltage variation above nominal. For reasons of protection of personnel against electric shock under fault conditions, safety standards limit the current that is allowed to flow continuously in the ground/earth conductor. Different standards have different maximum values, and the values depend on the way that the equipment is connected to the mains source. ----- these values for a few of the most important equipment safety standards. The commonly-used figure of 0.75mA for portable ground/earthed equipment results in a maximum capacitor value of 4.7nF between each phase and ground/earth, and many mains filters incorporate this value.

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Earth leakage current limits in some equipment safety standards

Standard

...... EN 60335"1 household Class I, portable, EN 60950 ITE

.... Class I, stationary

0.75mA 3.5mA

......

0.75mA 3.5mA EN 61010 meas't and control Class II

0.25mA EN 60601-1 medical Patient leakage

0.25mA Sinusoidal Non-sinusoidal DC

0.5mA 0.7mA 2mA Type B Type BF Type CF

0.5mA ground/earth leakage

0.1 mA 0.1 mA 0.01 mA

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From the systems point of view, it will often be the case that several items of equipment will be paralleled onto one mains supply. There is the real possibility that, even though an individual apparatus remains within the leakage current limits, the combination will not. However, the limit for portables does not apply to permanently- wired apparatus, so when such a combination is permanently wired in place the problem may be avoided. E.g., five appliances which meet the portable limit of EN 60950 can be paralleled if they will be connected in such a way as to render them "stationary". Heavy industrial equipment which has its own power supply does not fall under the rules of these standards, which can be fortunate, since it often needs much higher values of common mode capacitance to meet emissions requirements. On the other hand, patient-connected medical apparatus is only allowed 100~tA of ground/earth leakage current. This normally precludes any common mode capacitance in the filter at all, since such a low level can easily be generated by stray capacitances and leakage paths within the apparatus itself. Leakage current limits for certain medical systems can turn out to be particularly onerous when it comes to meeting the associated EMC requirement.

When a system is being built from a number of items of equipment which are known to be individually compliant both for safety and for EMC, the leakage current question must be properly addressed in the design. Special installation requirements apply where the leakage current exceeds 3.5mA, according to the IEE Wiring Regulations EN 60950. This provides a number of options for connecting single items of equipment to a final circuit, depending on the leakage current magnitude:

++ 3.5mA < I_l 1kg _< 10mA: permanent connection to installation, or connection by means of plug-socket

++ I_l kg > 10mA: permanent connection to installation (preferred), or connection by means of plug-socket combination plus additional 4mm ~ protective conductor, or by multicore with two protective conductors totaling 10mm^2.

(Safety of information technology equipment) requires a warning label for leakage current greater than 3.5mA, and under no circumstances is it acceptable for ground/earth leakage to exceed 5 % of the total input current per phase.

Options for a high-leakage final circuit If the final circuit is expected to carry a leakage current greater than 10mA, the following options are available:

++ high integrity protective single conductor not less than 10mm^2, or

++ separate duplicate protective conductors of not less than 4mm^2, or

++ duplicate protective conductors totaling not less than 10mm^2 (minimum 2.5mm^2), or:

++ ground/earth monitored circuit, or

++ circuit supplied by double wound transformer, or

++ "modified" ring final circuit.

The modified ring circuit is the most widely used. The ring circuit's protective conductor must have a minimum cross- sectional area of 1.5mm^2, connected in the form of a ring with each leg connected separately at the distribution board. The final circuit must have no spurs and only single socket-outlets may be used.

If the installation is designed to supply a large installed base of IT equipment or other items with significant leakage currents, both the requirements for final circuits and the integrity of the distribution circuit protective conductors must be considered.

Furthermore, RCDs (residual current devices) protecting a circuit should have a rated residual current of at least four times the total ground/earth leakage, to prevent nuisance tripping.

Residual voltage

The capacitors connected across the phases in mains filters can take quite high values, up to several microfarads being not uncommon in high power units. These capacitors must withstand and carry the phase voltage continuously. Apart from mandating a particularly reliable form of construction (which is normally specified by quoting a part to IEC 60364-14), if the supply is suddenly interrupted with the load disconnected, the instantaneous phase voltage remains on the capacitor. This voltage can be anywhere between zero and the peak value, depending on the supply phase at the instant of disconnection. With high values of capacitance, if such a voltage becomes accessible to personnel a severe shock hazard can exist.

The typical case in which this could happen is with pluggable apparatus which can be switched off on the load side of the filter; the hazardous voltage can remain between the live and neutral pins of the plug when it’s removed from the supply socket. For permanently wired systems apparatus, the threat is really only applicable to service and maintenance personnel. Even so, it should still be addressed. The normal solution is to ensure that a bleed resistor is wired across the phases to discharge the capacitors within an acceptable time when the supply is disconnected.

Fusing

Filter components are not perfect and can fail just like other components in the mains supply circuit of equipment. Additionally, transient suppression devices (typically, in mains circuits, metal oxide varistors or MOVs) can be connected both across phases and from phases to ground/earth, to meet the increasing requirement for surge immunity. If these are subject to a high level surge they may fail short circuit, putting a low impedance path across the supply.

To meet this threat it’s vital to include fusing of the proper value on the supply side of any filter or surge suppressor. An un-fused unit will present a potentially serious fire hazard in the equipment. Any bought-in items that already comply with safety standards should include this protection (but check!), but the principle must also be applied to any other discrete filtering or suppression that is put in place to implement the zoning concept within the system.

Screened enclosures

Creepage and clearance

The mechanical design of many electrical products relies on separation distances between live and accessible parts to maintain safety against shock. When such products are placed within screened enclosures, it’s necessary to ensure that such separation distances are not compromised. A typical problem might occur when a product in a plastic case with normally sufficient separation between hazardous and safe circuit connections, is bolted to a metal frame inside a screened enclosure; the separation distances from each set of connections to the metal frame may not now be sufficient. A related issue occurs when a plastic case is conductively coated for screening purposes.

The original designer may well have relied on the internal case geometry being insulating to maintain safety clearances. Adding a coating can breach these clearances by providing a new conducting path. The dangers of coating also extend to the possibility of flakes of coating becoming detached due to environmental stresses such as vibration or temperature extremes, and lodging in areas where they may bridge critical insulation.

Screening is not the only situation in which clearances may be affected: a poorly designed or installed mains filter or surge suppressor can create similar issues.

Thermal effects

To be fully effective a screened enclosure must be totally sealed with respect to electromagnetic penetration. I.e., ventilation openings are anathema to the EMC engineer, who will bend every effort towards minimizing or preferably eliminating them. It’s easy to carry this process too far, and design a well-screened enclosure which overheats due to internal dissipation and may create a fire hazard or cause insulation to degrade. Liaison is necessary between the EMC design aspects and the thermal design aspects of an enclosure. EMC-screened ventilation openings are available (such as honeycomb panels) which are highly effective, if somewhat more expensive to implement.

It’s also advisable to take into consideration the thermal properties of high current filters; any series choke will have appreciable resistance which is in series with the supply and which will cause additional dissipation. Very often this is the limiting factor in filter design. Expect a filter choke to run at elevated temperature, and check that it does not create a hazard in the worst case ambient temperature. Bear in mind that chokes are normally rated for RMS current, and that many electronic devices (particularly switch-mode converters) draw current with a high crest factor, which increases the I^2R loss over that expected with sinusoidal currents.

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