INFORMATIVE ANNEX DD  – Rationales for the microwave barrier and associated leakage tests

The standard measurement of microwave oven leakage
There are several commercial instruments on the market. Those that perform sufficiently well for the purpose have a small, reasonably isotropic (omnidirectional) sensor at the end of a plastic rod. The sensor reacts to the electric field only. There is also a non-disturbing sensor spacer which is used to determine a 50 mm minimum distance between the sensor and any part of the appliance as specified in the standard. Testing of instruments include calibration in the farfield (the inaccuracy is allowed to be about ±20 %), and one or two tests intended to show that the sensor is "electrically small" so that it does not itself cause interference (standing waves) to objects nearby.   
The scale on microwave leakage instruments is not in the same units as what is actually measured (V/m) but instead in W/m² (or mW/cm²). The conversion is correct only in the free space plane wave case, where the wave impedance is 377W and there is unidirectional propagation. Since a standing wave is the sum of two waves propagating in different directions, and the probe is not direction sensitive, the field impedance then becomes smaller or larger than 377W, so that the instrument reading becomes erroneous. Erroneous readings are also obtained in strongly curved nearfields and with the probe in a waveguide or similar where there is a single or multiple mode (having a different impedance).
The minimum 50 mm distance between the instrument sensor and any accessible part of the appliance was specified more than 35 years ago when the first microwave oven leakage standard was created. The major reasons were that it was found desirable to use the same type of instruments which were used for far-field exposure measurements. 
It was concluded that an electric field sensor instrument would not indicate a proper value for determining the outgoing power flux density if the probe was located:
a) where the field curvature was very significant (in comparison with the wavelength;
b) in the presence of any standing waves near the sensor
A reasonable compromise with the need to measure emission (i.e. in the source region, so that the "leaking spot" could be found) was found to be 50 mm for the 2 450 MHz ISM band. Even if it was noted in the instrument literature at the time that the same 50 mm distance would be less appropriate for the lower ISM band at 915 MHz, the matter was not considered so problematic that the specification was modified.
The historical reason for the choice of the maximum allowed level of 50 W/m² (=5 mW/cm²) was a result of an existing regulation on free space power flux density of up to 100 W/m² being acceptable in commercial and industrial environments, plus considerations of a possibility of two or more microwave ovens being located close to each other. Later, when household microwave ovens came on the market, the nature of door leakage was found to typically be from only some few leaking spots, so that the power flux density decreased almost quadratic with the distance away from these. There was no reason why the user would remain very near the closed door of an operating oven. Widely publicised investigations showed that the actual exposure of any part of the human body became very low, particularly in consideration of a reasonable averaging time of 5 to 10 minutes for hazard assessment. As a result, the 50 W/m² limit was applied also to household microwave ovens.
In the beginning of the 1970's, the US authorities responsible for radiation safety found some quality problems with some microwave oven models, and introduced a 10 W/m² "factory limit" for new unused ovens, in order to dampen any public concerns. Only one or two other countries followed.
In the meantime, the SC 61B oven safety standard was successively developed and the value 50 W/m² became the the worldwide limit after all tests. However, in empty operation and after a potentially destructive door test, 100 W/m² was instead required. The rationales for the higher value under no-load conditions were reported difficulties by some manufacturers, and the conclusion that no-load operation would typically be even more short-term and an also uncommon fault condition. 
In the 1980's, leakage measurements at covers for lamp replacement was dealt with IEC SC 61. The hole array in the cavity wall, at the lamp, can of course leak microwaves. The size of the cover may be such that the 50 mm distance to the nearest appliance part can be maintained also with the sensor almost inside the external housing from which the cover has been removed. A case had been reported where the instrument reading was quite high in this condition, but there was a very low reading with the whole housing removed. The reason for the high reading was that a standing wave inside the housing had been created. There was an electric field but no real leakage since the standing wave is the sum of an outwards- and inwards-going wave and may have no net power flux. In addition, if a finger would be put into the opening, the standing wave would disappear and only the real leakage becomes the possible hazard. SC 61B added a statement to the standard to the effect that the instrument sensor should not be closer to the opening plane than 50 mm, i.e. the region inside the cover should not be considered accessible with regard to the leakage measurement. The same principle is adhered to in this Standard, but the actual leakage situation is now really assessed, by the extended test in clause 32.101.

Microwave hazards – the basic restriction 
Microwave exposure is considered to be potentially hazardous if the heating of parts of the human body exceeds certain values. These are specified as SAR values (specific absorption rate) and are expressed in W/kg tissue. The  lowest SAR value of whole-body exposure where there may be some risks has been found to be 4 W/kg. A safety factor of 10 is subsequently applied for microwave workers (instructed persons), and a further safety factor of 5 for the general public (ordinary persons), resulting in the basic restriction of 0,4 and 0,08 W/kg in the two cases. Local, non-hazardous exposure limited to the head and trunk may be up to 10 W/kg and 2 W/kg, respectively. Twice this (20 W/kg and 4 W/kg) are considered non-hazardous locally in the extremities (including hands and fingers). The integration volumes are then over any 10 g body mass, and the time integration is over 6 minutes.

Microwave hazard evaluation – the free space exposure method 
For all practical exposure situations (except from communication devices such as mobile phones for which a total source maximum power concept may apply), two simplified verification methods are used in industry and for protection of microwave workers and the general public: a maximum allowed far-field power flux density far away from the source, and an emission standard for appliances such as microwave ovens.
The issue is now if the relaxation of SAR values for parts of the body, in combination with the integration volume, are compatible with the free space exposure method.
When parts of the human body having a small radius of curvature are heated, diffraction, resonant and other focussing or amplification phenomena may occur. In the case of 2 450 MHz, the internal wavelengths in tissues as well as the penetration depth limitation result in only fingers being of major interest. In principle, also bent knuckles and elbows could create focussing effects, but fingers are definitely much more problematic with regard to the effects discussed here. It is not assumed that other protruding parts of the body such as the nose, ears or penis are brought very close to microwave leakage sources in commercial or household heating equipment.
The following modelling results indicate the degree of compatibility between the basic restriction and the free space exposure method:
Numerical modeling using commercially available electromagnetic software was used. A finger with 13 mm diameter and typical dielectric data (homogeneous, with e=40–j10, where the loss factor (10) is lowered in consideration of bone and tendons) was exposed to 10 W/m² in free space. The strongest absorption occurred for TMz polarisation (i.e. with the impinging electric field parallell to the finger axis) and the mode in the finger then becomes of the TMz1 type, having two opposite axial zones of maximum heating intensity. The maximum power intensity becomes 5 W/dm³ and the average over the worst 10 cm³ becomes about 1,8 W/dm³.
If the finger would be exposed to a plane wave with a power flux density of 50 W/m²which is allowed from microwave ovens, etc., the maximum value would become 25 W/dm³ and the 10 cm³ integrated value would become 9 W/dm³. [Note: the density is set to 1 kg/dm³]
The conclusions are that:
·	The ordinary person basic restriction is exceeded. However, ordinary persons are with today's standards only exposed to microwave ovens with a door, where the leakage source is so small that the high intensity is over a significantly smaller volume of the finger. Additionally, there is no reason to keep the hand near the closed door of an operating microwave oven. There are numerous reports from experimental investigations in the 1970's which clearly indicate the averaged exposure level over several minutes is 10 to 100 times lower than 10 W/m². Therefore, the actual absorption is within the SAR limit.
·	The instructed person basic restriction is about the same as the actual SAR value. However, the actual situation with an operator occupied with load removal at the port of a continuously operating tunnel microwave oven for long periods is more onerous than with a microwave oven with a door, but the working hand can typically not be near the opening more than about half the time. An additional aggravating factor is that the tunnel opening is larger than an oven door as a leakage source, so that the region with a high microwave energy density may extend further out than from an oven door. Therefore, the construction of the tunnel end regions as well as the measurement method must ensure that SAR values of 20 W/kg in the human finger exceeding those under 50 W/m² far field exposure are not exceeded.
·	The operating condition of the tunnel oven shall be such that any higher average leakage levels do not occur. However, parts of a tunnel microwave oven can be operated empty with the operator still removing loads. Therefore, the 100 W/m² value for an operating empty oven with a door should not be applicable for tunnel ovens. 

Microwave hazards from openings in cavities, and from tunnel ends
The actually absorbed microwave power in a part of the human body is always very dependent on the field configuration, and the field configuration at the body part is also strongly modified by the part itself. This means that even a knowledge about the true power flux density or the electric field intensity cannot be used to assess the actual microwave absorption rate. It becomes necessary to establish a more complete scenario before any calculations of the absorption can be made. Hence, the leakage intensity measured as a quasi-plane free space wave at 50 mm or more away from the source will now not alone determine the level of hazard. The actual hazard also depends on:
·	any possibility of access into a region where there is microwave energy;
·	the size of the opening, which may determine the type of field characteristics, or allow several kinds of microwave field characteristics;
·	any objects, including a load to be heated or a part of the body in the opening, which may also determine the type of field characteristics.
The access situation is of course crucial and must be standardised in some ways so that reasonably simple and objective procedures and requirements can be established. Since only the arm, hand and finger are considered to be the parts of the body which may get in contact with or be inserted into openings in these appliances, two important issues can be directly quantified: 1) all geometric factors (by Test probe B, etc.), and 2) as addressed above, these parts of the body are less sensitive than for example the head.
An important principle is that a "hazard boundary" (called reference surface in this standard) is defined somewhere in the vicinity of the physical opening surface and that a leakage instrument reading of 50 W/m² is to apply for the tests. This means that what remains is to construct tests which will ensure, with reasonable certainty, that actual power densities (in W/m³, or SAR values in W/kg) in human fingers, hand or arm "contacting" the reference surface will not exceed those caused by a "normal" leakage source such as a microwave oven door region giving a power flux density reading of 50 W/m² at 50 mm distance from any part of the appliance.
The field configuration then becomes the issue, i.e. how to obtain realistic measurement results with the same type of instruments as are used for microwave ovens with a door. Clearly, there is a need for simplification and standardisation using some typical scenarios. The most important matter is then to consider cases where access would be more severe than in the normal door leakage case. These "onerous" cases are:
·	The field configuration is such that there is a very high intensity in a region, and the intensity diminishes very quickly with increasing distance, so that no reading may be obtained but there may still be a quite hazardous microwave energy density 50 mm or less from the instrument sensor. Structures creating non-radiating nearfields or strongly evanescent modes have this effect.
·	The field configuration is such that a microwave power flux is bound to a dielectric object. A load which is heated and is conveyed out of a tunnel oven is the most typical example, and a bound surface wave may then exist, and "transport" a quite large microwave power away from the opening. This may then be manifested as a measurable leakage 500 mm or more away from the opening, whilst no leakage can be measured (using the 50 mm sensor distance) at the opening. – A problem with this type of wave is of course that it must be assumed that the operator hand actually contacts the loads and then becomes a part of the scenario. Another problem with this kind of waves is that any measured leakage may become spurious and confusing, since it may not be discovered in the region where it emanates.
Cases where a non-hazardous condition exists but a high instrument reading is obtained are also undesirable. The lamp cover case addressed above is of this kind.
In this standard, a method of leakage extraction and non-shielding microwave barriers is used. One end of the metal rod may act as a receiving antenna and since the end can be located very close to parts of the oven and load it will also pick up nearfields, evanescent modes and surface waves when suitably oriented. A "spatial averaging" of the externally available microwave energy also results, since the instrument sensor is still not closer than 50 mm to any other object.
The tip of the rod may be inserted up to 50 mm into entrance and exit ports. This may be considered onerous, but is for disencouraging constructions with certain operator-accessible "curtains" intended to reduce leakage, and due to the particular need to compensate for the imperfections of the simple measurement method in view of the wide variety of objects in and geometries of the ports, and possible prolonged operator presence at these ports.

The time averaging 
There are only two time integration specifications in the existing international standards:
a) 6 minutes for whole-body exposure (probably including fingers) and
b) criteria for duty cycles in cases of very short pulses such as from radar transmitters. Additionally, in some national legislation on non-ionising radiation there is a ceiling value of exposure; a ceiling value of e.g. 250 W/m² and a 10 W/m² average may be interpreted as maximum 300/25= 12 sec isolated strong exposure being allowed during any 6 minute interval, with no exposure during the remaining 5 min 48 sec of the interval. 
The 6 minute integration time is quite compatible with typical cases of irradiation of parts of the body having a radius of curvature larger than about one free space wavelength of 2450 MHz microwaves. In such cases essentially a plane damped wave propagation can be assumed, as well as a depth of 30 to 40 mm in the tissue over which equilibration by heat conduction takes place. Using the heat conductivity data and the Fourier heat conduction equation then results in a time constant (i.e. about 63 % of the stationary conditions have occurred) of about 5 minutes. A useful comparison is with boiling of an egg in 100 °C water: it  takes about 5 minutes for the centre to reach a temperature of about 65 °C.
The most onerous heating pattern in a Ø 13 mm finger under plane wave 2450 MHz irradiation is uneven, with about 5 mm distance between the hot and cold areas. It can be shown that the overall microwave coupling is strongest for about Ø16 mm finger diameter. The corresponding distance between hot and cold areas then becomes 7 mm or less.
The Fourier heat conduction equation is spatially quadratic. Using the boiling of a Ø40 mm egg in 5 minutes having distance between the cold and hot regions is 20 mm as a basis, a 7 mm distance would be similarly equilibrated in (7/20)² of 5·60 sec, i.e. about 35 sec integration time is adequate.
There is, however, another factor to also consider: even a very localised heating rate should not be so high that there will be any risk of pain or injury during the time of integration. A suitable acceptable local temperature rise may be set to 5 K, in consideration of both that the skin area with heat-sensing nerves will be heated at least by conduction and that such a temperature rise under short term conditions will not cause any injury in the fingers. A normal person will feel and react to a temperature increase of the same order or less, about 3 K, within some very few seconds. 
A homogeneous SAR value of 20 W/kg (the basic restriction for instructed person fingers) will result in a temperature rise rate of about 0,5 K/min.
Now supposing that only e.g. the tip of a finger absorbs all power and the remainder of the 10 g absorbs no power. Such scenarios are actually not uncommon and may occur e.g. with the finger contacting damaged microwave oven seals and in some nearfield cases. The volume of that part of the tip that absorbs microwaves is now set to 0,5 cm³ (which is the volume of a hemisphere with Ø12 mm). Using this in relation to the 10 cm³ of the basic restriction, one obtains a 20 times faster "allowed" temperature rise rate of 10 K/min. This will also mean that the person will feel the heating of the finger within 20 sec. Since the equilibration by heat conduction has about the same time constant as above, one again arrives at about 30 sec suitable integration time.
There is an extreme case of the tip of the finger touching a leaking narrow slot in a metal surface. The local SAR value becomes very dependent on the dryness of the skin. As an example, a Ø13 mm finger tip with 1 mm dry skin is pressed against the centre of a 2 mm wide and 100 mm long slot. This has a leakage that would be measured to 50 W/m² at 50 mm distance (i.e. the electic field strength is 137 V/m) with no finger. The local SAR value then becomes about 30 W/dm³, over a 4 mm wide and 1,5 mm deep volume. This local value is in itself approximately within the basic restriction. If the finger is wet and the skin is thin, the local SAR value may be up to 50 times larger but the two small heated volumes contacting the slot sides are then only about 1 mm wide and deep. The thermal equilibration distance is now over only 2 mm, so the heat conduction has now a time constant of  (2/20)²·5·60= 3 sec. The local, thermally insulated heating rate could be up to 40 K/min. However, heat conduction would result in a stationary temperature rise of less than about 3 K, which is also acceptable. Hence, there is no need to have a shorter integration time than about 30 sec even in this most onerous case of high local SAR values in microwave cavity oven situations.

Conclusions and modifications of the standards for ovens with a cavity door
The 6 minute time of integration specified in existing international standards is inadequate for the purposes now under investigation by SC 61B.  A more realistic value should be 30 sec. There may be cases of open-ended microwave applicators intended for heating of a contacting load. Such applicators may cause almost instantaneous injury if contacted by any part of the body when in operation, and other provisions for safety must be applied. 
The existing emission standard for microwave ovens specifies an integration time of about 2 seconds for the measurement. This is for historical and practical rather than safety reasons. A typical household microwave oven has either a ceiling stirrer or a turntable, and with the specified circularly cylindrical test load the leakage variation periodicity will be comparable to or less than the specified integration time. Measurements are then correct and made easily and quickly with the present standard.
Since the doorless appliances considered in this Annex may behave quite differently and there is no reason to introduce limitations on construction which have no relevance to safety considerations, 20 seconds time of integration for leakage measurements shall be applied. The is shorter than 30 seconds, but also allows for faster measurements and easier integration. The most onerous 20 seconds interval is to be chosen, and the instrument integration time of 2 to 3 seconds shall be maintained.
For reasons given in here, the allowed leakage level shall not be 100 W/m² in empty operation, as for microwave ovens with a door. The regular value of 50 W/m² shall apply.
In addition, a maximum measured (integrated, ceiling) value of 500 W/m², consistent  with the instrument integration time of 2 to 3 seconds is introduced, to simplify instrument specifications and handling as well as the numerical integration in cases of highly variable leakage. Such strong variability may occur for example in appliances with a protective device consisting of a built-in leakage monitor coupled to a cut-out.
 
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