What is the purpose of a magnetron

Microwaves

security

Microwaves in the area

While electromagnetic waves from television and radar systems are emitted into the environment, with microwave ovens the waves should only work in the closed cooking space. What escapes undesirably to the outside is called leakage radiation. The leakage radiation permitted in accordance with the safety regulations, measured at a distance of 5 cm, of 5 mW / cm² when the device is under load, or 10 mW / cm² when the device is idle, is harmless. In practice, however, the devices offered today fall below these limit values ​​many times over. In comparison, for example, people at rest have their own heat output of around 10 mW / cm².

Device security

After switching off the magnetron at the end of a process or when opening the door during operation, no more microwaves are transmitted. This is why there are no more microwaves in the oven. Microwaves cannot escape when the door is open.

Safety devices

Like all electrical equipment, microwave ovens must meet strict safety requirements in order to exclude hazards for the user. According to the law for technical work equipment, all dangers for the user and third parties should be excluded when used properly. A certain misconduct on the part of the user is also taken into account.

It must not be possible to remove covers and housing parts without special tools if this makes live parts accessible. If, after removing a cover, microwave leakage occurs which exceeds the permitted limit value, the device manufacturer must affix a clearly legible label with the following: “Warning! Microwave energy. Do not remove cover ".

The devices are secured against the escape of microwave energy. Some microwave energy can only escape in the area of ​​the door joint, but its energy density is low and limited by the safety regulations (see safety checks).

Microwave ovens are equipped with the following safety devices:

Security against leakage of microwaves when the door is open
Several safety systems connected one after the other ensure that the magnetron is switched off immediately, even if the door is only slightly open, and that no microwaves can penetrate the outside. Safety switches have to monitor two different parts of the door, e.g. to limit the effects of a warped door to a minimum. At least one interlock is specially monitored. Failure will cause the device to be inoperable until customer service has repaired the device.

Security elements:

  • electric lock
  • mechanical locking (start button, locking mechanism)

Security against leakage of microwaves when the door is closed
In order to prevent uncontrolled leakage of the microwaves when the door is closed, the door edge must rest on the edge of the housing in an electrically conductive manner without a gap or have special chambers. The dimensions of these so-called quarter-wave chambers (λ / 4) are matched to the wavelength of the microwaves (λ = 12.25 cm), so that the joint between the door and the housing is in the so-called current minimum, which makes it easier to seal against microwave leakage.

Security elements:

  • sealing quarter-wave chamber
  • reflective perforated sheet in the window
  • microwave-absorbing seal around the door area (ferrite seal)

Safety against overtemperature on the magnetron and in the cooking chamber Safety elements:

  • Overtemperature protection on the magnetron
  • partly overtemperature protection in the exhaust air duct for exhaust air from the oven (e.g. when food catches fire due to extremely improper use)

Protection against excessively high starting current to protect the fuse and the power supply system Fuse elements:

  • electronic starting current limitation (triac)
  • Series resistor and relay

Safety hot surfaces
If portable microwave combination devices have touchable metal surfaces that show a temperature increase of more than 90 K during operation, they must be marked with the symbol for hot surfaces. ATTENTION: Hot surface.

Safety regulations in standards / guidelines

The following guidelines and standards are relevant for microwave ovens and microwave combination ovens:

EMC Directive 2014/30 / EU Electromagnetic Compatibility (of electrical and electronic products).

DIN EN 55011; VDE 0875–11: 2017–03
DIN EN 55014–1; VDE 0875-14-1: 2012-05
Series of standards DIN EN 61000

Safety of electrically operated devices
Low Voltage Directive - Directive 2014/35 / EU

DIN EN 60335-1; VDE 0700–1: 2012–10
Household and Similar Electrical Apparatus Safety - Part 1: General Requirements (IEC 60335–1: 2010, modified); German version EN 60335–1: 2012

DIN EN 60335–2–25; VDE 0700–25: 2016–09 Household and similar electrical appliances - Safety - Part 2–25: Particular requirements for microwave ovens and combined microwave ovens (IEC 60335–2–25: 2010 modified + A1: 2014 + A2: 2015); German version EN 60335–2–25: 2012 + A1: 2015 + A2: 2016

DIN EN 60335–2–6; VDE 0700–6: 2016–02 Safety of electrical appliances for household use and similar purposes - Part 2–6: Particular requirements for fixed cookers, hobs, ovens and similar appliances (IEC 60335–2–6: 2014, modified); German version EN 60335–2–6: 2015

DIN EN 60335–2–9; VDE 0700–9: 2011–10 Safety of electrical appliances for household and similar purposes - Part 2–9: Particular requirements for grills, toasters and similar portable cooking appliances (IEC 60335–2–9: 2002, modified + A1: 2004 + A2: 2006); German version EN 60335–2–9: 2003 + A1: 2004 + A2: 2006 + A12: 2007 + A13: 2010 + AC: 2011

Usability: DIN EN 60705; VDE 0705–705: 2015–11 Microwave devices for household use and similar purposes - Methods for measuring usability (IEC 60705: 2010 + A1: 2014 + A2: 2006); German version EN 60705: 2015

Security checks

Test of stability
With the exception of built-in devices, devices whose doors have hinges on the underside have a mass placed in the middle of the open door. The mass for portable devices is 3.5 kg and for stationary devices 7 kg. The device must not tip over.

Check the door seals, hinges
According to these tests, the cooking chamber door is operated 100,000 times, with the door seals being soiled with cooking oil after every 10,000 operating cycles in order to simulate the toughest operation that could occur in practice. This test of the appliance door corresponds to an assumed period of use of around 30 years, assuming it is used 10 times a day.

The safety of doors hinged below is checked with the help of a hardwood stick. The rod is placed on the hinges and an attempt is made to close the door with a force of 90 N. The stick corresponds, for example, to an item of cutlery that has been accidentally jammed.

Impact test
The microwave door is closed and three blows with an energy of 3 J (joules) each are applied to its outer surface. The impact is applied by means of a steel ball with a diameter of 50 mm and a mass of about 0.5 kg. The ball is hung on a string and dropped onto the surface of the door like a pendulum.

These explanations show part of the mandatory safety regulations. They are adhered to by the manufacturers during production by constantly checking the microwave devices. In addition, the dielectric strength of the device is tested with 1000 V.

Leakage radiation
Leak radiation is the energy that can still be measured outside the microwave device. According to DIN EN 60335-2-25; According to VDE 0700-25: 2007-04, the permitted leakage radiation for microwave devices at a distance of 5 cm from the door must not exceed 5 mW / cm during the service life (approx. 10 years)2 (Milliwatts per square centimeter) with load (water). When idling, 10 mW / cm2 not be exceeded. These values ​​are harmless. The body's own heat production served as a reference point for this security. The limit value was derived from the basal metabolic rate (amount of heat and calories that an empty body generates in 24 hours when completely resting) of the person (70 W) and his body surface (approx. 7000 cm2) calculated. This results in a heat emission of around 10 mW / cm for humans2. The mean radiation energy of the sun incident on earth is approx. 100 mW / cm2.

If different limit values ​​are given, e.g. from other countries, it must be taken into account that these are based on different measurement methods or that other definitions apply.

When comparing these different standards, a distinction must always be made between the permissible leak rate, i.e. the emitted microwave energy, and the permissible radiation dose, i.e. the microwave energy incident on humans. For the leak rate, in addition to the energy density, the distance from the leak point must be specified. For the permissible radiation dose, in addition to the energy density, the radiation time permissible for this energy density must be specified. If this is not adhered to, discrepancies will inevitably arise. In reality, however, the existing standards in the eastern and western countries hardly differ in practice.

The limit values ​​stipulated in Germany in DIN EN 60335-2-25; VDE 0700-25 are also valid at the international level in IEC-60335-25 (International Electronic Commission).

The limit of 5 mW / cm permitted outside of microwave ovens2 is undercut in practice by a factor of 10 and averages a maximum of 0.5 mW / cm2. Since, according to the laws of physics, the energy density decreases with the square of the distance, of this energy, e.g. at a distance of 0.2 m, only approx.0.03 mW / cm remain2. In production, the permissible leak rate is checked several times for each device.

Nameplate

Important information about the safety of electrical devices can be found on the type label, which is permanently and legibly attached to the device. It contains safety signs and essential characteristics for the correct connection and use of the device.

GS mark

GS = tested safety. The manufacturer or importer of technical work equipment may label it with the GS mark if it has been subjected to a type test by a notified certification body and all requirements of the GSG have been met. The GS mark may only be used in connection with a test center identification. z. B. VDE-GS mark, TÜV-GS mark, etc.

If the VDE test center carries out the test, the manufacturer can choose whether he wants to attach the VDE mark alone or the VDE-GS mark to his products under the same test conditions.

CE mark

The CE marking is mandatory for every type plate. The CE mark documents that the guidelines that apply to the device are complied with at the time of issue. The manufacturer applies these guidelines and creates documentation about them. The application of the guidelines is proven with a so-called "manufacturer's declaration" on request (e.g. for authorities). If the manufacturer has the necessary tests carried out in a qualified third-party laboratory (e.g. VDE), the manufacturer can also indicate this on the nameplate.

Effect of microwaves on food

Different foods are suitable for use in microwave ovens in different ways.

  • Microwave ovens can be used well for defrosting and warming food (no loss of liquid, no sticking).
  • Most foods can be cooked with little or no liquid added, i.e. the food is not so leached. The vitamins are spared.
  • There are limits to cooking meat and baked goods with microwaves. This includes typical pan-fried dishes, e.g. schnitzel, steaks, etc. During the short cooking time, no roasting substances can form and thus no crust can develop.
  • Crispy roasts and cakes can be prepared faster and more energy-efficiently with microwaves, which are combined in the microwave combination device with top and bottom heat, convection or convection grill.
  • All dishes that are to be steamed or steamed, e.g. ragout, sliced ​​meat and fish / vegetable dishes, can be optimally prepared with microwaves.
  • Large quantities of pasta are less suitable for preparation in a microwave oven because it saves neither time nor energy.
  • There is no such thing as a typical "microwave taste". The typical flavor of many foods, e.g. milk, fish and vegetables, is retained to a greater extent.
  • To kill pathogens such as parasites, trichinae and nematodes as well as the Salmonella bacteria, only the temperature and exposure time are decisive. It is necessary to heat the food evenly up to the core to 70 ° C – 80 ° C. It does not matter whether you cook with microwaves or in a conventional way (e.g. electric hob, grill, etc.).
  • Device manufacturer information on the maximum cooking quantity and layer thickness as well as the recommendation of the equalization time for temperature equalization are tried and tested orientation aids. Taking this information into account, a sufficiently high germ reduction can be achieved when cooking food with microwave ovens, corresponding to conventional methods.

When reheating dishes and using frozen ready-to-eat meals, attention must also be paid to the aforementioned intensive warming (equalization period). Simply heating such dishes to consumption temperature can be hygienically questionable. When cooking and reheating food and especially when defrosting raw food (e.g. frozen chicken legs), the general principles of kitchen hygiene must always be observed.