How to avoid EMS problems
Original article date: June 2000
Put these guidelines into practice when using enclosures and cases, and you’ll be OK with OKW!
Cases have quite an influence on the electromagnetic compatibility of a product. The main reasons for EMC interference are:
- Conductive – within low frequency range below 100MHz
- Radiation from Cables – within medium frequency range (30-300MHz)
- Penetration through material and openings – high frequency range > 300MHz.
Hence, in order to reduce interference radiation, shielding of cases for ranges exceeding 100MHz is of vital importance. Shielding is defined as the ratio of field intensity in front of and behind the shielding wall. The effect of electromagnetic shields is based on a combination of several effects: if an electromagnetic wave hits metal, part of the wave is reflected. Another part of the wave penetrates the metal layer and is transformed into heat. The remaining part penetrates the metal layer and finally represents the source of interference.
As almost every case is provided with openings and cable connections which reduce the shielding effect, it is important to observe the following recommendations. The interference energy which is released through the opening largely depends on the following factors:
- Maximum linear size
- Frequency of the interference energy
- Polarisation of the wave
- Distance from radiation
- Field type.
Experience suggests that an opening should have an attenuation performance of at least 20dB. This means:
| Frequency (M Hz) 30 100 300 500 1000 |
Max length of Slot (cm) 45.7 15.2 5.1 3.1 1.5 |
In order to achieve a shielding effect of approximately 40dB, OKW applies an aluminium layer which is exclusively attached to the inside of the case. Gaskets made of silver and silicone are also recommended, though these are unsuitable for humid conditions. Display windows should be provided with a conductive foil and should have a terminal lugs for connection to the case shield. To reduce the radiation between cable, plug and case, good design practice is to:
- Reduce the number of contacts
- Remove lacquer, colours and oxides
- Ensure direct and complete contact between cable shield and case
- Use shielded cables.
In order to maintain the protection class of the case and to improve the cable radiation, a suitable screw fitting for the cables is necessary. The use of external accessories may influence the protection class and can cause corrosion. For this reason, do consult the supplier before use.
- OKW
June 2000