Pilz on EMC compliance for automation
Earthing a shielded cable
on
EMC compliance for
automation:
A common problem encountered in modern automation is insufficient consideration given to EMC. But what is EMC?
EMC is not a formula by a German physicist; rather it stands for Electro Magnetic Compatibility.
What exactly does this mean? EMC refers to the ability of an electrical device to work satisfactorily in its electromagnetic environment without influencing the surrounding devices, or being influenced by the surrounding devices.
In essence EMC deals with the minimisation of Electromagnetic Interference (EMI) such as electrical noise or radio frequency interference (RFI).
EMC compliance is required to ensure that an installation is as noise-free as possible, or to minimise the impact noise may have on an installation.
Typical sources of electrical noise are devices that produce quick changes in voltage or current such as:
- Large electrical motors or drive being switched on or off
- Fluorescent light tubes
- Lightning strikes
- High voltage surges
- Welding equipment
- Non EMC compliant equipment or installation
It’s important to note that any EMC issues may not always be there – it is usually an intermittent problem, and the problem may not necessarily show itself until months after installation.
Possible EMC problems may arise from the following:
- Poor earthing
- High noise devices too close in the control cabinet
- Poor cable layout
- Suppressors not used on coils of AC contactors or valves
- Flyback diodes not present on DC contactors or valves
- Non-shielded/screened cable or shield/screen not connected to earth
- Drives not installed to specifications i.e. without filters
Some important items to note with earth connections are:
- A cable cross section of at least 2.5mm2
- As short as possible connections (to minimise impedance)
Industrial communication networks
For industrial communication networks the IEEE 518-1982:IEEE Guide for the Installation of Electrical Equipment to Minimize Electrical Noise Inputs to Controllers from External Sources standard lists a set of tables indicating separation distances for various classes of cables.
It is possible to differentiate between cables according to their function.
The following groups exist:
- Group 1: Data and supply lines for DC voltages below 60 V and AC voltages below 25 V
- Group 2: Data and supply lines for DC voltages from 60 V to 400 V and AC voltages from 25 V to 400 V.
- Group 3: Supply lines above 400 V
Cabling inside buildings:
- The cable groups listed above should be laid separately
- Cables of the same group can be laid within the same cable duct
- Cables from group 1 and group 2 should be laid in separate groups or in cable ducts which are at least 10 cm apart
- Cables from group 1 and group 3 should be laid in separate groups or in cable ducts which are at least 50 cm apart
- Data lines should be laid as close as possible to an earthed surface
Industrial communication cables are susceptible to noise due to the sensitive nature of the data they carry, and the fact there are frequently long cable runs. For this reason most industrial communication networks use shielded or screened cables.
The shield should always be connected to earth, otherwise it is rendered useless. It must only be connected to earth at one point in the network.
Any more than one earth point can create earth loops or circulating currents. Networks such as SafetyBUS p may use repeaters or Routers that create additional segments in the network.
In this case, each segment should have the shield earthed at one point.
The best way to connect a shield to earth is to pin it to the earth bar. This ensures a very good connection.
EMC compliance is a requirement for CE marking. Electronic products that can be sold in Europe should meet the EMC requirements.
10-Dec-2007