Measure and avoid electromagnetic shield currents
In industrial data communication, interference sometimes occurs due to high-frequency currents affecting the screen of the data line. By measuring the compensation currents on the data line screen, we can clarify that the fieldbus network is not the direct cause of the fault. It is important to identify the real cause of the error and take steps to correct it.
The leakage current clamp EMCheck® LSMZ I is specially designed to measure leakage currents and shield currents in the frequency band 50/60 Hz and 5 Hz – 1 kHz. The adjustable measurement range is divided into levels from 30 μA to 100 A, where the lower level is particularly interesting for measuring the shield current in the data cable.
EMCheck® LSMZ I has a large opening of 2.8 cm, which allows the cable to easily pass through for convenient measurements even in unfavorable installation conditions. It is possible to carry out permanent measurements, such as maximum currents, by using the "hold" function.
Based on experience from Indu-Sol, it has been shown that a screen current of <40 mA is sufficient regardless of machine specifications. In case of larger shield and leakage currents, additional measures must be taken to improve grounding or eliminate the source of the interference.
The official maximum electrical conductivity value for a shield is 150mA, and anything above that will result in reduced shielding power and electrical overloading of the devices. The limit value for grounding conductors (for safety reasons) is 300 mA. Protective conductors running inside a cable can carry a maximum of 5% of the flowing phase current.
