Measure and avoid electromagnetic shield currents
In industrial data communication, interference sometimes occurs due to high-frequency currents affecting the data line screen. By measuring the compensating currents on the data line screen, it can be clarified 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 counteract it.
EMCheck® LSMZ I is a measuring tool that is specially designed to measure leakage currents and shield currents in the frequency band 50/60 Hz and 5 Hz to 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® The LSMZ I has a large opening of 2.8 cm, which allows the cable to easily pass through for smooth measurement even in unfavorable installation conditions. It is possible to perform permanent measurements, such as maximum currents, by using the "hold" function.
Experience from Indu-Sol shows that a screen current of <40 mA is acceptable regardless of machine specifications. In case of larger shield and leakage currents, additional measures must be taken to improve the grounding or remove 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 earth conductors is (for safety reasons) 300 mA. Protective conductors running inside a cable can carry no more than 5% of the flowing phase current.
