Functional description SCT5xx
The Beckhoff SCT5xxx ring-type current transformer for differential current measure the instantaneous real-time value of both DC and AC residual current using the built-in transformer head with integrated balanced flux gate detector. To protect the function and especially the fire protection of the electrical system, the SCT converts the True RMS (TRMS) value of the residual current, which corresponds to the amount of heat dissipated in the insulation. To provide a high degree of flexibility in the use of the SCT and to be suitable for as many potential applications as possible, these devices are equipped with three user-selectable parameters that affect the final TRMS value, which is output as an analog value at the 4-20mA output and displayed at the relay output and status LED.
The "Frequency Range (FR)" parameter limits the frequency bandwidth of the measured differential currents. After the desired frequency range has been selected, the "Integration time (IT)" parameter determines the length of the integration window for the TRMS value. Finally, the TRMS value is scaled into two different ranges according to the selected "Rated RC limit" (details in the electrical specifications tables). After these intermediate steps, the TRMS value of the differential current is ready to be output to the analog 4-20mA output. However, the behavior of the relay output and the status LED is influenced by some additional processing blocks as described below.
When the level set by the "Rated RC Limit (RL)" is exceeded by the weighted TRMS, the relay output and status LED toggle the state. The weighted TRMS is obtained from the TRMS value by applying a weighting filter that attenuates the residual currents with higher frequencies while allowing the low frequency residual currents to pass completely in accordance with the applicable product standards.
The reason for this choice is that higher values of high-frequency residual currents typically occur in switch mode power supplies when rapidly changing voltages are applied to, for example, the cable insulation, the output dv/dt filters, the EMC input filter and the Y capacitors to ground.
The determination of the status of the relay output is done with a comparator with hysteresis, i.e. after activating the relay, the TRMS value of the residual current must decrease by a certain hysteresis before the relay output is disabled. In case of repeated activation and deactivation of the relay, it will be locked in the activated state and the test button must be pressed for at least 3 seconds to reset it.
