A Type B ground-fault circuit interrupter is the recommended choice when you need to protect an inverter-controlled pressurization unit, because ground fault currents can also be continuous.
In this response, we clarify the different classes (AC, A, and B), fault current waveforms, and selection criteria, with one key point: sensitivity and response time cannot be “guessed, ” but must be defined during the design phase based on the system’s configuration.
Table of Contents
What is a ground-fault circuit interrupter, and what does it “detect”?
A ground-fault circuit interrupter (GFCI) is a safety device that automatically cuts off the power supply when it detects a ground fault current exceeding a certain threshold.
There are various types of differentials on the market, which are classified according to different parameters. Among the most important is the “class” (or type), which is related to the differential’s ability to detect certain current waveforms.
Why does the inverter change the type of ground fault current?
The now widespread use of electronic devices in both civilian and industrial settings can result in leakage currents to ground with non-sinusoidal components in the event of an insulation failure.
In practice, in addition to the case of sinusoidal current, leakage currents may occur:
- unidirectional,
- buttons,
- continue.
In the case of an inverter-controlled pressurization unit, this is precisely the condition associated with a continuous leakage current. And this is where the choice of residual current device (RCD) becomes critical.
Classification: AC, A, and B differential types
With regard to the leakage currents to which the residual current device is sensitive, the rules classify them into three types: AC, A, and B.
The key point is not “which one is absolutely better,” but which one is consistent with the waveform of the possible ground fault currents that the system can generate.
When electronic equipment (such as inverter drives) is present, this check becomes even more important, because leakage currents may deviate from the “traditional” sinusoidal behavior.
Type B Ground-Fault Circuit Interrupter with Inverter: When and Why
If the pressurization unit is controlled by an inverter and a continuous leakage current condition is present, a conventional residual current device will not function properly.
Class B residual current devices, manufactured in accordance with IEC/EN 62423, are capable of:
- detect all waveforms normally detectable by Class AC and A differential meters;
- In addition, it can also detect continuous ground fault currents.
For this reason, their use is highly recommended in presence of inverter drives—that is, devices capable of generating continuous fault currents. In other words: if the inverter can cause a “continuous fault,” you need a residual current device that is sensitive to that type of fault as well.
Sensitivity and Response Times: What the Designer Must Determine
Even once the correct type has been identified (for example, a Type B ground-fault circuit interrupter), there are still two key aspects that should not be treated as “standard sizes” that are always valid:
- Sensitivity (nominal differential operating current)
- Response time
These parameters must be calculated by the electrical designer based on the specific configuration of the system. This is an essential step because it is the actual configuration—not a general concept of the system—that determines which combination of sensitivity and response times is consistent with the required protection.
Common Mistakes to Avoid When Choosing a Differential
When protecting an inverter-controlled pressurization unit, some common mistakes stem from underestimating the role of leakage current waveforms.
In particular, be careful not to:
- choosing a “traditional” residual current device without verifying whether it is suitable for use in the presence of continuous ground faults;
- confusing the “class” (AC, A, B) with a simple preference for a brand or model, while ignoring the ability to detect waveforms;
- determine sensitivity and timing without a calculation based on the system configuration (a decision that is up to the electrical designer).
Conclusion
For an inverter-controlled pressurization unit, the potential presence of continuous leakage currents makes the selection of the residual current device a matter of technical precision:a Type B residual current device (IEC/EN 62423) is highly recommended because it detects continuous earth fault currents in addition to those normally detected by AC and Type A devices.
