Voltage-dependent residual current devices (RCDs) generally employ an electronic amplifier to provide an enhanced signal from the sensing coil to operate a trip solenoid or relay. RCDs of this type are defined as “voltage-dependent” because they rely on a voltage source, derived from the mains supply, or an auxiliary supply, to provide power to the amplifier.
Figure 1: Structure of a voltage-dependent RCD
Since the design of voltage-independent RCDs is naturally immune to power supply voltage, some RCD manufacturers with a strong footprint on voltage-independent RCD business claim the risk of failure about voltage-dependent RCDs.
However, it might not be an objective view. The article discusses the impact of power supply voltage on voltage-dependent RCDs, which provides inputs for decision-making about whether to use voltage-dependent RCDs during electrical installation.
The following scenarios are often mentioned when discussing the voltage dependence of residual current devices. Thus, we will take an in-depth look at how voltage-independent RCDs perform in these scenarios.
Loss of neutral
The loss of neutral may happen for several reasons: defective breakers that do not close the neutral pole properly, loose connections, works on electrical lines or nearby electrical conductors etc. In case of loss of the neutral, hazardous voltage is still present but voltage-dependent RCDs without functional earth (FE) cannot trip in case of fault, with risk for human life.
To improve the level of protection provided by voltage-dependent RCDs, in the event of neutral breaking, the product is requested to be supplied by redundancy via the FE (line to FE supply instead of line to neutral) in some countries and regions, allowing it to continue to perform its protection function. The author of this article advocates installing voltage-dependent RCDs with FE in all countries where voltage-dependent RCDs are allowed for households and similar applications.
Loss of a wire for multi-phase RCD
If the electronic system of a multi-phase RCD only draws power from one phase and the neutral, it might not trip in the occurrence of loss of a wire. Even worse, the user cannot see any evidence of the problem, because loads supplied by uninterrupted wires will work properly.
Therefore, the RCD standards in many countries – such as the UK and China- require a multi-phase RCD with a voltage-dependent design that must work functionally in the occurrence of loss of any two-phase wires for multi-phase RCD.
Under voltage of supply lines
If the power supply voltage falls to a value lower than the operating value of the voltage-dependent RCD and greater than a dangerous level, the RCD will not trip but the risk of dangerous faults is present. Thus, the product and installation standards in many countries require voltage-dependent RCDs to work with a minimum voltage of 50V AC, which significantly mitigates this risk.
In China, an over-voltage/under-voltage (OV/UV) protection device must be installed at in-door distribution according to its national standards for electrical installation for residential buildings. The OV/UV protection device can switch off the power supply in case the main voltage drops below the defined threshold, which also mitigates the risk of RCD failure in the occurrence of under-voltage fault.
Ground fault in the system
A ground fault in the system can cause a decrease in the power supply voltage to the residual current devices. What level of the voltage decrease is further related to the installation position of RCDs. If a ground fault happens at the load and the RCD is installed close to the load, the voltage supply to the RCD can be reduced to 25% Un. Fortunately, The value of 25% Un is higher than the 50V AC – the minimum operation voltage of many voltage-dependent RCDs.
Figure 2: RCDs installed close to the loaded
The development of residual current devices (RCDs) in China, the US, and many Asian countries took the path of voltage-dependent types (also called electronic types in some articles). Research and operation worldwide have proved that a voltage-dependent design nowadays can provide completely comparable protection to a voltage-independent design.
To conclude, voltage-independent RCDs do have their advantages in handling abnormal power supply voltage, but it does not necessarily suggest that voltage-dependent RCDs cannot provide sufficient protection. Firstly, the standards for voltage-dependent RCD keep developing, and the devices are becoming more and more robust. Secondly, an RCD never works in isolation but always as part of an entire installation, wherefore it is never solely liable for the safety of the entire installation. Thirdly, voltage-dependent types nowadays protect against smooth DC leakage current, which is not feasible with voltage-independent types.