Protection against earth faults — unpacking the technical requirements of RCDs

Residual Current Devices (RCDs) have been used for many years in Australia as the front-line protection for people against the risk of a fatal electric shock. In modern electrical systems we utilise three layers of protection. The first layer is designed to prevent contact with live parts using passive measures like insulation, barriers or enclosures. The standards refer to this as basic protection.

Basic protection could be sufficient unless there is a fault in the insulation or equipment, or if the protection has been removed intentionally due to maintenance, etc. Basic protection does not provide a means to automatically disconnect supply in the case of a fault condition.

The second level of protection defined by the standards is fault protection. This can be achieved by a second independent layer of insulation, commonly known as double insulation. In the event of an earth fault, the fault protection can also be achieved by containing the live parts in an earthed metal housing. A fault current could flow from the live parts to the earthed enclosure and then return to the power source. A protective device can automatically disconnect supply in the event of abnormal current flow to earth. The value of the earth current will be dependent upon various impedances in the circuit.

Australian and international standards define the next level of protection as ‘additional protection’. This is used in final sub-circuits to provide automatic disconnection of supply in the case of an earth fault current that could be dangerous to people. The standards define 30 mA as the required protection level for people against faults in these final sub-circuits, as this has been proven to be the current value where irreversible damage to the human heart can occur if exceeded, often leading to death. In high-risk areas a higher sensitivity of 10 mA is required to provide the protection necessary. These areas can include hospital zones where medical equipment is in contact with patients.

Residual Current Devices come in many forms, from the physical construction, the technology incorporated into the design, the electrical characteristics, and the reaction of the device to various waveforms of earth leakage current. An earth leakage current is not always sinusoidal like the supply and is affected by the characteristics of the respective load. ABB’s latest whitepaper on protection against earth faults covers the technical details and requirements of RCDs available.

There are four main types of RCD available today. Type AC, where the earth leakage is sinusoidal having a frequency identical to the supply frequency, ie, 50 Hz. Typical loads are simple resistive, inductive, or capacitive loads, generally without electronics, such as tungsten and halogen lighting, ovens and heaters.

Type A RCDs have the same characteristics as a Type AC, with the additional capacity to detect residual currents that are only partially sinusoidal, or a pulsating DC. These current wave forms are typical of networks supplying power electronics components, such as diodes and transistors. With the increased adoption of electronic appliances, such as dimmers, computers and television sets, the currents and voltages in the network have experienced a progressively higher contribution from switched mode power supplies, etc, where thyristors control the power supply by alternately switching the current flow on and off through powered appliances. This explains the only partial sinusoidal nature of earth leakages in networks with a significant quantity of electronic loads.

The Type A RCD will detect residual currents with an AC waveform, pulsating DC waveform or a smooth direct current of up to 6 mA. In recognition of the increased presence of these loads in Australia, the standards are transitioning to requiring Type A RCDs as the minimum requirement from early 2023.

In addition to detecting residual current waveforms of the Type A and Type AC RCD, a residual current protective device of Type B can detect smooth DC residual currents. Type B RCDs are recommended for use with drives and inverters for supplying motors for pumps, lifts, textile machines, machine tools etc., since they can detect a continuous fault current with a low-level ripple and have tripping values defined up to 100 kHz.

Type F RCDs are an extension of Type A and are recommended for loads with single-phase inverters and similar equipment (eg, modern washing machines, air conditioners). For the Type F RCDs, additional tests have been added to those of a Type A, to simulate the ground fault in the presence of a single-phase inverter. These RCDs can detect residual currents with a harmonic content at frequencies higher than the power frequency, up to 1 kHz, typically found in networks with single phase power converters that modulate the power flow by shaping current and voltage according to very general waveforms.

RCDs are very sensitive by their nature as they are a protective device. Some installation issues that need to be understood are the causes of unwanted tripping, often called nuisance tripping, which can be the result of accumulated leakage from various loads.

A new whitepaper titled “Protection Against Earth Faults” from ABB Australia provides details of the history and operation of RCDs, providing insight into the various technologies and applications of RCDs.

Download the whitepaper here.