How Type B residual current devices protect against shock in the most challenging applications
Credit to Author: Natacha Lucius| Date: Thu, 09 May 2019 11:00:41 +0000
Shocks from contact with electrical conductors continue to occur every year in every country. For example, in the UK, the Health and Safety Executive documented that “Each year about 1000 accidents at work involving electric shock or burns are reported … Around 30 of these are fatal.”
In my first post in this two-part series, we looked at how new types of electrical loads are presenting challenges in protecting people against shocks. I described the four main types of residual current devices (RCDs) designed to provide shock protection in different types of applications. In simplified terms, you choose the type of RCD based on the expected residual current waveforms: pure sinusoidal (Type AC), pulsating DC (Type A), composite up to 1000 Hz (Type F), or non-sinusoidal and smooth DC (Type B). See my first post for examples of the types of loads that produce such currents.
In this post, we’ll have a closer look at Type B RCDs, as well as the SI type, that has been developed by Schneider Electric to improve continuity of service.
How do Type B RCDs work?
Type B RCDs are usually designed with two residual current detection systems. The first uses ‘fluxgate’ technology to enable the RCD to detect smooth DC current. The second uses a technology similar to Type AC and Type A RCDs, which is voltage independent. This means that, even if line voltage is lost, a residual current fault can still be detected and people are still protected.
The challenge of switch mode power supplies
The proliferation of switch mode power supplies (SMPS) inside equipment is due to their benefits of compactness and energy efficiency. However, these and other types of switching mode converters generate substantial levels of high frequency ‘capacitive earth leakage current’. On its own, this non-energetic current is not dangerous, but it can cause unwanted tripping of protection devices like RCDs.
Another factor is the ventricular fibrillation threshold for humans. The tripping threshold of the RCD needs to be below this threshold to ensure protection of humans, while at the same time be higher than the curve of capacitive earth leakage current to avoid any unwanted tripping.
To address this challenge, Type SI RCDs are designed for use anywhere switching mode converters are being used, such as SMPS, one phase variable speed drives, motors, pump, UPS, one phase photovoltaic (PV), and electronic ballast lighting. Type SI RCDs also provide protection against network transient disturbances like lightning.
Typical applications for Type B RCDs
Type B RCDs are ideal for use with 3-phase EV chargers, as EV manufacturers state that DC current leakage can occur while charging. Though a 6 mA detection device (RCD-DD) inside the charger can provide protection, a Type B RCD ensures better continuity of service and protection because it will detect DC current and its tripping value is much higher than 6mA DC. Unlike an RCD-DD, it will also detect earth leakage current at frequencies higher than 50/60Hz.
For 3 phase PV systems, a Type B RCD is also recommended to protect against electrocution, due to DC/AC converters being used.
3-phase drives can create earth leakage currents at various frequencies, as well as DC. This can include applications such as a crane powered from a mobile panelboard, or an elevator. These require a Type B RCD for protection, with the trip current rating selected based on the specific application.
Coordinating RCDs at different levels of a circuit
Selective-tripping coordination between RCDs is achieved either by time-delay or by a subdivision of circuits. Such approaches avoid the tripping of any RCD, other than the device immediately upstream of a fault position. To enable time-delayed operation a selective RCD, known as Type S, is able to withstand a residual current during a specified time, without tripping.
Through selection of different tripping thresholds and tripping times, discrimination between RCDs can be achieved. Also, in terms of sensitivity to DC current, by choosing RCDs with higher tripping levels for higher levels of a circuit, you will avoid any blinding effect by DC current. In other words, an RCD at a lower level of a circuit, closer to the earth leakage fault, will trip sooner at a lower threshold. This will isolate the fault, enabling continuity for the rest of the circuit.
Schneider Electric offers a complete range of RCDs, from add-on devices for circuit breakers to complete residual current circuit breakers (RCCB), including Type B and Type SI models. For more information about choosing the right RCD type, including relevant standards and other considerations, download the white paper “Why to Choose Type B Earth Leakage Protection for Safe and Efficient People Protection.”
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