New-TechEurope Magazine | November 2017 | Digital Edition

Sensors Special Edition

conductive, non-live exposed parts e.g. housings. Additional protection safeguards against dangerous electrical shock and situations in which basic and fault protections fails. Fire protection uses RCDs to prevent electrical fires caused by insulation faults. Types of RCDs RCDs are characterised by the residual current waveforms they can detect and respond to, if they are dependent or independent to the voltage, and whether their trip response is instant or delayed. The ability to respond to various current waveforms is important, and the chosen RCD has to be suitable for each waveform type. It is also very important to note the different tripping level for each waveform. Several types of RCDs are available, each for use with differing current waveforms. Type AC only detects sinusoidal fault currents, but these are not permitted in many EU countries. Type A detects both sinusoidal and pulsating DC residual currents. Type F RCDs are primarily used for single phase frequency inverter applications to handle the residual currents with frequency mixture up to 1kHz which typically occur on the output of a single phase frequency converter. They can also detect sinusoidal AC currents as well as pulsating DC currents. Type B devices can detect sinusoidal AC and pulsating DC as well as smooth DC fault currents. RCDs of this type are designed for use in three-phase systems. Type Bfq comply with Type B requirements while being designed for use in circuits that include powerful frequency converters for speed- controlled drives. Type B+ have a frequency tripping response defined up to 20 kHz and provide superior protection from fire risk caused by

ground fault currents in applications with electronic drives. In addition to the above, Type G, “Li” and S RCDs have a trip delay and hence a surge-withstand capability to avoid nuisance tripping. Classification of RCDs RCD is the general term for all types of residual current protective devices. A standard residual current circuit breaker is called an RCCB and some further types of RCD exist; these are described below. RCD relays are devices with a separate current transformer and contactor to handle higher current ranges up to 400 A. However, RCD relays combined with moulded case circuit breakers (MCCB) offer a solution up to 1800A. A residual current operated circuit breaker with overcurrent protection (RCBO) is a combination of an RCCB with a miniature circuit breaker (MCB). It provides overload, short circuit, shock protection and fire prevention from a single device. Many different combinations can be made from available RCD add-on

blocks and MCBs without having to stock a large number of products. This gives a high degree of application flexibility and makes it easy to customize the combination of RCD and MCB devices. RCD add-on blocks are also available for MCCBs to cover applications up to 250A. Digital RCDs Digital RCDs combine protection functionality with a set of digital features, working together to provide maximum circuit status information and increase the protected system or machine’s availability. The digital technology is applied to both RCCBs and RCBOs. Local pre-warning LEDs and remote pre-warning potential- free outputs can be provided in the RCCB. These pre-warnings allow maintenance staff to resolve creeping problems before they lead to interruptions or failures. Cost savings accrue due to the reduction in unscheduled service callouts and further savings arise because test intervals can be extended to once a year only.

Image 1: Direct and indirect contact risks

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