Protecting man and machine against damaging residual currents

How the correct choice of the residual current device ensures operator safety, machine uptime and profitability

Introduction

Technology continues to rapidly evolve, and machine builders are faced with designing products to enable their customers to produce faster, and more efficiently, at a lower cost.  Product lifecycles are becoming shorter, placing demands on machine builders to design and develop highly customized machines.  Exporting equipment remains complex, as electrical standards often differ from country to country.

Manufacturers and their end users are looking more towards their suppliers for support, due in part to in-house engineering team size reductions.  One aspect of this is electrical circuit protection, which can impact both safety and machine up-time.  Electrical faults can have devastating effects on the human body causing injury, and lead to machine failure and fire – affecting productivity.

The function of a residual current device

Residual currents in electrical systems are caused by either fault conditions or generated by system components such as frequency converters.  The role of a residual current device (RCD) is to automatically disconnect the power supply if the fault can pose a risk.

Residual currents can arise for many reasons and take many forms.  The biggest challenge is being able to differentiate between all the possible forms of residual current, while protecting people and equipment, but on the other hand avoiding nuisance tripping related to system caused earth leakage currents.  The solution is to recognise the different forms of earth leakage current that can occur, and design in the appropriate RCD.

Types of protection

Devices have to shield operators and technicians from the dangers of electric current, both during normal use and fault conditions.  Potentially life-threatening accidents can result from either direct or indirect contact.

Direct contact refers to when a person touches a live electrical part that is intended to be live during normal operation.  Indirect contact is a fault when a conductive, non-live exposed and touchable part becomes live due to an electrical fault.

There are multiple types of protection required; basic, fault, additional and fire.  Basic prevents contact with live parts through insulation or a cover.  Fault protection includes situations where an insulator (basic protection) fails and the RCD switches off before dangerous voltages can occur on 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.

Global standards

Protection and trouble-free operation depends on paying attention to compliance with all relevant local and global standards and guidelines.

Standard IEC/EN RCDs can be used worldwide except in the USA and Canada. In the North American market, UL standards are used instead of international IEC standards, and so for export to this region RCDs must be available in special UL approved versions.  Country specific approval is also required in a number of other countries.

Ensuring that the RCDs conform to international standards, such as IEC/EN 61008 or UL1053, and that they carry the corresponding marks, as Eaton’s do, is essential for guaranteeing safety.  By specifying one product that is a world market product and can be used globally will ultimately save time and associated costs when exporting.

Electrical interference problems and solutions

There are various electrical interface problems that can occur and machine builders need to consider when specifying RCDs.

Leakage currents are currents that conduct to ground during normal operation without any insulation fault. RCDs though cannot distinguish between leakage currents and fault currents, and will trip if the currents’ sum exceeds their tripping value.

Type F, U or Bfq RCDs have tripping curves that are set to be insensitive to system caused earth leakage currents. This prevents nuisance tripping errors in industrial applications with powerful frequency inverter controllers.

Dynamic leakage currents are transient currents to the ground conductor. To prevent this unwanted tripping, the use of short-time delayed RCDs Type G or Li are recommended.

Nuisance tripping in RCDs can also be caused by high currents from inductive loads. According to the product standard RCDs must tolerate up to six times their rated current to provide resistance to nuisance tripping.

Over-voltages created by thunderstorms can lead to nuisance tripping of the RCD. Eaton offers the Type G RCD specified according to ÖVE E 8601 to avoid this problem.

Summary

RCDs can protect both man machine from harm, eliminate the risk of fire and reduce machine downtime by detecting and reacting to residual currents. As these currents can arise for many reasons and take many forms, and it is essential to choose RCDs with tripping characteristics that ensure protection from genuine fault conditions, while avoiding lost production time due to nuisance tripping.

Machine builders must consider the relevant international legislation and installation aspects of power protection systems; including the type of earthing system being used, installation standards, and electrical interference problems and solutions.

It is part of Eaton’s commitment to providing circuit protection solutions, from initial design steps through to installation, maintenance anad spares holding. Eaton’s global organisation and portfolio of internationally approved, innovative components and technologies is complemented by local production facilities, expertise and support.

Image 1: Direct and indirect contact risks