Index

• Starting With the Names.
  ELCB , RCD , RCCB , RCBO , GFI
• Types of RCD Type AC , Type A , Type B , General Type , S Type , Differentiation based on mains supply conditions.
• Choosing RCDs AC or A , Discrimination in terms of I^n and Trip Times. , Response to abnormal supply conditions , Solid neutral?
• Problems with RCDs Nuisance tripping. , Compliance with standards. , Standing leakage currents , Filtering circuits , Other factors , Supply side fault appearing as load side fault.
• Voltage Dependent Vs Independent. Use of electronics in VI types. , Further reading
• International Trends in RCD's. Discrimination between upstream and downstream faults. , Distinguishing between resistive and reactive currents. ,Combination with monitoring circuitry.
• About the Author

Starting With The Names

Possibly the greatest confusion with regard to RCDs concerns the plethora of names attaching to these devices. Various names are used in countries all over the world, but in Ireland and the UK the following are the most frequently used terms.

RCD, RCCB, RCBO, ELCB

I am going to start with the name ELCB because it is the name that causes most confusion.

ELCB = Earth Leakage Circuit Breaker. There are two types of ELCB, the voltage operated device and the differential current operated device. For the convenience of this article only (and at the risk of causing even more confusion) I will refer to these as vELCB and iELCB. vELCBs were first introduced about sixty years ago and iELCBs were first introduced about forty years ago.
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The principle of operation of the vELCB is as follows. Under normal conditions the closed contacts of the vELCB feed the supply current to the load. The load is protected by a metal frame, such as in an electric cooker. The vELCB also has a relay coil, one end of which is connected to the metal frame and one end connected directly to ground. A shock risk will arise if a breakdown in the insulation occurs in the load which causes the metal frame to rise to a voltage above earth. A resultant current will flow from the metalwork through the relay coil to earth and when the frame voltage reaches a dangerous level, e.g. 50 volts, the current flowing through the relay coil will be sufficient to activate the relay thereby causing opening of the supply contacts and removal of the shock risk.

As can be seen from the above description, this type of ELCB is essentially a voltage sensing device intended to detect dangerous touch voltages. The level of shock protection provided by the vELCB was somewhat limited as these devices would not provide shock protection in the event of direct contact with a live part. An additional problem with the vELCB was its tendency to be tripped by earth currents originating in other installations.
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The iELCB was introduced in the late 1950's. It operates on the following principle. Under normal conditions the closed contacts of the iELCB feed the supply current to the load. The load conductors are passed through a current transformer (CT), a doughnut shaped device. The load conductors act as primary windings of the transformer. The CT is fitted with a secondary winding. Under normal conditions, the total current flowing from the supply to the load will be the same as the total current flowing back to the supply from the load. As the current in both directions is equal but opposite, it has no effect on the CT. However, if some current flows to earth after the iELCB, possibly due to an earth fault, the current flowing to the load and from the load will be different. This differential current will cause a resultant output from the CT. This output is detected and if above a predetermined safe level, it will cause the iELCB to trip and disconnect the supply from the load.
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For many years, the voltage operated ELCB and the differential current operated ELCB were both referred to as ELCBs because it was a simpler name to remember. However, the use of a common name for two different devices gave rise to considerable confusion in the electrical industry. If the wrong type was used on an installation, the level of protection given could be substantially less than that intended. To remove this confusion, IEC decided to apply the term Residual Current Device (RCD) to differential current operated ELCBs. Residual current refers to any current over and above the load current.

The RCD is now the preferred means of providing shock protection, and the term RCD has largely replaced ELCB within the industry. Unfortunately, the RCD industry has had considerable difficulty in shaking off the old association with ELCBs, and many electrical contractors still ask for an ELCB when in fact they want an RCD. Hopefully this article will remove some of the confusion.
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RCD = Residual Current Device. This is a generic term for the entire range of RCDs.

RCCB = Residual Current Circuit Breaker. This is basically a mechanical switch with an RCD function added to it. Its sole function is to provide protection against earth fault currents.

RCBO = Residual Current Breaker with Overcurrent Protection. This is basically an overcurrent circuit breaker (such as an MCB) with an RCD function added to it. It has two functions,

1. to provide protection against earth fault currents and
2. to provide protection against overload currents.

GFI = Ground Fault Interrupter. This term is mainly used in North America. The principle of operation is exactly as described for the iELCB above.
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Types of RCD

RCDs can be divided into two categories based on the means by which they detect and respond to earth fault currents. The two types are Voltage Independent (VI) and Voltage Dependent (VD). These are sometimes also referred to as electromechanical and electronic types respectively. The VI type uses the output energy from the CT to activate a relay which in turn activates a tripping mechanism causing the RCD to trip. The VD type uses electronic circuitry to detect the earth fault current and to activate a tripping mechanism causing the RCD to trip. The VI device derives its operating energy from the earth fault current whereas the VD device derives its operating energy from the mains supply.
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The type of protection provided by RCDs results in an additional two categories as follows.

• Type AC devices, which can only be used for protection against AC earth fault currents.
• Type A devices, which can be used for protection against AC and pulsating DC (rectified AC) earth fault currents.

There is also a Type B available which detects pure DC earth fault currents, but this is usually confined to special applications. (e.g. rectified three phase supplies)

It is now a requirement that all RCDs be marked with a symbol such that the user can determine which type it is, as follows.

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RCDs are also divided into two categories determined by their response time to an earth fault current, as follows.

• General Type - having a trip time < 300mS for fault currents of I^n and < 40mS for >5I^n.
• S Type - Having a trip time of 150 - 500mS for I^n, and 40 - 130mS for >5I^n.

As the name implies, General types are intended for general purpose use. However, S (Selective) types are normally used in conjunction with downstream General type RCDs. The S type effectively provides discrimination in terms of the response time to earth fault currents for upstream and downstream RCDs. For example, when two RCDs are connected in series the first RCD will often be an S type.
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Earlier, I explained that VI and VD RCDs are often referred to as electromechanical and electronic types respectively. However, there is an increasing use of electronic components in VI RCDs and the term "electronic" is becoming more of a misnomer.

Two factors have resulted in the increasing use of electronic circuitry in VI RCDs. These are:

1. Performance enhancement
2. Nuisance tripping immunity

A basic VI RCD will be a General and AC type with limited sensitivity. By adding electronic circuitry, the device can be made to provide a delayed response (S Type) and improved sensitivity. Problems of nuisance tripping can arise due to voltage or current surges which can induce a current into the CT. By adding electronic circuitry, the resultant output from the CT can be snubbed or delayed so as to prevent an instantaneous response to the surge. (See Problems with RCDs, below).
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Lastly, we can think in terms of the mains supply conditions for an additional set of responses, as follows.

1. Does not trip on loss of mains supply*
2. Trips on loss of mains supply and remains open on restoration of the supply.
3. Trips on loss of mains supply but recloses automatically on restoration of the supply

* These RCDs will not trip for any reduction or fluctuations of the supply voltage from 100% to 0% of the normal level. Within this category there are RCDs available which are intended to trip on loss of supply neutral alone. A loss of supply neutral condition may be considered to be hazardous and where this is of concern to the installer or user, this type of RCD can be used.

Types ii) and iii) are sometimes referred to as undervoltage release RCDs, and will trip automatically when the mains voltage falls below a specified level. The exact voltage level at which the RCD trips will be specified by the manufacturer, but can sometimes be set in accordance with customer requirements. In previous years, these devices tended to respond almost immediately to a low voltage condition, giving rise to problems of nuisance tripping. Following changes to RCD product standards, these devices will now generally not trip automatically unless the low voltage condition exceeds a specified period, typically 300 - 500mS. This ensures that the RCD will not trip in response to momentary dips caused by inrush currents.

Type ii) devices are usually selected for use on installations where a potential hazard could be caused by low voltage conditions. For example, unexpected reactivation of moving parts such as conveyors, motors, rotary blades, or reactivation of processes, (production, chemical) etc. could prove hazardous and may need to be prevented from happening.

The Type iii) device will open automatically when the supply voltage falls below a specified level or in the event of loss of phase or neutral, thereby isolating the load until full restoration of the supply.
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Choosing RCDs

Factors such as the rated voltage, load current, residual current and number of poles should be fairly obvious, so I will deal with some of the other discretionary factors.

1. AC or A

In order to choose an RCD, the user or installer should first of all decide on the type of protection required. As an absolute minimum requirement, all RCDs will provide protection against AC earth fault currents (Type AC RCDs). If protection against pulsating DC (rectified AC) fault currents is also required, the user should choose a Type A device. One problem that arises here is how the user or installer is to know where Type A earth fault currents are likely to arise. The answer is that any circuit where the mains supply is likely to be rectified will have the ability to produce Type A earth fault currents. This covers the use of power tools, motor speed controllers, etc. In some countries, Type AC RCDs are not permitted, therefore if you are supplying RCDs or equipment containing RCDs to another country it will be important to check their national rules to see if Type AC devices are acceptable. Whilst Type AC RCDs are permitted in many countries, the Wiring Rules of most countries warn installers to take into account the likely effects of electronic equipment which may produce pulsating DC fault currents. This is in line with IEC guidelines.
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2. Discrimination in terms of I^n and Trip Times.

An installation can comprise of a main circuit and several sub-circuits. If the installation is protected by a single RCD on the main circuit, that device will trip in response to a rated residual current on any part of the installation including the sub-circuits, thereby removing power from the entire installation. It is common practice to fit an RCD on the main circuit with additional RCDs being fitted on some or all of the sub-circuits. Such practice is referred to as discrimination. A rated residual current on any sub-circuit will trip the local RCD only. However, in the event of a major earth fault current or in the event of failure of a downstream RCD to trip, the main RCD will trip. As a rule, downstream RCDs will have a trip current and a trip time less than that of upstream RCDs. I.e., downstream RCDs will be more sensitive and faster responding than upstream RCDs. Downstream RCDs should never have trip currents or trip times greater than those of upstream RCDs.
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3. Response to Abnormal Supply Conditions

The next issue to be addressed is the response of the RCD to abnormal supply conditions. If low voltage conditions are not of concern, use type i) above. If low voltage conditions could prove hazardous, choose either

• Type ii) if you want the RCD to remain open until manually reclosed (avoids unexpected reactivation).
• Type iii) if you want the RCD to reclose automatically when normal supply conditions have been restored (prevents motor burn-out)

4. Solid or Opening Neutral.

Some RCDs are fitted with a solid neutral conductor which means that the neutral will not be disconnected from the load when the RCD trips. These devices are only available in RCBO form, not in RCCB form. (Remember, an RCBO is effectively a combination of an MCB and an RCD). These devices are sometimes referred to as single pole (SP) RCBOs. This term is not strictly correct because it is possible to have a three pole RCBO which also has a solid neutral. However, it is fair to say that the vast majority of RCBOs with solid neutral are SP types. SP RCBOs can come in single module (18mm) or two module (36mm) widths because their design is usually based on MCBs produced by the manufacturer. Confusion sometimes arises because a two module width RCBO may give the impression that it is a two pole device. This is not a problem where disconnection of the neutral is not required. However, where disconnection of the neutral is required, (for example on IT and TT systems), it is vitally important to ensure that a device with a solid neutral is not used.

The main advantages of single module SP RCBOs are that they can be used to replace a single pole MCB thereby adding RCD protection to a circuit with overcurrent protection, and they take up the minimum amount of space for an RCD. The use of SP RCBOs has grown considerably in recent years. IEC60364 specifies the conditions under which RCDs with solid neutral may be used.

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Problems With RCDs

As far as users are concerned, the single greatest problem with RCDs is nuisance tripping. This problem will either be due to the design of the RCD or due to the installation. Nuisance tripping can be a frustrating problem for the user and there have been cases where RCDs were strapped out, removed from installations completely or replaced with RCDs with a higher rated trip current. All of these types of actions are very dangerous and should be avoided. Where the problem lies with the RCD design, it is usually attributable to factors such as voltage spikes, surges, switching transients, noise, inrush currents, etc. Over recent years, the problem of nuisance tripping attributable to the RCD itself have been addressed by IEC. New tests have been introduced into the product standards to ensure that RCDs have a reasonably high immunity to nuisance tripping. Unfortunately, all RCDs do not comply with the new requirements. To minimise this problem, ensure as far as possible that the RCD has compliance to IEC61008 or EN61008 (RCCBs) or IEC61009 or EN61009 (RCBOs) and also to IEC61543 or EN61543 (EMC requirements for RCDs).
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Sometimes the problem of nuisance tripping is attributable to the installation. IEC recommend that the standing earth leakage current on the installation should not exceed 30% of the rated trip current of the RCD intended to be used on that installation. This means that for a 30mA RCD, the standing earth leakage current should not exceed 10mA. Given that a 30mA RCD may trip anywhere from 15 - 30mA, a 10mA standing leakage current will virtually prime the RCD to trip. In general, RCDs cannot tell the difference between a standing leakage current and an earth fault current. The sum of these two currents is the residual current seen by the RCD and if this aggregate current is greater than its rated trip current, the RCD will trip, unless it's faulty. (but that's another story)
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The problem of standing leakage currents is increasing. This is due to two factors:

1. equipment manufacturers filtering internally generated noise to earth
2. fitting of RFI suppression to provide immunity to mains borne noise.

Recently introduced European Directives on EMC require manufacturers of products, appliances, equipment, etc. to contain within specified limits the levels of RFI type emissions produced by their products. Manufacturers often have to resort to the use of filtering circuits to meet these requirements. The filtering circuits can divert high frequency signals to earth, but can also result in the flow of significant levels of leakage current to earth at the standard 50Hz frequency. Under IEC rules, electrical appliances may allow a standing leakage current of up to 3.5mA/50Hz to flow to earth. Such current levels coupled with other sources of standing leakage currents can give rise to nuisance tripping.
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Washing machines, oil or gas burners and even immersion heaters can contribute to nuisance tripping. Temperature and humidity may also be factors in nuisance tripping. Other factors contributing to nuisance tripping may be poor earth terminations, surge suppressers, neutral voltage rising above earth potential, etc. A very sneaky problem is where an earth fault current on the supply side of the RCDs manages to appear as a load side fault to the RCD, causing it to trip. This problem is more usually associated with IT systems, or on TT systems with high earth impedance. Before changing the RCD in response to a nuisance tripping problem, check the installation by measuring the standing leakage current and carrying out an earth loop test. Also try to identify equipment that is likely to contribute to standing or transient earth currents. A residual current monitor can be fitted to an installation to detect the level of the standing earth leakage current or even transient leakage currents.
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VD - v - VI RCDs

It must be said that RCDs, VD and VI types, have made an enormous contribution to electrical safety and each type will continue to play this vital role in the future. More and more manufacturers of VI types are now producing VD types to enhance their service to customers. There is no question of one type being completely displaced by the other, no more than petrol engine cars have displaced diesel engine cars. And as has always been the case with our four wheeled friends, long term success is dependent on performance, reliability, price and ongoing technical improvements.

A key development in recent years has been the increasing use of electronics in VI RCDs. In some cases, this has resulted in manufacturers changing over from electromechanical (VI) types to all electronic (VD) types. However, that is not to say that VD types are better than VI types per se although they can provide some advantages. For example, the SP RCBO in a single module (18mm) width can only be produced in VD form because the VI device cannot cope with the space constraints. Ultimately, it is a matter of choice for the user and the fundamental requirements remain the same; performance, reliability and price. Readers wanting more information on this topic are recommended to read the following interesting articles.

• "Why electronic and not electromechanical RCDs" by Viv Cohen of Circuit Breaker Industries in S Africa, published in Elektron May 1996.
• "Safer Alternative" by Ray Lewington of Square D Co in the UK, published in Electrical Review (UK) August 1997

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International Trends in RCDs

Customers provide the greatest force for change in any free market. In recent years, RCD customers have demanded improved performance, improved immunity to nuisance tripping and improved reliability, and taking all of those as given, they also want the product at a lower price. Manufacturers and installers who want to be successful have no option but to respond positively to these demands. There is an unstoppable demand for enhanced performance RCDs.

Earlier in this article, I referred to a problem where the RCD can be caused to trip by an upstream fault appearing to come from the load side of the RCD. RCDs have already been developed which can discriminate between upstream and downstream fault currents and are not fooled by this type of fault.
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I also explained that the problem of nuisance tripping is often attributable to current flowing to earth through filtering circuits. These can be standing or transient currents. Such currents usually flow to earth via filter capacitors and are therefore reactive currents. They are also generally harmless but nonetheless can cause the RCD to trip.

Resistive earth fault currents present a greater risk of fire as well as being a shock hazard. Conventional RCDs cannot distinguish between resistive and reactive residual currents. As the problems of filtering currents increases, so too will demand increase for RCDs that can ignore such currents.

With conventional RCDs, the user has no way of knowing when the RCD is likely to trip and is only aware of the presence of residual currents after the RCD has tripped. (Think of the standing leakage currents priming the RCD). RCDs are now being fitted with monitoring circuitry which can provide a continuous or pre-warning indication of the level of residual current on the installation. This enables the user to identify loads or areas of the installation contributing to residual currents, and provides the possibility to carry out repairs when convenient.
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About the author.

Pat Ward is managing director of Western Automation R & D based in Ballinasloe. The company is a specialist designer and manufacturer of RCD products. Pat has been an active member of technical committees in ETCI, IEC and CENELEC since 1991 and has been involved in the drafting of several international standards for RCD & RCMs. He has also participated in working groups within IEC addressing problems of reliability and EMC requirements for RCDs.
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