UN

DER

STA

ND

ING

RC

DS

10

IET Wiring Matters | Summer 06 | www.theiet.org

This article is based on information from ‘The RCD Handbook, The BEAMA Installation Guideto the Selection and Application of Residual Current Devices’

IN THIS ARTICLE we discussResidual Current Device (RCD)operation, which can occur due toelectrical faults both downstream andupstream of the device. An RCD, whencorrectly selected and installed, canprovide protection against electricshock due to indirect contact. RCDsare also used to provide supplemen-tary protection against direct contactand protection against thermal effects.Operation of an RCD can occur due toa downstream fault such as a crossedneutral on a split-load board, highprotective current or an upstreameffect such as mains-bornedisturbances.1. Indirect contact. An RCD may beused to provide protection against

UNDERSTANDING RCDsBy John Ware

Faults existing downstream of the RCD

1 Faults existing Indirect contact2 downstream Direct contact3 of the RCD Incorrect application4 No discrimination between series-connected RCDs5 Loose connection6 Crossed neutral on split load board7 Neutral to Earth fault8 Nails and picture hooks 9 High protective conductor current10 Mineral insulated cables11 Moisture ingress12 Double pole switching13 Faults existing Loose connections14 upstream Mains-borne disturbance including lightning strike15 of the RCD Site machinery or plant and installed services16 Overhead lines

UN

DER

STA

ND

ING

RC

DS

IEE Wiring Matters | Summer 06 | www.theiet.org

11

electric shock due to indirect contactin an installation in accordance withthe requirements of Regulation 413-02-16(for a TN system) or Regulation 413-02-20 (for a TT system) of BS 7671.

2. Direct contact. An RCD may be usedto provide supplementary protectionagainst direct contact in accordancewith the requirements of Regulation412-06 of BS 7671.

3. Incorrect application. An RCD mustbe correctly selected and erected forthe particular application. Forexample, protecting an entireinstallation using a single high-sensitivity RCD can, in many cases,lead to unwanted tripping,particularly in industrialenvironments where inductive loadswill cause greater transientovervoltages and where longer cableruns will result in larger values ofcapacitance to earth.

4. No discrimination between series-connected RCDs. A fault downstreamof two series-connected RCDs mayresult in operation of either device.Fig 1 illustrates two RCDs connectedin series. A fault downstream of thesecond device will be ‘seen’ by bothdevices. Inconvenience may result ifthe upstream device operates.Discrimination, where required, mustbe ensured by means such as selectinga time-delayed device for theupstream device (Refer to Regulation531-02-09).

5. Loose connections. A looseconnection downstream of an RCDmay cause it to operate due totransient voltages or capacitiveeffects. Every connection must beproperly constructed (Regulation 133-01-04) of durable electricalcontinuity and adequate mechanicalstrength (526-01-01), correctly selected(526-02) enclosed (526-03) andaccessible, where required (526-04).

Fig 1: Series-connected RCDs

Fig 2: Crossed neutral on a split-load board

UN

DER

STA

ND

ING

RC

DS

12

IEE Wiring Matters | Summer 06 | www.theiet.org

6. Crossed neutral on split loaddistribution board or consumer unit. Inthe diagram, fig 2, the external lightingcircuit has been incorrectly connected.The miniature circuit-breakersupplying the circuit is connected tothe non RCD-protected side of the split-load board but the neutral conductorfor the circuit has been inadvertentlyconnected downstream of the RCD. Assoon as the outside light is energized,the RCD will ‘see’ a large imbalanceand will operate. The neutral must betaken from the same final circuit, neverfrom another circuit as has happenedin this example. Circuits must be keptseparate (Regulation 314-01-04). In thisexample a ‘borrowed neutral’ situationexists presenting a potential shock riskfor an electrician attempting totroubleshoot the problem.

7. Neutral-to-Earth fault. A neutral-to-earth fault or inadvertent connection ofneutral to earth downstream of an RCDwill probably result in the deviceoperating as part of the neutral currentwill flow in the circuit protectiveconductor resulting in the RCD ‘seeing’an imbalance (see fig 3, above). Aneutral-to-earth fault can be caused by:

■ A neutral conductor touching anearthed mounting box or earthedmetal conduit

■ Reversed neutral and earthconnections at an accessory or itemof current-using equipment

■ Withdrawal of a fuse or switchingoff a circuit-breaker in a finalcircuit resulting in an RCD trippingas the neutral is normally notinterrupted.

The most effective way of testing forearth faults in the wiring orequipment is by measuring theinsulation resistance as described inSection 713-04 of BS 7671.8. Nails and picture hooks, screws and

power drills. A floorboard nail drivenbetween the neutral and earthconductors creates a neutral to earthfault which is likely to cause anupstream RCD to trip. The fault can belocated by insulation testing. Thedamaged cable must be replaced andeither relocated to avoid furtherdamage or protected. Requirements toprotect cables from impact andpenetration are given in Regulation522-06 of BS 7671.

9. High protective conductor current.Equipment such as variable speeddrives, computer equipment and highfrequency luminaires often incorporatefilters that cause a certain amount ofearth leakage or protective conductorcurrent. Protective conductor currentlevels below the operating current ofthe RCD cannot be ignored. The RCDsensitivity is effectively increased andbecomes the difference between theRCD trip current and the standingprotective conductor current. Forexample, an RCD with a rated residualoperating current of 30mA will have atypical trip current of 22mA. If thestanding protective conductor currentdue to computer equipment is 10mA itwill only take an earth fault current of

12mA to operate the RCD. This couldlead to unwanted tripping.

The electrical contractor can obtainan approximate measurement ofprotective conductor current by usinga sensitive clamp-on a.c.milliammeter encircling the phaseand neutral conductors. Theinstrument ‘sees’ the same currentthat an RCD would see at the samepoint in the installation.

The RCD selected to provideprotection must be selected to takeaccount of protective conductorcurrents, or, alternatively, the circuitsmust be subdivided so that anyprotective conductor current likely tooccur during normal operation of theconnected load is unlikely to causeunnecessary tripping of the device(Regulation 531-02-04).

10. Mineral insulated cables. Mineralinsulated cables can absorb moistureif not correctly terminated resultingin reduced insulation which maycause an RCD to trip as a certainamount of outgoing phase currentwill return through the MI cablesheath causing the RCD to see animbalance. Insulation testing shouldidentify the problem.

Fig 3: Neutral-to-Earth fault

UN

DER

STA

ND

ING

RC

DS

IET Wiring Matters | Summer 06 | www.theiet.org

17

11. Moisture ingress can cause reducedinsulation resulting in RCD operation.Reduced insulation can result from wetplaster, condensation or water entry intoaccessories. Similarly some appliancesmay exhibit reduced insulation causingRCD operation. Certain installedservices such as heating elements incookers can have reduced insulationwhen cold; the insulation increasingwhen hot. Manufacturer’s instructionsshould be consulted.

12. Double-pole switching. Double-pole switching within the fixed wiringis known to trip an RCD whenswitching off or on due to capacitiveeffects. Changing over from double-pole to single-pole switching canovercome the problem, where suchreplacement is permissible and safe.

Faults existing upstream of the RCD13. A loose connection upstream ofthe RCD such as at the main switch orat the busbar connections can causethe device to operate.

14. Mains-borne disturbances such asspikes, voltage surges and dips, alightning strike and the operation ofdistribution network switchgear andprotective devices combined withcapacitance to earth within theinstallation can cause unwanted RCDoperation. A filter may be ofassistance.

15. Site machinery or plant andinstalled services can cause mains-borne interference. Motors such as liftmotors, control gear for dischargelighting and transformer inrush

currents can cause unwanted RCDoperation. Although significanttransients can arise within aninstallation they would normally onlyoccur under fault conditions. Theymight, however, travel to otherinstallations where they could causeunwanted tripping of an RCD.

16. Overhead lines. Unwantedtripping may occur more frequentlyin an installation supplied byoverhead lines compared to onesupplied by an undergroundconcentric cable. An undergroundconcentric cable is, by its very nature,a good attenuator of transientovervoltages. Spurious tripping maybe avoided by installing a filterupstream of the RCD at the origin ofthe installation.

Fig 4: Protective conductor current caused by computer equipment.