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Residual Current Devices

Selection and Nuisance tripping

Why’s and wherefore’s

Gary Busbridge

February 2014

The Residual Current Device

●The RCD is a device that disconnects a circuit

whenever it detects that the electric current is

not balanced between the energised conductor

and the return neutral conductor.

Schneider Electric 2- Division - Name – Date

Designed to Trip

●Should the primary means of protection, the

insulation, fail, the RCD is designed to trip

before…

a set time and at a current so low as to


prevent permanent injury or death from an

electric shock.

●Tripping may be as low as half the related

sensitivity but must trip at its rated sensitivity.

An RCD or a Safety Switch?

●Introduced into the market as an Earth Leakage

Circuit Breaker.

●Heavily promoted to the installers and

standards groups as a supplementary safety

protection


protection

●Entrepreneurial retailer in SA dubbed the unit

as a Safety Switch so that he could penetrate

the consumer market

●Standards and industry speak now refers to the

units as RCD’s

“Normal” Residual Current

●A sustained residual current which is above the

predetermined level.

●Using a real-life scenario, the leakage on a

circuit made up of a fridge, freezer and electric

kettle


kettle

●Each of these may have its own sustained

residual current and when added together could

be above the threshold of the RCD and..

as such will trip the breaker, ceasing the

flow of that residual current.

The Residual Current DeviceA path to Earththru tapware

Appliance becomes faulty

RCD on socketor switchboard

Human body becomes path to Earth


●30mA and 300ms is the cutoff before heart

fibrillation

The Residual Current DeviceA path to Earththru tapware

Appliance becomes faulty

RCD on socketor switchboard

Human body becomes path to Earth


●10mA and 40ms is the cutoff before muscular

contraction

RCD’s & Protection

●The use of RCDs with a residual current not

exceeding 30 mA, is recognized as providing

additional protection.


additional protection.

●The use of RCDs is intended only to augment

other measures of basic protection.

RCD’s & Protection

●RCDs do not provide protection against:

Faults between live conductors

Voltages imported into the installation

earthing system through the supply neutral.


●The use of such devices is not recognized as a

sole means of protection and does not obviate

the need to apply the protective measures

Disturbances : nuisance tripping

�earth-leakage protection can operate

incorrectly and cause:

• Power outages,

• production downtimes or data losses

• Lighting outages


• Lighting outages

• End-user dissatisfaction

“Abnormal” Residual Current

●Any residual current that flows after an initial

surge or impulse and then automatically ceases

is a non-sustained residual current.

●This is not associated with insulation


●This is not associated with insulation

breakdown and therefore not sustained, and…

is largely due to lightning surges, reactive

load switching and momentary flows to earth.

General principle of nuisance tripping●Although not dangerous, a high

frequency current is seen as a fault

by the residual current circuit-

breaker


Causes of the phenomenon●It seems that electro-magnetic compatibility

can be partly attributed to this recent

phenomenon as..

manufacturers are including mains input

filters in their electronic equipment and..

High frequency ballasts into fluorescent


High frequency ballasts into fluorescent

lighting.

●Manufacturers of these devices are

capacitively coupling current to earth to

reduce radio frequency signals emitted into

the atmosphere.

Causes of the phenomenon

●The switch-on phase in appliances provide a

brief period where the capacitors are passing

more leakage to earth than usual or..

in the case of lighting using electronic

starters, there is an ignition current of hundreds


starters, there is an ignition current of hundreds

of amperes with a duration of a few

milliseconds..

which result in large capacitive current flow

to earth and a lot of harmonic noise for a similar

period.

Causes of the phenomenon

●This current is not returning to earth through

the RCD via the neutral but still causes the RCD

to see a difference in the current out and


to see a difference in the current out and

current in.

●Often the difference exceeds the allowed

threshold and hence it trips the breaker.

What does the Installer need to

understand?

●The installer must understand the practical

requirements of the circuits being installed and

consider the effects of this residual


consider the effects of this residual

current in relation to circuit protection.

The phenomenon and the effect on

computers

●Any computer, whether new or old, may have a

residual current of between 1 and 2mA.

●It would seem that the number of computers

that may be connected to an RCD may be

between 7 and 10.


between 7 and 10.

●Public Schools Departments in Australia have

limited the number of computers on one RCD to

seven.

Microcomputers, servers, data Microcomputers, servers, data Microcomputers, servers, data Microcomputers, servers, data

centerscenterscenterscenters●Each PC amplifies the sensitivity of the RCD;

the more PCs there are, the greater the risk of

tripping during high-frequency interference


30 mA

13 mA

1.5 mA

The phenomenon and effect on

lighting

●Between 0.3mA and 1mA can be expected on

any electronic ballast for fluorescent lighting

●It would seem that the maximum number of

fluorescent lamps with ballasts protected by


fluorescent lamps with ballasts protected by

one RCD should be limited to 15 and 20.

●Consideration should be given to battens with

more than one fluorescent lamp fitted, as each

tube carries its own ballast.

Very low voltage Very low voltage Very low voltage Very low voltage

fluorescent lightingfluorescent lightingfluorescent lightingfluorescent lighting

●When banks of lighting units are turned off,

the high-frequency oscillations will be seen

as a fault


100 kHz - 10 MHz

Transformer Lighting unit

200 pF/m

1 nF

Fluorescent lighting with electronic Fluorescent lighting with electronic Fluorescent lighting with electronic Fluorescent lighting with electronic

ballastballastballastballast●Upwards of 20 lighting units, there is a risk of

tripping due to the stray capacitances


10 -100 kHz

The Phenomenon and effect on other

devices

●Similar principles should apply to other

devices but the non-sustained residual current

may vary..

and will need to be checked before

determining the maximum number of devices


determining the maximum number of devices

to be protected by any one RCD.

●Be aware that even though the residual

current on any device may not be sufficient to

trip the RCD it may have an overall effect on

the background leakage.

EarthEarthEarthEarth----leakage protection leakage protection leakage protection leakage protection

downstream a UPSdownstream a UPSdownstream a UPSdownstream a UPS

●Slow and damped oscillations may cause

nuisance tripping

N


Un

100 V

-100 V

N

Un

Activating a capacitor bankActivating a capacitor bankActivating a capacitor bankActivating a capacitor bank

●Activating the capacitor bank causes an

overvoltage with high-frequency oscillations,

which can cause nuisance tripping

I∆


U

U 1000 VI

∆

I∆U

Variable speed drives with frequency Variable speed drives with frequency Variable speed drives with frequency Variable speed drives with frequency

convertersconvertersconvertersconverters

●Square voltage modulated between 2 and 15

kHz according to the frequency of the motor:

This presents no danger, but the personnel

protection devices may trip2-20 kHz


EMC

M

2-20 kHz

LightningLightningLightningLightning

●The lightning pulses flow to ground, and the

RCD will trip


10 kA

50 µs

Quantified Effect

1mA 2mA


0.5mA 1mA

0.5mA

0.5mA

0.5mA

Around 1mA

0.75mA

1mA

1.5mA

What does the Installer need to do?

●Be aware of the type of load being put on the circuit.

●Divide the circuits to prevent a surplus of load on any one circuit.

●Limit the number of ballasts in the lighting circuit.


circuit.

●Use a selective type RCD (Class A Type Si) on those circuits that may be affected. (These types of RCD’s are specifically designed to be less sensitive to the effect of high frequency disturbances whilst at the same time ensuring personal safety.)

Si type RCDsSi type RCDsSi type RCDsSi type RCDs

NS

2 Signal processing devices

1 - Current transformer (toroid)

3 Electromech. actuator


NS

Transient filter

- waveform types :

8/20 µs- 0,5µs 100 kHz

Pulsed current detector

HF filter

Trip signal check

SI

Energy accumulator

Common Myths

●Testing instruments are required to check the

activation of an RCD.

WRONG! Field testing has verified that the Test button operation

is sufficient

●RCD’s cannot be used to protect Cookers

because of nuisance tripping


because of nuisance tripping

WRONG! There is now sufficient evidence from Cooker

manufacturers that this is not a problem

●RCD’s protect against active to neutral faults

WRONG! Common misconception amongst home-owners

Common Myths

●The new rules for 3 circuits per RCD are

sufficient

RIGHT & WRONG! It’s safe enough to install in this manner but

how do you fault find without having 1 circuit per RCD

●RCD’s have been added to lighting circuits

and if more than one then shared across


and if more than one then shared across

RCD’s for extra safety

RIGHT & WRONG! It’s helps keep the ceiling cavity safe but it

also ensures that if one RCD trips then there is still a chance that

the light circuit on the other is still working. Elderly concern!

In Summary

●Being more aware of the practical

requirements of the circuit being designed and

installed..

●May well be the nuisance value in the future..

and perhaps go a long way in negating the


and perhaps go a long way in negating the

nuisance trip problems that installers and

designers face today.

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