Background Image
Table of Contents Table of Contents
Previous Page  180 / 211 Next Page
Information
Show Menu
Previous Page 180 / 211 Next Page
Page Background

180

TECHN I CAL DATA

C I RCU I T P ROT ECT I ON

SPECIFICATION

l

BS EN 61008 RCCBs

EN 61009-1, IEC61009-1 RCBOs

BS 7288 SRCDs

Range of current ratings

13–100A

Range of sensitivities

10–300mA

Pole configurations

SP&N, DP & 4P

DP SRCD – 230V

Voltage ratings

1 & 2 Module SP & N – 230V

2 & 3 Module DP – 230V

4 Module 4P – 400V

Frequency rating

50Hz

Tripping principle employed Electro-mechanical (2, 3 & 4 Module RCCBs)

Electronic (1 & 2 Module RCBOs)

OPERATION

l

The RCD employs the current balance principle which involves the supply

conductors to the load (phase and neutral) being wound onto a common

transformer core to form the primary windings. The secondary winding of the

current transformer is then connected to the trip mechanism, either an electro-

magnetic relay in the case of 2, 3 and 4 module RCCBs or an electronic relay

in the case of 1 and 2 module RCBOs. Under healthy circuit conditions, the

current in the phase conductor is equal to the current in the neutral and the

vector sum of the current is zero. In the event of an earth fault, an amount

of current will flow to earth, creating an out of balance situation in the

transformer assembly. This out of balance is detected by the secondary

winding of the transformer and at a pre-determined level of out of balance

will activate the trip mechanism.

Single phase and neutral or three phase and neutral units (suitable for 3 or 4

wire systems) are available, the latter being suitable for balanced or

unbalanced 3 phase loads.

The RCD trip mechanism will operate at a residual current of between

50–100% of its rated residual operating current (sensitivity).

TEST BUTTON

l

A test button is provided on all RCDs to enable the operation of the device to

be checked.

It is recommended that an RCD is tested at least quarterly. (See BS 7671

Regulation 514-12-2).

RESIDUAL CURRENT DEVICES (RCCBs, RCBOs and SRCDs)

l

TERMINAL CAPACITIES

l

Lifestar RCCBs

50mm

2

Lifestar SRCDs

3 x 2.5mm

2

3 x 4mm

2

2 x 6mm

2

Starbreaker RCBOs 16mm

2

Loadstar RCBOs 16mm

2

APPLICATIONS

l

a) Residual Current Devices (RCDs) may be required to ensure the compliance

of an installation with BS 7671, formerly the IEE Wiring Regulations.

An RCD (30mA) meeting the requirements of Regulation 415.1.1 must be

used for circuits and cable installations covered by Regulation 411.3.3 (socket

outlets), 522.6.6, 522.6.7, 522.6.8 (wiring systems), and 701.411.3.3

(locations containing a bath or shower). Where a high earth fault loop

impedance disqualifies the use of overcurrent protection devices as a means

of providing the necessary automatic disconnection in the case of a fault, an

RCD may be used to satisfy the requirements of Regulation 411.3.2.2

(411.4.9). To comply with Regulation 411.5.3 the earth fault loop

impedance in Ohms multiplied by the rated tripping current of the RCD in

Amperes must not exceed 50(V). With the RCD having a sensitivity of 30mA,

the maximum permissible earth fault loop impedance is calculated as follows:

Z

S

(max)=50/0.03=1666 Ohms

Rated residual

Maximum earth fault loop Impedance

Z

S

Ohms

operating current (mA)

120V<Uo

230

30mA

1667

100mA

500

300mA

167

(b) to provide a higher level of protection than that given by direct

earthing, against fire or shock risks caused by earth leakage currents.

Overcurrent protection devices cannot detect earth fault currents below their

operating current. If they are the only means of earth fault protection, it is

possible for sufficient earth fault current to flow undetected to constitute a

fire risk.

By using an RCD, the flow of the sustained earth fault current, above the

tripping current of the RCD, is prevented. The shock risk associated with these

earth fault currents is also greatly reduced.

To provide complete personnel protection, a high sensitivity RCD to a Type A

classification with a maximum tripping current of 30mA should be used. This

is particularly important with portable appliances where there is a danger of

losing earth continuity due to damage or fatigue.

Residual current devices are completely selective in their operation. They are

unaffected by parallel earth paths and are thus ideally suitable for the

protection of installations in modern high density dwellings or office blocks.

They are virtually tamperproof and provide a predetermined level of

protection. Even if earthing conditions deteriorate substantially, they will

continue to provide a higher level of protection than would have been given

by direct earthing.

SENSITIVITIES

l

10mA

provides the highest degree of personal protection, for use in sensitive

areas such as laboratories, schools and workshops where potential hazards

exist from electrical faults caused through misuse, accidental damage or

failure of electrical appliances.

30mA

provides a high degree of personal protection, satisfying the

requirement of Regulation 415-1 for additional protection. (when an

operating time not exceeding 40ms at 5 times rated residual operating current

is proven.

100mA

provides a high level of fire risk protection and a degree of fault

protection.

300mA

provide fire risk protection.

TRANSIENT EARTH LEAKAGE CURRENTS

l

All Crabtree residual current devices incorporate a high level of immunity

to tripping when subjected to transient earth leakage currents.

Such transients can occur when there is a significant level of capacitance to

earth as can result from cable capacitance (particularly MICC) or RF filter

networks. Crabtree RCDs are therefore less susceptible to nuisance tripping

due to transient earth leakage currents.

N 5 3 1

L (N)

L (N)

two pole

N (L)

N (L)

N 6 4 2

N 5 3 1

L1 L2 L3

L1 L2 L3

R

three pole

N 6 4 2

N 5 3 1

L1 L2 L3 N

L1 L2 L3

four pole

N 6 4 2

N

RCD circuit diagrams (four pole)

Tripping

Device

Test button

(interlocked)

1/2 3/4

2/1 4/3

Test

resistor

Current balance

transformer

Fault sensing

winding

RCD circuit diagram (two pole)

Note

Exposed installation

metal work must be

earthed.

RCCB in a three-phase system without a neutral conductor:

The N terminal should be connected to terminal 5 or 6 via the R

resistor, depending on the supply side, in order to keep 230V

power supply voltage of the test circuit. Test current is wrong

if the value of the R resistor is incorrect or if only a wire

connection is used instead.

Note

Exposed installation

metal work must be

earthed.