5.9.2 - The principle of the residual current device

The RCD is a circuit breaker which continuously compares the current in the phase with that in the neutral. The difference between the two (the residual current) will he flowing to earth, because it has left the supply through the phase and has not returned in the neutral (see {Fig 5.22}). There will always be some residual current in the insulation resistance and capacitance to earth, but in a healthy circuit such current will he low, seldom exceeding 2 mA.

Fig 5.22 The meaning of the term residual current

The purpose of the residual current device is to monitor the residual current and to switch off the circuit quickly if it rises to a preset level. The arrangement of an RCD is shown in simplified form in {Fig 5.23}. The main contacts are closed against the pressure of a spring, which provides the energy to open them when the device trips. Phase and neutral currents pass through identical coils wound in opposing directions on a magnetic circuit, so that each coil will provide equal but opposing numbers of ampere turns when there is no residual current. The opposing ampere turns will cancel, and no magnetic flux will be set up in the magnetic circuit.

Residual earth current passes to the circuit through the phase coil but returns through the earth path, thus avoiding the neutral coil, which will therefore carry less current. This means that phase ampere turns exceed neutral ampere turns and an alternating magnetic flux results in the core. This flux links with the search coil, which is also wound on the magnetic circuit, inducing an e.m.f. into it. The value of this e.m.f. depends on the residual current, so it will drive a current to the tripping system which depends on the difference between phase and neutral currents. When the amount of residual current, and hence of tripping current, reaches a pre-determined level, the circuit breaker trips, opening the main contacts and interrupting the circuit.

For circuit breakers operating at low residual current values, an amplifier may be used in the trip circuit. Since the sum of the currents in the phases and neutral of a three-phase supply is always balanced, the system can be used just as effectively with three-phase supplies. In high current circuits, it is more usual for the

Fig 5.23 Residual current circuit breaker

phase and neutral conductors to simply pass through the magnetic core instead of round coils wound on it.

Operation depends on a mechanical system, which could possibly become stiff when old or dirty. Thus, regular testing is needed, and the RCD is provided with a test button which provides the rated level of residual current to ensure that the circuit breaker will operate. All RCDs are required to display a notice which draws attention to the need for frequent testing which can be carried out by the user, who presses a test button, usually marked T. {Table 5.10} shows the required notice.

This installation, or part of it, is protected by a device which automatically switches off the supply if an earth fault develops. Test quarterly by pressing the button marked 'T' or 'Test'. The device should switch off the supply, and should then be switched on to restore the supply. If the device does not switch off the supply when the button is pressed, seek expert advice

Table 5.10 - Periodic test notice for residual current device

The test circuit is shown in {Fig 5.23}, and provides extra current in the phase coil when the test button is pressed. This extra current is determined by the value of the resistor R.

There are currently four basic types of RCD. Class AC devices are used where the residual current is sinusoidal - this is the normal type which is in the most wide use. Class A types are used where the residual current is sinusoidal and/or includes pulsating direct currents - this type is applied in special situations where electronic equipment is used. Class B is for specialist operation on pure direct current or on impulse direct or alternating current. Class S RCDs have a built-in time delay to provide discrimination (see below).

It must be understood that the residual current is the difference between phase and neutral currents, and that the current breaking ability of the main contacts is not related to the residual operating current value, There is a widely held misunderstanding of this point, many people thinking that the residual current setting is the current breaking capability of the device. It is very likely that a device with a breaking capacity of 100 A may have a residual operating current of only 30 mA.

There are cases where more than one residual current device is used in an installation; for example, a complete installation may be protected by an RCD rated at 100 mA whilst a socket intended for equipment outdoors may be protected by a 30 mA device. Discrimination of the two devices then becomes important. For example, if an earth fault giving an earth current of 250 mA develops on the equipment fed by the outdoor socket, both RCDs will carry this fault current, and both will become unbalanced. Since the fault is higher than the operating current of both devices, both will have their trip systems activated. It does not follow that the device with the smaller operating current will open first, so it is quite likely that the 100 mA device will operate, cutting off the supply to the complete installation even though the fault was on a small part of it. This is a lack of discrimination between the residual current devices. To ensure proper discrimination, the device with the larger operating current has a deliberate delay built into its operation. It is called a time delayed RCD.