Residual Current Devices are used to provide human protection against electric shock that can happen under a faulty condition of an Electrical Installation. Here, the electric shock is the effect of the current flow through the human body due to the direct or indirect contact of the live parts of an electrical installation. As per the IEC 60479 Standard, four main zones are defined in a time- current curve to show the effect of AC current when flowing through human body.
According to the above curves the body current must be limited not to enter to AC-3 region or beyond to avoid from human injuries. Therefore, access to the hazardous live parts of an Installation causing electric shocks shall be prevented and that can be achieved by following measures according to the BS7671 Standard.
- Basic Protection against direct contact
- Insulation of live parts
- Placing live parts out of arm reach range
- Isolation live parts within an enclosure
- Fault Protection against indirect contact
- When the expose conductive parts are properly earthed, Residual Current Devices are used as Automatic Disconnection Devices to establish fault protection.
Residual current Devices are called as RCDs in short term and they are capable of disconnecting the circuit automatically when an earth fault current is detected in the circuit. The common reasons behind the earth faults are lack of proper maintenance and insulation failures within the installation. In order to ensure the protection of the user, RCD must be highly sensitive and fast responsive.
Operation of RCD
Though there are two pole, three pole and four pole RCDs available to use for an application, the operation principle of each type RCD is similar. If we consider the operation of a two pole RCD, a toroidal magnetic core inside the RCD is wounded with three current carrying conductors where the two coils for positive current carrying and negative current carrying and the third coil for residual current carrying. When the circuit is healthy, vector sum of positive current (I1) and negative current (I2) is equal to zero and that will not create any flux in the magnetic core. But when there is an earth fault current returning to the source via the ground or earth conductor, a magnetic flux is created due to current imbalance in the magnetic core. This magnetic flux generates a current known as residual current (I3) and two technologies are involved in operating the tripping device of the RCD using this this residual current.
- Voltage Independent Technology
When the residual current exceeds the value required to operate the tripping device in the RCD, the energy to operate the trip mechanism is directly obtained through the residual current independent of the line voltage. Even with the neutral breakdown or voltage drop, this technology works without an issue.
- Voltage Dependent Technology
When the residual current exceeds the value required to operate the tripping device in the RCD, an electronic circuit send a signal to operate the trip mechanism. Here, the energy to operate the electronic circuit and trip mechanism is obtained from the line voltage separately. When the supply voltage is below the operating voltage of the electronic circuit and trip unit, RCD will not be able to operate even though it detects an earth fault.
There is a test button on the front of the device to check whether the RCD is healthy. Once the test button presses it creates a bypass current path in the main circuit resulting a residual current due to the current imbalance in the toroidal core.
Types of RCDs
Different loads can create different residual current waveforms when there is an earth fault situation in the system. By considering that IEC 60755 defines mainly four RCD types to provide suitable protection.
- Type AC
Type AC RCD can detect the residual current in sinusoidal alternating currents and this type is used for most of the general applications.
- Type A
Type A RCD can detect sinusoidal residual currents as well as residual current in pulsating DC currents. These current waveforms can be seen on diode, thyristor based electronic loads and commonly used to provide the protection for single phase electronic loads. Some of the Type A RCD applications include power supplies, LED drivers, Induction cooker and etc.
- Type F
Type F RCD are capable of detecting the characteristics related to type A RCDs and further, it is specially designed to provide the protection for the frequency converter based applications like washing machines, air conditioners, power tools and etc. The residual current waveform consists of multi- frequency current components due to the mixture of different frequencies in the equipment.
- Type B
Type B RCD can detect all the residual currents covered in above three types and in addition to that, this type is sensitive to operate for smooth DC residual current as well. Three phase rectifier circuits can generate pure DC residual currents and therefore it is recommended to use Type B RCDs for three phase rectifier load applications. Common applications of Type B RCDs include inverters, Uninterrupted Power Supplies, elevators, escalators and etc.
Common Parameters of RCDs
According to the IEC 60755 Standard, following common parameters are considered for RCDs.
- Type of RCD
As we discussed previously, this is the classified type of the RCD according to the operating characteristic with DC current components in the residual current.
- Rated Current – In
This is the value of the current assigned to RCD by the manufacturer that the RCD can carry continuously without any interruption.
- Rated Residual Operating Current (Sensitivity) – I∆n
This is the r.m.s. value of sinusoidal residual operating current assigned to RCD by the manufacturer at which the RCD operates under specified condition.
- Rated Voltage – Un
This refers to the r.m.s. value of the voltage assigned to RCD by the manufacturer for which RCD performance relates.
- Rated Frequency – f
This the value of the frequency at which RCD operates correctly and RCD is designed for.
- Rated Making and Breaking Capacity – Im
This refers to the r.m.s. value of the prospective current that the RCD can carry while closing the contacts, carry for opening time and break under specified condition without making any functional issue. Here the prospective current means the current that could flow in the main circuit when the RCD and over current protective device (if any) were replaced by a conductor of negligible impedance.
- Rated Residual Making and Breaking Capacity – I∆m
This refers to the r.m.s. value of the residual prospective current that the RCD can carry while closing the contacts, carry for opening time and break under specified condition without making any functional issue.
High Sensitivity RCDs
As per the IEC 60364 standard, the term called additional protection is defined as the protection achieved by high sensitivity RCDs and it is provided for the following circuits.
- A circuit with maximum 10 number of socket outlets and the circuit rated current less than or equal to 32A.
- A circuit that supplies the current for outdoor mobile equipment with rated current less than or equal to 32A.
- Final circuits consisting of indoor lighting.
Here the high sensitivity RCDs can be identified as the rated residual operating current is less than or equal to 30mA.
Coordination of RCDs
As we know the breaker discrimination of an electrical installation is essential to ensure the reliability of the entire electrical distribution system. Therefore, RCDs shall be properly coordinated to operate the closest local RCD before the upstream RCD operates for an earth fault situation.
The selectivity of RCDs shall be carried out according to the BS7671 standard or the guideline provided by the RCD manufacturer. Following are the conditions to be satisfied for the correct selection of RCDs as per the standard.
- The upstream RCD is of selective type which means Type S or time-delayed for appropriate time delaying.
- The ratio of the rated residual operating current of upstream RCD to that of the downstream RCD is at least 3:1. This condition is not necessarily to be satisfied since some manufacturers provide RCDs with better earth leakage current measuring accuracy.
Following table shows the time-current performance criteria for RCDs according to BS7671 standard.
