All electricians are familiar with the Residual Current Device or RCD. Specified in BS7671 to provide for either fault protection, or more commonly, ‘additional protection’ the RCD can be found in most modern consumer units or distribution boards. What is not so commonly known is that there are different RCD types available.
All RCD types continuously monitor the line and neutral AC currents which under normal conditions should be equal and opposite in direction of flow i.e. flowing from the line supply conductor through the load and returning via the neutral conductor. In the event of a fault causing current to flow via the earth then this creates an imbalance of currents between the line and neutral conductor currents causing the RCD to trip and isolate the supply from both line and neutral conductors.
RCDs with a Tripping point of 30mA and an operating time of 40ms when the earth current equals 150mA, are defined in BS7671 as a means of ‘additional protection’ in the event of a person coming into contact with a live conductor.
BS7671 specifies additional protection using 30mA RCDs to be used for protection on socket outlets, buried cables, sockets for equipment used outdoors and special locations such as bathrooms. The type of RCD used in these instances is generally an ‘AC’ type of RCD. However, in some instances due to the nature of the current flowing in the circuit a standard AC type RCD may be prevented from operating correctly.
AC voltage equipment that contains non-linear loads produce non-linear load currents. In the event of a fault the residual earth currents will also contain the non-linear currents such as pulsed DC, pure DC of high frequency AC currents. These currents may affect the tripping point or delay the operation of the RCD or even prevent it operating at all. For this reason, manufacturers provide different RCD types to be able cope with the different currents, so they continue to provide the required level of protection.
Residual current devices are classified as Type AC, Type A and Type B and operate as follows:
Type AC. Ensures tripping for residual a.c. currents,
Type A Ensures tripping for residual a.c. currents and pulsating d.c. currents,
Type B Ensures tripping for residual a.c. currents, pulsating d.c. currents and smooth d.c. currents.
Figure 1 shows the symbols used for the different types of RCD, which can be found on the front of the device.
Figure 1 RCD Type Symbols
For most applications Type AC devices are the most suitable. Where loads produce DC currents such as Solar PV inverters and electric vehicle charging points then Type A or Type B RCDS will need to be considered. The decision to fit a Type A or B will depend on the potential level of DC current, and the manufacturer’s data for the equipment would need to be consulted.
In addition to the types AC, A and B, some manufacturers are now producing Type F RCDs. This type of device protects from electric shocks in the case of residual currents that can result with electrical loads such as washing machines and pumps, for example. These RCD types can generate residual currents with mixed frequencies. Type F RCDs are designed to detect mixed frequency residual currents 10Hz< 1kHz as defined in BSEN62423. They also incorporate a 3kA transient resistance and 10mS short time delay, to reduce nuisance tripping due to in-rush currents associated with this type of load.
With modern electrical installations the standard AC type RCD may not be sufficient to provide the required level of protection. The designer must consider the nature of the load in order to select the correct type of RCD.
For more information on RCD types and electrical training courses that help you understand them in even more depth, please don’t hesitate to get in touch.
