With the emphasis on the need for RCD protection in BS 7671:2018, RCBO’s (residual current operated circuit-breakers with integral overcurrent protection) are being installed at a much higher rate than ever before. It is now more important than ever to truly understand the RCD
function of this device and the different types available to the installer.
With ever growing number of residential installations which may have DC fault currents, (such as electric vehicle or photovoltaic installations) it is important to understand that there are some RCDs that can detect DC fault currents and other which cannot.
So before we can answer whether Type A RCBO’s should be considered compulsory, we must first evaluate the correct type of RCBO from the type selection available with the RCD function in mind. With an imminent deadline of April 1st 2021 set for all rental properties to have a satisfactory Electrical Installation Condition Report, this is a topic many installers will need to understand and address with property landlords over the next six weeks.
Why RCBO’s should be considered as compulsory?
RCBO devices are a combination an RCD and an MCB in one device. An RCD also detects earth leakage which has become another keen topic within the 18th edition wiring regulations. Currently in residential installations it is common to find one or more RCD’s used alongside MCB’s within the Consumer unit. These are grouped together to protect multiple circuits. Commonly what happens in the event of an earth fault on one circuit is that the entire group of circuits, including the unaffected are tripped and switched off.
With situations such as this, choosing RCDs and MCBs in groups would not comply with aspects of the IET’s 17th Edition Wiring regulations. If you study chapter 31 - division of installation, regulation 314.1, which requires every installation to be divided into circuits as necessary:
A - To avoid danger in the event of a fault
B - To facilitate safe inspection, testing and maintenance
C - To take account of hazards that might arise from the failure of single circuit e.g lighting circuits
D - To reduce the possibility of unwanted tripping of RCDs (not due to fault)
The above requirements cannot be fully provided for if groups of circuits which are all connected to one or two RCDs, are in a single consumer unit.
If we evaluate point A and C further, a fault caused by an appliance or shower would then effect the lighting circuit and force the homeowner into darkness. It would not be safe for the resident to move around the house especially in high risk environments such as bathrooms and kitchens, directly impacting against points A and C.
When looking at point B, that communicates around the topic of testing and inspections, grouped circuits make this more difficult to achieve. For instance if cutting the circuit to lighting is required to test the socket outlets.
Point D is in our opinion the most relevant with modern installation as there are an increasing amount of appliances that place a heavier demand upon circuits than ever before. Unwanted, or nuisance tripping is far more likely to occur in the residence if there is on 30mA RCD protecting multiple circuits. Electrical equipment and devices are set to continue to increase in modern homes, the likelihood of cumulative circuit protector currents and earth leakage reaching a level that will be sufficient enough to trip a 30mA device will become a common risk.
With this evaluation alone, it is clear that to remain compliant with Chapter 31 you must avoid grouping circuits together on one or two RCDs and instead provide separate electrical protection by individual RCBOs for each individual circuit. Under these circumstances all will remain in operation.
Selection of Types of RCD
As we are now aware, RCDs exist in various different forms and will react completely differently in the presence of DC components, frequencies and faults. The different available types of RCDs are:
Type AC - General purpose use, RCD can detect and respond to AC sinusoidal wave. Communally used in all residential installations as standard.
Type A - Can be used for general purpose and for equipment incorporating electronic components. The RCD can detect and react as Type AC as well as accommodate pulsating DC components.
Type F - Used for appliances containing synchronous motors and some class 1 power tools. Some air conditioning controllers and other frequency controlled equipment .
Type B - Used for three phase electronic equipment such as inverters for speed control and EV charging where the current is .6mA and PV systems.
Why choose Type A RCBOs?
Type AC RCDs are standard in RCBOs, but as discussed above the use of Type AC RCD devices must be carefully considered when looking to future proof an installation for a modern home. Most appliances within a modern home will risk producing earth leak faults, so an RCBO on each circuit can eliminate this, yet the distinction between whether a Type AC should be upgraded to a Type A lies in the evolution of residential properties in the coming future.
As we know, when installations which may have DC fault currents are installed, the possibility of a DC component causing a fault on an AC device is highly likely. The most common DC circuits on residential properties currently are EV cars, an area that has seen a 28% increase in the last 2 years alone. With legislation in the automobile industry pushing manufacturers to a renewable source of travel and combustion engines soon to be made illegal, future proofing the electrical installation so that it can continue to comply with points B and C of Chapter 31 would mean installing a Type A device.
The requirements for RCDs used in any EV charging installation are given in Regulation 722.531.2.101. In this Regulation it requires that protective measures are to be taken against DC fault current greater than 6mA. This means that at the charger itself a Type B device should be used. This however, will not stop a pulsing DC current arriving at the distribution board and therefore, this device needs to be allocated as a Type A as a minimum requirement and to ensure that the installation can continue to comply with section D of Chapter 31.
We believe that an RCBO installation is the safest and most compliant installation available to the installer for residential properties. The question of whether a Type A should be used or even made compulsory is a matter of affordability and awareness. With the cost of RCBOs now considerably lower than in years gone by and the cost difference between Type A and AC not being a vast amount, future proofing your installations to ensure they comply long after installation would see Type A become best practice.
With our VCP range we aim to support this best practice approach, it is with this in mind that we have made our Type A RCBO a standard RCBO within the range. To further this, ALL RCDs within the Verso Circuit Protection range are Type A. This way installers can provide their customers with peace of mind knowing no matter the requirement in the future, their consumer unit is fit for purpose. At Verso we have chosen to absorb the additional cost difference between Type AC and Type A to ensure that neither the installer nor the end user is faced with an inflated installation cost of the ever changing parameters of Chapter 31 that are due to technological and regulatory advancements.