In a modern consumer unit there will be one or more RCD devices. These are usually RCCBs or RCBOs. Whilst an RCBO is effectively an RCD introduced to each circuit individually, more commonly an RCCB will introduce RCD safety with multiple circuits at a time. The picture below shows a standard split load consumer unit with two separate RCCB devices. Each of these RCCBs then protects 4 separate circuits in the house for example; the cooker, the downstairs lights, the upstairs sockets and the smoke alarms. Effectively the RCCB offers adequate RCD protection to all 4 of these separate circuits but in a combine fashion. This means that if a fault develops on just 1 of these circuits for example; the downstairs lights, then the RCCB will trip which switches off ALL four of the circuits.
With this in mind when an RCCB trips and you are unable to switch it back on, we need to identify exactly which circuit is causing the problem. One way to do this is to switch off all 4 of the individual breakers protected by the RCCB which has tripped and then try and reset the RCCB. It should be noted that this will only be successful for LIVE to EARTH faults and not NEUTRAL to EARTH Faults which is explained further below.
If you manage to get the RCCB set again, then we can now switch on the four separate circuits one by one until one of them makes the RCCB trip again. This circuit is going to be the one causing the fault. You can now leave this circuit OFF and use power in the rest of the installation. You still need to get an electrician to take a look to identify the exact fault but at least you may be able to get your heating back on.
Because of the way in which an RCD (Residual Current Device) works, it protects against faults throughout the system for various types of faults; LIVE to EARTH faults, or NEUTRAL to EARTH faults. Either of these will cause differences in current flow between the two LINE conductors (LIVE and NEUTRAL). This means that even if an appliance is plugged into an outlet but is Switched OFF, it can still affect an RCD through a NEUTRAL to EARTH Fault. This is because fuses in a fuse board, fuses in a plug, switches on a socket outlet and even light switch are classed as ‘single pole devices’ and ONLY isolate the LIVE conductors of the appliance and wiring. The NEUTRAL or return path is still connected via the RCD and so if the NEUTRAL develops a fault to Earth, the RCD will detect this.
With this in mind the next thing you can do, in order to try and track down exactly which Light or Socket is causing the problem, is to completely unplug all your appliances (If it is a socket circuit) or switch OFF all the lights (If it is a lighting circuit). This may allow you to re-energise the particular circuit with the fault thus getting even more of the installation back up and running before your electrician can get out to fix the fault. This assumes the fault is with an appliance or a light fitting itself rather than the wiring to it. If you have a wiring fault you might not be able to get any further narrowing down where the fault is and your electrician may have to do more of this themselves. Also, as outlined above, if the fault is between NEUTRAL and EARTH then it is likely conductors will need to be disconnected from the consumer unit in order to track down the fault. We do not advise you to do this – you should let your electrician take over at this point.
If you have a consumer unit which utilises RCBO devices (As shown above), which incorporate RCD protection into each circuit individually as shown below, then only the circuit containing the fault should be affected. This is much more convenient during the development of a fault as all the rest of the circuits will be unaffected. The downside is simply the cost of having individual RCBOs is far greater than the cost of using an RCCB to protect ,multiple circuits. The regulations say that as long as every circuit is protected by an RCD then it does not matter how, and how many devices are used to do this. Effectively you could install one single RCD device before the fuse board and this would be within the regulations. However, the whole installation would be isolated for any fault on just a single circuit which can be very inconvenient. Many installations with older style fuse boards used this method in order to get within the regulations very cheaply as it did not involve having to replace the consumer unit. This method is now seen as not very effective due to the inconvenience it can cause during fault conditions.