As I often say (e.g. in relation to freezers, there's a bit of swings and roundabouts here.
Having just one circuit, maybe a 'dedicated' circuit, on one RCD/RCBO will obviously decrease the risks associated with the circuit failing whilst one is doing something important (like the credit card transaction you mention). However, it also increases the risk of a failure of the circuit when one is not doing something important going unnoticed, so that one doesn't discover the problem until one next needs to use the circuit 'for something important'!
Kind Regards, John
That is very true, personally I have a rechargeable torch in same supply as freezers, so if I loose power the torch auto switches on, one can easy miss some thing not showing like no display on freezer, less likely to miss a torch shining.
The bigger problem is the designer often does not consult the user, with the cheap re-wire of mothers house, one sub-consumer unit in kitchen was going to be left untouched, and it had 4 RCBO's in it, the electrician doing the job was going to put the SWA feed to this CU on one of the pair of RCD in main consumer unit until I said no, there is no need for the SWA to have RCD protection. Why he as the designer felt it was correct to have the RCD's doubled up I don't know.
So often the answer to why did you do that is well we always do it that way.
Clearly with most new builds the user is not available at design stage, so he has to use a genetic design to suit most users, but so often a consumer unit is charged without really taking into account what the user wants. Only after the RCD trips does the user find out there were options.
If we think back to the 80's with TT supplies it was common to fit a 100 mA RCD and they rarely had nuisance tripping, it was only when we went to 30 mA the problem raised its head. Today with a TT supply we can use double pole single width RCBO's allowing one circuit to be isolated for electrical work, with a TN supply no need to isolate neutral so only talking about a TT supply. But the cheaper consumer units like fuse box don't do double pole single width RCBO's so looking at some think like Wylex, so around £32 each. So typical 10 way consumer unit gets expensive, not only due to RCBO cost but also the SPD cost, which jumps from £30 to £100, so £20 extra for each RCBO Plus £80 extra for the SPD so with a TT installation RCBO protection gets rather expensive, at least £250 more than a simple twin RCD box.
Most users would flinch at the extra, they don't consider the cost of a lost freezer full of food should it trip when no fault on that circuit, or the problems finding a fault before you can reset the RCD. Even when you know the fault, bath over flowed for example, drying out before all power is restored takes time, but this is rarely pointed out to the user, the designer by this time is long gone.
So we went from
BS7671:2001 said:
314-01-01 Every installation shall be divided into circuits as necessary to:
(i) avoid danger and minimize inconvenience in the event of a fault, and
(ii) facilitate safe operation, inspection, testing and maintenance.
314-01-02 A separate circuit shall be provided for each part of the installation which needs to be separately controlled for compliance with the Regulations or otherwise to prevent danger, so that such circuits remain energised in the event of failure of any other circuit of the installation, and due account shall be taken of the consequences of the operation of any single protective device.
to
BS7671:2008 said:
314.1 Every installation shall be divided into circuits, as necessary, to:
(i) avoid hazards and minimize inconvenience in the event of a fault
(ii) facilitate safe inspection, testing and maintenance (see also Section 537)
(iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit
(iv) reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation
(v) mitigate the effects of electromagnetic interferences (EMI)
(vi) prevent the indirect energizing of a circuit intended to be isolated.
314.2 Separate circuits shall be provided for parts of the installation which need to be separately controlled, in such a way that those circuits are not affected by the failure of other circuits, and due account shall be taken of the consequences of the operation of any single protective device.
And the designer has to decide if twin RCD protection complies with the current regulations, sorry don't have 2018 version, but since 2008 it is debatable as to if twin RCD's comply.
Yes I know if we get a power cut we loose all power anyway, so the take account of danger that may arise from the failure of a single circuit such as a lighting circuit seems a bit odd, but the take account could be emergency lights does not need to be split into circuits, but a RCD is an over current device, it does not measure current in the circuit but it does measure the current imbalance so since it is an over current device it does form a circuit.
If you accept the RCD forms a circuit then it is down to the designer to consider 314.1 and since it says "reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation" it is clear IET considered the RCD as forming a circuit, so the designer has to consider "facilitate safe inspection, testing and maintenance" so needs to provide a method to isolate.
In commercial premises this is considered, I have worked in many an office with a suspended ceiling where each lamp with plugged in with a klik plug in ceiling rose so we could isolate one lamp and change it without turning off all the lights,
even in just the one room. Some one in the design stage had considered maintenance.
So with where I work there was clearly an error, no one had considered what would happen if internet lost when the router was installed, so now we know the error, a UPS will be fitted, the problem is the wrath was aimed at the person who caused power cut, not the designer who did not allow for a power cut.