It seems that they did for some users. However, as I've often said, for well over 20 years I have lived with many (probably about a dozen) multi-MCB RCDs and, during that period, have experienced almost no 'trips for no good reason'.
From what you've often said, it sounds as if you have been particularly unlucky in experiencing many of these 'trips for no good reason', so I would be interested to know how many others have had similar experiences.
Kind Regards, John
I think you have to remember I fitted all RCD protection back in early 90's when they were quite new, so maybe my RCD's are not to modern standards and more like to trip with a spike, and fact resetting one RCD could cause the other one to trip if I did not turn off the MCB's first also points to a spike causing the trips.
I have been lucky in a way that in 30 years of use, only lost around 4 freezer full of food, 9 times out of 10 we have been able to reset in time, or at least think we reset them in time.
Today my freezers if they lose power show the temperature reached before power was restored, so without opening doors to check, I know if de-frosted, except for the chest freezer, and to be frank a chest freezer can be without power for nearly 24 hours and OK, in the main because they are not frost free. The problem with the frost free upright freezer is the time it can be without power depends where it is in regards of the de-frost cycle. It may last 10 hours if just before de-frost cycle is about to start, but if just when de-frost cycle is about to conclude then lucky if it lasts an hour.
Now when you find the RCD has tripped you clearly reset it and don't open the freezer door as want the heat kept out until it has had a chance to cool, so there is a danger food has defrosted but you have not noticed, once power is lost the circulating fans in the freezer stop, so food at top can defrost but food at bottom still OK, so there is a very real chance that an inspection will miss the fact food at top has defrosted.
I used an old fridge/freezer to brew beer in, and was surprised how long it took in freezer compartment to cool beer at 24°C to 19°C talking about a couple of hours or more. Yes it was 40 pints so a big lump, but when you lose power with a full freezer and it does warm up, it also takes a long time to freeze again, and there is a very real danger of damage to the food.
In the early days of RCD protection it was permitted to have selected designated sockets without RCD protection for things like freezers, but that has been removed.
When I fitted my RCD's in the 90's they were new, and we had not really worked out what should be done before fitting them, I did use an insulation tester but these use DC, today we know we should measure the leakage, as with AC there is always some leakage, and in use a 30 mA RCD should not with no fault supply circuits with over 9 mA of leakage, this is with every thing turned on. I will hold up my hands and admit I have never done this, simple reason my clamp on ammeter will only go down to 100 mA, so I have never been able to measure leakage of whole circuit, the PAT tester measured it for individual items, but not as a group.
I know from the PAT tester some filters have resulted in a high leakage even with new items, which will not show up with insulation testing as that uses DC, so the leakage is often luck, and we have all in the early days of PC's had problems where multi PC's were used in one room. I can not recall a single 100 mA RCD tripping without good reason, even when whole house on one RCD, so it would seem even if 3 x 30 mA RCD's supplied whole house so total of 90 mA it is unlikely they will trip without reason, so 30 mA must be just on the edge, do remember 30 mA is between 15 mA and 30 mA with most RCD's I know the X-Pole RCD claimed tripped between 90% and 100% but the standard is between 50% and 100%.
My RCD tester was ½, full, and x5 there was no ramp test, so I don't know if my RCD's were tripping at 16 mA or 30 mA I had no way to test. I do know where a RCD was tripping and could not find any fault with either RCD or installation swapping it cured the problem, hind sight was likely tripping at 18 mA and new one at 26 mA and had I had a clamp on meter able to measure 1 mA then I would have seen the problem, or a RCD tester with a ramp option.
Clearly part of the problem with just twin RCD's is down to testing, and this has been highlighted with the discussions on type AC, A, F, and B RCD's, we have been always told to test with no load, we test the RCD not the circuit as a whole, and clearly with some equipment we don't want to turn off the supply without logging off. And it is the equipment which requires logging off from which is likely to cause problems, either with unwanted tripping or failure to trip.
So we have always said don't put all your eggs in one basket, be it equipment/s which stops a RCD tripping or causes it to trip when it should not, if the CU is like mine with 16 RCBO's the risk of not tripping with fault, or tripping without a fault is reduced. So if my TV is drawing a DC component stopping the RCD tripping, then the other circuits are still RCD protected.
So I have one circuit which I have found feeds just one FCU, I would assume originally supply to shed, if I use that to charge an electric car, it may be a 16A type AC RCD but there would be a RCD built into the charging pod, so there would be around 2 meters of twin and earth which if damaged when a faulty car was on charge may not trip the RCD, the risk of that happening must be very very low. But if one RCD feeds multi outlets then not 2 meters of unprotected cable as a result of fault on car, but 100's of meters, so with RCBO's the risk of DC stopping them tripping is very low, but with just 2 RCD's the risk is much higher.
But in real terms until we are required to test RCD's with a DC component, and test for a DC component on the supply, it is all guess work, yes we have all seen the demo videos, but in real life how often does the RCD protection require the 40 mS or the 30 mA for genuine wanted tipping two main causes are human error, not checking for cables, and water, and with the latter a 100 mA type S would trip and protect one, the speed is not really required, and with the former even 40 mS gives one a nasty belt, it makes no never mind if 30 mA or 300 mA it still takes around 20 mS to trip, and you get full current during that time.