I've read that current practice is to use several RCDs in a domestic installation. A time delayed 100mA RCD to protect the whole installation and
30mA RCDs for sockets, outside wiring etc. My question is why can't the main RCD be set at a safer 30mA sensitivity since it's time delay should prevent it from tripping before any other ones? Also if you install a garage unit with a 30mA RCD on an installation which is protected by another 30mA RCD at the CU, which one trips if an earth fault occurs?
Time delay 30ma is a rare beast & it's possible these days for there to be enough cummulative (but normal) leakage in a system to make it a nuisance. Whole installation RCDs are to protect against fire not human contact.
Both of these shorthand formulations have the potential to mislead. The specific reason for putting in 100mA RCDs on TT installations (those where the electricity company provides no earth, and there's a local earth rod) is to kill the power in the event of a fault-to-earth quickly enough. The reasoning goes like this: with a good, i.e. low-resistance, path to earth in the case of a fault-to-earth, a big enough current will flow to cause any fuse or MCB to blow/break in a Short time to avoid the voltage on nominally "earthed" equipment being elevated for Too Long a time. "Short" and its kissing-cousin "Too Long" are defined quite precisely in the Regs, with the two most relevant times being 5 seconds for fixed stuff (lighting circs, immersions, cookers) and 0.4s for socket circs - 'cuz hand-held stuff gets plugged in to those. For low-resistance earth paths, quite enough of a current flies through the fault path to blow the protective devices in good time; for installations relying only on a local earth rod, the earth impedance may be too high and variable according to soil conditions to be reliable. So you stick an earth-leakage protection device in to assure disconnection times even with a sometimes- too-high earth rod resistance.
There *is* a fire/cable-heating aspect to these disconnection times: while there's a fault current flowing, it's massively more than the current which the earth conductor (that's the Circuit Protective Conductor in carefully-correct current terminology) could ever carry for a Sustained Time. Again, the Tables in the Regs - based on the known resistance of copper cables - account for this quite carefully (i.e. it's not just a handwave, there's a quite detailed model of how much temperature rise will happen, incorporating the eminently reasonable assumption that none of this heat can get out of the cable in the short times we're considering). If the fault current were to last too long, the cable would be dangerous for future use (e.g. insulation would've got soft and conductors would touch or nearly-touch which are supposed to be kept apart by that insulation) - long before the cable actually Catches Fire in the manner beloved by Hollywood special-effects people! The Tables also account for how hot different cable types can safely get while carrying such fault currents - so, for example, at two extremes of that range are normal PVC cables (insulation gets unacceptably soft at, what, 90 degrees C?) versus MICC cables (bare copper live-and-neutral conductors bedded in a mineral insulation, copper sheath on the outside as the earth, often seen surface-run in churches and other buildings where you have to surface-run and want cables as thin and un-ugly as possible: a Win for these calculations both because the mineral insulation doesn't go soft at wimpy temperatures like mere water-boiling-point, and because the earth conductor is of bigger cross-sectional area in these cables than the live conductor).
So: the disconnection time is what the ELCB is there to assure. The disconnection time in turn is important for two reasons: one, to limit the duration of the exposure of any person to higher-than-earth voltages on anything nominally "earthed"; and secondly, to limit the time during which cables have to carry fault current.
My guess would be if three are 3 other sub rcd in the house with a 30ma sensitivity and a 11 ma leakage each then the main rcd would always trip. only a guess but plausible.
Because 30mA isn't very much leakage current, modern appliances with SMPSUs and other things with suppression components "leak" to earth quite a bit. ISTR that you shouldn't have more than 10 such appliancies on a ordinary 30mA RCD because of the leakage level causing nusiance trips.
Only certain types of installation need a 100mA time delayed whole house RCD. An 100mA delayed RCD is required when the earth loop impedance is (or maybe) to high to allow large enough fault currents to flow enabling the protective devices (fuses or MCBs) to disconnect in the required times.
Already been said "indeterminate". But the garage should be fed from it's own MCB and RCD at the CU not just be part of one of the houses RCD protected ring mains.
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