how does this rcd work?

I understand the principle operation of an rcd, phase and neutral both connected through the rcd and if one becomes unbalanced by a fault to earth the rcd trips BUT what about the rcd's I got from Screwfix.They are plug in types to replace re-wirable fuses on old ccu, only plug into phase conductor so how are they balanced with the neutral?I am studying my 236 part II at present so be as technical as you want. Mark
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I assume you mean RCBOs, which are RCD and MCBs combined which can replace a fuse.
They work because they also have neutral connections. A flying lead (sometimes supplied hard wired, sometimes with a Supply Neutral screw terminal) goes to the neutral bar. Both the Live and Neutral from the load connects to the load terminals on the RCBO.
If you don't mean an RCBO, please post the screwfix item number so we know what you are talking about.
Christian.
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Mark Atherton wrote:

Are you sure you have an RCD? The mini-trips that Screwfix sell are just MCBs.
How do MCBs work? I can't find it on google.
Ben.
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Two separate mechanisms, in fact. One thermal, to detect "modest" overcurrent, trips at around 1.4 times the nominal rating after a good few seconds or more (precise curves in manufacturer's data); another electromechanical, trips quickly (a few mains cycles only, maybe 0.2s) given a Hefty overload of about 5 times the nominal for a B-type, 10 times for a C-type: again, figures illustrative rather than precise.
Stefek
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snipped-for-privacy@hp.com wrote:

That's interesting to know. The electromechanical bit -- this is presumably just a relay coil wound and arranged so as to pull the switch open at the appropriate current? If so, isn't it a worry that a massive overcurrent will melt the wiring closed-circuit, so that the next time the MCB is required it doesn't work?
I suppose the manufacturers have thought of that, however :).
Ben.
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They, and those nice real-engineer types at the IEE. The mains supply itself has a finite impedance, limiting the "prospective fault current" (I think I remember the jargon aright). The combination of the quick action and the "you won't get more than, say, 6kA out of a normal domestic main, even with a dead short" allows the mfr to size the coil and so on of the MCB so that the total energy dissipated won't heat up the device beyond use. In practice, the current passing in a "dead" short is further limited by the resistance of the cables between the MCB and the point where the short is - shorts to neutral or earth usually happen somewhere Out There in the house, not a few inches from the MCB with a massive lump of copper between the MCB output terminal and the earth or neutral busbar.
"Unless", as we say in homage to She Who Allegedly Named RCDs, "you know better"...
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The magnetic portion normally trips within half a mains cycle, i.e. at the next zero-crossing point.

Yes. Each MCB has a breaking capacity -- the maximum current flow it can break. A typical value for a modern one is 6000A, and it will sometimes be shown as M6 in the rating data for the RCD. If you do exceed this, the MCB might be damaged by the current, and might fail to break the circuit (e.g. because the arc fails to extinguish). Generally, you have to make sure that the supply impedance will limit any fault current to within the MCB's breaking capacity. Not many domestic installations will have a prospective short circuit current of 6000A (or even near. However I have seen a picture taken inside a flat where this did happen (it was immediately next to the building transformer). The MCB and the main fuse both failed to break the fault current. The wire had exploded out of the wall, leaving the channel handy for rewiring the circuit.
--
Andrew Gabriel

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Whatever they are, they're not RCDs: as you've worked out, those just gotta have both a phase and a neutral incoming and outgoing connection - 4 in all. The only plug-in things to replace old rewireable fuses on the venerable Wylex boards which I've come across are "mini-trip" MCB's, just like DIN-rail mounting MCBs, single-pole-only, but with the physical connections and ratings to be a plug-in for the old nailXXXXfuse carriers.
Did Screwfix describe them as RCDs? The only thing I see in their catalogue described as and RCD is one CU incomer (80Aload/30mA sensitivity), with the expected 2-in 2-out wiring; they also sell 2-module wide RCBOs, an RCD and MCB combined in one device, again with the expected phase-and- neutral connections for both input and output - these have a flying lead to get the incoming N from the common N bar in the CU.
Stefek
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Stefek, you are right. Page 182 of Screwfix mag. They are mini trips, my mistake. Still not completely sure how they operate, presume they trip with earth fault current only not an imbalance. Mark
wrote:

earth
in
conductor
at
sensitivity),
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Indeed. You'll need lots of it too. Whilst an RCD would trip with an earth current of typically 30mA (0.03A), the mini trip requires 5 x rating (i.e. 150A for a 30A Type B MCB). You can see why an additional RCD would be beneficial.
Christian.
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In which case there's no earth-leakage detection about them. At all. None. They're exactly an overcurrent protection device, as a fuse would be, and the relevance to earth faults is only as the D element in EEBAD. As you say, they trip only with a (big) earth fault current.
("EEBAD" is IEE-speak for the normal way installations deal with a fault-to-earth: it stands for Earthed Equipotenial Bonding and Automatic Disconnection. Which means, in much simpler terms, "when a live wire touches a metal bit, a big fault current flows because the metal bit's well earthed, and the big fault current blows a fuse or trips the circuit-breaker.")
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