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.
I assume you mean RCBOs, which are RCD and MCBs combined which can replace a
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.
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.
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 :).
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
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.
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, 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.
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
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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