Is there a case for protecting the upstairs with an RCD ?
The follow-on question: what happens if you stand on a carpet (or wooden
floor) with an uninsulated live cable (and only this cable) in your hand --
(1) will it trip the RCD, and (2) will you feel anything ? I know what
happens if you stand on a wet lawn!
It all depends on the resistance between you and earth - which
depends on how insulating you carpet/wooden floor is.
If the resistance is more than 300k, you'll get less than a mA
through you, which will tingle but not do much harm. If the
resistance is more than a meg, you're unlikely to feel anything.
If the carpet/wood is dry, and you're wearing rubber soles, your
resistance is going to be pretty high. So holding the live wire
(just the live, not live + neutral/earth) will result in only a
few uA flowing through you, which you won't feel.
The RCD will only trip if you pass through a current greater
than it's trip current (e.g. 30mA).
Thanks - and sorry to persist...
What about capacitance - what if I hold in the other hand a large metal
ladder standing (on the same carpet) on rubber feet, will current flow in
and out (at 50 Hz) of the ladder, and how much current ? And if it does
(and exceeds 30mA) will it trip the RCD ?
The capcitance of the ladder will be way to low to support any
significant current flow - will be in nF.
So the reactance of the capacitor will be:
R = 1/(2Pi.F.C) (F = frequency, C = capacitance)
Let's assume 10nF (this is generous)
R = 1/(6.3x50x10E-9)
R = 317k
Which will only let say a mA through.
But it's more than enough to put your muscles into a 50Hz spasm and it
will fing hurt. Even it you don't come to any lasting harm from the
direct effects of the electric shock, the jolt may well make you
withdraw you arm rather rashly and bash your elbow or fall of the
ladder which can cause serious injury...
Do you volunteer to have the mains applied to your anatomy to prove
Dave. pam is missing e-mail
How do you know you've had 'em if the don't hurt? B-)
I've lost count but I know damn well that they all hurt! The worst
having pain/stiffness at the elbow for the following 8hrs combined
with viral type ache in the muscles from the shoulder down. The point
of contact was the hand somewhere. That must have been an up the arm
and out somewhere else jobbie most others have been across parts of a
hand and no were near as nasty.
Dave. pam is missing e-mail
Thanks for your replies, do I take it that the opinion is that if you touch
the live only, possibly while standing on, and holding on to, a large
A-frame metal ladder, but otherwise (you AND the ladder) insulated from
earth and neutral, that:
- it will hurt somewhat, but the RCD will not trip (<30mA)
Roughly, yes. The reason that RCDs for personal shock protection are
rated at a nominal 30mA (and in fact required (a) not to trip below
15mA, and (b) required fer-sure to trip by the time the current imbalance
is 30mA, within 0.2seconds, and (c) to trip within 0.04seconds with a
current imbalance of 150mA) is that this figure is chosen to balance a
low probability of serious injury against nuisance tripping.
So, if the path to earth is high resistance enough not to trip it, your
experience will be of discomfort and a feeling of stupidity (including the
particularly galling feeling that there's no-one else to blame. Unless the
cat happens to be passing, of course ;-). If the path to earth is of low
enough resistance to trip the breaker, your experience will be of greater
discomfort, some pain, but little likelihood of your beneficiaries collecting
on the life or disability insurance!
Obviously, only the teenaged lads among us will be daft enough to do the
trial "for real", 'cos it's a dumb idea to play games with mains electricity.
And whole-house 100mA RCDs will let significantly more damaging currents
through for longer before tripping, if at all.
HTH - Skefet
I think but am not sure that Stefaks post (above) has explained
Cos I've been looking for the term 'imbalance' in previous posts
I thought I understood that these operate on an 'IMBALANCE' of
current between the live wire and the neutral wire?
So may I ask, as follows?
Let's suppose an electrical appliance is drawing, say, 10 amps
(2300 watts) through a circuit equipped with an RCD.
Suppose then an imbalance occurs, for some reason, whereby the
current between live and neutral differs by more than 15
milliamps, somewhere say around a 30 m/a 'difference'. That
imbalance will cause the RCD to trip the circuit within 0.2
seconds (12 cycles); yes?
Then, supposing the same circuit is not supplying any load
current to an appliance at all; somebody gets onto a ladder and
what with body capacitance, damp slippers on a metal ladder,
holding onto an uninsulated live wire in their teeth etc. or
other nonsense, a current greater than 15 milliamps, say around
30 m/a flows in the live conductor only. Again we have an
imbalance. The RCD will again trip the circuit within 0.2
Or have I missed something? Cheers. Terry.
PS. At low DC voltages holding say test probes in my calloused
(read 'hard working, son of the soil' etc.) my skin resistance is
around 100,000 ohms. however at higher voltages and or with minor
cuts there could easily be a break down of that figure. Got a few
scars to prove it. Worst was discharging a 2000 volt DC medium
size capacitor across my knuckles about year 1950. Yup. small
scar still there!
Right on the money, again. An RCCD (of the currently-available current-
operated sort) doesn't care whether the circuit it's controlling has
40A flowing round it or 0.4A; what it cares about is that "what goes out
comes back" - i.e. that the current flowing out through the Live terminal
is within a gnat's whisker of the current coming back to its Neutral
terminal. Though more recent ones use electronics for this differential
sensing, the classic construction method is to use two coils with the
same number of turns but wound in opposite directions around an iron toroid
(doughnut-shape) - these are the "sense coils". When the currents are
in balance, there's no nett magnetic field set up in the toroid, as the
sense coils are wound in opposite directions. But if some of the current
going out of the live terminal finds a different way back to earth, the
two fields no longer cancel, and the nett field is used by another coil
- the "search" coil - to pull back a solenoid which is all that's stopping
the breaker spring from disconnecting the circuit. Pop.
So, back to your two scenarios: both result in an imbalance in the
currents in the two sense coils, so both create a magnetic field, which
the search coil turns back into electrickery to operate the disconnection
This same current-imbalance sensing is what causes multi-circuit RCDs
to trip when you short a neutral to earth on a circuit you've "disconnected",
i.e. pulled-the-fuse/manually-tripped-the-MCB on. The other circuits
are still live, and the sum of current being drawn by them is neatly
balanced on the way out and on the way back. Then you - to take a purely
hypothetical example - let the neutral wire of the immersion-heater cable
you're working on brush against the (uninsulated) copper tank. Pop, whole
house in darkness (yes, this place had a single whole-house RCD; no longer
Regs-compliant, but given the crappy state the electrics were in, a Win
for safety on balance!). That's because you've just given all those
eager electrons coming down the neutrals a second way back to Earth than
through the N terminal of the RCD - enough of them like this New Way back
that the RCD is unbalanced, and pops. (Remember all the Ns are commoned
up together back at the consumer unit, right? And pulling the fuse only
breaks the N connection on the circuit you're working on.)
We had a related case just the other day on this group - someone had
wired their new circuit so it took a live feed from the RCD-protected side
of their CU, but had connected the return N back to the non-RCD side.
Unsurprisingly, the moment the load was switched on, the imbalance made
the RCD trip.
HTH - Setfek [determined to keep up the inner anagramming, while keeping
the "Two Es" feature of the name unchanged and thus subliminally reinforcing
the most-frequently-gashed feature of his moniker :-]
(blush) yeah well, the intelligent reader knows I *ought* to have written
"breaks only the L connection" instead; the unintelligent will believe
the literal stuff as written and select themselves out of the gene pool
soon enough ;-)
Thanks Stefak for a very complete reply. Also you mentioned that
whole house tripping RCD question and explained it for me.
Yes here too we have the earth/ground supposedly connected to the
neutral once only; at the main circuit breaker panel (CU). In
older homes RCD type breakers are not yet commonly in use AFIK.
But outlets so equipped are now common and required for all
'damp' locations e.g. a bathroom and/or for use of 'outside'
appliances such as an electric mower, hedge clipper etc. and are
called GFCI (Ground fault circuit interrupter is, I think, the
There are two basic types of GFCI duplex outlet; one type
'protects' itself and any other outlets downstream of it. The
other only protects itself and anything plugged into it.
For example we have a GFCI in the garage which also 'protects'
one downstream outlet on an outside wall. Out thre we have
plugged AC mains to our slide into truck camper unit. Every so
often there is some kind of imbalance that trips the GFCI; it's
probably the old fridge in the camper! Although the camper unit
is not grounded (oops, sorrry, earthed!) Doesn't matter because
the camper is only stored there for the winter with power used
The major difference is that we have a 115-0-115 volts, three
wires; with 230 volts across the two outside wires (hence two
pole breakers for 230 volt circuits). Typical house service is
200 amps (230 x 200 = 46 kVA). especailly if electric heating
(Cost equiv. to about 3.5 to 4.0 pence per unit). The
earth/ground is provided by an earth rod connected to both the
ground bar and the incoming power utility neutral only once at
the incoming electrical service and incidental connection to any
available metallic water supply etc. back at the transformer tha
middle 'zero' potential wire neutral is also connected to earth
and via what is called a multi grounded neutral MGN to everything
else in sight, including guy wires telephone company cable
Your info/discussion much appreciated. My regards. Terry.
Point of detail, Mr. Z: the coils are wound in the same direction, it's the
currents that go in opposite directions - like this:
L --------UUUUU----->----- L
supply .- - - load
N --------UUUUU-----<----- N
- - - i2
wind'g | | (residual current = |i1-i2|)
Hmm, basket faze, bakez feast: ze fab skate... (s'pose you've seen em all
D'oh! Totally bleedin' obvious now you point it out. That'll teach me
to engage brain (and perhaps make the Maxwell three-finger-gesture ;-)
One of the better ones an obviously-underemployed colleague at previous
employer (s/w house just south of Oxford; hello any Joint European
Tourist types ;-) came up with in a fit of story-writing in which he
insisted on anagramming all our names was that arabic variant, Fez Atabesk.
Must get myself a new Blunkett made out in that moniker ;-)
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