I'm after a bit of advice as to why I've just got an electric shock. i
have an old but quality lathe with a seperate switch with on/off
buttons. The switch has a metal case. The live wire from the plug had
chafed and come into contact with the box and when I went to switch it
on I got a shock.
The bit I don't understand is why nothing tripped. The live and neutral
wires are both wired into the contactor and the earth from the lathe and
from the feed from the plug both connect to an earth terminal on the
switch casing and were done up tight. The socket in the garage is fed
from a dedicated 16 amp breaker in a modern MK consumer unit with an
RCD. Should that not have protected me?
I don't understand electricity as well as I should and would be grateful
The RCD will only trip, if sufficient current passes through you, to
earth. Possibly there was not a sufficiently low resistance path
through you to ground to trip it. Another possibility is that the RCD
is faulty or seized up. All are fitted with a test button, does it trip
if that button is pressed?
From the description, I assumed the chafed live was touching the
earthed case, so shouldn't be waiting for anyone to touch anything
before a trip...
Maybe the earth in the garage (exported from the house?) is faulty.
Also of course, are you absolutely sure that the earth itself is real, IE is
the wire at the switch end and the supply end actually connected? You may
laugh, but screws have a habit of working loose and wires bent at right
angles in a short space can also break if quite old. I had an ally one break
to a cooker like this some years ago.
There has to be a reason as otherwise when the live shorted and the earth
on the box formed a circuit some protection somewhere would have tripped.
This newsgroup posting comes to you directly from...
30mA is a modest current for you to feel. I suspect the current you felt
was less than this. It shouldn't trip at a current less than 15mA.
It sounds like your lathe has a faulty earth connection. As soon as the
live had made contact with the 'box', which I assume is metal and should
be earthed, the RCD should ha tripped.
As another poster has suggested, it might be a good opportunity to test
Yes it should. So it suggests you have one or more faults somewhere.
Generally the way earthing works is to force the supply to disconnect by
operating a circuit protective device - typically a fuse, or circuit
breaker or RCD. The objective being to limit the duration of any shock
you can receive so that its is not long enough cause serious injury.
You also have an additional protection from the RCD. That looks at the
current flowing in both live and neutral at the same time, and trips
when it sees anything more than a tiny mismatch in those currents (the
theory being that if some current is flowing to earth somewhere in the
circuit (say from a live switch casing, though you to earth), then it
will see and imbalance and trip.
So lets look at the earthing first:
So with your lathe, the metalwork of the whole machine and its switch
box should be well connected to earth. When that live wire made contact
to the metalwork, you should have had a very large current (often called
a "Fault current") flow between live and earth, and that should have
tripped an upstream protective device.
So assuming the lathe actually works, it seems likely that you have a
good connection on both live and neutral. So the most likely problem is
that your earth connection is either broken completely, or is has a high
electrical resistance for whatever reason. Its important you find where
the fault is, since it is safety critical.
Disconnect the lath from the mains at the plug
You need a basic multimeter to carry out some tests. With the multimeter
set to measure resistance, connect one lead to the earth pin (the top
one) on the plug, and touch the other to the metal case of the lathe.
You should see a nice low resistance - under 1 ohm typically. Check you
still keep the good connection when the flex is flexed a bit (its not
uncommon for wires to break in the flex where they get lots of movement).
If the flex and plug look good, then it may be the socket or its supply
circuit which is faulty.
First a visual check. Turn off the circuit at the CU. Unscrew the socket
faceplate and have a peek behind - check the earth connections look
good. Give a tug on the wires, and make sure they do not pull out of the
terminals. If that looks ok, replace the socket and we need to move on
to some electrical tests.
With the mains on, and your multimeter set to measure AC volts (on a
range were the maximum voltage is more than 240V), you should be be able
to measure 240V (ish) volts between the Live (right hand pin on the
socket) and the earth. You will need to use the probe in the earth pin
to force open the shutters on the live pins. If you can't, then that
suggests that the earth is broken somewhere between the socket and the
CU. (and if the socket is part of a ring circuit, then its broken
twice!) - if this seems to be the case, post back for detailed advice on
The other question is why did that not trip. There are two likely
possibilities: either it is not working correctly, or the shock you
received did not actually pass enough current to reach its trip
threshold. (its even possible the device installed has too high a trip
So first step, check the writing on the front of the RCD and make sure
it has a 30mA (or 0.03A) trip threshold.
Now verify it really is protecting the circuit the lathe is on by
turning it off and machine sure something plugged into the lathe's
socket will not work.
Now try the test button - it should trip the RCD immediately. If it does
not, it needs replacing.
If all those pass, then its likely the shock current was too small to
cause a trip. There are more detailed tests that you can do, but again
post back for more if required.
Have you got a DVM? Try measuring the resistance between the metal part
where you got the short and the earth pin on the plug. Not a definitive
test by any means, but a poor connection between earth lead and the
machine metalwork is pretty likely with age.
*There are two kinds of pedestrians... the quick and the dead.
Dave Plowman email@example.com London SW
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