Ronald Raygun wrote:
The Natural Philosopher wrote:
John Rumm wrote:
On 07/11/2010 10:00, Ronald Raygun wrote:
Dave Osborne wrote:
An RCD that trips at 17mA is likely to be noticeably more of a nuisance
than an RCD that trips at 27mA.
I wouldn't have thought so. Surely if there is a leakage (sorry,
residual) current of 17mA then there is something seriously wrong
somewhere and it's worth finding it and fixing it.
It depends a bit on what you have on the circuit. Lots of IT kit and
other electronic items for example will typically "leak" a small amount
due to the mains input filtering. With enough items on the circuit the
cumulative "normal" leakage can add up and take a lump out of your
available trip threshold.
And more to the point, if there is any step surge or interference on
the mains input, can transfer quite large currents resulting from the
harmonics higher than 50Hz.
I don't find either of these two answers particularly illuminating.
Would you guys mind explaining why mains input filtering, or harmonics,
would "leak" current to earth? Surely *no* equipment should have *any*
connection to earth other than from the enclosure, and there should be
no current path to earth from any of the "gubbins". And why would
interference harmonics cause a leak?
Doesn't in all circumstances "what goes in must come out" apply? In
other words why should the live current ever (at any instant) differ
from the neutral current (except in direction)?
Or is it that there is never a real leak, but the breaker is fooled into
thinking there is, in which case what is the mechanism for fooling it?
OK, so the RCD measures the current flowing in the live wire and
measures the current flowing in the neutral wire and compares the two.
Notionally, if the current in the neutral wire is less than the current
in the live wire, then some of the current from the live wire *must* be
flowing to earth, as the neutral is bonded to earth at the substation
and therefore earth provides an alternative return path to neutral for
the current from the live wire. If the neutral current is lower than the
live current by a threshold value, then the RCD trips out the supply.
Some types of equipment can inject some nasty signals into the mains
which cause interference with other equipment, so to comply with the EMC
directive, suppression components have to be installed to stop the
interference from getting out of the unit.
One of the techniques for limiting the interference is to connect small
capacitors between live and earth and between neutral and earth. These
are called Class Y capacitors[1].
Now, some small current flows in these capacitors. As the nominal
potential difference between live and earth is 240V and the nominal
potential difference between neutral and earth is 0V, then the current
flowing from live to earth is 240V/Xc and the current flowing from
neutral to earth is 0V/Xc (i.e. zero), where Xc is the impedance of the
capacitor.
So, with this type of interference suppressor, current (say 0.5mA)
legitimately flows from live to earth and this current is detected by
the RCD. These currents all add up, so, for example, thirty pieces of IT
equipment[2] may have a leakage of 15mA. If the nominal 30mA RCD
actually trips at 25mA then effectively, RCD protection is sensitised to
9mA of unacceptable fault current instead of 24mA of unacceptable fault
current.
Now, the impedance of the suppression capacitor (Xc) is a function of
the frequency of the signal passing through it. As you will see from
http://en.wikipedia.org/wiki/Capacitor the impedance is inversely
proportional to the the frequency, which is another way of saying that
when the frequency rises, the impedance falls.
So if the device being protected generates harmonic interference (i.e
interference at multiples of the mains frequency), then the harmonic
interference is going to cause an even higher current to flow between
live and earth than that just due to the 50Hz mains.
In summary, there are circumstances where a legitimate current flows
from live to earth; these circumstances are commonplace in a modern
house and they cumulatively serve to "sensitise" the tripping
characteristic of the RCD.
[1]
http://my.execpc.com/~endlr/line-filter.html
[2] How many pieces if IT equipment in your home?
Computers?
Laptop power supplies?
TVs?
DECT phones?
DVD players?
Satellite boxes?
Printers?
Scanners?
Monitors?
ADSL modem?
Phone chargers?
Microwave oven?
etc.