Earth Fault Indicator

Actually,
since these devices are frequently in wet conditions, the screw and/or
lug used to connect the earth wire to the device frame corrodes
gradually and ends up breaking or causing a faulty contact. The remedy
is then worse than the problem because the user, thinking that he/she is
protected by earth, does not take special precautions and risks
his/her life. However, all that’s needed is an extremely simple system
to automatically detect any break in the earth connection; so simple
that we ask ourselves why it is not already included as part of all
factory production for appliances that carry any such risk, as we have
discussed above.

We propose it as a project for you to build
using this schematic. The live wire (L) of the mains power supply is
connected to diode D1 which ensures simple half-wave rectification
which is sufficient for our use. The current which is available is
limited to a very low value by resistor R2. If the appliance earth
connection to which our circuit is installed is efficient, this current
is directed to earth via resistor R1 and the rest of the circuit is
inactive due to insufficient power. If the earth connection is
disconnected, the current supplied by D1 and R2 charges up capacitor
C1.

When
the voltage at the terminals of the capacitor reaches about 60 volts,
neon indicator light La1 is turned on and emits a flashing light which
discharges capacitor C1 at the same time. This phenomenon is reproduced
indefinitely as long as the earth connection has not been restored,
and the neon light continues to flash to attract attention in case of
danger. Building the project is not particularly difficult but, since it
is a project aimed at human safety, we must take the maximum of
precautions concerning the choice of components utilised. Therefore, C1
must have an operating voltage of at least 160 volts while R2 must be a
0.5-watt resistor, not for reasons of power dissipation, but in order
to maintain the voltage.

The neon light can be any type,
possibly used, or it may be part of an indicator light to make it
easier to attach to the protected appliance. In the second case, we
must obviously get rid of its series resistor which would prevent
proper operation here. During installation of the circuit in the
appliance to be protected, we should also clearly mark Live (L) and
Neutral (N) (for example, seek Live with a simple screwdriver) because
inverting these two wires at this point will disable proper operation.
The final point, which is self-evident considering the principle used
here: the earth connection for our setup must be hooked up to the frame
of the appliance to be protected at a different point than where the
normal earth wire is connected.