Moving into a new house, I felt I needed a way to test the RCDs and wiring
on ring mains. It was an excellent opportunity to over-engineer something for the
sake of it. What I came up with was a device, that:
Can trip the power from a phone using a bluetooth (serial) connection
Is powered by the mains, and therefore needs/has a UPS
Is an awesome prank device
So far, other than surprising everyone during an unrelated countdown at a
party, I detected faulty earth wiring in my new house with it. It turned out
that there was a non-connection in (at least one) of the earthing points behind
one of the sockets for one ring main. Fixing this, the device was then able to
trip the power.
Here’s a video of it in action. Note that in this particular house (my old
house) every circuit loses power as there is a global RCD. Some houses,
including my current house, only protect the ring mains by RCD; which means the
lights don’t go out – making it much less annoying/fun.
You can use picoterm on a laptop or Bluetooth SPP on an android phone. The
device used is a UART (9600 baud, arbitrarily) HC-05 CY-MCU bluetooth board.
How does it work?
By shorting earth and live. I know that sounds explosive, and you’d be right if
I didn’t use a ballast resistor to limit the current. In this case, a 2K2
resistor rated for 50W. This limits the current to around 100mA, depending on
your mains voltage. 50W is overkill, considering the output is only triggered
for 300ms or so. Furthermore, most RCDs are specified to trigger within
something like 20ms worst case. Nonetheless, the resistor can tolerate full
mains voltage over it for some time before things are a problem – using
P=V^2/R the RMS dissipation is P = 250^2⁄2200 = 28.4W which is within
the rating of the resistor, if not necessarily that of its enclosure.
The nano is connected to a 5v power board from dx.com. The board allows 5v
charging of a 3v7 li-po cell, providing a boosted 5v output in the case of lost
power. This 5v power is provided by the somewhat-OK switch-mode 5V @ 2A power
supply, also from dx.com.
I used a mains solid state relay in the end. This was instead of the original
mechanical relay which was much too loud – it would reveal its location when
thrown. There was an interesting problem caused by this, however – the SSR has
some inherent capacitance that is present even in the off state. Combined
with the original 1K resistor, the resultant RC network was actually enough
to produce a current significant enough to trip the breaker when the device was
plugged in. Switching to the 2K2 resistor fixed this problem.
Here is the old version, with a mechanical relay.
I photocopied the components and stuck them on with contact adhesive to lay out
the board and drill holes. Lazy, but quick and accurate enough for