On Monday, June 24, 2013 12:28:10 PM UTC+1, Andy Wade wrote:
On 23/06/2013 18:47, wrote:
On Sunday, June 23, 2013 12:03:49 PM UTC+1, John Rumm wrote:

or use a flasher in the feed

I am sure you don't mean an old git in a rain coat, so could you elaborate?

If you pull the fuse or switch the mcb off, and clip a flashing
light bulb across it, you then get a mains feed that goes on and off
frequently. You can thus tell with your ground probe how much
difference the mains feed is making, it just means you can do one
sweep not 2. Its a classic old fashioned way to trace circuits.

I assumed that's what you meant, but
(a) is there any sort of two-terminal 'flashing bulb' available that
will work into the load here - predominantly the cable capacitance in
parallel with a highish resistive leak?

You can switch a filament lamp on to give it some load.

I havent seen thermal flashers in decades, you could probably make an equivalent using a relay though, with RC charging.


(b) I suspect that the flickering display on a DMM might be difficult to
interpret, especially if other time-varying voltages are present due to
other leakages, PME diverted neutral currents, etc.

Sure, you need either audio detection or an analogue meter.


I've a better idea now: instead of injecting mains, inject an audio tone
(1 to 2 kHz, say) at as high an amplitude as you can manage. You'd need
an audio sig gen and and a power amplifier with a 100V line o/p, or a
standard amplifier plus a step-up transformer.

mains transformers are fine at that freq.

(Do ensure that the
capacitive load and the low primary DC resistance of any transformer
don't upset the amp!)

At least 5ohms dc for an 8 ohm amp, preferably 8.

You can now use an inherently logarithmic detector to look for the
ground leakage signal, i.e. headphones + brain. The 'cans' would need
to be high impedance (Sennheiser 414's come to mind) or used with a
suitable preamp.

A simple opamp can get you a wide range of gain

High-pass filtering can be used to get rid of all the
50 Hz (and its harmonics) 'noise'.


yes, if needed. Even if 50Hz drove the opamp to clipping it wouldnt stop the tone indication working.

You might just use a self oscillating relay direct on the mains, with a series lamp to limit i and a 2nd parallel lamp to limit V_out to 50v max. A small relay chopping mains at 200Hz or so would get you a very different sound to 50Hz. Hi tech stuff.


NT