On 02/09/2010 19:16, Fredxx wrote:
"Dave Plowman wrote in message
...
In article
,
Toom wrote:
What I don't understand is how to find impedance down earth pipe and
at socket with a multimeter.

In practice, you can't. You need a proper tester which just looks a bit
like a multi-meter. It puts a very large load to earth on the circuit for
a fraction of a second and measures the current flow, and from that
calculates the impedance.
They are quite expensive devices and need regular calibration.


Wouldn't a 3 bar electric fire between live and earth do the job? Where you
measure the voltage (change) between earth and neutral?

Resistance = voltage / current through fire?


That would work well, I think, in the absence of more sophisticated
equipment.

Many years ago I was having problems with hum in a particular piece of
equipment. The problem only occurred first thing in the morning, so I
swapped out the obvious part for further examination. When I'd done so,
every thing was perfect, as anticipated.

Until the following morning!

I missed that day's display but turned up bright and early the following
day to witness the problem at its most severe. It defied all logic and I
was desparate but luck was on my side. It was an office in old building
and the corner I was working in wasn't very well lit - it was mid winter
and natural light was virtually non-existent.

As I disconnected an earthed cable I saw a minute spark between the
connector and the earthed chassis. I plugged it back in again and,
almost immediately, I noticed the fault condition sharply reduce in
intensity - and realised that a girl was just returning to her desk
after turning down the fan heater they were using to supplement the
central heating ...

I immediately realised that earth and neutral must be reversed somewhere
along the line and connected a meter between the 'earthed' chassis and
the cable that I knew was most definitely earthed down in the basement.
Each kilowatt step on the fan heater produced a voltage drop of 500mV or
so and the hum was obviously being injected by the hefty AC current
passing through the screen of the cable.

I was deceived on my first visit, of course, because, by the time I'd
got the kit going again, the temperature had risen sufficiently for the
temporary fan heater to be switched off completely for the rest of the
day ...

--

Terry