[clug] Measuring residual current

Neil Pickford neilp at goldweb.com.au
Sat Jun 23 03:43:50 MDT 2012

As a first step get a decent Multimeter that can measure AC Volts and 
Mega Ohms (resistance).

Turn off the circuit in question (trip the RCD) but leave the now dead 
gear connected.

Double check there is no voltage on the circuit with the multimeter on 
the AC Volts range between all pins (Active, Neutral and Earth) of an 
unused power point (Socket) (Make sure the power point switch is ON).

If there is any voltage present (above about 1 volt) - DO NOT Proceed to 
try and measure resistance (and blow up the multimeter)

Measure Active to Earth Resistance of the DEAD circuit.

It is most probably the Active to Earth resistance that is of most 
interest, however Neutral to Earth resistance may also be a factor but 
can only be measured if the Neutral is also broken at the RCD breaker 
(since Neutral and Earth are tied together at the main switchboard [~0 
Ohms] in the MEN system)

The Active to Earth resistance should be around or above 1 Mega Ohm
If you measure 10s or 100s of kilo Ohms then you have observed the 
problem.  Most general RCD's have a guaranteed residual trip current of 
30 ma which equates to around 8000 ohms.

Unplug each item of equipment until you get readings back in the Mega 
Ohms.  The last one you unplugged is probably the culprit.

Unfortunately if the leakage is occurring via an active switch mode PSU, 
this test may not reveal the culprit, since the equipment will not be 
operating with the mains supply off.  However, this test is still a 
valid first test.

You could also unplug each item on the circuit and do the test at it's 
plug, however measuring the entire dead circuit and unplugging stuff one 
at a time will show if there are dramatic changes occurring on the 

If you have unplugged everything and it hasn't improved it will also 
show if the permanent power circuit, it's self, has a fault (Damage, 
Moisture or Vermin [spider / cockroach / mouse] attack).

You should not need to disassemble any power equipment to do this test.

Neil Pickford

On 23/06/2012 9:26 AM, Peter Barker wrote:
> On Fri, 22 Jun 2012, Mike Carden wrote:
>> Well you could get one of these:
>> http://www.sparkydirect.com.au/p/888262/clipsal-3-pin-plug-top-safety-shiel
>> d-heavy-duty-439shdtr-.html
>> and wire the active and neutral across to one of these:
>> http://www.sparkydirect.com.au/p/888263/clipsal---rewireable-cord-extension
>> -10amp-pvc-socket-438str.html
>> and break out the earth to your trusty AC ammeter.
>> What Could Possibly Go Wrong?  :)
> Or, of course, leave out the last step (anything to do with Earth). That
> would certainly solve my immediate problems :-)
> But yes, you're quite right here. I've been thinking about this in terms
> of Residual Current measuring, because it's an RCD tripping. I expect we
> have bigger problems if we are leaking current anywhere *but* to the
> Earth line, so perhaps I should redirect my searches to "inline earth
> leakage ammeter" or something (making up our own device as you suggest
> above is probably not an option...)
>> MC
> Thanks,

More information about the linux mailing list