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G8MNY  > TECH     16.05.19 08:00l 102 Lines 4441 Bytes #6 (0) @ WW
BID : 5029_GB7CIP
Subj: Edgcume Peebles Earth Loop Z
Sent: 190516/0559Z @:GB7CIP.#32.GBR.EURO #:5029 [Caterham Surrey GBR] $:5029_GB

By G8MNY                                        (New Jan 08)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

This is an old tester the engineers used to use for testing the mains socket &
veracity of earths on sockets etc. It is quite useless nowadays with RCD/ELCB
protected supplies as it WILL ALWAYS TRIP THEM!
납 .'~'. 납  Range 0.2 - 80
납\      邂  Peak Current 30 Amps
납o \     o납  Wander probe for
냅컴컴컴컴켸  Exterior earth tests.
   ~~~~~     Often in a Leather case.
             No batteries as it
             uses the mains.

The meter provides a single half wave pulse of current (30A) from live to earth
through a known resistor. The peak voltage across the resistor will equal the
supply voltage less the loss due to earth line resistance.

           o   _____________
       Pulse/ 쿘ains Offset &  ______   _____
           쳐캑 Peak Detector쳐퀯uffer쳐퀾eter
           R  읕쩡컴컴쩡컴컴켸  ~~~~~~   ~~~~~

If the Earth & Live resistance equals R the voltage across R will be halved.
This voltage is compared to the mains so that variation in the mains does not
alter meter reading. Because this is fast pulse test (10mS) the earth circuit
"impedance" to the pulse rather than just the DC resistance is measured. Also
by using a full 30A peak current the capability of the wiring is actually
tested. E.g. fine wire strands that may pass the resistance test would be blown
up passing this current! This makes the test, what is required for a PROPER
SAFETY TEST, even though it hazards other it plugged in at the time.

                                 fet쳐d         Cal   5k6         
                                 g>           25k18k __ZD1 +  
                                   쳐퓋 +/~~~\   旼>1k /_\'  === __D4
      5A                      -20V   쳐캑Meter쵬  퀊ero    10u  /_\
Live훟-o컫컴컫컴쩡컴>L              47k  \___/       1k8      60v 
        220k  __                   읕컴컴컴컴컴)컴컴좔컨컴컴컨커_)|| __ L
 L-E旼컴캑    \_/SCR Test charge                 -11V        16V   ||(
  Neon 100k    냅컫100훟 o캑<쳐)컴컴컴컴컴컴컴컴)컴컴컴컴컴컴컴__ ||(
    읕컴캑 220k        /\  D1  쳐컴컴컴컴캑쳐컴컨컴쩡컴캑>쳐컴커__)||(__ N
               240     +      쳐<쳐     u22           D3    8V)||
         쿗-N      47u===        D2 쳐33R컴쩡컴컴캑-11V        0V
          Neon           Reset픬<컴캑           ===             
                         Test o         100k    10u           
Probe커       쳐컨컴컴컴좔컴컴컴컴컴컴)컴컴컴좔컴컴컨컴컴컴컴컴컴컴
      o      10R 30W                 1k

On connecting the mains the 2 neons should light, indication the mains is SAFE
to test. The 5A fuse handles the 30A pulse OK.

The transformer's 16V winding charges up the 47uF capacitor via D1 ready to
trigger the SCR. The 8V winding via D3 makes a -11V mains reference rail.

On testing, a single half cycle +ve peak voltage appears across the 30 Watt 10R
(one test per min MAX) & it is attenuated with 1k & 33R & compared to the 11V
rail, the resultant peak voltage is fed via D2 (not shorted in test mode) to
the u22 storage cap.

The voltage across the cap is then read with the FET buffered meter, with its
floating regulated power supply. Zero offset & meter gain presets finish off
the calibration.

To quickly blow a 13A fuse, a current > 26A must flow. So @ 220V that needs a
total loop resistance < 9. However with high current main ring circuits of 30A
to blow a ring fuse quickly needs a current > 60A, which needs loop resistance
< 3. These are much lower values than you can get with external earths.

I once tested a large unused radio tower & earth system with a simple if
dangous electric kettle test (no kit on tower) & I was horrified to find it was
as high as 14!

See my buls "Earths", "Edgcume Peebles Megger & Low R". "Maplin Mains Meter
2000MU-UK", & also "Mains Power Protection".

Why don't U send an interesting bul?

73 de John G8MNY @ GB7CIP

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