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G8MNY > TECH 21.11.24 10:27z 103 Lines 4468 Bytes #150 (0) @ WW
BID : 20337_GB7CIP
Read: GUEST
Subj: Edgcume Peebles Earth Loop Z
Path: SR1BSZ<IW0QNL<JH4XSY<N3HYM<CX2SA<GB7CIP
Sent: 241121/1018Z @:GB7CIP.#32.GBR.EURO #:20337 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Upated Apr 23)
(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!
___________
³ÚÄÄÄÄÄÄÄÄÄ¿³
³³ .ú'~'ú. ³³ Range 0R2 - 80R
³³ú\ ú³³ Peak Current 30 Amps
³³o \ o³³ Wander probe for
³ÀÄÄÄÄÄÄÄÄÄÙ³ Exterior earth tests.
³ [~~~~~] ³
Þ ~~~~~ Ý Often in a Leather case.
Þ Ý No batteries as it
Ý Þ uses the mains.
Ý Þ
³_________³
PRINCIPLES
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/impedance.
LiveÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄ¿
o _______³______
Pulse/ ³Mains Offset &³ ______ _____
ÃÄÄ´ Peak DetectorÃÄ´BufferÃÄ´Meter³
R ÀÄÂÄÄÄÄÄÂÄÄÄÄÄÄÙ ~~~~~~ ~~~~~
EarthÄÄÄÄÄÄÁÄÄÄÄÙ ³
³
NeutralÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
If the Earth and 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 kit plugged in at the time.
METER CIRCUIT
ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÂÄ2K2ÄÂÄÄÄ¿+80V
fetÃÄÙd Cal 5K6 ³ ³ ³
gÚ>³ Ú25KÄ18K´ _³_ZD1 ³+ ³
³ ÃÄ¿s +/~~~\ ³ ÚÄÄ>1K /_\' === _³_D4
5A -20V³ ÃÄÄ´MeterÃÙ ³zero³ ³ 10u³ /_\
LiveÄo-oÄÂÄÄÄÂÄÄÂÄÄÄ>L ³ 47k \___/ ³ 1K8 ³ ³60v³
220K ³ _³_ ³ ÀÄÄÄÄÄÄÄÄÄÄÄÄ)ÄÄÄÄÁÄÄÁÄÄÄÄÄÁÄ¿_)|| __ L
L-EÚÄÄÄÄ´ ³ \_/SCR Test charge ³ ³-11V 16V ||(
Neon 100K ³ ³ÀÄÂÄ100Äo oÄ´<ÃÄÄ)ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ)ÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿__ ||(
ÀÄÄÄÄ´ 220K ³ ³ /³\ D1 ÃÄÄÄÄÄÄÄÄÄÄ´ÃÄÄÄÄÁÄÄÂÄÄÄÄ´>ÃÄÄÄÄ¿__)||(__ N
³ ³ ³ 240R ³+ ÃÄ´<ÃÄ¿ u22 ³ D3 8V)||
³L-N³ ³ ³ 47u=== ³ D2 ÃÄ33RÄÄÂÄÄÄÄÄÄ´-11V 0V³
³ Neon ³ ³ ³ ResetÀo<ÄÄÄ´ ³ === ³
³ ³ ³ ³ ³ Test o ³ 100K ³10u ³
ProbeÄ¿ ³ ³ ÃÄÄÁÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄ)ÄÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
o ³ ³ 10R 30W 1K
EarthÄo<ÄÁÄÄÄ)ÄÄÁÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
³ 220K
NeutralÄÄÄÄÄÄÁÄÄÄÁÄÄ>N
CIRCUIT OPERATION
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) and 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.
Z VALUES
To quickly blow a 13A fuse, a current > 26A must flow. So at 220V that needs a
total loop resistance < 9 ohm. However with high current main ring circuits of
30A to blow a ring fuse quickly needs a current > 60A, which needs loop
resistance < 3 ohm. 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 ohm!
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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