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G8MNY > TECH 23.11.08 10:21l 94 Lines 3630 Bytes #999 (0) @ WW
BID : 9280_GB7CIP
Read: GUEST
Subj: Earths
Path: IZ3LSV<IQ3GO<SR1BSZ<ON4HU<ON0BEL<ZS0MEE<GB7CIP
Sent: 081123/0158Z @:GB7CIP.#32.GBR.EU #:9280 [Caterham] $:9280_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (New Nov 08)
(8 Bit ASCII Graphics use code page 473 or 850)
Z VALUES
To quickly blow a 13A fuse, a current > 26A must flow. So @ 220V that needs a
total loop resistance < 9ê. However with high current main ring circuits of 30A
to blow a ring fuse quickly needs a current > 60A, which needs loop resistance
< 3ê. These are much lower values than you can get with external earths.
AC TESTING A TOWER EARTH SYSTEM
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
originally about 40ê!
Note do not attempt this method if there are ELCB trips.
Test
LÄo\ÄÄÄÂÄÄÄÄÄÄÄÄÄ¿
³ ³ This high current & voltage test overcomes some
2kW AC Volt of the errors of low currnet & voltage testing
EÄÄÄ´Kettle meter due to corrosion etc. As direct mains is envolved
³ ³ some basic safety needs to be considered. And if
ÃÄÄÄÄÄÄÄÄÄÙ you have just filled the kettle are U wet !
/
NÄÄÄÄo/ oÄÄTest Earth (Z)
CALCULATIONS
Assuming the mains is good!
Vm - Vt
Unknown Z = ------- x R
Vt
Where Vm is On load mains across kettle.
Vt is Volts Live to test.
R is Resistance of Kettle (e.g. 24ê)
MEASUREMENTS from old GB3HV mast..
Z CALCULATION
Live to Neutral 240V on load 0 ê reference
240-150
Mast to live 150v. Rm = ------- x 24 = 14.4 ê
150
240-169
New Stake to live 169v. Rs = ------- x 24 = 9.9 ê
169
4.95 x 14.4
Theoretical total with 2 earth stakes Rt ----------- = 3.7 ê
4.95 + 14.4
Mains Earth tested at 0 ê.
P.M.E.
With UK's Perminant Multiple Earthing system on modern housing, a mast or
outside earth can be condidered "dangerous". This is because there is no
safety earth provided, just "neutral" called earth in the house. all appliances
you can touch are at this potential so your safe. This is a cost cutting scheme
introduced by the electric supply companies & they effectively deemed it safe.
Substation Supply Shack
L-----------------LÄÄÄÄLÄÄÄ DC ÚÄÄÄ Ant
NÂ----------------NÄÂÄÄNÄÄÄ PSU :::::::RIG
_|_ ÀÄÄEÄÄÄ etc Ù ÀÄÄ¿ RF Earth
Substation _³_Outside
Earth
Now suppose the Neutral is not at ground, say 100V off ground due to unbalanced
load 3 phase loads or someone is blowning a fuse. The 100V now sees your path
to earth as an alternate rout to the substation. How many amps can your rig
take from aerial to PSU & PSU mains lead? None of this is fused all can just
burn out/catch fire!
Solutions.
1/ Connect a heavy 100A lead between the external earth & the mains earth
point, this may save your kit, but ensure the RF earth is noisy!
2/ Fuse the external earth. This will protect your kit, but if it blows will
you know?
3/ Use an isolation transformer to feed the sack. This overcomes many of the
problems but does introduce non earthed safety issues.
4/ Ask your supplier to provide a suitable proper earth system.
Why don't U send an interesting bul?
73 de John G8MNY @ GB7CIP
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