G8MNY > TECH 15.05.19 11:00l 99 Lines 3817 Bytes #5 (0) @ WW
BID : 4902_GB7CIP
Sent: 190515/0845Z @:GB7CIP.#32.GBR.EURO #:4902 [Caterham Surrey GBR] $:4902_GB
To : TECH@WW
By G8MNY (New Nov 08)
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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 easily 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 14ê!
Note do not attempt this method if there are ELCB trips.
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)
Assuming the mains is good!
Vm - Vt
Unknown Z = ------- x R
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..
Live to Neutral 240V on load 0 ê reference
Mast to live 150v. Rm = ------- x 24 = 14.4 ê
New Stake to live 169v. Rs = ------- x 24 = 9.9 ê
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.
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, so all
appliances you can touch are at this potential so you are 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
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 route to the substation. How many amps can your rig
take from aerial to PSU & PSU mains lead? 500A? None of this is fused all can
just burn out/catch fire!
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 now noisy!
2/ Fuse the external earth. This will protect your kit, but if it blows will
3/ Use an isolation transformer to feed the sack. This overcomes many of the
problems, but it does introduce non earthed safety issues.
4/ Ask your supplier to provide a suitable proper earth system.
See my bul on "Constant Voltage Transformers", "Edgcume Peebles Earth Loop Z",
"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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