Hello Costas, hello all,
thank you for your interest on my isolation
transformer solution and the suggestions you made.
CK> The only problem with this approach is that in case of
CK> electrical fault or lightning strike one of the two
CK> earths conductors is at a high potential (voltage)
CK> in respect to the other. As the earth conductors are
CK> connected also to various metal objects (water pipes,
CK> equipment chassis etc) this situation can easily cause
CK> electrical shock or fire.
[use monospaced font for best view]
P -------W S S W----------EMF-----p"P"
W S S W
N -------W S S W
S S-CT
G ---------+ | W
| W
| W----------EMF-----p"N"
|
+---------------------pG
P = phase conductor of standard mains
N = return conductor of standard mains
G = safety conductor of standard mains
S = shields of transformer
CT = center tap
W = windings of transformer
p"P" = private phase conductor
p"N" = private return conductor
pG = private safety conductor
EMF = electromagnetic fuse (double sense, double throw)
Note: p"P", private phase conductor and p"N", private
return conductor in fact are more or less symmetrical
in potential referred to pG and (capacitively) to G.
Regarding case of electrical faults:
fault-1 (p"P" touches pG)
fault-2 (p"N" touches pG)
fault-3 (p"P" touches p"N")
The EMF will detect these 3 situations and will
shut off both, p"P" and p"N" conductors.
I think electrical shock is impossible as long as
all connetions made with conductor pG are low ohmic
and as long as EMF does not fail (when did you check
resistance of the safet conductor in your house
wiring last time? ;-).
Regarding case of lightning strike:
Lightning cannot strike G, nor can it strike pG
directly (those conductors are underneath earth
surface).
Lightning can however strike outdoor equipment
such as the antennas. Antennas are protected with
safety spark gap (approx. 15 kV flashover voltage)
connected to mentioned private grounding rod
(direct electrical continuity with pG, indirect
electrical continuity with G via soil).
Antennas are connected via LC circuits with
coaxial cables. Shields are directly connected
with private grounding rod. Center conductors
have arrestors outdoors and at the shack end
(protection levels 1kV/250 V respectively).
Coaxial cables run in steel conduit, 70 cm below
soil surface for 7m length, before entering the
house.
I do not see any chance of built up of lethal
voltages in between conductors G, pG and local
soil potential. In case of direct hit of a
lightning strike into antenna, the conductor
pG and all metallic objects connected to it
will jump to a potential Istrike times
ground rod to earth resistance (6 Ohms in
my case). This will happen as well if I directly
connect conductors pG and G. So where is the
benefit of using conductor G exclusively?
CK> A few years ago somebody tried a similar isolation
CK> system but conencted together the two earth
CK> conductors with an inductor made of thick
CK> electrical wire on a ferrite toroid. The coil
CK> had very small reactance at power frequency
CK> (50/60 Hz) but blocked conducted noise at radio
CK> frequencies.
This is a good idea as such and was taken into
consideration as well. Finally I did not implement
this for the following reason:
Receiption on my QTH is/was impaired by common mode
(noise) currents coming along the safety wire,
flowing on the outside of equipment chassis,
over coax shield surface and being coupled into
the coax at the antenna. Those currents seem to
origin from relative high impedance voltage sources.
Preventing them from flowing along a conductor,
an even more higher series impedance is required.
To be effective in LF range, large values of
inductivity are required. Large inductors have a
great amount of self capacity which in return
again provides a conducting path for frequencies
above LF.
The cure in my case was to provide a private
earth connection at the antenna (shorting out the
noise voltages from shield to ground) and break up
the conducing path from G to pG, leaving the
leakage capacity in between both as the residual
path for noise currents.
best 73 de Gamal
-----Original Message-----
From: Costas Krallis [SMTP:[email protected]]
Sent: Tuesday, January 23, 2001 2:44 PM
To: [email protected]
Subject: Re: LF: Mains Borne Noise
At 11:44 23/01/2001 +0100, you wrote:
>>> Has anybody on the list got a "whole house" mains filter
>>> installed at their QTH?
>
> Not a "whole house" mains filter, but a radio room
> mains "filter".
>
> All main supply to my radio equipment is run via a
> single phase isolation transformer,...
The only problem with this approach is that in case of electrical
fault or lightning strike one of the two earths conductors is at
a high potential (voltage) in respect to the other. As the earth
conductors are connected also to various metal objects (water pipes,
equipment chassis etc) this situation can easily cause electrical
shock or fire.
A few years ago somebody tried a similar isolation system but conencted
together the two earth conductors with an inductor made of thick
electrical wire on a ferrite toroid. The coil had very small reactance
at power frequency (50/60 Hz) but blocked conducted noise at radio
frequencies.
Costas
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