[AMPS] Re: Surge voltage, HV connectors.

[Ian White, G3SEK]

Tom Rauch wrote:
>> > both sides of the line float with no CT reference to earth, although you
>> > have a safety ground. I had a specific conversation about that with two
>> > ZL's. Was I misinformed?
>> 
>> Very much so. I hope those guys aren't practising electricians in ZL2....
>> The mains here is multiple earthed neutral, typically 235 V, 50 Hz.
>
>If the mains are earthed, why couldn't I measure zero volts from 
>one side of the line to ground? If they are earthed at the center tap, 
>why aren't the lines 120 V from each side of the line to earth?
>
>Ian, what's the status in GB? 

We exported our mains supply system all over the British Empire and most
of continental Europe. If the Americas had waited until electric power
was invented before declaring independence, you'd be using it too   :-)

The output from the local distribution transformer is three-phase, Y-
connection, 400V between phases, 230V to neutral. Each house receives
one 230V phase plus neutral. The company decides which phase you get, in
order to spread the load among all three. (I'm told that three-phase
distribution direct to individual houses is also common in Scandinavia,
for heating purposes.)

The neutral is close to earth potential, so the line voltage really is
230V with respect to dirt, not + and - 115V like you have in the States.
All appliances are 230V, from big loads like the cooker right the way
down to a neon indicator.

There are actually two systems sharing the name "multiple earthed
neutral". I recently confirmed that modern distribution practice is for
the company to multiple-earth the neutral. In the traditional UK
distribution system, each house also has its own earth spike and there
is no connection between earth and neutral at the house. With Protective
Multiple Earthing, the company supplies all the earth protection - each
house does not have its own earth spike, but has a neutral-earth
connection on the supply side of the RCD instead.

In both systems, almost all the house wiring uses three wires. Live and
neutral are only for power, and earth is only for safety. Circuit
breakers or fuses protect the live-neutral circuit from excess current
in the normal way. 

Also, in all modern installations a residual current breaker (RCD)
monitors any unbalance between the live and neutral currents. That
includes leakage to either mains earth (eg a transformer primary to the
frame, which is connected to mains earth) or conduction to real earth
through someone accidentally touching the live. A typical household RCD
will trip on a 60mA difference between currents in the neutral and live
(eg 60.00A and 60.06A) and you can get 30mA RCDs for situations of
higher risk such as outdoor installations.

So, in a cable containing mains and *any* other conductors, the RCD
should catch any problems due to shorts. 

However, the real issue is that mains and HV inside the same cable is
lousy engineering practice! It would be contrary to product safety
regulations world-wide, which also explains why suitable connectors are
not available. 

HV inside a separate coaxial line with a grounded sheath is much safer,
even if tie-wrapped together with other cables.

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)
                           http://www.ifwtech.demon.co.uk/g3sek

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