[TowerTalk] CQ article error(?)

[Ian White G/GM3SEK]

Michael Tope wrote:
>----- Original Message ----- From: "Ian White G/GM3SEK" 
><gm3sek at ifwtech.co.uk>
>
>> Interesting... the power companies in the UK take a different view. 
>>In the event of a neutral break in their local distribution system 
>>(230V single-phase, neutral close to earth potential) they'd rather 
>>have the return current flowing back through earth than create a shock 
>>due  to the neutral becoming hot.
>>  Therefore they almost always earth the neutral at the transformer, 
>>and/or anyplace else they conveniently can. Having made that policy 
>>decision a long time back, they have created the expectation that 
>>neutral will be safe to touch when doing household wiring work, so now 
>>they're perpetually locked into that policy.
>>
>As I recall, there are also rules in the UK (or at least there were
>10 years ago) about shields on CATV drops being isolated between the 
>customer premises end and the rest of the plant. Presumeably this is to 
>keep neutral currents from the AC mains from flowing back to the source 
>through the CATV plant. When I worked for a CATV manufacturer I had the 
>opportunity to play with a number of these ground isolators. It turns 
>out to be a real design challenge to break the shield at 50 Hz while 
>providing a nearly continuous shield at RF frequencies. Most of the 
>designs I tested leaked RF like a sieve.

That would be in an apartment block where the power company provides 
earthing for all the apartments via the earthed neutral.
Each individual apartment would have its own inlet panel ("consumer 
unit") where a local protective ground is derived from the incoming 
neutral. They also bond the incoming water and gas services for each 
apartment to that same earth-neutral point. This isn't at true earth 
potential, of course, but it creates a safe equipotential zone inside 
each apartment so you can't get a shock by touching two supposedly 
grounded objects.

Each apartment is also protected by its own individual residual current 
breaker (RCD) at the panel, which will trip if the live and neutral 
return currents are not equal. The circuit feeding the CATV distribution 
amps would have a separate RCD.

The requirement for CATV ground isolators is mostly to prevent the CATV 
drops from introducing their own different circuit-ground potential into 
every apartment. In that case you could have neutral currents flowing 
between the cable shield and a nearby central heating radiator... 
through the occupier. If even one shield found a solid contact to a 
local apartment ground, it would probably trip the RCD feeding the CATV 
equipment.

All this makes sense for a multiple-occupancy apartment building. The 
real trouble starts when the "company provides your earth" system is 
applied to individual houses - especially those occupied by hams who 
want to add their own earth connections to the real dirt outside. If 
they don't take special precautions, a neutral break in the supply 
system could leave a large share of their own and their neighbors' 
neutral current flowing through the shield of their RG58... or through 
themselves if they unplug the antenna cable.

G3RZP wrote a warning article about this for RadCom, several years ago, 
but I fear a lot of UK hams are not aware of it.



-- 
73 from Ian G/GM3SEK