[SML] wire colors

[e-mail frank.wood95]

We, on our side of the pond, do it differently. The last thing that
the street distribution sees is a sub-station, with a star-wound
transformer. The star point, which is the neutral, is connected to
earth here, and NOWHERE ELSE! The earth plate at the sub-station  is
connected to the steel wire armouring of the buried distribution
cablse. That is the usual urban system.

Rural distributions sometimes use a Protective Mutual Earth (PME)
system. This involves earthing the neutral at each and every pole in
the overhead wires. Doing it otherwise can create problems,
particularly when the soil is of low conductivity.

I remember doing a show at the Minack Theatre, in Cornwall. The
sub-station was some way off, and the soil of low conductivity. The
whole site had, then, a 60A 230V supply. If you were running near the
limit, the dressing room making a cup of tea could trip the breaker,
producing a total blackout.

It had other effects. I needed to fire some pyros, and used spare
cables of the permanent lighting system for this, one firing circuit
being the live to earth, and the other being the neutral to earth.
What I didn't know was that the fool who did the wiring had bonded all
the cable neutrals to the incoming neutral, rather than allowing them
to find it when patched. At an early rehearsal, when I faded up the
stage lights, half the pyros went off, and the main breaker tripped
out. Subsequent investigation revealed that, with a big load up, the
neutral and earth were 90V apart, with sufficient current available to
blow a pyro fuse.

I have often been worried by the electricians on location filming, who
often make their earth connection to a big generator by parking the
rubber tyres of the lorry on a bared earth cable on a tarmac road.
Given the possible instaneous earth currents with a major
live-to-earth fault, which are many kiloamperes, I have never seen
this as adequate protection. I have also heard of complex OB rigs,
where the outer of one single co-ax cable has been providing the sole
safety earth for one whole van.

On 28 June 2015 at 20:22, Bruce Purdy via Stagecraft
<stagecraft at theatrical.net> wrote:
> Frank Wood wrote:
>>
>> All I know about US codes and practices has been gleaned from this
>> list. I know that they tend to treat Neutral and ground with a
>> disturbing degree of interchangeability, which would lead to the
>> wholesale condemnation of whole installations by an UK inspector.
>
>         No, Frank ... Neutral and Ground are NEVER treated as "interchangeable."  The neutral (White on this side of the pond) is bonded TO the ground at the entrance to the building, so it is sometime referred to as the groundED connector, whilst the Ground (carried on separate Green wires) is also known as the groundING connector. Note the 'ed" and 'ing' suffixes, they make a big difference even though the terms are similar. Again, they are never interchangeable!
>
> Bruce
> -----
> Bruce Purdy
> Central New York Magic Theatre Co.
> http://cnymagic.com
>
>
>
>
>
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-- 
Frank Wood