[Gllug] OT: Mains electricity colour coding.

[Chris Bell]

On Wed 27 Oct, Robert Newson wrote:
> 
> On 26/10/10 20:15, Chris Bell wrote:
> ...
> >     Correct, the neutral current goes in the reverse direction if it is
> > single phase, but what about a 3-phase plus neutral installation?
> In three phase if the load is properly balanced between the phases there 
> will be zero current along the neutral - the sum of the voltages in 
> three phase at any one time is zero*, and current of the phase(s) which 
> has(ve) positive voltage is cancelled out by the current of the
> phase(s) which has(ve) negative voltage.
> 
> If the loading of the phases is not balanced, ie there is more current 
> drain on one of the phases, then the inbalance travels along the neutral.
> 
> * In the ideal situation; the three phases being 120 degrees out of 
> phase means that the sum of the voltages is zero - assuming no load.
> 
> 
> 
   A 3-phase RCD senses the total current through each of the three phases
plus the neutral. If this is not zero then any residual current must be
leakage to earth. There will always be some leakage through stray
capacitance, but it should be very low.
   I used to work as an engineer on BBC TV Outside Broadcasts, with frequent
changes of supply. We could run our control vehicle on any mix of 1, 2, or 3
phases, 63 amps for each of 3 cables, fed from either generator or temporary
mains supply. We always checked mains safety every time we switched on, and
there was continuous monitoring. Common faults found during testing were
poor earth connections due to paint or other contamination, and faulty
switchgear. I had a reputation for discovering faults during the early
morning switch-on, and when the rest of the crew arrived they found me
bypassing burnt out switchgear including when we were covering the Wimbledon
Centre Court on the mens final day. We also had problems with local power
distribution through screw connections where the connections worked loose
and burnt the plastic insulation, almost always on the neutral connection,
which then tracked to earth.
   We also had to be prepared for lightning strikes, and had earth spike
cables connected to all vehicles, although they were not much use when we
were parked on hard standing. Dropping the spike down a drain only connected
us to the U-bend, and often the best we could do was park the vehicle on top
of the earth spike. The engineer on top of the 200ft crane aiming the dish
receiving the signal from the Oxford v Cambridge boat race Umpire Launch
survived a lightning strike just before the start of the race. The crane was
brought down using the controls on the lorry, the engineer was dragged off
and replaced, and the crane was sent back up to cover the race. The original
engineer recovered and stopped shaking after a few hours. 
   Another common problem was single 400 amp lighting cables laid at the
same time as other cables because it was too easy to end up with a single
400 amp turn around almost anything else, causing induced current when the
lights were powered on. We sometimes had a single 1000 amp generator for
technical supplies plus several 1000 amp generators for lighting.  

-- 
Chris Bell www.chrisbell.org.uk (was www.overview.demon.co.uk)
Microsoft sells you Windows ... Linux gives you the whole house.

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