T&T: loose neutral

[Peter Bennett]

Thursday, May 3, 2007, 2:26:03 PM, Pierre wrote:

P> Hi friends,

P> we frequentely read "loose neural" on subject about inverters or 110V
P> onboard. Can someone lead us to a clear definition of what is a loose
P> neutral and what is the difference of approach between the European 230V
P> and the North American 115V for the use of the neutral?

P> That could very much help as more and more boat travel both side of the
P> Atlantic and as AC tend to generalize onboard.

I don't know much about the European 230V system, but I don't think
they have a neutral in the same sense that we do.

In normal North American residential wiring, the power company's final
distribution transformer has a 240V secondary winding with a center
tap (connection to the center of the secondary winding).  There is
120V between this center tap and either end of the transformer.  The
center tap is grounded at the transformer, and becomes our "neutral"
(the white wire).  The two end terminals of the transformer become two
"hot" wires (typically black and red).

240 volt loads such as a water heater or electric stove will be
connected between the two hot wires, with no connection to the neutral.

120 volt loads (lights and most things we plug in) connect between one
hot wire and the neutral.  They will be distributed between the two
hot wires in an attempt to keep the loads on the hot wires somewhat
similar.

If you connect a 1200 watt heater between black and neutral, and
another between red and neutral, you will have 10 amps flowing in each
hot wire, and no current in the neutral, as the currents from the two
hot wires will be 180 degrees out of phase and will cancel.  In this
case, the neutral can be removed with no ill effect.

However, if you connect a 1200 watt heater between black and neutral,
and a 120 watt lamp between red and neutral, you will have 10 amps in
the black wire, and only 1 amp in the red wire.  In the neutral the 1
red amp will cancel 1 black amp, leaving 9 amps in the neutral.  So,
if you remove the neutral, the voltage drops in the two loads will
change so that each will try to pass the same current.  (Kirchoff's
Law says that the current is the same at all points in a series
circuit.) The result will (briefly) be about 20 volts across the
heater, and 220 V across the lamp, which will quickly burn out.

If you have a loose (high resistance) neutral connection, the neutral
voltage at the loads will move towards the higher-power load, putting
excessive voltage on the lower power load.

-- 
Peter Bennett, VE7CEI    Vancouver, B.C., Canada
Lien Hwa 28 (AKA Polaris 30) "Sea Spray"
GPS and NMEA info: http://vancouver-webpages.com/peter 
Vancouver Power Squadron: http://vancouver.powersquadron.ca