| m much confused in fundamentals of neutral. I want help of u ppl
| regarding that.
| My que is when neutral gets open in distribution network i.e aftr xmer
| secondary at far end or last leg of the distribution cable .nd
| distribution network is earth at multiple point with neutral grounded.
| Then wht will be the phenomena ? Over voltage at consumer load side
| will happen bt wht exactly happes technicaly that is wht i need. Nd wht
| are the protection which can help in this case.suggest also a link or
| site that can give drawing nd calculation 4 that. Urgent pls
If there is only one hot wire in the circuit, open neutral means the
current stops flowing.
If there are two or three hot wires in the circuit, the open neutral
is no longer able to balance out the currents and voltages.
Each load or group of loads that are connected to the two or three hot
wires would, without the neutral, be operating in series between those
hot wires. If the resistances of the loads are equal, then it happens
to be in balance and there would not be current on the neutral even if
it were not open. But if the resistances are different, then the load
with the higher resistance gets a higher voltage because the other load
is not resisting enough.
There are 2 hot wires, black and red (USA color standards). There is
one neutral, white. A 60 watt light bulb is between black and white
and gets 120 volts (USA voltage standard). A 15 watt light bulb is
between red and white and gets 120 volts in the opposite phase. The
resistances of these at operating temperature is 240 ohms for 60 watt
and 960 ohms for 15 watt. For the sake of description, I will assume
the resistances will remain constant (really, they won't, but they
will be somewhat close to the same proportion). There is 0.5 amps of
current through the 60 watt bulb and 0.125 amps through the 15 watt
There is 240 volts between the black wire and red wire.
Now disconnect the neutral. What we have now is two bulbs in series
connected to 240 volts. The resistance of both in series is 1200 ohms.
At 240 volts that will be 0.2 amps of current through both bulbs. In
the bulb with 240 ohms, since 0.2 amps is flowing through, the voltage
drop across the bulb is 48 volts. Now it is dissipating a mere 9.6 watts.
It would be very dim. The other bulb with 960 ohms also has 0.2 amps of
current flowing through it. Both have to have the same current if they
are in series. But in this case the voltage drop across 960 ohms for
0.2 amps is 192 volts. And that means the bulb is dissipating 38.4 watts.
The sum of the voltage drops, 48+192, is 240, our supply voltage between
the black and red wires.
But the 15 watt bulb is overloaded at 38.4 watts and the 60 watt bulb is
underloaded at 9.6 watts. The 15 watt bulb is likely to burn out real
I hope you can see that when you consider various appliances in this kind
of situation, when the loads are out of balance, some get more and some
get less voltage.
Now consider what if instead of a 15 watt bulb, there were 2 7.5 watt bulbs.
Those two would be splitting the 38.4 watts between them at 19.2 watts each.
If you saw this kind of thing happening, you might well be tempted to shut
off the first overloaded bulb you see. But that would actually be bad for
the 2nd overloaded bulb because this makes the out of balance even worse.
If you ever encounter a case where you suspect a broken neutral and there
are many appliances on and some are overloaded and some don't have enough
voltage (which can even damage them that way, too) ... DO NOT ... DO NOT
shut off any appliance individually. Instead, immediately go to the MAIN
shutoff switch for the building and shut off ALL the power at the same time.
DO NOT shut off individual breakers. ONLY shut off the main breaker. Then
call a qualified electrician to diagnose the problem and find where the
neutral break is.
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
Click to see the full signature.