connecting fuses to phases alone and why we are not connecting for neutral?

| why we are connecting fuses to phases alone and why we are not | connecting for neutral?

When fuses are involved, the fuse blowing out on the neutral can create a very unsafe condition. Search for "broken neutral" and "loose neutral" and read about hazardous conditions when just the wires come loose. You could be subjecting a 230 volt device to 400 volts when the neutral is no longer able to conduct, but at least 2 other phases can.

It is usually permitted to disconnect the neutral via a circuit breaker. The circuit breaker must be designed for such a purpose. It must ensure that it disconnects the neutral last when opening, and connects it first when closing. Fuses cannot do this. Only special 4-pole breakers can.

Normally the neutral is grounded and does not need to be disconnected. If you have a situation where there could be excessive neutral current, such as harmonics, that needs to interrupt the power, this should be done by using a current transformer sensor on the neutral that trips breakers on the phase lines. Note that some 4-pole breakers may include current sensing on the neutral, and some may not.

for safety purpose

one of our staff said that the neutral is nothing but a current returning path, that means just a path to make a closed circuit and he added that only a part of the current(for eg.10A entering means only

3A will flow thro neutral path)will reach the neutral path.....but how this is possible...?if it is a closed circuit means the entering and leaving currents are same only.....then how he said a part of the current alone returned?if we are considerin that it will b dissipated as powerlosses will this condition satisfied....

On Thu, 27 Mar 2008 05:21:17 -0700 (PDT) VIDHYA wrote: | On Mar 26, 7:06?pm, buttered wrote: |> VIDHYA wrote: |> > why we are connecting fuses to phases alone and why we are not |> > connecting for neutral? |>

|> for safety purpose | | one of our staff said that the neutral is nothing but a current | returning path, that means just a path to make a closed circuit and he | added that only a part of the current(for eg.10A entering means only | 3A will flow thro neutral path)will reach the neutral path.....but how | this is possible...?if it is a closed circuit means the entering and | leaving currents are same only.....then how he said a part of the | current alone returned?if we are considerin that it will b dissipated | as powerlosses will this condition satisfied....

The neutral normally carries the imbalance current between 2 or 3 phases.

If you have a 40 amp resistive load on phase A, and a 40 amp resistive load on phase B, and a 40 amp resistive load on phase C, there will be no current on the neutral. If you shut off the load on 2 of those phases, then you will have a 40 amp load on the neutral. But one thing a lot of people will not realize with 3 phase electricity is that if you have the load on two of the phases (only shut off one phase), you still get 40 amps on the neutral.

And there are cases where you can get _more_ than 40 amps on the neutral, involving non-resistive loads. This will be harmonic or reactive loads.

It might be simpler to first get a grasp of the concept by studying the type of single phase wiring used in the USA. It involves 3 wires which include a neutral and what some people might say is 2 phases which are 180 degrees apart. It is simpler to study this because phase angles let you avoid doing the trigonometry. It's simple addition and subtraction of the current. If "phase" A has 40 amps and "phase" B has 30 amps, the neutral carries the difference of 10 amps.