[SOLVED] Neutral and Ground Connection?

[RHeTTRoNiCS]

Just a bit confuse...need help from you guys...We have 220AC 3phase line (L1,L2,L3,Ground, & Neutral). But I wanted only to use 110AC to power-up my 110AC power supply. So I tried to measure Neutral and live (either L1, L2, L3) and I can get 110AC. Same goes to Ground and live (either L1,L2,L3).
BUT when I connected the power supply using Ground and live the circuit breaker trips.
When using Neutral and live, the power-supply powered-up fine. When I ohm check neutral and ground can read short.
So why the power supply trip by using ground and live? Why neutral and live is ok?
Ground and Neutral ohm check shorted. Any idea? Thanks.

 

[barry]

Maybe some kind of ground-fault detection going on. Is it a grounded plug on your power suppy? (I assume it's the power supply breaker that's tripping, not the mains breaker).

 

[red_alert]

That's the purpose of a differential current breaker, to protect people on accidental touch of a high voltage (mains) potential.

This is how it works: the differential breaker checks the current flowing on both live and neutral wires (not ground!).

During normal operation, both currents are identical. When someone touches a live connected wire (or there's any current leakage from live - this is your situation) then the breaker pops.

The differential current is usually only 10 mA (the maximum allowable before causing any hurt to our body).

That's why any home appliance case are connected to earth (ground). If any internal component connected to live is touching the case, there will be a leakage current to the ground so the breaker will pop.

As a conclusion: yes, the neutral and ground MIGHT have been shorted (actually, they should not) but the differential circuit breaker it's only monitoring the neutral (not ground) so any current flowing through live but not through neutral will break the equilibrum.

 

[RHeTTRoNiCS]

Thanks barry and red_alert....thanks for the answers....so probably for my final question is....what is making the neutral and ground shorted? Does they have some kind of inductor between them or low-ohm resistor or some kind of special metal material? Thanks.

 

[red_alert]

Neutral and ground could effective be shorted but BEFORE the differential breaker. Actually, before ANY breaker. For safety purpose, the ground should never pass through any breaker.

 

[RHeTTRoNiCS]

Still not answering my question...

Does they have some kind of inductor between them or low-ohm resistor or some kind of special metal material?

 

[Surendhar M]

Combined neutral and ground conductors are commonly used in electricity supply companies' wiring and occasionally for fixed wiring in buildings and for some specialist applications. Neutral wires are usually connected at a neutral bus within panelboards or switchboards, and are "bonded" to earth ground at either the electrical service entrance, or at transformers within the system. For electrical installations with split-phase (three-wire single-phase service), the neutral point of the system is at the center-tap on the secondary side of the service transformer. For larger electrical installations, such as those with polyphase service, the neutral point is usually at the common connection on the secondary side of delta/wye connected transformers

 

[chuckey]

Bearing in mind that the mains always comes from a BIG transformer somewhere, if the neutral was not connected to earth, both the neutral and live wires and the transformer secondary could float to any voltage, say a lightning strike would make both the wire jump to 20 KV, Not very safe!!, so the Neutral is always earthed at the transformer. The knock on effect is that at a consumers outlet the neutral may not be exactly 0V, due to the IR volt drop down the conductor.
Frank

 

[red_alert]

Still not answering my question...

Does they have some kind of inductor between them or low-ohm resistor or some kind of special metal material?

No, it's just a plain connection between neutral and ground (as the previous posters did mention, too).

The knock on effect is that at a consumers outlet the neutral may not be exactly 0V, due to the IR volt drop down the conductor.

That's why is mandatory to have (and use) in-situ grounding electrodes.

 

[Dan Mills]

Depends actually, there are systems that place a deliberate impedance between the star point and earth.

The UK (In a new installation) usually bonds neutral and earth inside the suppliers termination unit in a building and then bonds all the exposed metalwork to this point in order to reduce the danger in the event of an upstream neutral fault. In the US the neutral/earth bond is the responsibility of the consumer and is usually done in the main panel.

The details of how all this works are VERY country dependent and are even sometimes installation dependent, in the UK for example we have TN-S (Separate earth wire right back to the star point), TN-C-S (Combined neutral and earth conductor on the suppliers side, often also pinned to the body of earth at multiple points), TT (No explicit earth conductor back to the transformer, uses earth rods and usually needs an RCD to cope with the rather high loop impedance), IT (There is a deliberate impedance in the earth path, sometimes seen in hospital operating rooms where having things continue to work in the presence of a single fault is desirable).

TT is soften seen on farms and such as well as often being the best option for radio transmitter installations where there is significant accessible metalwork outside the building (Towers and such).

Note that in UK practise there is no case where a neutral/earth bond is the responsibility of the consumer, and indeed anything injecting meaningful current into the earth conductor downstream of the company demarcation point is almost certainly faulty.

Note also, that both phase and neutral should be viewed as being 'live' conductors for the purposes of insulation and protection from accidental contact, the neutral should be treated with as much respect as the phase conductor.

Regards, Dan.

 

[FvM]

Supplementary to Dan's explanation, see this Wikipedia article about Earthing https://en.wikipedia.org/wiki/Earthing_system

 

[RHeTTRoNiCS]

Thanks to All...I'll mark it as solved.