Neutral and Ground

[weerachay]

Hi,
I have some question to ask an expert.
If people touch an equipment that have electricity leakage, an electric current will flow through their body to the ground because we have connect the ground to be a part of electric circuit, Does my understanding is correct?
If so, What is the reason for connecting ground to the neutral point of the star connection of Delta-Star transformer in power distribution system? (Star connection is in the secondary side)
And if we disconnect ground from the neutral point, Does it affect to electrical equipment?
I have to apologize if my English is not quite good.
Looking forward to the answer.
Thank you in advance.

 

[cuky2000]

I belive that your understanding is correct with some clarification.
a- If the chassis of the equipment became accidentally in contact with a phase conductor the current could trip the protective device if the impedance to ground is zero or very small. On the other hand if the equipment is insulated from ground the current generated may not be high enough to trip the protective device. If a person touch the chassis of the equipment the voltage imposed in the person will be as follow:
Vchassis=(Zeq/(Z1+Zeq))Vconductor.
Were:
- Z1 = Impedance between the faulty phase and the chssis of the equipment.
- Zeq = Impedance in parallel between the person and the chassis of the equipment to ground.

Notice that if the system is effectibly grounded Zeq is virtually zero and the current circulated in the person body is negligible.

Therefore, the grounding of the equipment will protect personnel in case of accidental contact with the phase conductor and the current generated will induce the tripping of protective device to isolate the faulty circuit.

There are a major difference between the purpose of ground and the neutral conductor.
Ground is not a current carrying conductor during normal operation will conduct current for short time during fault conditions.

Neutral conductor, on the other hand, is the return current path. In a three phase system happen that will carry the umbalance and harmonics current to ground.

Equipemnt ground is used for safety primarly for Safety reasons. Most coutries the use of equipment ground is enforced by electrical code, local lows, standard recommendation or voluntarily following good engineering practice.

System grounding configurations are selected for other reasons different than equipment grounding that include reliability and system performance. There are a large variety of system grounding such as ungrounded system, high impedance grounded, resistance/reactance grounded. resonant grounded, etc.

 

[jbartos]

Suggestion to the original posting marked ///\\\:
Hi, I have some question to ask an expert. If people touch an equipment that have electricity leakage, an electric current will flow through their body to the ground because we have connect the ground to be a part of electric circuit, Does my understanding is correct?
///Almost. The current flowing through the body will be a portion of the leakage current since the body creates one resistance or impedance from the conductive surface, which they touch, to the ground. The second resistance or impedance will stay the way it was and the current through it will be smaller since the portion of it flows through the body.\\If so, What is the reason for connecting ground to the neutral point of the star connection of Delta-Star transformer in power distribution system? (Star connection is in the secondary side)
///This ground is called "system ground" apart from the one where the leakage may be and flow through the body. That ground is called "equipment ground." The system ground is created for proper functioning of protective devices that protect equipment and power distribution and ultimately personnel around them.\\And if we disconnect ground from the neutral point, Does it affect to electrical equipment?
///Yes, it does, since the voltage in the power distribution system and in equipment will be increased due to arcing, switching, potential resonances, etc. This increased voltage has impact on the insulation, develops more heat in the equipment, and impacts equipment functioning.\\I have to apologize if my English is not quite good. Looking forward to the answer.
Thank you in advance. ///You are welcome.\\