How should we understand the ground of the power distribution system?

乱世煮酒论天下

How should we understand the ground of the power distribution system? [Copy link]

 

In the TT power distribution system, if the power supply end is grounded and the load end is also grounded, can the resistance between the two grounds be considered to be 0, or very small?

If it is an IT power distribution system, if the load is unbalanced, or in a TN power distribution system, the neutral point is not grounded, how does it cause the midpoint potential to drift? Why is there a voltage on the neutral line if the power supply is a three-phase AC power supply, and the voltage of the live line to the ground increases or decreases if the three phases are unbalanced?

maychang

[If the power supply end is grounded and the load end is also grounded, can the resistance between the two grounds be considered to be 0, or very small? ]

It depends on your definition of "very small". Is 10 ohms considered "very small"? How about 1 ohm?

maychang

There are too many questions. I suggest you read "Electrical Engineering". All these questions are answered in "Electrical Engineering".

qwqwqw2088

The so-called TT power distribution system has a system grounding on the power supply side with a grounding resistance value Rb not greater than 4Ω, and a protective grounding on the electrical equipment side with a grounding resistance value Ra generally ranging from 10 to 30Ω, and there is no electrical connection between the two.

qwqwqw2088

Also, when the three phases are unbalanced, it is actually because the currents in each phase are not equal, which causes the current at the neutral point to be non-zero. It is easy to understand why the voltage of the live wire to the ground increases or decreases.

EFT

There are too many questions. I suggest you read "Electrical Engineering". All these questions are answered in "Electrical Engineering".

乱世煮酒论天下

maychang posted on 2024-5-4 16:28 [If the power supply end is grounded and the load end is also grounded, can the resistance between the two grounds be considered to be 0, or very small? ] It depends on your understanding of...

The resistance is less than 10 ohms, but why are these two grounds not connected? I don't understand. There is another question: the neutral point of the transformer is grounded, and the cable insulation layer is grounded at the load. Why are these two grounds not connected?

乱世煮酒论天下

qwqwqw2088 posted on 2024-5-4 22:07 The so-called TT power distribution system, the system grounding on the power supply side, the grounding resistance value Rb is not greater than 4Ω, and the protective grounding on the power equipment side, the grounding resistance value Ra ...

The neutral point of the transformer is grounded, the cable shield is grounded at the load end, and no power is supplied. When measuring the insulation of the cable shield and the cable core wire, can it be extended to measuring the internal resistance between the phase power supply and the shield or the earth when there is no power? What should this value be?

maychang

Luan Shi Zhu Jiu Lun Tian Xia published on 2024-5-31 18:20 The little finger is less than 10 ohms, but why are these two grounds not connected? There is another question, the neutral point of the transformer is grounded, and the cable is insulated...

[Another question: the neutral point of the transformer is grounded, and the cable insulation layer is grounded at the load. Are these two grounds not connected? ]

The resistance between these two "grounds" is at most a few ohms.

maychang

Luan Shi Zhu Jiu Lun Tian Xia published on 2024-5-31 18:20 The little finger is less than 10 ohms, but why are these two grounds not connected? There is another question, the neutral point of the transformer is grounded, and the cable is insulated...

[But why are these two places not connected? I don’t understand. ]

Who said these two places are not connected?

乱世煮酒论天下

maychang posted on 2024-5-31 18:50 [There is another question, the neutral point of the transformer is grounded, and the cable insulation layer is grounded at the load. Are these two grounds connected? ] These two "grounds"...

What is the resistance between the phase line of the power supply and the ground!

maychang

Talking about the world in a chaotic world published on 2024-5-31 19:26 What is the resistance between the phase line of the power supply and the ground!

[What is the resistance between the phase line of the power supply and the ground! ]

You want to use a multimeter to measure the resistance between the phase line and the earth? The circuit breaker in your laboratory must have tripped.

maychang

Talking about the world in a chaotic world published on 2024-5-31 19:26 What is the resistance between the phase line of the power supply and the ground!

[What is the resistance between the phase line of the power supply and the ground! ]

Not to mention the phase and neutral lines of the AC mains, even if it is an ordinary lead-acid battery in a car, have you tried measuring the resistance between the positive and negative poles?

qwqwqw2088

乱世煮酒论天下published on 2024-5-31 18:23 The neutral point of the transformer is grounded, the cable shield is grounded at the load end, and no power is supplied. Can the insulation of the cable shield and the core wire be measured?

Measuring the insulation resistance between the cable shield and the cable core is equivalent to measuring the insulation performance of the cable.

including the insulation material of the cable and any air gaps that may exist,

qwqwqw2088

乱世煮酒论天下published on 2024-5-31 18:23 The neutral point of the transformer is grounded, the cable shield is grounded at the load end, and no power is supplied. Can the insulation of the cable shield and the core wire be measured?

According to electrical safety standards, such as IEC 60227 or national standards, the insulation resistance of the cable should meet specific minimum requirements.
For low-voltage cables, the insulation resistance is usually required to be no less than 0.5MΩ/km.

qwqwqw2088

乱世煮酒论天下published on 2024-5-31 18:23 The neutral point of the transformer is grounded, the cable shield is grounded at the load end, and no power is supplied. Can the insulation of the cable shield and the core wire be measured?

By the way, please remind me to use proper measurement method.

Use an insulation megohmmeter to measure the insulation resistance of the cable.

乱世煮酒论天下

maychang published on 2024-5-31 19:34 [What is the resistance between the phase line and the ground of the power supply! ] Let alone the phase line and neutral line of the AC mains, even if it is an ordinary lead-acid battery in a car, you...

What I mean is that the neutral point of the transformer is grounded, the shield layer of the cable end is grounded at the load end, without power, measure the insulation of the cable core at the load end to the shield layer, that is, the ground. What factors are related to the size of this insulation? If the resistance between the power supply end grounding and the load end grounding should be considered?

maychang

乱世煮酒论天下published on 2024-5-31 23:11 What I mean is that the neutral point of the transformer is grounded, the shield layer of the cable end is grounded at the load end, without power, measure the load end cable core to the shield layer, that is, the ground...

[I mean the neutral point of the transformer is grounded, the shielding layer of the cable end is grounded at the load end, without power, measure the insulation of the cable core at the load end to the shielding layer, that is, the ground]

Blind guess.

Simply [turning off power] is absolutely not enough, the cable core wire must be disconnected from the transformer, leaving only the cable itself, and then short-circuit the cable core wire and the shielding layer (it should not be called shielding layer, it should be called armor layer or outer covering layer) first, and then remove the short-circuit line, and then as qwqwqw2088 on the 16th floor said, use a megohmmeter (shaking meter) to measure.

As for adding a guard ring to avoid the influence of surface leakage when measuring with a megohmmeter, the principle is a little complicated and will be ignored here.

The cable will not be connected directly to the transformer, it must go through some kind of circuit breaker. Measure the resistance between the cable core and the shield (whatever you call it) and the circuit breaker should not be included.

maychang

乱世煮酒论天下published on 2024-5-31 23:11 What I mean is that the neutral point of the transformer is grounded, the shield layer of the cable end is grounded at the load end, without power, measure the load end cable core to the shield layer, that is, the ground...

[How should the resistance between the power supply ground and the load ground be considered? ]

I really don't understand what you said. [Between the ground at the power supply end and the ground at the load end] is the attributive of [resistance]. How can an attributive be accompanied by [if]?