Electricity

Three Phase Electricity

You can use more than one phrase. But most homes supply only 1 phrase.

Measure the voltage difference between any two phrases will have the same result.

Since adding the vectors of 3 phrases add up to zero, we can use one phrase as power and the other two as neutral to generate energy.

1 Hz is one cycle of generator rotation, different countries use different frequency standard.

Usually in AC, we use $V_{RMS}$ as the voltage. Note that this is not maximum voltage:

The reason we use $V_{RMS}$ instead for AC is because the power transmission is equivalent for that voltage in DC.

Single Phrase Transmission

The general transmission to household is as following:

generator -> step-up-transformer -> transmit -> substation-step-down-transformer -> split phrase -> household service cable

It wires from magnetic power plant to a transformer to transform generated current to high voltage to decrease power losses during transmission. High voltage current then arrive and we lower the voltage by a transformer.

The service cable has one phase line and one neutral line inside a armoured sheaf.

Then the electricity go to main fuse of a house to limit the current coming in. Then current enters electricmeter for cost calculation and then to consumer unit - fuse board.

Main Switch: for cutting main power.

Residual Current Device (RCD): check whether current in phase line is equal to neutral line. Otherwise, if difference is large enough 30mA, cut the power for fault protection.

• Miniature Circuit Breaker (MCB): protect overload and short circuit, with limited current. Each for one plug.

• There are usually multiple MCB connected to single RCD. And multiple RCD for one main switch.

There are multiple possible setup for ground:

• connect ground to neutral (neutral fault)

• connect ground to metal sheaf (fault in transformer)

• connect ground to earth ground (no fault)

Single Phase in Practice

Although ground can sometimes be wired directly to neutral, shorting ground with neutral is not a good idea.

Because there might be unintended phrase shift after passing through transmission lines, especially at the consumer sites, neutral line can have voltage difference with actual ground because 3 phrases can't balance out. Such voltage might be large enough to cause heavy shock depending on the voltage usage at the moment.

The phrase shift isn't bad at power plant because if there is so, other power plant's frequency will correct the phrase shift by magnetic induction backward onto generator motion. But the phrase shift control isn't so good at client side. (by magnetic induction, wires' resistance, etc) Usually there is $\pm 2 V$ between ground and neutral. If "grounded" with neutral line, items will be charged.

This article and the comment section or this explains the concept well.

The long high voltage transmission line usually don't contain neutral line, and all neutral line in power plant and tramsformers actually connect to earth ground. But the client side can't connect to ground because they usually have a bad ground.

It will be very dangerous to connect the items to hot line without connecting to neutral line. The item will be charged, and if we are grounded and touch item, we will get shocked. Therefore it is important for neutral to be grounded.

Using AC is more likely to get shocked than DC because there is a capacitor between ground and human body.

There are more interesting questions like 直流电源正极或者负极接地会不会产生电流？

Here is a way to test grounding