Ground in AC circuit

Question

In DC circuits, we say that the negative terminal of battery is considered as ground(0V). But in AC circuit, the polarity of votage change continuously but what is ground in AC circuits with respect to which polarity changes? Please also explain the concept of ground in circuits in little bit more detail. In books, this concept of ground is not taken into account.

Answer 1

Ground as a concept comes from referencing earth as a baseline voltage potential, that is, the ground. If you think about how mains wiring works, the AC signals vary with respect to each other, but they also refer to the earth as well - by design. That’s why there’s a grounding rod at the power feed to your house, that is tied to the neutral return.

To summarize, a single-phase AC feed will have four connections at the panel:

• Line 1
• Line 2 (inverse of line 1)
• neutral (carries any imbalance between line 1 and 2)
• earth ground (bonded together with neutral at the panel)

That said, there’s no specific reason for a DC or AC circuit to have a defined, fixed voltage with respect to earth - in fact, many systems don’t, they have a ‘floating ground’, which is kind of an oxymoron, but it means that the reference is only local to the circuit.

By convention, a local ‘ground’ is usually chosen from the voltage node that serves as the return path for all the voltages in the system.

And that reference can be literally any voltage node. For example, modern cars tie the (-) battery terminal to the car body as ground. But it wasn’t always this way - early cars and many British vehicles until the 1980s or so tied the battery (+) to the car body and called that ‘ground’.

Answer 2

In AC supplied power circuits, if the supply has an isolated/floating output, like directly coming from a AC transformer secondary, then it has no voltage reference to anything set beforehand so on the circuit board you can freely select your ground reference point, as all you get is AC voltage between two wires.

What it is will depend on is what you are going to use it for.

Sometimes you may want to consider one wire as 0V "neutral" and the other wire as being "live" with 24 VAC.

Sometimes you may want to consider both wires being "live" with 12 VAC so the 0V point is selected like it was coming from a center tapped transformer.

Sometimes all you do is rectify the AC input to DC output with a full bridge rectifier, so you would set the rectified negative to be 0V, and so AC input would not be fixed to any stable potential because it would be ground referenced via the bridge diodes and diodes have voltage drop based on current over them.

Answer 3

In ac circuits power is transmitted at high voltage along a transmission network with no neutral only 3 phases .when the voltage is stepped down at a transformer for distribution to buildings the transformer type is a delta star transformer or delta Y transformer ( look it up ) the delta side (primary winding) is the 3 phase power from the power grid . The star side (secondary winding ) still 3 phase power but voltage is much reduced , this is where the neutral is "created" the 3 windings are all connected directly to earth and neutral under the transformer so the potential of neutral is 0 volts. If the winding was not connected to earth at one end and you measured voltage across the winding when energised it would read 0 volts because the winding is effectively the same point. There would be no potential difference .

Earthing is basically maintaing a good connection with the earth at the transformer through any metal work in an electrical installation and any appliances connected. The safe disconnection of a circuit breaker requires the earth fault path to have a low resistance as in the event if a fault it would act as a circuit back through the earth cable or the earth if you have a earth grounding rod back to the transformer and through the phase into your breaker and as long as enough current is flowing to overload it it will trip and stop anyone getting a shock.

Answer 4

The term 'ground' is generally loosely applied to a common or reference point in isolated AC / DC circuits.

In AC / DC mains circuits 'ground' / 'earth' refers to an actual connection to earth.

In a 3 phase star-connected system, say at a distribution transformer, the star point is earthed and referred to as a 'neutral'. In such a system the line-to-line voltage is called the line voltage and line-to-neutral voltage the phase voltage. Line voltage is equal to √3 * Phase voltage. The earthed neutral ensures that exposure of personnel, coming in contact with a line, is limited to phase voltage only. The earthed neutral facilitates tripping of the system during an earth fault.

Some standards mandate connection of the neutral to earth at the user end while others forbid that.

Answer 5

In DC circuits, we say that the negative terminal of battery is considered as ground(0V). >But in AC circuit, the polarity of votage change continuously but what is ground in AC >circuits with respect to which polarity changes? Please also explain the concept of ground >in circuits in little bit more detail. In books, this concept of ground is not taken into >account.

The first thing to understand is Electricity always returns to its source. In DC this may be a battery and in AC it is generally a transformer.

The ground in AC is Zero volts (0v). The current in AC will always travel back to its source using the wire which is at Zero volts (0v). In a single phase system this will be the Neutral wire. The Neutral is kept at Earth Potential (0V) by being directly connected to the Earth.

In a multiple phase system the current will return because the phases are at Zero volts (0v) potential to the other phases.

To further explain multiple phase systems each phase is kept out of cycle with each other such that the phases when summed together equal zero volts (0v) which allows the current to return to its source.

Answer 6

n DC circuits, we say that the negative terminal of battery is considered as ground(0V).

Sometimes.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. Two 9 V batteries series connected but with various grounding options.

Even in a DC circuit we can call any point ground. It's just a reference point from which we take readings.

But in AC circuit, the polarity of voltage change continuously but what is ground in AC circuits with respect to which polarity changes? Please also explain the concept of ground in circuits in little bit more detail. In books, this concept of ground is not taken into account.

For AC measurements your meter only measures magnitude of an AC voltage. Again when taking measurements we need a reference point. This is usually the neutral which has been "neutralised" by connecting it to Earth at the supply transformer.

^{simulate this circuit}

Figure 2. For AC we have the same situation as with DC but with no sign - just the voltage magnitudes.

When working with AC, therefore, it is common to measure the voltages from the neutral point. In Figure 2 we can connect any one of the three terminals of a centre-tapped transformer to Earth and then take voltage readings at the other terminals.

Answer 7

It may come surprising to you, but in any microwave oven, the anode of the magnetron tube –and thus, the + of the DC part of the circuit— is tied to the case and so, grounded.