- The role of grounding in a three-phase network
- How the ground loop works
- Difference between grounding and zeroing
- Types of grounding systems
- Key points of electrical installation
Some aspects of electrical safety are not entirely clear to the layman, and this is what distinguishes him from a professional who has permission to install electrical networks. Today we will talk about the most important components of any electrification system – grounding and neutralization..
The role of grounding in a three-phase network
Any electrical system is built on a three-phase AC network or is a part of it. Without going too deeply into theory, we recall the basic definitions of the operation of any three-phase system..
A voltage of 380 V occurs between any two phases taken 50 times per second. Specifically, at this moment in time, one of the conductors turns into ground – a source of free electrons, and the other conductor accepts these electrons.
The same phenomenon occurs in the other two pairs of phases, but the time difference between how the phases “switch” is about a third of the oscillation period in one of them. This scheme of work owes its appearance to the most popular type of electrical machines. If you arrange the phases in a circle in the right order, then the occurrence of current in them would also follow in a circle and would be able to push the round core of the motor. In the simplest version of electrical connections, all three phases must be connected at one point, while at a particular moment in time, only two of them will be in peak power.
The main problem is that the resistance of the working elements (motor windings or heating coils) included in each of the phases cannot be absolutely equal. Therefore, the current in each of the three circuits will always be different, and this phenomenon needs to be compensated in some way. Therefore, the convergence point of all three phases is connected to the ground in order to take the residual electric potential into it..
How the ground loop works
Any entrance to a multi-storey building can be modeled according to the same scheme. But apartments, distributed over the three available phases, consume electricity at random, and this consumption is constantly changing. Of course, on average, at the point of connection of the house cable in the distribution point (RP), the difference in currents in the phases is no more than 5% of the rated load. However, in rare cases, this deviation can be higher than 20%, and this phenomenon promises serious problems..
If for a moment we imagine that the electrical riser, or rather, its frame part, on which all the neutral wires are screwed, turned out to be isolated from the ground, such a high difference between the consumption of apartments at different phases results in the following pattern:
- At the most loaded phase, a voltage drop occurs in proportion to the load.
- On the remaining phases, this voltage, respectively, increases.
The neutral wire, connected to the ground loop, serves as a spare source of electrons for just such a case. It helps to eliminate load asymmetry and avoid overvoltages on adjacent branches of a three-phase circuit..
Difference between grounding and zeroing
If during the operation of a single pair of phases the load on them is not the same, a positive electrical potential will certainly arise at the point of convergence. That is, if, upon a break in the grounding circuit, a person grabs the driveway housing, he will be shocked, and the strength of this shock will depend on the degree of asymmetry of the loads.
Most electrical machines are designed in such a way that the loads are evenly distributed across all three phases, otherwise some conductors will heat up and wear out faster than others. Therefore, the connection point of the phases in some devices is brought out into a separate fourth contact, to which the neutral conductor is connected.
And here is the question: where to get this same zero conductor? If you pay attention to the poles of high-voltage power lines, there are only three wires on them, that is, three phases. And for the transportation of electricity, this is quite enough, because all transformers at step-down substations have a symmetrical load on the windings and each is grounded independently of the others..
And this fourth conductor appears at the latest transformer substations (TS) in the chain of transformations, where 6 or 10 kV turns into 220/380 V we are used to, and a non-illusory probability of an asynchronous load arises. At this point, the beginning of the three windings of the transformer are connected and connected to a common grounding system and from this point the fourth, neutral wire originates.
And now we understand that grounding is a system of rods immersed in the ground, and grounding is the forced connection of the midpoint to the ground to eliminate dangerous potential and asymmetry. Accordingly, the neutral conductor is connected to the grounding point or closer, and the protective earth wire is connected directly to the ground loop itself.
Types of grounding systems
Have you noticed that the neutral wire in a three-phase cable has a smaller cross section than the rest? This is quite understandable, because not the entire load falls on it, but only the difference in currents between phases. There must be at least one ground loop in the network, and it is usually located near the current source: the transformer in the substation. Here, the system requires mandatory zeroing, but at the same time the zero conductor ceases to be protective: what happens if the zero is “burnt out” in the TP is familiar to many. For this reason, there can be several grounding loops along the entire length of the power transmission line, and usually this is the case..
Of course, re-grounding, unlike grounding, is not at all necessary, but it is often extremely useful. By the place where the common and repeated zeroing of the three-phase network is performed, several types of systems are distinguished.
In systems called I-T or T-T, the protective conductor is always taken regardless of the source, for this the consumer arranges his own circuit. Even if the source has its own grounding point, to which the neutral conductor is connected, the latter has no protective function, and does not contact the consumer’s protective circuit in any way.
Systems without consumer grounding are more common. In them, the protective conductor is transmitted from the source to the consumer, including through the neutral wire. Such schemes are designated by the TN prefix and one of three postfixes:
- TN-C: protective and neutral conductors are combined, all grounding contacts on the sockets are connected to the neutral wire.
- TN-S: protective and neutral conductors do not contact anywhere, but can be connected to the same circuit.
- TN-C-S: the protective conductor follows from the current source itself, but there it is still connected to the neutral wire.
Key points of electrical installation
So how can all this information be useful in practice? Schemes with consumer’s own grounding are naturally preferable, but sometimes they are technically impossible to implement, for example, in high-rise apartments or on rocky ground. You should be aware that when combining neutral and protective conductors in one wire (called PEN), the safety of people is not put in priority, and therefore equipment with which people come into contact must have differential protection.
And here, novice installers make a whole bunch of mistakes, incorrectly determining the type of grounding / grounding system and, accordingly, incorrectly connecting the RCD. In systems with a combined conductor, the RCD can be installed at any point, but always after the place of combination. This error often occurs when working with TN-C and TN-C-S systems, and especially often if in such systems the neutral and protective conductors do not have the appropriate marking..
Therefore, never use yellow-green wires where it is not necessary. Always ground metal cabinets and equipment cases, but not with a combined PEN conductor, on which a dangerous potential arises in case of a zero break, but with a PE protective conductor, which is connected to its own circuit.
By the way, if you have your own circuit, it is very, very not recommended to perform unprotected grounding on it, unless it is a circuit of your own substation or generator. The fact is that with a zero break, all the difference in the asynchronous load in the citywide network (and this can be several hundred amperes) will proceed to the ground through your circuit, heating the connecting wire to white.