Why is the neutral wire heated

Heating the neutral wire can cause it to burn out and cause a power accident. Most often this happens when the loads are unevenly distributed in phases in a three-phase power network and due to poor contact. In this article we will explain why the zero wire is heated and what to do in this situation.

Three-phase current

In order for the reasons for heating zero, you need to understand how a three-phase network works. The load in the three-phase network can be connected by a star and a triangle, and the windings of the supply transformer can also be connected. The winding has two conclusions - the end and the beginning.

If the ends of the windings of a three-phase transformer are connected at one point - then they say that this is a star connection diagram. According to Kirchhoff’s laws, at the point of their connection (O), the current will always be zero, that is, flow from phase to phase. If the load in each of the phases (a, b, c) is the same, then the voltages at the beginning of the windings (A, B, C) as well as the current in them will be equal. What is illustrated in the vector diagram below, where the phases of currents and voltages are indicated by vectors and are shifted by a third of the period relative to each other (120 degrees).

R1 = R2 = R3

I = I1 + I2 + I3 = 0

Note:

Symmetric is called such a three-phase load, in which the load resistance (respectively, the consumed current or power) of each of the three phases is the same.

But as soon as the current in the phases begins to differ, when the load in the phases is different in power, then the voltages in the phases begin to differ from each other. This is called phase imbalance.

To solve this problem, the connection point of the load star is connected to the connection point of the transformer star. This is called neutral, or neutral wire, or simply zero.

Power supply for dummies at home

We have smoothly come to practice, when connecting single-phase consumers to a three-phase network, the loads are often unequal, that is, asymmetrical.

This is often found in apartment buildings. Three phases and zero start up in the house, one phase and zero start up in each apartment. In one apartment, only a refrigerator and a light bulb are turned on, a powerful electric heater works in the other, and in the third, nothing is turned on at all. That is, the loads in the phases are not the same. Currently, three-phase input is often found in apartments, but the situation does not change from this.

In private homes, the situation is similar - on the street a three-phase power transmission line passes along the poles, and 1-3 phases and zero are started in the house.

Still, why is it warming up

As a result of the uneven distribution of the load across the phases in the houses and apartments along the neutral conductor, current flows. Have you noticed that in thick 4 core cables there are 3 "phase" conductors with the same cross-sectional area, and the fourth core is "zero" or "earth" usually thinner?

This is precisely due to the fact that with a symmetrical load, no current will flow through it, and with a non-symmetrical load, the current should be less than in a phase conductor. But this does not always happen.

With non-linear loads, as well as loads that consume intermittently current (switching power supplies, and they are now used everywhere) the currents in the phases do not cancel each other, in addition, they are saturated with various harmonic components ... All this is the reason that the currents at the junction point of the star are simply not compensated and it may turn out that the current is in zero the wire will be more than in phase.

When the electric current flows, the conductor heats up, this is the impeccable work of the Joule-Lenz law in practice. It says that the greater the resistance of the conductor and the longer the electric current flows, the more heat will be released on it.

We also recall that the smaller the cross section of the conductor and the greater its length, the greater the resistance.In addition, the quality of the contacts on the connection of terminals and wires also depends transition resistance. In simple words, the larger the contact area of ​​the contacts and the stronger they are pressed against each other - the lower the transition resistance and the less their heating.

In such a contact, as in the figure below, the surfaces are flat, the area will be equal to the area of ​​the tip touching the washer, plus the resistance of the washer itself and the area of ​​its contact with the copper bus. If all the components are in good condition, do not have oxides and soot, the resulting transient resistance will be low.

If the surfaces are burnt, oxidized or rusty, the contact is obtained as shown in the illustration below. It is clearly seen here that touches occur at individual points, and not over the entire area.

IN VAGO terminal blocks and other spring terminal blocks, the contact area of ​​a plate with a round conductive core is quite small, therefore the main field of application of such terminal blocks is circuits with a current of 8-16 Amps, in rare cases when the terminal block is structurally capable of passing a larger current.

In screw terminal blocks and tires, the contact area is more determined by the area of ​​the screw that presses the conductive core. Below you see the terminal blocks in a plastic sheath.

A sleeve made of material similar to brass and two screws is located inside the polyethylene housing. Due to the design, bare stranded wires cannot be connected with screw terminal blocks. They need to be tinned or crimped with the tips of the NShVI.

Therefore, with a similar principle of operation, the terminal blocks on the carbolite base provide better contact due to the square washer plate. In addition, you can make a ring from the wire and wrap it with a screw or use tips like NKI.

If you are interested in ways and means for connecting wires - write in the comments and we will do an overview of all types listing the advantages and disadvantages of each of them.

Where is warm

Why the zero is heated, we figured out, and now let's figure out where this happens most often. First of all, zero can burn out in the switchboard at the entrance to the building. This is the most common situation, because in this place the load from all apartments and from all three phases falls on the zero wire.

Further, problems often arise on the zero bus in the drive electrical panel. If there are any buses at all, and is not connected as in the photo below.

Often the bus is mounted directly on the body of the access electrical panel, then it looks like the one shown below.

In the terminal blocks of the circuit breakers, zero is heated, up to the carbonization of parts of its case.

If you have old wiring and plugs with fuses or traffic jams, then pay attention to both the screw terminal blocks and the plug base itself. The thread and central contact may oxidize and burn, as illustrated in the figure below.

Common tires are very often prone to zero burn problems. This is due to their device and compliance with the rules for working with them. The screw way of connecting the conductors, although it is certainly convenient, but such contacts need to be revised at least occasionally - to strip and stretch, otherwise you will get what is shown in the figure below.

And in normal condition, it should look like this:

The solution to the problems caused by heating is simple - strip the contacts, conductors and stretch again. If the terminal block was very overheated - replace it, if the wire was heated in the machine, the machine may also need to be replaced!

What happens next and how to avoid the consequences?

As it heats up, contact begins to burn and deteriorate. Screw clamps are weakened due to thermal expansion and subsequent cooling after unloading. This causes an avalanche-like process of growth of resistance and heating of the compound. As a result, zero sooner or later burns out completely.At the same time, it may seem outwardly that he is still in the terminal strip, but in fact all adjacent surfaces will be covered with a layer of oxides and soot.

After which the phenomenon that we spoke about at the beginning of the article phase imbalance.

Note:

The fact that zero will soon burn off can be indirectly judged by the frequent drawdowns and voltage increases, especially if you have a three-phase input and installed voltmeters or voltage relays and an indication of the voltage in the network. If the voltages are constantly stable (or deviations are insignificant) - then you are okay with the wiring.

With a phase imbalance, the load, in our case, private houses or apartments turn out to be connected in series at 380 volts. Voltages will be distributed according to Ohm's law - where a larger load is turned on - the voltage will drop out (load resistance is small), and in the apartment where a minimum of electrical appliances is turned on, the voltage will increase (load resistance is high).

The consequence of phase imbalance in the best case will be the burning out of the conductors at the input, knocking out the machine, etc. In the worst case, due to the increased current, the insulation of the wiring may melt and a fire may occur.

To protect your home from the effects of burning off zero, we recommend that you install voltage monitoring relaybetter yet paired with SPD. The voltage regulator at the entrance to the apartment in this situation may not solve the problem and itself fail.

You can see the voltage relay connection diagram below.

As such devices, we can recommend popular models:

• UZM-50TS (combined device with the function of a volt-ampere meter);

• Digitop VA-32 (inexpensive, but reliable option, the model may differ depending on the rated current);

• RN-106.