Isolated neutral: what it is and where it is applied

Currently, an isolated neutral is difficult to find in everyday life, you will never come across it if you do wiring in apartments. While it is actively used in high-voltage lines, as well as in some cases in 380V networks. In more detail about what a network with an isolated neutral is and what its features are, we will tell you in simple words in this article.

What it is

The definition of "isolated neutral" is given in Chapter 1.7. PUE, in clause 1.7.6. and GOST R 12.1.009-2009. Where it is said that an isolated neutral is called a neutral at a transformer or generator, not connected to grounding device in general, or when it is connected through protection devices, measurements, alarm.

Neutral is the point at which the windings of transformers or generators are connected when connected in a "star" circuit.

There is a misconception among electricians that the abbreviated name for isolated neutral is

IT system, according to the classification of p. 1.7.3. Which is not entirely true. The same paragraph says that the designations TN-C / C-S / S, TT and IT are adopted for networks and electrical installations with voltages up to 1 kV.

In the same chapter 1.7 of the PUE there is clause 1.7.2. where it is said that with regard to electrical safety measures, electrical installations are divided into 4 types - isolated or solidly grounded up to 1 kV and above 1 kV.

Thus, there are some differences in the safety and application of such a network in different voltage classes and it is at least incorrect to call a 10 kV line with an isolated neutral "IT system". Although schematically - almost the same.

In networks up to 1 kV

General information

Let's figure out where, how and in what cases an insulated neutral is used in electrical installations with voltages up to 1000 V, the so-called IT system. In the PUE chapter 1.7. NS. 1.7.3. a definition similar to the one given above is given, but it is slightly different. It says that housings and other conductive parts in IT installations must be grounded. Let's see how it looks in the diagram.

Since the neutral of the IT network transformer is not connected to earth, then, in simple terms, we do not have a dangerous potential difference between earth and phase conductors. And accidentally touching 1 live wire in IT system is safe. Due to the relatively low voltage, the capacitive conductance of the phases is neglected here.

In networks with an isolated neutral, there is no pronounced phase and zero - both conductors are equal.

The current through the human body is equal to:

I_h = 3U_f/(3r_h+ z)

U_f - phase voltage; r_h - the resistance of the human body (taken 1 kOhm); z is the total insulation resistance of the phase relative to earth (100 kΩ or more per phase).

The current in this case returns to the power supply through the insulation of the wires, and not to the ground, as is the case with TN.

Since the insulation resistance is more than 100 kOhm per phase, the current through the body will be units of milliamperes, which will not cause harm.

The next feature of this system is that the leakage currents to the frame and the short-circuit currents to earth will be low. As a result, protective automatics (relay or circuit breakers) do not work in the way we are accustomed to in networks with a solidly grounded neutral. But the insulation resistance monitoring system is triggered.

Accordingly, with a single-phase closure of a three-phase line, the system will continue to function. In this case, the voltage on the two remaining wires increases relative to the ground. If a person touches the phase wire, he falls under line voltage.

In connection with this design, in a network with an isolated neutral, there are no two types of voltage, in contrast to a solidly grounded one, where between the phases U_linear (in everyday life 380V), and between phase and zero U_phase (220V). To connect a single-phase load to the IT network with a voltage of 380V, you can use step-down transformers of the 380/220 type and connect devices between the two phases to line voltage.

Scope of application

Let's talk about where such a solution is used. This power supply system was used in domestic power grids to transmit electricity to residential buildings during the Soviet era. Especially for the electrification of wooden houses, where the use of a solidly earthed neutral increased the risk of fire during ground faults.

From the point of view of electrical safety, the difference between an isolated and a solidly grounded neutral in the power supply of houses is that if in an IT network one of the conductors will touch grounded conductive parts, for example, wall fittings or water pipes, the network will continue to function due to low currents leaks.

Accordingly, neither the residents, nor anyone else will know about the problem, until, while someone touches one of the wires and the pipeline, someone will not be electrocuted.

In a system with a solidly grounded neutral, at least the differential protection will work, and in case of a "good" metal circuit, the circuit breaker will turn off. With the beginning of the mass construction of panel houses (the so-called Khrushchevs), it was abandoned and in the 60s-80s they switched to TN-C, and at the end of the 90s on TN-C-S, read about the reasons below.

Currently, isolated neutral is used wherever it is necessary to provide increased safety or it is not possible to do normal grounding, namely:

• At sea - on ships, oil and gas production platforms, where the use of the platform hull as grounding is impossible due to anodic protection, and in places where current flows into the water, it will begin to rust intensively and rot.
• In mines and other places of mining (with a voltage of 380-660V).
• In the underground.
• On lighting and control circuits in stationary cranes, etc.
• Also, in household gasoline, gas or diesel generators, it is the isolated neutral at the output terminals.

It can be found not only in the form that we have shown in the diagram above, but also in the form of step-down and isolation transformers, which are used to power portable lighting devices (no more than 50V or 12V PTEEP clause 2.12.6.) and other equipment or tools, including those with which they work in closed and damp premises.

Let's summarize

We figured out why an isolated neutral up to 1 kV is needed, now we list the advantages and disadvantages of a power supply system with an isolated neutral for kettles in electrics.

Benefits of using:

1. Greater safety.
2. Greater reliability, making it suitable for hospital lighting, for example.
3. The economic factor - in a three-phase network with an insulated neutral, electricity can be transmitted using the minimum possible number of wires - three.
4. The system will continue to operate in the event of single phase earth faults.

Disadvantages:

1. In the event of an earth fault, the danger of use is increased as the power supply continues.
2. Small short-circuit currents.
3. There are no sparks in the primary short circuit.

In networks above 1000 V

Currently, insulated neutral is most often used in networks with medium voltage class (1-35 kV). For a network of 110 kV and above - deafly grounded. Due to the fact that with a short circuit to the ground, the voltage, as it was said, increases to linear, so in a 110 kV power transmission line the phase voltage (between the ground and the phase wire) is 63.5 kV. This is especially important in the case of an earth fault, and allows you to reduce the cost of insulating materials.

By the way, in a transformer substation with a higher voltage up to 35 kV, the primary windings of the transformers are connected in a triangle, where there is no neutral as such.

Low short-circuit currents and the ability to work with single-phase short-circuits on overhead lines are especially important in distribution networks and allow you to organize an uninterrupted power supply. In this case, the shift angle between the phases remaining in operation remains unchanged - at 120˚.

At voltages of thousands of volts, the capacitive conductance of the phases cannot be neglected. Therefore, touching the wires of overhead power lines is dangerous to human life. In normal mode, the currents in the source phases are determined by the sum of the loads and the capacitive currents relative to the ground, while the sum of the capacitive currents is zero and the current does not pass through the ground.

If we omit some details in order to present it in a language understandable for beginners, then with a short circuit to the ground, the voltage relative to the ground of the damaged phase approaches zero. Since the voltages of the other two phases increase to linear values, their capacitive currents increase by √3 (1.73) times. As a result, the capacitive current of a single-phase short-circuit is 3 times higher than the normal one. For example, for a 10 kV overhead transmission line with a length of 10 km, the capacitive current is approximately 0.3 A. When a phase is closed to earth through an arc, as a result of various phenomena, dangerous overvoltages up to 2-4U occur_f, which leads to a breakdown of the insulation and phase-to-phase short circuit.

To exclude the possibility of occurrence arcs and elimination of possible consequences, the neutral is connected to the ground through an arc suppression reactor. In this case, its inductance is selected according to the capacity at the place of the short circuit to the ground, and also so that it ensures the operation of the relay protection.

Thus, thanks to the reactor:

1. Decreases much I_kz.
2. The arc becomes unstable and fades out quickly.
3. The voltage build-up after arc extinguishing is slowed down, which reduces the likelihood of re-arcing and switching current.
4. Negative sequence currents are small, therefore, their effect on the rotating rotor of the generator has no significant effect.

Let's list the pros and cons of high-voltage networks with isolated neutral.

Advantages:

1. For some time it can work in emergency mode (with a short circuit to the ground)
2. In places of malfunction, an insignificant current appears, provided that the current capacity is low.

Disadvantages:

1. Trouble detection is more complicated.
2. The need to isolate installations for line voltage.
3. If the short circuit continues for a long time, then a person may be injured by electric shock if he falls under step voltage.
4. With 1-phase short-circuits, normal operation is not ensured relay protection. The magnitude of the closing current directly depends on the branching of the circuit.
5. Due to the accumulation of insulation defects from the effect of arc overvoltages on it, its service life is reduced.
6. Damage can occur in several places due to insulation breakdown, both in cables and in electric motors and other parts of an electrical installation.

This concludes the overview of the principle of operation and features of networks with isolated neutral. If you want to supplement the article or share your experience - write in the comments, we will definitely publish it!

Related materials:

• Causes of a short circuit
• How to make grounding in a private house
• What is the difference between grounding and grounding