A broken zero wire in a three-phase circuit (or its burning off) is a common phenomenon that is familiar to most specialists and operational personnel of substations. He is also faced by electricians serving apartment buildings in which this damage occurs at the entrance to the riser or directly in the apartment. Usually it manifests itself as a cliff of the "earth" vein. To understand the essence of the phenomenon, you first need to deal with the causes of its occurrence.
The formation of supply chains and the causes of breakage
The principle of forming 380 Volt supply circuits is based on the fact that each phase has its own group of consumers (houses, entrances or apartments). A zero break in three-phase networks occurs when the load distribution is interrupted, connected, like the windings of a station transformer, according to the "star" scheme - they must be connected evenly. With the correct distribution, the current components are mutually compensated, and the total value in the neutral wire is close to zero. Therefore, a neutral core is made of a smaller cross section than phase wires - theoretically, it can be excluded altogether, since the current should not flow here.
Any deviation from this requirement leads to phase imbalance and the appearance of stray currents in a neutral conductor.
Since on the consumer side the number of switched on household appliances and light bulbs per phase can be arbitrary, no power line can do without deviation from the norm.
A current always flows through the neutral wire, slightly shifting the phase nodes in one direction or another. On the corresponding diagrams, this looks like the approach of the zero point to one of the phases. With a strong distortion, the cross section of the neutral wire in the supply networks may not be enough to withstand the increased current through it. Over time, constant overload leads to its burning.
When switching from three-phase chains to linear branches (their formation takes place at the entrance to the access riser), the situation is completely different. Problems with zero burning in single-phase networks can arise for the following reasons:
- Bad contact or damage to the zero core on the linear tap. It settles down on the entrance switchgear of the entrance.
- Disappearance of the corresponding contact in the floor plate. In some houses, it is installed on each site.
- Disruption of connections in the "earth" wire at the entrance to the apartment or inside it.
The malfunction first manifests itself as a short-term loss of electricity, which can not be immediately found. Over time, when the contact at the point of connection of the zero core is completely destroyed, household appliances will completely stop working, and the light will turn on.
Possible consequences
The effects of a zero cut in a three-phase network are sometimes excessively dangerous. Regardless of the grounding system used, when firing a zero core, high potentials appear in the apartments connected to such a cable. Due to a strong skew on some lines of the wiring, voltages reaching 380 volts will appear. On the other branches of the 3-phase input, on the contrary, they can drop to almost zero.
Zero-wire breaks are dangerous in that they, above all, pose a threat to household appliances connected to outlets. This can threaten the complete failure of expensive equipment or the ignition of an old aluminum wiring that could lead to a fire.On the other hand, if the house is connected via a TN-C system with combined PE and N conductors, a break in the common PEN core will lead to the loss of the protective function that protects against electric shock. In the absence of re-grounding, the consumer will be defenseless when the PEN wire is broken, even if an RCD is installed in his apartment, which cannot work without a zero core.
If a zero breakage occurred on one of the apartment lines, protected by a separate machine, first of all, all electrical devices connected to it will stop working. In addition, if there is no zero and there is a phase in the network, a dangerous potential of 220 volts through constantly connected loads will fall to the ground terminal. As a result, another phase will appear in the outlet, which is very dangerous in the absence of a normal grounding.
In case of any accidental breakdown of insulation in the washing machine, for example, the hazardous potential will have nowhere to drain, since the earth wire is broken. For a consumer standing on a concrete floor connected to the ground, this poses a great danger, since all current flows through it.
Burnout or zero cut protection
A study of the consequences of disturbances in the operation of three-phase lines and their branches showed that it is necessary to take some measures to prevent these phenomena. Reliable zero break protection in a single-phase network allows you to:
- keep household appliances intact;
- provide user protection against electric shock;
- to prevent accidental ignition of a decrepit wiring and the occurrence of a fire.
To protect against phase failure, modern electrical equipment is used, which includes special relays, as well as line overload protection devices (SPD). The first are available in two versions, one of which is designed for 3-phase circuits, and the second allows you to protect single-phase branches. The principle of their work is to instantly turn off the power supply in the event of a voltage deviation in it in excess of the established norm.
The second device for phase loss protection is usually used in private households in order to disconnect loads in the event of a dangerous situation. The principle of its operation is to reduce the conductivity of internal circuits with significant potential drops. The most effective way to prevent dangerous consequences in three-phase networks is to use re-grounding, the device of which in apartment buildings is associated with great difficulties.
In rural areas and private suburban buildings, this approach is implemented very simply. It is enough to equip a grounding device in the area adjacent to the house and connect it via a copper bus to a separate contact mounted in the input shield.
As another tool that can protect against a zero-wire break, you can use a residual current device or, in short, an RCD. Its variety is a differential device that combines the functions of an RCD and a typical machine. For these purposes, ordinary products that need a whole zero core for normal operation are not suitable. Mounting in branch lines is allowed only for those devices that specifically provide for the function of protection against zero breakage.
