Safety shutdown- high-speed protection that provides automatic shutdown of the electrical installation in the event of a danger of electric shock in it.

Such a danger may arise, in particular, when a phase is shorted to the electrical equipment case; when the insulation resistance of the phases relative to the ground drops below a certain limit; the appearance of a higher voltage in the network; touching a person to a live part that is energized. In these cases, some electrical parameters change in the network: for example, the case voltage relative to earth, phase voltage relative to earth, zero sequence voltage, etc. can change. Any of these parameters, or rather, changing it to a certain limit, at which danger arises electric shock to a person, can serve as an impulse that causes the operation of a protective shutdown device, i.e. automatic shutdown of a dangerous section of the network.

Residual current devices(RCD) must ensure the shutdown of a faulty electrical installation for a time not exceeding 0.2 s.

The main parts of the RCD are a residual current device and a circuit breaker.

Residual current device- a set of individual elements that respond to a change in any parameter of the electrical network and give a signal to turn off the circuit breaker.

Circuit breaker- a device used to turn on and off circuits under load and in case of short circuits.

RCD types.

RCD responding to case voltage relative to earth , are intended to eliminate the danger of electric shock in the event of an increased voltage on a grounded or grounded housing.

RCDs responding to operational direct current , are designed to continuously monitor the insulation of the network, as well as to protect a person who has touched the current-carrying part from electric shock.

Consider a circuit that provides protection when voltage appears on the case relative to ground.

Rice. Residual shutdown circuit at voltage on

hull relative to the ground.

The scheme works as follows. When the button P is turned on, the power circuit of the winding of the MP magnetic starter is closed, which turns on the electrical installation with its contacts and self-locks along the circuit composed of normally closed contacts of the “stop” button C, protection relay RZ and auxiliary contacts.

When a voltage appears relative to the ground on the case U z, equal in magnitude to the long-term permissible contact voltage, under the action of the RZ (KRP) coil, the protection relay is activated. RZ contacts break the MP winding circuit, and the faulty electrical installation is disconnected from the network. The artificial circuit circuit, activated by the K button, serves to monitor the health of the shutdown circuit.

It is advisable to use protective shutdown in mobile electrical installations and when using hand-held power tools, since their operating conditions do not allow ensuring safety by grounding or other protective measures.

What is a safety shutdown used for?

The danger of electric shock is determined by the voltage of contact (£ / doya1, V) and then by the strength of the current that can pass through the human body (/ "A). As you know.

where /? A is the resistance of the human body, Ohm.

If the touch voltage at the moment a person touches the body or network phase exceeds the permissible value, then there is a real threat of electric shock and the degree of protection in this case can only be a break in the current circuit, disconnecting the corresponding section of the network. To accomplish this task, a safety shutdown is used.

A protective shutdown is a fast-acting protection that provides automatic shutdown of an electrical installation in the event of a danger of electric shock to a person.

Grounding and zeroing do not always guarantee the safety of people. Protective shutdown disconnects the damaged section of the installation much faster than zeroing, than more guaranteed protection of people from electric shock.

When is a safety switch used?

Protective shutdown is used only in electrical installations with voltages up to 1000 V as independent protection or simultaneously with grounding:

in mobile electrical installations with isolated generator neutral;

in stationary installations with isolated neutral for the protection of those working with hand power tools;

in stationary electrical installations with a dead-earthed neutral on separate high-power consumers remote from transformers, on which zeroing protection is ineffective;

where there is an increased risk of electric shock. The scope of application of residual current devices is practically unlimited. They can be used in networks of any purpose and with any neutral mode. However, they are most widely used up to 1000 V, especially where it is difficult to carry out effective grounding or zeroing, when there is a high probability of accidental contact with live parts (mobile electrical installations, hand-held power tools).

What are the requirements for a protective shutdown and what functions does it perform?

Protective shutdown can be used as the main type of protection or together with grounding and zeroing.

The following requirements are placed on the residual current device: self-control, reliability, high sensitivity and short turn-off time.

Protective shutdown, alone or in combination with other means of protection, performs the following functions:

protection in case of a short circuit to the ground or the equipment case;

protection against dangerous leakage currents;

protection during the transition of higher voltage to the lower side;

automatic control of the circle of protective grounding and zeroing.

How is a safety shutdown performed?

The protective shutdown is carried out by very sensitive and fast protective emerging devices. The sensitivity and transient action of them significantly exceeds automatic switches or other measures of the elements.

In electrical circuits, protective shutdown devices use sensitive elements that respond to the appearance of current in the neutral wire, voltage on the case of damaged electrical equipment, etc.

Protective shutdown devices operate in 0.1-0.05 s, while zeroing takes 0.2 or more seconds. With such a short duration of the passage of current through the human body, a current of even 500-600 mA will be safe. Considering that the resistance of the human body is 1000 ohms, then the current of the reduced value can flow through the human body only if its voltage is 500-650 V, and there cannot be such a voltage in electrical networks with a voltage of 380/220 V with a grounded neutral even in emergency mode in emergency situations.

Protective disconnection is also used in cases where the grounding device will cause significant difficulties (rocky soils) or will be impractical due to the moving front of the work.

Therefore, protective disconnecting devices are reliable protection of people from electric shock.

One of the safety measures in electrical installations is the use of low voltages of the order of 36.34.12 V or less: for local lighting lamps at machine tools; for portable lamps (12 V); power supplies for electric soldering irons, electric drills and other electric tools.

Safety shutdown- this is a high-speed protection that provides automatic shutdown of an electrical installation when there is a danger of electric shock to a person in it.

Currently, protective shutdown is the most effective electrical protective tool. The experience of developed foreign countries shows that the massive use of residual current devices (RCDs) has provided a sharp reduction in electrical injuries.

Protective shutdown is increasingly used in our country. It is recommended for use as one of the means to ensure electrical safety by regulatory documents (NTD): GOST 12.1.019-79, GOST R 50571.3-94 PUE, etc. In some cases, the mandatory use of RCDs in electrical installations of buildings is required (see GOST R 5066.9 -94). The objects to be equipped with AEO include: newly built, reconstructed, overhauled residential buildings, public buildings, industrial facilities, regardless of ownership and ownership. The use of RCDs is not allowed in cases where a sudden shutdown can lead, for technological reasons, to situations that are dangerous for personnel, to turn off fire, burglar alarms, etc.

The main elements of the RCD are a residual current device and an actuator - a circuit breaker. Residual current device- this is a set of individual elements that perceive the input signal, react to its change and, at a given signal value, act on the switch. Executive device- a circuit breaker that provides shutdown of the corresponding section of the electrical installation (electrical network) upon receipt of a signal from the residual current device.

Primary requirements, applied to RCD:

1) Speed ​​\u200b\u200b- the shutdown time (), added up from the time of the device (t p) and the time of the switch (t in), must meet the condition

The existing designs of devices and devices used in protective shutdown circuits provide a shutdown time t o tkl = 0.05 - 0.2 s.

2) High sensitivity - the ability to respond to small values ​​of input signals. Highly sensitive RCD devices allow you to set the settings for the switches (the values ​​​​of the input signals at which the switches operate), ensuring the safety of a person touching the phase.

3) Selectivity - the selectivity of the action of the RCD, i.e. the ability to disconnect from the network that section, where there is a risk of electric shock.

4) Self-monitoring - the ability to respond to own faults by turning off the protected object is a desirable property for RCDs.

5) Reliability - the absence of failures in operation, as well as false positives. Reliability must be sufficiently high, since RCD failures can create situations associated with electric shock to personnel.

Application area RCDs are practically unlimited: they can be used in networks of any voltage and with any neutral mode. RCDs are most widely used in networks up to 1000 V, where they provide safety when a phase is shorted to the case, the insulation resistance of the network relative to the ground drops below a certain limit, a person touches a live part that is energized, in mobile electrical installations, in power tools, etc. Moreover, RCDs can be used as independent protective devices, and as an additional measure for grounding or protective grounding. These properties are determined by the type of RCD used and the parameters of the protected electrical installation.

Types of residual current devices. The operation of the electrical network in both normal and emergency modes is accompanied by the presence of certain parameters that may vary depending on the conditions and mode of operation. The degree of danger of human injury in a certain way depends on these parameters. Therefore, they can be used as input signals for RCDs.

In practice, the following input signals are used to create an RCD:

Hull potential relative to earth;

earth fault current;

Zero sequence voltage;

Differential current (zero sequence current) ;

Phase voltage relative to ground;

operational current.

In addition, combined devices are also used that respond to several input signals.

Below is a diagram and operation of a residual current device that reacts on the potential of the hull relative to the ground.

The purpose of this type of RCD is to eliminate the danger of electric shock to people in the event of an increased potential on a grounded or grounded case. Usually these devices are an additional measure of protection to grounding or grounding. The device is triggered if the potential φ k that has arisen on the body of the damaged equipment is higher than the potential φ kdop, which is selected based on the highest continuous allowable contact voltage U pr.dop.

The sensor in this circuit is the voltage relay RN,

Fig.28. Schematic diagram of an RCD that responds to

potential of the housing connected to earth with the help of an auxiliary earthing conductor R op

When a phase is shorted to a grounded (or zeroed) case, protective grounding first acts, which reduces the voltage on the case to the value U k \u003d I s * R s,

where R s is the resistance of the protective earth.

If this voltage exceeds the relay setting voltage RN U set, then the relay will operate due to the current I p, opening the power supply circuit of the MP magnetic starter with its contacts. And the power contacts of the magnetic starter, in turn, will de-energize the damaged equipment, i.e. RCD will do its job.

Operative (working) switching on and off of the equipment is carried out by the START, STOP buttons. The contacts of the BC of the magnetic starter provide its power after the START button is released.

The advantage of this type of RCD is the simplicity of its circuit. The disadvantages include the need for auxiliary grounding, the lack of self-monitoring of serviceability, the non-selectivity of shutdown in the case of connecting several cases to one protective ground electrode, and the variability of the setting when R op changes.

Next, consider the second circuit that responds to differential current (or zero sequence current) - RCD (D). These devices are the most versatile, and therefore are widely used in production, public buildings, residential buildings, etc.

A protective shutdown is understood as a quick, within a time of not more than 200 ms, automatic disconnection from the power source of all phases of the consumer or part of the wiring in case the insulation is damaged or there is another emergency that threatens a person with electric shock.

Protective automatic power off- automatic opening of the circuit of one or more phase conductors (and, if required, the neutral working conductor), performed for electrical safety purposes.

Protective shutdown can be both the only and main measure of protection, and an additional measure for grounding and zeroing networks in relation to electrical installations with an operating voltage of up to 1000 volts.

Assignment of a protective shutdown- Ensuring electrical safety, which is achieved by limiting the time of exposure to dangerous current on a person.

Safety shutdown- high-speed protection that provides automatic shutdown of the electrical installation in the event of a danger of electric shock in it. This risk can arise when:

phase short circuit on the body of electrical equipment;

when the insulation resistance of the phases relative to the ground drops below a certain limit;

the appearance of a higher voltage in the network;

touching a person to a live part that is energized.

In these cases, some electrical parameters change in the network: for example, the case voltage relative to earth, phase voltage relative to earth, zero sequence voltage, etc. can change. Any of these parameters, or rather, changing it to a certain limit, at which danger arises electric shock to a person, can serve as an impulse that triggers a protective shutdown device, i.e., automatic shutdown of a dangerous section of the network.

Current devices protective shutdowns were usually used in electrical installations of four types:

Mobile installations with an isolated neutral (in such conditions, in principle, the construction of a full-fledged grounding device is problematic). Protective disconnection is then applied either in conjunction with earthing or as an independent protective measure.

Fixed installations with isolated neutral (where the protection of electrical machines with which people work is necessary).

Mobile and fixed installations with any type of neutral where there is a high risk of electric shock or if the installation operates in explosive environments.

Stationary installations with solidly grounded neutral on some high power consumers and on remote consumers, where grounding is not enough for protection or where it is not quite effective as a protective measure, does not provide a sufficient multiplicity of the phase-to-earth fault current.

To implement the protective shutdown function, special protective shutdown devices were used. Their schemes may differ, the designs depend on the features of the protected electrical installation, on the nature of the load, on the neutral grounding mode, etc.

Residual current device- a set of individual elements that respond to a change in any parameter of the electrical network and give a signal to turn off the circuit breaker. A residual current device, depending on the parameter to which it reacts, can be attributed to one or another type, including types of devices that respond to case voltage relative to earth, earth fault current, phase voltage to earth, zero sequence voltage, current zero sequence, operational current, etc.

Here, a specially installed protection relay can be used, which is arranged in the same way as highly sensitive voltage relays with breaking contacts, which are included in the power circuit of a magnetic starter, say, an electric motor.

The purpose of the protective shutdown is to implement a combination of protection or some of the following types of it with one device:

from single-phase short circuits to the ground or to electrical equipment normally isolated from voltage;

from incomplete short circuits, when a decrease in the insulation of one of the phases creates a danger of human injury;

from damage when a person touches one of the phases of electrical equipment, if the touch occurred in the protection zone of the device.

An example is a simple residual current device based on a voltage relay. The relay winding is connected between the body of the protected equipment and the ground electrode.

Under conditions when the relay winding has a resistance that is much higher than that of the auxiliary grounding switch, which is placed outside the protection grounding spreading zone, the K1 relay winding will be energized with respect to the ground.

Then, at the time of an emergency breakdown to the case, the voltage will be greater than the relay operation voltage and the relay will operate, closing the shutdown circuit of the Q1 circuit breaker or opening the power supply circuit of the Q2 magnetic starter winding by its operation.

Another option for a simple residual current device for electrical installations is (overcurrent relay). Its winding is included in the break of the ground wire, due to which the contacts open the power circuit of the magnetic starter winding in the same way if the power circuit of the winding of the circuit breaker is closed. Instead of the relay winding, by the way, it is sometimes possible to use the winding of the switch - release as an overcurrent relay.

When a residual current device is put into operation, it is mandatory to check it: scheduled full and partial checks are carried out to make sure that the device works reliably, that shutdowns occur when necessary.

Once every three years, a full scheduled inspection is carried out, often along with the repair of associated circuits of electrical installations. The inspection also includes insulation tests, verification of protection settings, tests of protective devices and a general inspection of the apparatus and all connections.

As for partial checks, they are carried out from time to time depending on particular conditions, but they include: insulation check, general inspection, protection tests in action. If the protective device does not work quite correctly, a deeper check is carried out using a special algorithm.

In our time, the protective shutdown has become most widespread in electrical installations used in networks with voltage up to 1 kV with a grounded or isolated neutral.

Electrical installations up to 1 kV in residential, public and industrial buildings and outdoor installations should, as a rule, be powered from a source with a solidly grounded neutral. To protect against electric shock from indirect contact in such electrical installations, automatic power off must be performed.

When performing automatic power off in electrical installations with voltages up to 1 kV, all exposed conductive parts must be connected to a solidly grounded neutral of the power source if the TN system is used, and grounded if the IT or TT systems are used. At the same time, the characteristics of protective devices and the parameters of protective conductors must be coordinated in order to ensure a normalized time for disconnecting a damaged circuit by a protective switching device in accordance with the rated phase voltage of the supply network.

Protection is carried out, which, working in standby mode, constantly monitors the conditions for electric shock to a person.

RCDs are used in electrical installations up to 1 kV:

in mobile email installations with an isolated neutral (especially if it is difficult to create a grounding device. It can be used both as independent protection and in combination with grounding);

in stationary electrical installations with isolated neutral for the protection of hand-held electrical machines as the only protection, and in addition to others;

in conditions of increased danger of electric shock and explosion hazard in stationary and mobile electrical installations with different neutral modes;

in stationary electrical installations with a dead-earthed neutral on separate remote consumers of electrical energy and a consumer of high rated power, on which zeroing protection is not effective enough.

The principle of operation of the RCD is that it constantly monitors the input signal and compares it with a predetermined value (setpoint). If the input signal exceeds the setting, the device operates and disconnects the protected electrical installation from the network. As input signals of residual current devices, various parameters of electrical networks are used, which carry information about the conditions of electric shock to a person.

A protection system that provides automatic shutdown of all phases or poles of the emergency section of the network for a total shutdown time of not more than 0.2 s is called protective shutdown.
Regardless of the state of the neutral of the supply system, any single-phase short circuit to the body leads to the appearance of voltage relative to earth on the electrical equipment cases. This circumstance is used in the construction of universal protection, which ensures that the damaged electrical equipment is switched off by automatic machines when a certain given potential difference appears between the case and the ground. Such a system is identical to grounding and is based on the automatic shutdown of the power receiver, if the latter appears on its metal parts, which are not normally energized. Protective shutdown is used for systems with isolated and dead-earthed neutral.

Rice. one. Schematic diagram of the protective shutdown:
1 - housing of the electrical receiver; 2 - disconnecting spring; 3 - contacts of the network contactor; 4 - latch; 5 - coil core; b - trip coil; 7, 8 - ground electrodes; 9 contact

Consider the action of a protective shutdown when voltage occurs on the case of a single power receiver as a result of damage to its insulation. Two cases are possible here: the power receiver is not grounded and the power receiver is grounded.
The first case corresponds to the open position of contact 9 (Fig. 1). At some distance from the protected power receiver, ground electrode 7 is driven into the ground (in the event that there are no natural ground electrodes that should not have electrical connection with the housing / power receiver). The protective switch allows breaking the power supply circuit with the contacts of the mains contactor when voltage is applied to coil 6.
When the coil 6 is de-energized, its core 5 holds the latch 4, preventing the spring 2 from opening the contacts 3 (the contacts are shown in the diagram as open, although the core holds the latch). One end of the coil winding is connected to the housing 7 of the electrical receiver, the second - to the remote ground electrode 7. In case of damage to the insulation between the electrical receiver housing and the remote earth electrode 7, a phase voltage will appear. The trip coil 6 will be energized, and current will flow through its winding. The core 5 will retract and release the retaining latch 4. The spring 2 will open the contacts 3 of the network contactor, and the power supply circuit of the electrical installation will break. The touch voltage on the body of the electrical receiver will disappear, contact with it will become safe.
The second case, when the body of the power receiver is grounded, corresponds to the closed position of contact 9. In the event of an insulation damage, a voltage will appear on the body of the power receiver, the value of which will determine the voltage drop in the ground electrode, equal to the ground fault current multiplied by the grounding resistance of the ground electrode. There is no fundamental difference in the protection action in the first and second cases.
The basis of protection using a protective shutdown is the quick disconnection of a damaged electrical receiver.

Rice. 2. Residual current circuit with isolated neutral

According to the PUE, protective shutdown is recommended for use in the following installations: electrical installations with isolated neutral, which are subject to increased safety requirements (in addition to the grounding device). The scheme of such a protective shutdown is shown in fig. 2. When a ground fault current appears in the KA relay coil, its NC contact in the coil circuit of the KM contactor opens and the contactor disconnects the electric motor M from the mains with its main contacts;
electrical installations with a solidly grounded neutral with a voltage of up to 1000 V, the cases of which are not connected to a grounded neutral wire, since such connection is difficult;
mobile units, if their grounding cannot be performed in accordance with the requirements of the PUE.
Protective shutdown is versatile and fast, so its use in networks with both dead-earthed and isolated neutral is very promising. It is especially advisable to use it in networks with a voltage of 380/220 V.
The disadvantage of the protective shutdown is the possibility of disconnection failure in the event of burnt contacts of the switching device or wire breakage.