Gate and fence 

C. Safety shutdown. Protective Disable Devices responsive to case voltage to ground

Protective shutdown is a protection system that automatically turns off the electrical installation in the event of a danger of human injury. electric shock(in the event of a ground fault, reduced insulation resistance, grounding or zeroing fault). Protective shutdown is used when it is difficult to ground or neutralize, and also in addition to it in some cases.

Depending on what is the input value, to which the protective shutdown reacts, protective shutdown circuits are distinguished: on the case voltage relative to the ground; for earth fault current; for zero sequence voltage or current; to the phase voltage relative to earth; for direct and alternating operating currents; combined.

One of the protective shutdown circuits for the case voltage relative to the ground is shown in fig. 13.2.

Rice. 13.2. Residual trip circuit for case voltage relative to ground

The main element of the circuit is a protective relay RZ. If one phase is shorted to the housing, the housing will be energized above the permissible voltage, the RZ relay core is drawn in and closes the coil power circuit circuit breaker AB, as a result of which the electrical installation is turned off.

The advantage of the scheme is its simplicity. Disadvantages: the need to have an auxiliary ground RB; non-selectivity of shutdown in case of connection of several cases to one grounding; setpoint variability with changes in resistance RВ. Residual current devices that respond to zero-sequence current are used for any voltage, both with grounded and isolated neutral.

Fires and explosions

Fires and explosions are the most common emergency events in modern industrial society.

Most often and, as a rule, with severe social and economic consequences, fires occur at fire and fire hazardous objects.

The objects at which explosions and fires are most likely include:

Enterprises of the chemical, oil refining and pulp and paper industries;

Enterprises using gas and oil products as raw materials for energy carriers;

Gas and oil pipelines;

All types of transport transporting explosive and flammable substances;

Fuel stations;

Food industry enterprises;

Companies using paints and varnishes and etc.

EXPLOSIVE AND FIRE HAZARDOUS substances and mixtures are;

Explosives and gunpowder used for military and industrial purposes, manufactured at industrial enterprises stored in warehouses separately and in products and transported various types transport;

Mixtures of gaseous and liquefied hydrocarbon products (methane, propane, butane, ethylene, propylene, etc.), as well as sugar, wood, flour, etc. dust with air;

Vapors of gasoline, kerosene, natural gas on various vehicles, fuel stations, etc.

Fires at enterprises can also occur due to damage to electrical wiring and energized machines, furnaces and heating systems, containers with flammable liquids, etc.

There are also cases of explosions and fires in residential premises due to malfunctions and violations of the rules for the operation of gas stoves.

Characteristics of combustible substances

Substances that can burn on their own after the ignition source is removed are called combustible, in contrast to substances that do not burn in air and are called non-combustible. An intermediate position is occupied by hardly combustible substances that ignite under the action of an ignition source, but stop burning after the latter is removed.

All combustible substances are divided into the following main groups.

1. COMBUSTIBLE GASES (GH) - substances capable of forming flammable and explosive mixtures with air at temperatures not exceeding 50 ° C. Combustible gases include individual substances: ammonia, acetylene, butadiene, butane, butyl acetate, hydrogen, vinyl chloride, isobutane, isobutylene , methane, carbon monoxide, propane, propylene, hydrogen sulfide, formaldehyde, and vapors of flammable and combustible liquids.

2. FLAMMABLE LIQUIDS (FLL) - substances that can burn independently after the ignition source is removed and have a flash point not higher than 61 ° C (in a closed crucible) or 66 ° (in an open one). Such liquids include individual substances: acetone, benzene, hexane, heptane, dimethylformamide, difluorodichloromethane, isopentane, isopropylbenzene, xylene, methyl alcohol, carbon disulfide, styrene, acetic acid, chlorobenzene, cyclohexane, ethyl acetate, ethylbenzene, ethyl alcohol, as well as mixtures and technical products gasoline, diesel fuel, kerosene, white spirit, solvents.

3. COMBUSTIBLE LIQUIDS (GZh) - substances that can burn independently after the ignition source is removed and have a flash point above 61 ° (in a closed crucible) or 66 ° C (in an open one). Flammable liquids include the following individual substances: aniline, hexadecane, hexyl alcohol, glycerin, ethylene glycol, as well as mixtures and technical products, for example, oils: transformer, vaseline, castor.

4. COMBUSTIBLE DUST (FP) - solid substances in a finely dispersed state. Combustible dust in the air (aerosol) is capable of forming explosive mixtures with it. Dust (airgel) deposited on the walls, ceiling, equipment surfaces is a fire hazard.

Combustible dusts are divided into four classes according to the degree of explosion and fire hazard.

1st class - the most explosive - aerosols with a lower concentration limit ignition (explosive) (NKPV) up to 15 g / m3 (sulfur, naphthalene, rosin, mill dust, peat, ebonite).

2nd class - explosive - aerosols having a LEL value from 15 to 65 g / m3 (aluminum powder, lignin, flour, hay, shale dust).

3rd class - the most flammable - aerogels with a LEL value greater than 65 g / m3 and a self-ignition temperature of up to 250 ° C (tobacco, elevator dust).

4th class - flammable - aerogels with a LEL value of more than 65 g / m3 and an autoignition temperature of more than 250 ° C (sawdust, zinc dust).

In accordance with NPB 105-03, buildings and structures that house production facilities are divided into five categories.

Room category Characteristics of substances and materials located (circulating) in the room
And explosive and flammable Combustible gases, flammable liquids with a flashpoint of not more than 28 ° C in such an amount that they can form explosive vapor-gas-air mixtures, upon ignition of which an estimated excess explosion pressure in the room develops in excess of 5 kPa. Substances and materials capable of exploding and burning when interacting with water, atmospheric oxygen or one with the other in such an amount that the calculated overpressure of the explosion in the room exceeds 5 kPa.
B explosive and flammable Flammable dusts or fibres, flammable liquids with a flash point of more than 28 ° C, flammable liquids in such an amount that they can form explosive dust or vapor-air mixtures, upon ignition of which an estimated overpressure of the explosion in the room develops in excess of 5 kPa.
B1 - B4 fire hazardous Combustible and slow-burning liquids, solid combustible and slow-burning substances and materials that can only burn when interacting with water, atmospheric oxygen or one with the other, provided that the premises in which they are available or circulated do not belong to categories A or B
G Non-combustible substances and materials in a hot, incandescent or molten state, the processing of which is accompanied by the release of radiant heat, sparks and flames, combustible gases, liquids and solids that are burned or disposed of as fuel
D Non-flammable substances and materials in a cold state

EXAMPLES of production facilities located in premises of categories A, B, C, D and D.

Category A: shops for the processing and use of metallic sodium and potassium, oil refining and chemical industries, warehouses for gasoline and cylinders for combustible gases, premises for stationary acid and alkaline battery installations, hydrogen stations, etc.

The nature of the development of a fire and the explosion that follows it largely depends on the fire resistance of structures - the properties of structures to maintain their bearing and enclosing capacity in a fire. In accordance with SNiP 2.01.02.85, five degrees of fire resistance of buildings and structures are distinguished: I, II, III, IV, V.

The fire resistance of building structures characterizes the following parameters:

1) the minimum fire resistance limit of a building structure - the time in hours from the beginning of the impact of fire on the structure until the formation of through cracks in it or reaching a temperature of 200 ° C on the surface opposite to the fire.

2) the maximum limit for the spread of fire along building structures visually determined size of damage in centimeters, which is considered to be charring or burnout of materials, as well as melting of thermoplastic materials outside the heating zone.

All Construction Materials according to flammability, they are divided into three groups: FIRE-RESISTANT, FIRE-RESISTANT and COMBUStable.

The non-combustible materials and structures include metals and inorganic mineral materials used in construction and products made from them: sand, clay, gravel, asbestos, brick, concrete, etc.

FIRM-RESISTANT materials include materials and products made from them, consisting of combustible and non-combustible components: adobe brick, gypsum dry plaster, fibrolite, lenolium, ebonite, etc.

COMBUSTIBLE materials include all materials of organic origin: cardboard, felt, asphalt, roofing material, roofing felt, etc.

Basic concepts of fires and explosions.

FIRE is an uncontrolled burning outside a special focus, causing material damage.

BURNING - a chemical oxidation reaction, accompanied by the release of a large amount of heat and usually glow. For combustion to occur, it is necessary to have a combustible substance, an oxidizing agent (usually atmospheric oxygen, as well as chlorine, fluorine, iodine, bromine, nitrogen oxides) and an ignition source. In addition, it is necessary that the combustible substance be heated to a certain temperature and be in a certain quantitative ratio with the oxidizing agent, and the ignition source would have sufficient energy.

EXPLOSION - an extremely rapid release of energy in a limited volume, associated with a sudden change in the state of matter and accompanied by the formation of a large amount of compressed gases capable of producing mechanical work.

An explosion is a special case of combustion. But in the usual sense, it has in common with combustion only that it is an oxidative reaction. An explosion is characterized by the following features:

High rate of chemical transformation;

A large number of gaseous products;

Powerful crushing (blasting) action;

Strong sound effect.

The duration of the explosion is about 10-5...10-6 s. Therefore, its power is very high, although the reserves of internal energy of explosives and mixtures are not higher than those of combustible substances that burn under their usual conditions.

When analyzing explosive phenomena, two types of explosions are considered: explosive combustion and detonation.

The first includes explosions of air-fuel mixtures (mixtures of hydrocarbons, vapors of petroleum products, as well as sugar, wood, flour and other dust with air). characteristic feature such an explosion is a burning velocity of the order of several hundred m/s.

DETONATING - a very rapid decomposition of an explosive (gas-air mixture). propagating along it at a speed of several km / s and characterized by features inherent in any explosion indicated above. Detonation is typical for military and industrial explosives, as well as for fuel-air mixtures in a closed volume.

The difference between explosive combustion and detonation lies in the rate of decomposition, in the latter it is an order of magnitude higher.

In conclusion, three types of decomposition should be compared: ordinary combustion, explosive and detonation.

NORMAL BURNING processes proceed relatively slowly and at a variable speed - usually from fractions of a centimeter to several meters per second. The burning rate essentially depends on many factors, but mainly on external pressure, increasing noticeably with an increase in the latter. In the open air, this process proceeds relatively sluggishly and is not accompanied by any significant sound effect. In a limited volume, the process proceeds much more energetically, is characterized by a more or less rapid increase in pressure and the ability of combustion gases to do work.

EXPLOSIVE COMBUSTION, in comparison with the usual one, is a qualitatively different form of the propagation of the process. Distinctive features explosive combustion are: a sharp jump in pressure at the site of the explosion, a variable speed of propagation of the process, measured in hundreds of meters per second and relatively little dependent on external conditions. The nature of the action of the explosion is a sharp blow of gases to environment, causing crushing and severe deformation of objects at relatively short distances from the explosion site.

DETONATION is an explosion propagating with the maximum possible speed for a given substance (mixture) and given conditions (for example, mixture concentration), exceeding the speed of sound in a given substance and measured in thousands of meters per second. Detonation does not differ in the nature and essence of the phenomenon from explosive combustion, but is its stationary form. The detonation velocity is a constant value for a given substance (a mixture of a certain concentration). Under detonation conditions, the maximum destructive effect of the explosion is achieved.

Windows Defender is a built-in component of the operating system that helps protect your computer from malware such as viruses, spyware, and other potentially unsafe applications.

In fact, Windows Defender is the same antivirus, only free, if you do not take into account the cost of the operating system itself. So why turn it off if it performs such useful features, for it you do not have to pay extra and install separately?

The fact is that Windows Defender only performs basic protection computer. Third-party antiviruses do a much better job of protecting your PC. You can see for yourself by looking at where the Defender is located according to the research of the AV-Test laboratory (the image is clickable).

On the other hand, if you are a “diligent” computer and Internet user, do not visit suspicious sites, do not download or use pirated software, use only trusted storage media, then Windows 10 Defender will be enough for you to ensure minimal security.

But back to the main topic of the article. How do you turn off Windows 10 Defender?

First of all, it should be noted that Defender turns itself off automatically when installing an additional antivirus software, provided that the system correctly recognizes third-party software.

Next, consider an option that I deliberately did not include in the general list of ways to deactivate Defender. The thing is, it's only temporary. After some time or after restarting the computer, the defender will return to the working state. This is a feature of Windows 10. In Windows 8.1, this method could completely disable the built-in antivirus.

  1. Open your computer settings ( Windows + I).
  2. Go to the section " Update and Security».
  3. Select " Windows Defender» in the menu on the left.
  4. Disable the " Real time protection»

Now let's look at ways that completely disable Defender.

Disable Windows 10 Defender permanently

Method 1 - Through the registry

1. Open the window " Run» ( Windows+R), enter the command regedit and press " OK».

2. Navigate to the following branch of the registry:

HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows Defender

3. Right-click on the empty space on the left and create a DWORD (32 bit) value named .

4. Double-click to open the newly created parameter, assign it a value 1 and press " OK».

Now you can close the registry editor and check the effect of this method through the computer settings. There you can make sure that all settings related to Defender have become inactive. You can also try running the built-in antivirus by clicking on the link at the very bottom " Open Windows Defender».

As a result, you will get a message that Windows 10 Defender has been disabled by group policy.

If you want to reactivate a disabled Windows 10 Defender, then just delete the DisableAntiSpyware setting or change its value to 0.

Method 2 – Using the Local Group Policy Editor

1. Run the command gpedit.msc through the window " Run» ( Windows+R).

2. Proceed to the next section:

Computer Configuration -> Administrative Templates -> Windows Components -> Endpoint Protection

In some versions (assemblies) of Windows 10, this section may be called Windows Defender or Windows Defender.

3. In this section on the left, find the item "" and open it.

4. Activate this option as shown in the image below and click " OK».

Close the Group Policy Editor and you can, as in the first method, check if Defender is disabled.

If you need to turn Windows Defender back on, follow all the steps above and set the value to " Not set". In this case, a reboot may be required to activate the built-in antivirus.

Method 3 - NoDefender Program

If the methods above don't help, you can try utilities specifically designed to disable Windows Defender. One of these is NoDefender.

Attention! Use this method only as a last resort. Programs of this kind are not officially supported by Windows developers, and therefore no one gives any guarantees that they will not affect the performance of the operating system.

Be sure to back up your system before using NoDefender. It is also worth noting that the process of disabling the defender using this utility is irreversible. At least the functionality of the program does not allow you to turn on Defender back.

2. Unzip the resulting archive and run the program.

3. In the first window of the program, click " Next».

5. Disable the following options: real-time protection, cloud protection and automatic sample submission.

7. Then click " Next" and in the last step " exit».

All. Windows 10 Defender is disabled. Now if you try to activate Defender, the message " The application is disabled and does not monitor the computer».

The developers of the application claim that restarting NoDefender allows the defender to be activated again. I didn't manage to do it.

Protective shutdown is the automatic shutdown of electrical installations when a single-phase contact is made with live parts that are unacceptable for humans, and (or) when a leakage current (short circuit) occurs in the electrical installation that exceeds the specified values.

The purpose of the protective shutdown is to ensure electrical safety, which is achieved by limiting the time of exposure to a dangerous current on a person. Protection is carried out by a special residual current device (RCD), which ensures electrical safety when a person touches the current-carrying parts of the equipment, allows for constant monitoring of the insulation, turns off the installation when the current-carrying parts are shorted to the ground. To protect people from electric shock, RCDs with a trip current of not more than 30 mA are used.

Scope of protective shutdown: electrical installations in networks with any voltage and any neutral mode.

The protective shutdown is most widely used in electrical installations used in networks with voltage up to 1 kV with a grounded or isolated neutral.

The principle of operation of the RCD is that it constantly monitors the input signal and compares it with a given value. If the input signal exceeds this value, the device disconnects the protected electrical installation from the network. Various parameters are used as input signals for residual current devices electrical networks, which carry information about the conditions of electric shock to a person.

The RCD reacts to the “leakage current” and cuts off electricity within hundredths of a second, protecting a person from electric shock, it catches the slightest current leakage and opens the contacts.

Structurally, RCDs are of two types:

electronic, dependent on the supply voltage, their mechanism for performing the trip operation needs energy received either from a controlled network or from an external source; electromechanical, independent of the supply voltage, they are more expensive than electronic RCDs, but have greater sensitivity. The source of energy necessary for the operation of such RCDs is the input signal itself - the differential current to which it responds.

All RCDs according to the type of input signal are classified into several types:

responding to the voltage of the case relative to the ground; responding to differential (residual) current; reacting to the combined input signal; responsive to earth fault current; responsive to operational current (DC; AC 50 Hz); responsive to zero sequence voltage.

The use of RCDs must be carried out in accordance with the Electrical Installation Rules (PUE).

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 that people work with 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.

    Fixed installations with solidly earthed neutral on some high power consumers and on remote consumers where earthing is not enough for protection or where it is protective measure not quite effective, does not give 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 set 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 coverage area of ​​\u200b\u200bthe protection 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 variant simple device protective shutdown 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 protection 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 is most widely used in electrical installations used in networks with voltages up to 1 kV with a grounded or isolated neutral.

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

While doing automatic shutdown power supply in electrical installations with voltage 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 protective devices and the parameters of the protective conductors must be coordinated in order to ensure the normalized disconnection time of the damaged circuit by the 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.

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.