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close this bookIntroduction to Electrical Engineering - Basic vocational knowledge (Institut für Berufliche Entwicklung, 213 p.)
close this folder9. Protective Measures in Electrical Installations
close this folder9.2. Measures for the Protection of Man from Electric Shock
View the document9.2.1. Protective Insulation
View the document9.2.2. Extra-low Protective Voltage
View the document9.2.3. Protective Isolation
View the document9.2.4. Protective Wire System
View the document9.2.5. Protective Earthing
View the document9.2.6. Connection to the Neutral
View the document9.2.7. Fault-current Protection

9.2.1. Protective Insulation

For electrical household appliances and portable tools (e.g. electric hand-drilling appliances), the electrical protective insulation is a very effective protection from electric shock. The emergence of a contact voltage is practically avoided by the special arrangement of the insulation. Fig. 9.2. shows two methods of arranging a protective insulation. For example, an electric hand-drilling appliance can be provided with a plastic enclosure (protective insulating covering) and the metallic gear and drill chuck can be connected with the motor by means of a clutch of insulating material (protective intermediate isulation). For this purpose, special requirements must be specified for the mechanical, thermal, electrical and chemical stability of the insulating materials to be used. At accessible points of live parts, which do not belong to the service circuit, a protective conductor must not be connected. Devices with protective insulation may be marked with two squares, an outer one and an inner one. ()

Fig. 9.2. Protective insulation


a) protective insulating covering


b) protective intermediate insulation

1

Operational insulation

2

Metal enclosure

3

Protective insulation

9.2.2. Extra-low Protective Voltage

If a safe protection against accidental contacts of the live parts in operation is not possible or if tools have to be used in an environment where particular hazards are given (e.g. in narrow boilers), an expedient protective measure is the use of extra-low protective voltage. In the no-load state, it must not be greater tan 50 V. For electrical toys for children lower values are specified.

When extra-low protective voltage is used, no live conductor must be earthed in operation. The extra-low protective voltage is generated in special transformers or by motor generators. The circuits of extra-low protective voltage have to be installed in such a way that they are safely separated from other circuits. For this purpose, only such plug-type connections or connectors have to be used which safely prevent an inadvertent connection of the tools and the like to other voltage sources. Like with protective insulation, conductive parts of the tools and the like, which do not belong to the service circuit, must not be connected with a protective conductor.

9.2.3. Protective Isolation

Like the extra-low protective voltage, the protective isolation is a galvanic separation from the feeding supply network but the output voltage may be greater than 50 V. This protective measure is applicable, for instance in television repair shops, in work to be done in the open or in boiler houses.

In this case only one load may be connected to an output winding of a transformer. Fig. 9.3. shows an otherwise dangerous situation in the case of double body contact and connection of two devices to one winding. The using plug-type connectors, switches and a tool or the like with protective conductor connections (protective class 1), all protective conductor connections must be connected with each other. If work is to be done in narrow space on metallic floors and the like, then the isolating transformer should be provided with protective insulation and arranged outside of this space. In addition a potential equalisation line must be arranged between protective conductor connection and tool. In this way any danger due to a damaged lead is safely avoided.


Fig. 9.3. Impermissible connection of two devices to one winding of a protective isolating transformer

IF = fault current;
UB = contact voltage

9.2.4. Protective Wire System

The measures described in 9.2.2. and 9.2.3. ensure a high protective effect against electric shock but they are very expensive. When in a room many electrical appliances are put into operation at the same time and when, for reasons of safety, it should be avoided that, in the case of body contact of one of the devices, the whole installation is switched off, then the protective wire system is the adequate protective measure. These conditions may occur, for example, in an operation theatre.

No conductor of the service circuit must be earted, not even the neutral conductor-that is a condition of the protective wire system. On the other hand, all of the conductive parts not belonging to the service circuit and all conductive parts of the building (water pipes, other pipe lines, metallic structures of the building) must be connected together in any case. There is a puncture cut-out between neutral conductor (H) and earth. In the case of a simple body contact, there is no danger of an occurrence of a dangerous contact voltage. However, in the case of a double body contact, there is the danger of a dangerous occurrence of contact voltage.

If in an installation - as has been mentioned above -, with an suddenly occurring body contact no danger to man shall occur and no switching off is to be effected, then a switching monitoring continuously the isolation resistance to earth of the service circuit (e.g. 25 W/V) is employed, releasing a fault signal (Fig. 9.4.). This type of monitoring will reliably indicate a first body contact. When signalling is not desired, care must be taken that, in case of a double body contact, the breaking current Ia will flow and the installation is reliably and quickly switched off. The conditions for the breaking current Ia will be explained in Section 9.2.5.


Fig. 9.4. Protective wire system with monitoring of the insulation resistance

1 - Monitoring relay for insulation resistance to earth with signalling the fault by a horn
2 - Water pipes
3 - Conductive parts of the building
4 - Other pipe lines

9.2.5. Protective Earthing

In protective earthing, all of the conductive parts not belonging to the service circuit are connected with the protective earthing device by means of the protective conductor. Since the fault current is conducted through the earthing resistance, no impermissibly high contact voltage must occur in the form of voltage drop in this resistance until the breaking current is reached. Therefore, it is necessary to realise a sufficiently small earthing resistance (see formula 9.1.)


where:

UB perm

maximum permissible contact voltage (e.g. 65 V)

Ia

breaking current

RS

earthing resistance

Fig. 9.5. shows an example of a fault-current circuit


Fig. 9.5. Fault circuit involved in the protective measure called protective earthing

IF - fault current
RS - earthing resistance
RB - operational earthing resistance

Depending on the fuse used, the breaking current must be selected in such a way that it is higher by the factor k than the rated current of the fuse (formula 9.2.)

Ia = k In

where:

Ia

breaking current

k

switching off factor

In

rated current of the fuse

In this way it is to be achieved that, in case of a fault, the circuit is interrupted within a adequately short time. High values of k ensure an increased safety due to a quicker response of the fuse but in many cases they cannot be realised by an economically justifiable expense. The minimum values of k are specified, in special regulations in dependence of the type of fuse connected.

For the return flow of the fault current, the water pipes may be used if permission is given. But this is rarely used today because no-metallic water pipes are increasingly employed.

9.2.6. Connection to the Neutral

A protective measure which can be realised, easily and at low expediture is the connection to the neutral; it provides a good protective effect. All conductive parts and units not belonging to the service circuit are connected with the protective conductor (PE) which is connected with the earthed neutral conductor (N). Protective conductor and neutral conductor may in this case be formed by a common conductor (PEN) (protective conductor carrying current in operation) or they may be installed separately (protective conductor not carrying current in operation).

In the case of a breakage of the PEN conductor at the supply side, a high contact voltage may occur at the protective conductor under unfavourable conditions (danger of Life!). Therefore, additional comprehensive regulations must be observed for the installation of the protective conductor carrying current in operations, especially in overhead local transmission lines. Further, a connection of the PEN conductor to the earth bus at the feeding point and sometimes also in the network spurs is required. An effective potential equalisation must be provided, in the customer installation.

If, for the connection of safety plugs, a two-core lead is used (protective conductor carrying current in operation), the lead (also known as supply line) must be connected to the protective contact first and then it must be brached off to the current carrying connection (see Fig. 9.6.). The fault-current circuit for body contact of L1 with the motor casing shows that the fault current flows through the fuse. For the breaking current the equation 9.2. also is applicable in this case.

Under certain conditions, monitoring of the voltage at PEN to reference earth may be required; in this case an all-pole cut-out is effected when a maximum permissible voltage is exceeded.


Fig. 9.6. Connection to the neutral (also known as multiple protective earthing)

1 - Protective contact socket
2 - Pipes and other conductive parts in buildings
IF = fault current
RB = operational earthing resistance

9.2.7. Fault-current Protection

In the fault-current protection system, a special switching device - the FI protective switch or the FI relay - is connected in series with the loads. All accessible conductive parts must be earthed Fig. 9.7. shows the fault-current protection device.

A summation transformer is arranged in the FI protective switch which monitors the current flowing into and out of the installation. In the faultless condition, the sum of these currents is always equal to 0 according to the first Kirchhoff’s law, even in the case of an unsymmetrical load.

The magnetic fields produced in the current transformer neutralise each other and the secondary of the transformer is not excited. If, in case of a fault, body contact occurs, the fault current does not flow via the summation transformer but via ground.


Fig. 9.7. Fault-current protection system with earthing the protective conductor connection

1 - Summation transformer in fault-current protective switch
2 - Fault-current protective switch
3 - Pipe lines as earthing resistance
RS = earthing resistance

Consequently, the sum of the inflowing and out flowing currents in the transformer is unequal to 0. The voltage generated in the secondary of the summation transformer causes the triggering of the switch and the all-pole switching off of the installation within a very short time (about 20 ms).

The FI protective switches differ by the height of the tripping current (rated, fault current Ifn) Ifn should be designed, in such a way that it is three times the leakage current to be expected. The earthing resistance must be so small that the rated fault current causes a maximum voltage drop of UB perm at the most (formula 9.3.)

RS = UB perm/Ifn

where:

RS

earthing resistance

UB perm

maximum permissible contact voltage

Ifn

rated fault current of the FI protective switch

For a protective switch with Ifn = 50 mA and UB perm = 65 V, an RS of 2.15 kW is obtained. This value can be reached without great difficulties. Also, for a protective switch with Ifn = 500 mA, an earthing resistance of 130 W is sufficiently small. When a tool and the like is with necessity connected with an earth lead (water pump, electrical thermal storage water heater), this method of earthing will suffice when the required earthing resistance is ensured.

In connection with the protective measure known as connection to neutral, the FI protective system can be used to advantage according to an IEC recommendation (Fig. 9.8.). The advantage over the connection to the neutral consists in the fact that - in case of a relatively low fault current which is considerably lower than the rated current of the fuse connected in series - a quick switching off of the faulty installation is effected. The problems of connection to the neutral associated with the realisation of the switching-off factor k (equation 9.2.) are avoided, the total switching-off time is shorter.

In order to protect man from the dangerous effects of an electrical shock, various protective measures can be taken in dependence of the concrete conditions given. Besides the protective measures without protective conductor (extra-low voltage, protective insulation), there are protective measures with protective conductor (protective isolation, protective conductor system, protective earthing, connection to the neutral). The measures of the second group differ with respect to protective effect and costs. It is possible to apply several protective measures at the same time (connection to the neutral with FI protective system). The selection of the suitable protective measure is dependent on the type of three-phase network given and the dangers that may occur in the handling of electrical tools and the like. Further, it must be decided whether or not several tools and the like may be switched off in cause of a fault current (e.g. in case of connection to neutral) or only the defective tool (e.g. in case of separate FI protective switch for each tool and the like). Further, it must be decided whether in case of simple body contact it should only be signalled and the work can be finished without endangerment (e.g. protective conductor system with monitoring of the insulation resistance).


Fig. 9.8. Connection to neutral combined with FI protective switch

1 - FI protective switch
2 - Protective contact socket
3 - Connection of PE to conductive parts in the building

Questions:

1. Explain the protective effect of the various protective measures!

2. What protective measure is suitable for dwelling

installation? Start from the consideration of the different three-phase current mains!

3. What are the advantages of the connection to neutral with FI protective switch over the connection to neutral?

4. What has to be observed when using the protective isolation?

5. Why have special connection to be used for the protective measure known as extra-low protective voltage?

6. Why should the earthing resistance not exceed a maximum value when the protective measure known as protective earthing with FI protective system is used?

7. Why should the breaking current value be higher than the rated current of the fuse connected in series?