3.2 Protective Measures which Prevent that Hazardous Contact Voltages Continue to Exist

Protective earthing

Mode of action of the protective earthing - When it comes to fault current, the fault-current circuit is closed by

· earth or
· via metallic conductors, such as water pipes or cable sheathings

Figure 9 - Protective earthing (return flow of the fault current through the ground) - 1 resistance of the service earthing, 2 devices, 3 protective conductor, 4 resistance of the protective earthing system, 5 fault current

Figure 10 - Protective earthing (return flow of the fault current through the water pipe system) - 1 water pipe system, 2 fault current

Criteria of protective earthing

- All metallic casings and metallic parts of electrical equipment must be connected with one another through a protective conductor.

- The protective conductor must be connected with the earth electrode.

- With the return flow of the fault current through the earth, the resistance must not exceed a certain value (Fig. 9./Legend 4).

This has to be calculated according to the following equation:

The symbols have the following meaning:

Rs	Resistance of the protective earthing
UB	Permissible contact voltage
k	Factor for the switching off
	k = 3.5 for fuses up to 50 A
	k = 5 for fuses over 50 A
In	Amperage of the uses of the protected device

With the return flow of the fault current over the water pipe system, the resistance must not exceed the value which is calculated by the following equation:

U_LE - stands for voltage between conductor and earth.

Where is the measure of protective earthing applied?
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Disadvantages of the protective earthing

- The earth transition resistance is subject to climatic influences.

- The required earthing resistances are achieved only with great difficulties.

- The unipolar switching off of devices consuming three-phase current may lead to damage to the electrical equipment.

Checking of the protective measure of protective earthing

- The required connections of the protective conductor are checked by visual inspection.

- By a continuity test the connections of the metallic pans between each other are checked.

- The earthing resistance has to be calculated and tested by measuring.

Connection to neutral

Mode of action

- Instead of being returned to the earthed mains point through the earth or the water pipe system, the fault current is returned through the "neutral conductor with protective function" (PEN) - the neutral conductor.

- A body contact that might occur as a result of a fault is turned into a unipolar short circuit by the connection between casing and neutral conductor.

Figure 11 - Protective measure of connection to neutral in a threephase four-wire system - 1 resistance of the service earthing, 2 pipe system, 3 motor, 4 protective conductor, 5 fault current, 6 socket with protective contact, 7 protective contact

Criteria of the connection to neutral

- All metallic casings of electrical equipment are connected to the neutral conductor through the protective conductor.

- The neutral conductor has to be earthed at the neutral point of the transformer, at the ends of the mains and evenly distributed in the mains.

- With cables with metallic sheathing, the latter has to be connected with the neutral conductor.

- With plastic cables, the neutral conductor has to be earthed at the casing connection.

- With the connection of the neutral conductor to electrical equipment, the protective function must be accomplished first. Only then it is connected to the operating contact.

- The protective measure is accomplished, if - with short circuit between external conductor and neutral conductor - a current flows that is 2.5 times the rated current of the fuse connected before.

The neutral conductor, at the electrical equipment, must be bridged from the protective contact to the operating contact.

Where is the connection to neutral used?
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Checking of the protective measure of "connection to neutral"

- It has to be inspected whether the protective conductor is connected according to the required conditions.

- The fault current with short circuit between external conductor and neutral conductor can be measured.

Fault-voltage protective system

Mode of action

- Similar to the protective earthing. Only that here the release, in the case of fault, is not actuated directly through the connection of the parts of the plant to be protected with the earth but all-polarly over a trip coil.

- The fault current flows through the voltage coil and excites it. The coil actuates the control element, and the part of the plant which is at risk is switched off in an all-polarly way.

Figure 12 - Fault-voltage protective system in a threephase system - 1 resistance of the service earthing, 2 resistance of the auxiliary earthing, 3 protective switch, 4 voltage coil, 5 fault current, 6 test resistance, 7 test key, 8 protective conductor, 9 auxiliary earth lead, 10 motor

Criteria of the fault-voltage protective system

- The protective conductor has to be connected with the casing of the equipment and with the voltage coil.

- The protective conductor has to be insulated against earth.

- The protective conductor has to be led separately from the auxiliary earth.

- The auxiliary earth has to be connected over an auxiliary earh lead.

- The resistance of the auxiliary earthing, with a highest admissible contact voltage of 24 V must not exceed 200 ohms and with 65 V or 100 V 800 ohms.

- The cross section of the protective conductor and of the auxiliary earth lead with fixed protected laying must not be greater than 1.5 mm² copper or 2.5 mm² aluminium and with fixed unprotected laying not greater than 4 mm² copper or 10 mm² aluminium.

- Pipe systems may be used as auxiliary earthing, if they provide the required resistance value.

Figure 13 - Testing of the fault-voltage protective system by probe I probe, 2 voltmeter for test voltage, 3 test resistance, 4 test current, 5 amperemeter for test current

Where is the fault-voltage protective system used?
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Checking of the protective measure of "fault-voltage protective system"

- Inspection whether the required conditions are fulfilled.

- Checking of the fault-voltage protective switch by actuating the test key.

- Measuring the resistance of the auxiliary earthing with the help of a probe.

Fault-current protective system

Mode of action

- If the operating current of a consumer is led through the summation transformer of a fault-current protective switch, the sum of the currents flowing in both the directions is normally zero.

- In the case of fault, the equilibrium of the currents is disturbed and the trip coil of the fault-current protective switch causes the all-polar switching off of the part of plant that is at risk.

Figure 14 - Fault-current protective system in a threephase system - 1 resistance of the service earthing, 2 protective switch, 3 test key, 4 current coil, 5 test resistance, 6 protective conductor, 7 devices, 8 resistance of the protective earthing system, 9 current transformer

Criteria of the fault-current protective system

- All metallic casings of the electrical devices have to be earthed by a protective conductor.

- The protective conductor has to be layed separately.

- The switching-off time must not exceed 0.2 seconds.

- The resistance of the earthing system of the fault-current protective system must not exceed a certain value.

This is calculated with the help of the below equation:

The letters read:

UBzul	highest admissible contact voltage
Ifn	rated fault current of the fault-current protective switch

Where is the fault-current protective system used?
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Checking of the protective measure of "fault-current protective system"

- Inspection as to the required conditions.

- Testing of the functioning of the fault-current protective switch by repeated actuation of the test key.

Isolating fault-current protective system

Mode of action

- Similar to that of the fault-current protective system.

- In the case of fault, the fault current flows through the current coil of the isolating fault-current protective switch, the coil is excited and causes the all-polar switching-off of the plant.

Figure 15 - Isolating fault-current protective system in a threephase system - 1 isolating fault-current protective switch, 2 isolating transformer, 3 capacity between conductor and ground, 4 ohmic resistance, 5 coil, 6 current coil, 7 test resistance, 8 test key, 9 protective conductor, 10 test socket, 11 resistance of the protective earthing system, 12 motor

Criteria of the isolating fault-current protective system

- The plant is operated through an isolating transformer.

- The neutral point of the secondary side of the transformer is earthed through a coil.

- This coil is switched as a voltage divider and limits the fault current to 7 mA.

- The switch switches the plant off all-polarly with a fault current of

6 mA with alternating current and
10 nA with direct current.

- All conductive pans of the plant which do not belong to the service circuit have to be connected to a protective earthing.

Service life conductors, with this protective system, must not be earthed.

- The protective conductor has to be earthed near the supply device. (With plants on ships, it has to be connected with the metallic body of the ship.)

On principle, the protective conductor has to be separated from leads before the isolating transformer!

- The resistance of the protective earthing must not exceed 500 ohms.

- The resistance of the insulation between a conductor and the protective conductor must not fall below 20 kiloohms.

- The capacity between conductor and earthing must not exceed 100 nanofarads.

Where is the isolating fault-current protective system used?
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Checking of the protective measure of "isolating fault-current protective system"

- Inspection as to the required criteria.
- Function test - the protective switch must switch the plant off.

Necessary measures:

- The isolating transformer has to be separated from the supply system at its primary side.
- A release current of 6 mA a.c. or 10 mA d.c. has to be applied to the protective conductor.