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Week3 Flashcards

1
Q

What are two basic ways of receiving an electric shock by contact with conductive parts made live due to a fault:

A
  1. Via parts of the body and the general mass of earth (typically hands and feet)
  2. Via parts of the body and simultaneously accessible exposed and extraneous conductive parts (typically hand to hand)
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2
Q

How protective devices like circuit breakers work?

A

It provides low impedance paths for earth fault currents, that main protective bonding is carried out, then the magnitude and duration of earth faults will be reduced to such a level as not to cause danger

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3
Q

What is earth electrode ?

A

conductor/s in intimate contact with earth , and providing electrical connection with earth

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4
Q

Define earth

A

Huge conductive mass , its electric potential at any point is conventionally taken as zero

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5
Q

Electrically independent earth electrodes

A

Earth electrodes located at distance from each other that the maximum current likely to flow through one of them does not affect the potential of the others

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6
Q

What is earth electrode resistance?

A

The contact resistance of an earth electrode with the earth

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7
Q

Earthing conductor

A

A protective conductor connecting the main earthing terminal of an installation to an earth electrode or other means of earthing

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8
Q

Exposed conductive part

A

A conductive part of equipment which can be touched and which is not a live part, but can be a live part under fault conditions

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9
Q

Protective conductor

A

A conductor used for protection against electric shock and intended for connecting :
1- Exposed conductive parts
2- Extraneous conductive parts
3- The main earthing terminal
4- Earth electrode
5- Earthed point of the source / artificial neutral

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10
Q

Extraneous conductive parts

A

Conductive part liable to introduce a potential (generally earth potential) and not forming part of the electrical installation like:
1- non-insulated floors or walls, metal framework of buildings
2- metal conduits and pipework (not part of e..ins..) for gas, water, heating , compressed air

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11
Q

Bounding conductor

A

A protective conductor providing equipotential bounding

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12
Q

Main earthing terminal is:

A

The terminal or bar provided for the connection of protective conductors, including equipotential bounding conductors, and conductors for functional earthing. To the means of earthing

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13
Q

What is the aim of the equipotential bounding?

A

To ensure that even if incoming extraneous conductors have been raised to some potential due to a fault external to the building, no difference of potential can occur between extraneous conductive conductors within the installation .

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14
Q

Where should the bonding affected ?
What about connections to earth of metallic sheaths of communications cables ?

A

As close as possible to:
1- the points of entry into the building
2- connected to the main earthing terminal

They require the authorization of the owners of the cables

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15
Q

Connection of exposed conductive parts to the earth electrode

A

The connection is made by protective conductors with the object of providing a low resistance path for the fault currents flowing to earth

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16
Q

What is Protective earthing system?
What the letter refer to?

A

TT system directly grounds the metal housing of an electric device.

The first T indicates that neutral point of the power system is directly grounded
The second T indicates that the conductive part of the load device that is not exposed to the live body is directly grounded.

Customer has to create their own earth

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17
Q

What TN system is

A

The source is earthed like TT system
In the installation, all exposed and extraneous conductive parts are connected to the neutral conductor

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18
Q

What is TN-C system?

A

Neutral is also used as a protective conductor and referred to as PEN (protective Earth and Neutral)
Combined with PE

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19
Q

What is the proper size for TN-C system?

A

Not permitted for conductors of less than 10 mm^2 or for potable equipment.

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20
Q

Which system requires an effective equipotential environment and why?

A

TN-C

It requires an effective equipotential environment within the installation with dispersed earth electrodes spaced as regularly as possible since the PEN conductor is both the neutral conductor and at the same time carries phase unbalanced currents as well as 3rd harmonic currents. The PEN conductor must therefore be connected to a number of earth electrodes in the installation .

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21
Q

What is TN-S system?

A

5 wires , the protective conductor and neutral conductor are separated . On underground cable systems where lead sheathed cables exist, the protective conductor is generally the lead sheath.

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22
Q

What is the size of TN-S systems?

A

The use of separate PE and N conductors is obligatory for circuits with cross sectional area less than 10 mm^2 for portable equipment.

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23
Q

TN-C-S

A

The TN-C must never be used downstream of the TN-S , since any accidental interruption in the neutral on the upstream part would lead to an interruption in the protective conductor in the downstream part and therefore a danger

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24
Q

What is IT system

A

Isolated neutral (transformer is not connected to the ground) or impedance-earthed neutral

No intentional connection is made between the neutral point of the supply source and earth. Exposed and extraneous conductive parts of the installation are connected to an earth electrode

25
What does the first letter represent?
The relationship between the power system and the ground T= neutral point is directly grounded I= power supply is isolated from the ground , or one point of the supply is connected to the ground via a high impedance like 1000ohm .
26
What does the second letter represents?
The electrically conductive device exposed to the ground T= the device shell is grounded, doesn’t have direct relation with any other grounding point in the system. N = the load is protected by zero
27
The third letter indicates to?
The combination of working zero and protective line. C= the working neutral line and the protection line are one S= the working neutral line and the protection line are strictly separate, so the PE line is called a dedicated protection line
28
Characteristics of TT (5)
1- simplest to design and install , used in installations supplied directly by the public LV distribution network 2- does not require continuous monitoring during operation (periodic check on RCD is necessary) 3- protection is ensured by special devices, RCD prevents risk of fire when set to <= 500 mA 4- each insulation fault results in an interruption in the supply of power, but outage is limited to the faulty circuit by installing the RCD in series (selective RCD) or parallel (circuit selection) 5- Leads or parts of the installation which during normal operation cause high leakage currents require special measures to avoid nuisance tripping. Supply the loads with a separation transformer or use specific RCD
29
Characteristics of TN (5)
Requires the installation of earth electrodes at regular intervals throughout the installation. • Requires that the initial check on effective tripping for the first insulation fault be carried out by calculations during the design stage, followed by mandatory measurements to confirm tripping during commissioning. • Requires that any modification or extension be designed and carried out by a qualified electrician. • May result, in the case of insulation faults, in greater damage to the windings of rotating machines. • May, on premises with a risk of fire, represent a greater danger due to the higher fault currents.
30
Characteristics of TN-C
1- at first glance will appear to be less expensive (elimination of a device pole and of a conductor) 2- requires the use of fixed and rigid conductors is forbidden in certain cases : • premises with a risk of fire • for computer equipment (presence of harmonic currents in the neutral)
31
Characteristics of TN-S
1- may be used even with flexible conductors and small conduits 2- due to the separation of the neutral and the protection conductor , provides a clean PE ( computer systems and premises with special risks)
32
Characteristics of IT system (6)
• Solution offering the best continuity of service during operation • Indication of the first insulation fault, followed by mandatory location and clearing, ensures systematic prevention of supply outages • Generally used in installations supplied by a private MV/LV or LV/LV transformer • Requires maintenance personnel for monitoring and operation • Requires a high level of insulation in the network (implies breaking up the network if it is very large and the use of circuit-separation transformers to supply loads with high leakage currents) • The check on effective tripping for two simultaneous faults must be carried out by calculations during the design stage, followed by mandatory measurements during commissioning on each group of interconnected exposed conductive parts
33
How to determine the best systems?
All systems equivalent in safety so the selection don’t depend on safety. The selection is by combining all requirements in terms of regulations , continuity of service, operating conditions and the types of network and loads
34
The quality of an earth electrode depends on? And the resistance is?
Resistance as low as possible Depends on: 1- Installation method 2- Type of soil
35
What are the three common of installation methods?
1- Buried ring 2- earthing rods 3- vertical plates
36
Explain butied ring method
For existing buildings , the electrode conductor should be buried around the the outside wall of the premises to a depth of at least 1 meter. All vertical connections (at least 4) from an electrode to above ground level should be insulated for the nominal LV voltage (600-1000 v)
37
Buried ring conductors material
Copper: bare cable (>= 25 mm^2) or multi strip (>= 25 mm^2 and >= 2 mm thick) Aluminum with lead jacket : cable (>= 35 mm^2) Galvanized-steel cable : Bare cable (>= 95 mm^2) or multiple strip (>= 100 mm^2 and >= 3 mm thick)
38
Material of rods in earthing rods method
• Copper or (more commonly) copper-clad steel. The latter are generally 1 or 2 meters long and provided with screwed ends and sockets in order to reach considerable depths, if necessary (for instance, the water-table level in areas of high soil resistivity) • Galvanized steel pipe >= 25 mm diameter or rod >= 15 mm diameter, >= 2 meters long in each case.
39
The total resistance in earthing rod
The total resistance (in homogeneous soil) is equal to the resistance of one rod divided by the number of rods in question .
40
Explain vertical plates method
Rectangular plates each side of which must be >=0.5 meter, are commonly used as earth electrodes, being buried in a vertical plane such that the center of the plate is at least 1 meter below the surface of the soil.
41
The material of vertical plates
The plates may be • Copper of 2 mm thickness • Galvanized steel of 3 mm thickness
42
How is the resistance of the electrode/earth interface affected ?
It rarely remains constant , and affected by: 1- Humidity of the soil 2- Frozen earth 3- aging 4- Oxidation
43
What must be done to be able to test the earth electrode and why?
there must always be one removable links to isolate an earth electrode , this links allow the earth electrode to be isolated from the installation for the periodic tests. The purpose of the tests is to establish that resistance of the soil surrounding an earth electrode is suitable and that the electrode makes contact with the soil
44
What are the methods used to measure the resistance of earth electrode?
1- Ammeter method 2- use of a direct-reading earthing resistance ohmmeter
45
Distribution switchboard Definition التركيبة والمكونات
The point at which incoming power supply divides into separate circuits, each is controlled and protected by the fuses or switchgear of the switchboard. It’s divided into several functional units , each comprising all the electrical and mechanical elements that contribute to the fulfillment of a given function
46
The distribution switchboard enclosure provides double protection
• protection of switchgear, indicating instruments , relays , fuse-gear. Against mechanical impact vibration and other external influence likely to interfere with operational integrity • the protection of human life against the possibility of direct and indirect electric shock
47
How are the distribution switchboards may be different? And what are the types? Note about two types
May differ according to the kind of application and the design principle adopted Principal types : 1- the main LV switchboard - MLVS 2- Motor control centers - MCC 3- sub-distribution switchboard 4- final distribution switchboard The final distribution switchboard and sub distribution switchboard are generally distributed throughout the site.
48
Explain functional distribution board
Dedicated to specific applications, made of functional modules that includes switchgear devices together with standardised accessories for mounting and connection, ensuring a high level of reliability and a great capacity for last minute and future changes.
49
Fixed functional units disadvantage How to minimize it?
Fixed functional units cannot be isolated from the supply so that any intervention for maintenance and modification requires the shutdown of the entire distribution switchboard. To minimize the shutdown times and improve the availability of the rest of the installation, a plug-in or withdraw-able devices can be used.
50
Disconnectable functional units
Each functional unit is mounted on a removable mounting plate and provided with a means of isolation on the upstream side (busbars) and disconnecting facilities on the downstream (outgoing circuit) side. The complete unit can therefore be removed for servicing, without requiring a general shutdown.
51
Drawer-type withdrawable functional units
The switchgear and associated accessories for a complete function are mounted on a drawer-type horizontally withdrawable chassis. The function is generally complex and often concerns motor control.
52
53
What is a conductor
It comprises a single metallic core with or without an insulating envelope
54
What is a cable
Several conductors, electrically separated but mechanically joint. Enclosed by a protective flexible sheath.
55
Cableway Definition Examples (4)
Conductors and/or cables together with the means of support and protection 1- cable trays 2- ladders 3- ducts 4- trenches
56
Conductors in a cable are identified either by their color or number, but there are three rules
Rule 1: Double green and yellow are strictly reserved for PE and PEN protective conductors Rule 2: •Circuit with neutral conductor & cable have > 5 conductors: must be light blue or marked 1. •circuit without neutral conductor & cable > 1 conductor: light blue can be used for phase conductor Rule 3: Phase conductors my be any color except: • green and yellow • green • yellow • light blue
57
Direct reading earthing-resistance ohmmeter : Distance? (4) Which connected to which? (3) Final results?
•distance: 1- Distance between earth electrode and test spice 1 should be *10 times the length of the tested electrode 2- T1 30-50 m from electrode 3- T2 15-25 m from electrode (half T1) 4- for the readings , T2 is moved 10% of T1’s distance •connections: 1- electrode connected to c1,p1 from ohmmeter via long lead 2- T2 connected to p2 3- T1 connected to C2 •results: Ra= average of 3 readings in ohm Tolerance +_ 5% Min= average -5% Max= average +5%
58
Installation system consists of (2)
1- distribution switchboard 2- cables and busways
59
Functional distribution boards
The switchgear and associated for a complete function are mounted on a drawer type horizontally withdraw able chassis. Its complex and often concerns motor control

Low Voltage (6 decks)

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