Download WEEK TWO LISBURN 2391

WEEK TWO and three 2391
IMPORTANT REMINDER
Your exam fees must be paid by Friday 15th April 2005 for the June exam. If this deadline is missed City
and Guilds levy an additional fee!
It may be prudent to remind ourselves that our inspection and testing procedures are required to confirm
compliance with BS7671 2001 which, as we have seen, will help demonstrate compliance the statutory
obligations of the duty holders of the installation concerned under the EAW Regulations 1989.
Part One of BS7671 2001, entitled Scope, Object and Fundamental Principles, requires that protection is
required for three distinct elements. State them.
The particular hazards fro the use of an electrical installation are also identified in Part One of BS7671
2001. State them.
We have begun by looking at the methods of protection against electric shock. These were identified in
the Week One workbook. It is, however, important to ensure that these other aspects of protection are
fully appraised in our inspection.
We continue now to look at one very important method of protection against indirect shock. EBADOS is
by far the most common method employed to provide such protection. It should, however, be emphasised
that this system will need support from the recognised methods of protection against direct contact in
order to have an effective shock protection regime.
5
Firstly, some clarification may be required in the terminology.
State the meaning of indirect contact as it refers to electric shock.
State three examples of exposed conductive parts in a typical installation
1
2
3
State three examples of extraneous conductive parts that are required to be connected to the main earth
terminal.
1
2
3
State the meaning of the acronym EBADOS.
Outline the basic principle of EBADOS.
6
In the space below, sketch the drawing on the board, which shows the fault voltages that might exist in
the event of a fault to earth in a typical circuit in a typical installation.
The importance of appropriate main equipotential bonding is emphasised in BS7671 2001 for both TN
and TT systems. The effect of such bonding then is to minimise fault voltages between exposed
conductive parts and extraneous conductive parts in the event of an earth fault. These fault voltages will
only be in existence for the time it takes to operate the protective device. The Regulations dictate the
required maximum disconnection times. Three common maximum disconnection times are identified for
TN and TT systems operating at 230volts to earth. The maximum time depends on whether the circuit
supplies fixed equipment, socket outlets or portable equipment and where that equipment is to be used.
Complete the table below.
CIRCUIT
Ring final circuit in a dwelling
DISCONNECTION TIME REQUIRED
Shower circuit in a dwelling
Lights in a dwelling
230v supply to a lighting circuit on a construction
site
230v sockets where livestock might be kept in a
farm.
Three phase 400v motor in a joiners workshop.
It is obvious that the disconnection time is dictated by the fault current which in turn is dictated by the
impedance of the circuit. The voltage responsible for the fault current is not the nominal voltage to earth,
7
Uo, at the terminals of the installation but the open circuit voltage to earth, Uoc, at the distribution
transformer. Uoc is taken to be 240v for a nominal supply voltage of 230v.
State this relationship in terms of the maximum tolerable earth fault loop impedance.
The operating time of a protective device can be found by reference to the Time Current Characteristics in
Appendix 3 of BS7671 2001. From these curves we can determine the maximum value of earth fault loop
impedance.
Lets look at the previous table this time with a stipulated protective device. Complete the table below.
DEVICE
In
CIRCUIT
DISCONNECTION CURRENT
TIME REQUIRED REQUIRED Ia
MAXIMUM
EARTH FAULT
LOOP
IMPEDANCE
Zs=Uoc/Ia
BS1361
30A
BSEN60898B
40A
BS1361
5A
BS88
10A
Ring final
circuit in a
dwellingcircuit
Shower
in a dwelling
Lights in a
dwelling
230v supply to
a lighting
circuit on a
construction
site
BS3036
230v sockets
20A
where livestock
might be kept
in a farm.
BSEN60898C Three phase
40A
400v motor in a
joiners
workshop.
Check your results against the maximum values of earth loop impedance found in Tables 41B1, 41B2 and
Table 41D in BS7671 2001. You should find a direct correlation (providing you have used 240v as Uoc).
The value of Zs is obviously important and there are a number of ways of determining it. The general
formula is;
Zs = Ze + (R1+R2)
8
We could find the value by measurement, calculation and to some extent by enquiry. The latter is used to
establish the maximum value of Ze from the DNO in situations where perhaps a design is being
considered and the supply is not yet available. Thus we can design or indeed test circuits using the R1 and
R2 values
9