Contract R405/4 – MEP WORKS OF AL ROUGH TUNNEL –
EAST BOUND TUBE
Earth Conductor for Fire pump Room
Project Code: 17-8-00001
Report
Earth Conductor Sizing For Fire Pump Room
31/05/2018
Calculation Summary
Contents
1 Introduction 1
1.1 Project Background 1
1.2 Objective 1
1.3 Reference Documents 1
2 Codes and Standards 1
3 Earth Conductor Calculation 1
3.1 Basis of Calculation 1
3.2 Key Assumptions 3
3.3 Earth Conductor Sizing 3
3.4 Result and comments 3
Appendices
1.IEEE: 80:2000 –Reference pages :42,43,44
2. ITB: 850.17 -Volume :II
Earth Conductor Sizing
1 Introduction
1.1 Project Background
The Al-Rough Tunnel is a twin tube dual two lane carriageway tunnel system with cross passages at 180m
Spacing. The civil works of the tunnel are largely completed with the handover to public.
The MEP fit out of the Tunnel is divided into two phases, the Staged Handover and the Full Fit Out. The
Staged. Handover targets to establish a road access to Shis Village only, whereas the Full Fit Out will
incorporate the Al-Rough Tunnel into the Diftah to Khor Fakkan Road with its four more tunnels.
1.2 Objective
The document covers the design philosophy and calculations of Earth Conductor of the Fire pump Room as
Per BS-7430 /IEEE-80:2008.
1.3 Reference Documents
1. Section 850.17 Specified in ITB Vol.II
2 Codes and Standards
The Earth Conductor system complies with all the currently applicable statutes, regulations and safety codes
in the locality where the equipment will be installed.
Standards issued by IEEE/British Standards have been considered for the Earth Conductor.
Code Description
IEEE Std 80-2000 Standard for safety in AC grounding system
Code of practice for protective Earthing of electrical installations
BS-7430
3 Earth Conductor Calculation
3.1 Basis of Calculation
The Earth Conductor is required to be sized for the Fire pump room.
Determining the minimum size of the Earthing conductors is necessary to ensure that the Earthing will be
able to withstand the maximum earth fault current. Like a normal power cable under fault, the Earthing grid
conductors experience an adiabatic short circuit temperature rise. However unlike a fault on a normal cable,
where the limiting temperature is that which would cause permanent damage to the cable's insulation, the
Earth Conductor Sizing
temperature limit for the Earthing conductors is the melting point of the conductor. In other words, during the
worst case earth fault, the Earthing grid conductors starts melting.
The minimum conductor size capable of withstanding the adiabatic temperature rise associated with an earth
fault is given by re-arranging IEEE STD 80 Equation 37:
Where A is the minimum cross-sectional area of the Earthing grid conductor. (mm2)
is the energy of the maximum earth fault (A2s)
is the maximum allowable (fusing) temperature (ºC)
is the ambient temperature (ºC)
is the thermal coefficient of resistivity (ºC - 1)
is the resistivity of the Earthing conductor (μΩ.cm)
Ko is (1/αr) – Tr in °C
is the thermal capacity of the conductor per unit volume. (Jcm - 3ºC - 1)
The material constants Tm, αr, ρr and TCAP for common conductor materials can be found in IEEE
STD 80 Table 1.
Table: Extract from IEEE Std 80-2000 – Table 01 Page number 42.
Earth Conductor Sizing
3.2 Key Assumptions
The key assumptions made for the calculation of Earth conductor of Fire Pump room
The type of Earth conductor considered is Copper (annealed soft-drawn) conductor.
The Earth conductor is sized based on short circuit current of 25KA, as per the Single Line Diagram
given. Hence, the same value of fault current is considered to size Earth conductor.
The duration of fault current is considered as 1 second.
The ambient temperature is 50°C.
3.3 Earth Conductor Sizing
The fault current assumed is 25KA for duration of 1 second.
According to the equation:
The following values are substituted in the above equation. (From IEEE 80:2000 Table.1)
i2t is the energy of the maximum earth fault (A2s) = 625
Tm is the maximum allowable (fusing) temperature (ºC) = 1083
Ta is the ambient temperature (ºC) = 50
TCAP is theThermal capacity [ J/cm3. ºC] = 3.42
-1
αr is the thermal coefficient of resistivity (ºC ) = 0.00393
ρr is the resistivity of the Earthing conductor (μΩ.cm) = 1.72
K0 is (1/ αr) – 20 °C = 234
Area of the conductor (mm2) = Sqrt. (625(0.00393x1.72x10 4)/3.42)/(ln[1+(1083-50/234+50)]))
=89.74 mm2
The Earth conductor size is obtained from above equation is 89.74 mm 2 copper (Annealed soft-
drawn) conductor.
3.4 Result and comments
Based on result the selected conductor value should be more than 90 mm2 and mentioned project
specifications (850.17.4) main earth conductor should be minimum 150 mm 2. So we are concluded
150 sq.mm cable can meet the all requiremnets.
