La teoría de la Relevancia y el modelo ostensivo-inferencial

Where,

Vd

I

= permissible voltage drop (mV/A/m)

n

VD = volt drop (V)

= the device current of the circuit (A) L = length of cable (m)

It is pertinent to point out that the cable rating must always be higher than (or equal to) that of the fuse or circuit breaker that is supposed to protect that part of the installation.

7.6 Type of cables commonly used

It used to be a standard practice in CKE to use PVC/PVC cables for non conduit wiring (surface and concealed). However these two (2) types of wiring are not used anymore in CKE. All wiring must now be in conduit using PVC insulated cables.

MSB SSB

2% 1.5% DB CL 0.5% Final Point

4%

Underground cables that are widely used:

a) PILCDSTAS Cable (Paper Insulated, Lead Covered, Double Steel Tape Armoured and Served Cable).

This is use conjunction with cable-box termination and is available only in the 3 phase 4 core types, normally used when the 3 phase line current exceeds 40 A.

b) PVC/SWA/PVC Cable (Polyvinyl Chloride / Steel Wire Armoured / Polyvinyl Chloride Cable).

This cable is used with cable gland terminations and is available both in the 3 phase 4 core as well as in the single phase 2 core. It is generally used when the current demand is less than 60 A.

c) XLPE / SWA / PVC Cable. (Crosslinked Polyethylene / Steel Wire Armoured / Polyvinyl Chloride Cable)

Normally used for 35 mm² and above.

For more information on other types of cables refer to manufacturers. Unless otherwise required all cables used for JKR projects shall be copper.

7.7 Practical Design Example

Figure 7.3

Allowable voltage drop:

2.5% = 10.375 V 3 phase 1.5% = 6.225 V

Iz > In > Ib

L1

L2

IMD + 30% = 1122.08A Rating MCCB = 1600A TPN TCL = 127.76kW (209.0A)

MD = 83.76kW (137.09A) Rating MCCB = 250A TPN

TCL = 15.49kW (25.35A) MD = 12.39kW (22.28A) Rating MCCB = 40A TPN

2.5%

1.5%

MSB NE DB GAL

SSB G1

SSB G2

SSB G3

MD = 527.36 kW IMD = 863.14A

i) Calculate cable size used and voltage drop from SSB G to DB GAL

a) Consider copper conductor used e.g. single core PVC insulated cable and installation method is enclosed in conduit on a wall where L2 = 60 metre.

I_z > I_n > I_b

Refer to table 4D1A from IEE Wiring Regulation Sixteenth Edition, Iz = 50 A Iz

50 > 40 > 22.28 > 40 > 22.28

From value of Iz we get cable size of 10 mm².

From table 4D1B voltage drop (per ampere per metre), V_d = 4.4 mV/A/m Voltage drop (VD) = Vd x L2 x In

= 4.4 x 60 x 40 / 1000 / 1000

= 10.56V

Therefore cable to be used is 4 x 10 mm² PVC in conduit.

b) Say L2 is longer i.e. L2 = 80 metre

Voltage drop (V_D) = V_d x L₂ x I_n

= 4.4 x 80 x 40 / 1000 / 1000

= 7.84 V (exceeding allowable voltage drop of 6.225 V) Thus, 4 x 10 mm² PVC in conduit cannot be used.

Check again the Iz value.

Refer to table 4D1A from IEE Wiring Regulation Sixteenth Edition, Iz = 68 A I_z

68 > 40 > 22.28 > 40 > 22.28

From table 4D1B voltage drop (per ampere per metre), Vd = 2.8 mV/A/m

ii) Calculate cable size used and voltage drop from MSB NE to SSB G

a) Consider copper conductor XLPE/SWA/PVC cable, installed underground where L1 = 110 metre

I_z > I_n > I_b

Refer to table 4E4A from IEE Wiring Regulation Sixteenth Edition, Iz = 251 A I_z

251 > 250 > 137.09 > 250 > 137.09

From value of I_z we get cable size of 70 mm².

From table 4E4B voltage drop (per ampere per metre), Vd = 0.60 mV/A/m Voltage drop (V_D) = V_d x L₁ x I_n

= 0.60 x 110 x 137.09 / 1000 / 1000

= 9.05 V

Therefore cable to be used is 4 core 70 mm² XLPE/SWA/PVC laid in the ground.

b) Say L1 is longer i.e. L1 = 155 metre

Voltage drop (V_D) = V_d x L₁ x I_n

= 0.60 x 155 x 137.09 / 1000 / 1000

= 12.75 V (exceeding allowable voltage drop of 10.375 V)

Thus, 4 core 70 mm² XLPE/SWA/PVC cannot be used.

Check again the Iz value.

Refer to table 4E4A from IEE Wiring Regulation Sixteenth Edition, Iz = 304 A I_z

304 > 250 > 137.09 > 250 > 137.09

From value of Iz we get cable size of 95 mm².

From table 4E4B voltage drop (per ampere per metre), Vd = 0.45 mV/A/m Voltage drop (V_D) = V_d x L₁ x I_n

= 0.45 x 155 x 137.09 / 1000 / 1000

= 9.56V

Therefore cable to be used is 4 core 95mm² XLPE/SWA/PVC laid in the ground.

c) Say L₁ is longer i.e. L₁ = 250 metre

Voltage drop (VD) = Vd x L1 x In

= 0.60 x 250 x 137.09 / 1000 / 1000

= 20.56 V (exceeding allowable voltage drop of 10.375 V)

Thus, 4 core 70 mm² XLPE/SWA/PVC cannot be used.

Check again the Iz value.

Refer to table 4E4A from IEE Wiring Regulation Sixteenth Edition.

Refer next value of Iz and calculate the volt drop until the acceptable volt drop is achieved. In this case the Iz that result in an acceptable volt drop is Iz = 406 A

Iz

406 > 250 > 137.09 > 250 > 137.09

Therefore cable to be used is 4 core 150 mm² XLPE/SWA/PVC laid in the ground.

8.1 Introduction

The effects of a low power factor in an installation are well known. These include:- i) A penalty charge in the electricity bill

ii) Extra losses in the feeder cable

iii) A significant voltage drop in the cables

iv) A reduction of the effective capacity of the cables v) A reduction in the power available at the transformer

vi) A significant voltage drop at the secondary of the transformer vii) Significant losses in the transformer

There are basically two types of equipment for improving the power factor of an installation:-

a) By rotary phase advancers, synchronous condensers or synchronous motors b) Static capacitors.

For normal installations, the capital cost of rotating machinery, both synchronous and phase advancing, makes its use uneconomical and, in addition, the wear and tear inherent in all rotary machine involves additional expenses for upkeep and maintenance.

Capacitors, on the other hand, have a very low initial cost, have minimal upkeep costs and can be used with high efficiencies on all sizes of installations. They are compact, reliable and convenient to install and thus is the more satisfactory equipment for power factor improvement.

The sitting of the capacitors in an installation depends on whether each piece of equipment, example a motor, is being individually corrected or the plant/installation as a whole is being corrected as a block (bulk or central correction).

As a rule of thumb, switchboards exceeding 200 A shall be installed with power factor correction board.