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1. Explain the difference between a Category A and a Category B MCCB (Moulded-Case Circuit Breaker BS EN 60947-2)
(b) Why do we impose an upper limit on the earth fault loop impedance (ZS), for each circuit and what can we do in situations where we cannot meet this?
(c) How can we check discrimination between the various protective devices?

(d) When protecting the live conductors of a cable against the effects of overload and short-circuit, how do we take ambient temperature and the grouping of circuits into account?
(e) How do we ensure that the circuit protective conductor is adequately sized so as to be protected against the thermal effects of fault current?

For this part of the assignment you can answer either of the following. You do not need to do both…

Either:

Draw a flow diagram to show how you would select a cable size that satisfies the BS 7671 protection requirements for Overload, Short Circuit, Indirect Contact and Voltage Drop. Your flow diagram should note all the necessary calculation steps and decision points for each of the above four parameters (i.e. Overload, Short Circuit,
Indirect Contact and Voltage Drop) and the diagram can be split across several pages.
Or:
Provide a time-based discrimination analysis on a suitable log-log scale graph to show how discrimination is achieved for a theoretical three-phase short circuit from the
incoming 33kV ring-main network supply through to the largest of the MCCBs on the MCCB panel. Details of the network relays and their settings are appended.

3. (a) Draw a single-line schematic diagram for the electrical distribution.

(b) Determine the approximate voltage drop along the length of Busbar 2 (East) You can assume that ground floor distribution boards are connected 2 metres from the bottom of the bus bar and that all other boards are connected at 4 metre intervals across the remained of the busbar length.
Busbar Details:
Current Rating: 250A per phase
Impedance (at warm operating conditions): R1 = 0.38mΩ/m; X1 = 0.13mΩ/m
(c) Hence determine the core size of XLPE/SWA 4-core cable from the MCCB panel to the busbar necessary to keep the voltage drop at the top of Busbar 2 (East) to 3%. State any assumptions made.

4. (a) If a standby generator were installed to support the main switchboard, how might your design be affected?
(b) If the load currents included a high harmonic component, how would your design be affected?
 

MCCB Categories and Short-circuit Protection

1. Categories of A and B of MCCB ( Molded Case Circuit Breaker)

Without an international short time delay for choosiness for conditions of short circuit and hence without a short circuit withstanding rating of current. For this category, they are actually miniature circuit breaker which operates at the end of the socket outlets and the final distribution. If a short circuit occurs, they will trip immediately (Bird, 2012).  

This category of MCCB has withstood breaking ability. Hence they do not essentially trip in case of any short circuit.  This will make the downstream circuit breakers to switch off.

We always impose an upper limit on the earth fault loop impedance to ensure that all the faults are covered within that range (Hambley, 2017).  With the upper limit most electrical appliances if not all are fully protected by the earth loop impedance protection. There is a different circuit with different loads and rating (socket outlets and lighting circuits’ protections are different). In cases where we cannot attain the upper limit, we employ the correct cable sizes calculations to ensure that a suitable and a perfect cable size is employed. 

Discrimination is an act of choosing a protective device and adjusting their settings in order to limit interruption to electrical installations under fault conditions (Hesse, 2011).  This can be checked through evaluation of current and time discrimination.

Time discrimination: This is checked as the circuit breaker are put to withstand electrodynamics effects and thermal effects of the current fault at the time of the delay (delay period).

Current discrimination:  This has two kinds of discrimination (overload discrimination and short circuit discrimination).  The overload is checked through a selection of an upstream device having a higher current rating and a pickup level for the next downstream device.  While the short circuit is checked through setting a circuit breaker to a limit which doesn’t cause unnecessary trip (Ojwach, 2012).

Ambient temperature has a direct impact on the conductor sizing. The temperature to be considered is that of the air around the cables used during the installation. Depending on the ambient temperature set, the size of insulation will be chosen for a particular ambient temperature. For our case, the ambient temperature is 400and 300 for the rest of the rest of the building. The PVC insulation which will be required is   0.77 mm all-round the cable.

Grouping of circuits together will lead to a reduction current carry capacity and this is suitable for correction factors hence it will ensure that there is no overheating in the cables (PRASAD, 2014). 

Sizing Cables for Overload and Short-circuit Protection

Overheating occurs in a conductor when the current through the passing through the conductor is too high for the size of the cable, therefore, the correct size of the cable is obtained through calculation. Choosing the accurate cable during installation is vital to ensure an acceptable conductors’ life and insulation subjected to the thermal effects of carrying current for long periods of time in ordinary service (Smith, 2013).

And the calculation of the cable size is done using the below equation. 

S = 

(Ia²t)

………………………………………………………………………………1

k

where

S  is the minimum protective conductor cross-sectional area (mm2)

Ia  is the fault current (A)

t   is the opening time of the protective device (s)

k is a factor depending on the conductor material and insulation, and the initial and maximum insulation temperatures (Tharaja, 2013).

2. Option A

Draw a flow diagram to show how you would select a cable size that satisfies the BS 7671 protection requirements for Overload, Short Circuit, Indirect Contact and Voltage Drop 

For the selection of the conductor size in order to avoid overheating the following are taken into account;

The type of the insulation which will define the maximum permissible temperature during the operation, and according to CIBSE the insulation of EPR can withstand 900 C while the PVC can withstand 700 C (Tonardo, 2013).  Therefore from the formula of getting the overload thermal as

V=I2R . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  . . . . . . . . . . . . . . . . 2

So when a given voltage is pushing a current then the resistance can be obtained, lets assume that the resistance is 0.2 ?

R= . . . . . . .  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

And taking the conductor to be copper ? = 1.72 × 10 -8 

The length can be taken as 100 m

0.2=  

A= 1176470588 m2

A=

r= 1.9 mm 

Cable size according to the voltage drop.

 The cable size can be selected from the voltage drop, and this is done through some calculations (John, 2013).

Assuming that

Current rating= 250 A/ ampere

Calculating Voltage Drop and Cable Size Selection

R1= 0.5 m ?/ft

X1= 0. 4m ?/ft

The total length =72ft

Power factor = 0.9  

The voltage drop

V= I (R1cos ? + X1sin ?)

Where V is the voltage drop in volts, I is the current in ampere/ phase, R1 is the conductive resistance in ohm/ft, X1 is conductor inductive in ohm/ft, ? is the angle obtained from power factor and L is one way length of the circuit source to load in kft (Couling, 2015).  

Power factor = cos ?

?= cos-1 0.9

? = 25.840

L=

L= 0.072 kft

V= I (R1cos ? + X1sin ?)

V= 1.73× 250 [ (0.5 × cos 25.84) + ( 0.4 × sin 25.84 )] × 0.072

V= 433× 0.6243× 0.072

V = 19.46 volts

Voltage drop 6 %

The size of the cable is obtained by the following equation

r=   

r=

r=

r= 1.92 mm

D= 3.89 mm

3. The ground floor distribution is connected 2 meters while the other five floors have 4 meters each.

Current rating= 250 A/ ampere

R1= 0.38 m ?/m

X1= 0.13m ?/m

The total length is given by 2+ (4×5) = 22 m

The voltage drop along the length of Busbar 2 (East) can be given by the following equations

V= I (R1cos ? + X1sin ?) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  2

Where V is the voltage drop in volts, I is the current in ampere/ phase, R1 is the conductive resistance in ohm/ft, X1 is conductor inductive in ohm/ft, ? is the angle obtained from power factor and L is one way length of the circuit source to load in kft (Katz, 2010).  

Power factor = cos ?

?= cos-1 0.9

? = 25.840.

 And from 1 ft= 0.3048m

R1= 0.38 ×0.3048

R1= 0.115824?/ft 

X1= 0.13 ×0.3048

X1= 0.0396 ?/ft

L= 22 m

L= 22/0.3048

L=

L= 0.072 kft

V= I (R1cos ? + X1sin ?)

V= 1.73× 250 [ (o.1158 × cos 25.84) + ( 0.0396 × sin 25.84 )] × 0.072

V= 433× 0.12149× 0.072

V = 3.7875 volts 

Voltage drop at the top 2 east is given as 3 %

The size of the cable is obtained by the following equation

Reducing Harmonics in Electrical Installations

r=  

r=

r=

r= 0.9622 mm

D= 1. 92 mm

Assumption made

For this calculation, their current is assumed to be flowing uniformly in the cross-section of the conductor (no skin effect). The resistivity of the conductor is also taken to be equal through the conductor (Baessler, 2010).

4. The standby generator is installed beside the building and the power from the generator is then connected to the switch room where the main switch box. These generators are automatic and will sense when there is power blackout and pick up immediately to supply the power. The electrical characteristics of component parts of assemblies shall apply (Keisler, 2012).   When the components are mounted in their enclosures, appropriate derating factors having been allowed for the effect of the enclosures, other components, and interconnections. The cables sizes would apply for supply as 6mm2.  The standby generator would be able to supply the five floors (Philip, 2015). 

Harmonics are currents or voltages with frequencies that are integer multiples of the fundamental power frequency. If the fundamental power frequency is 60 Hz, then the 2nd harmonic is 120 Hz, the 3rd is 180 Hz. This harmonic will hence result in overheating of equipment and conductors (Mitchell, 2013). Therefore it must be reduced in the design, the following designs would be employed to do away with the harmonics: 

A suitable design to reduce harmonics would support a load of many PC like a call center, would specify the neutral wiring to be more than the phase wire capacity by a factor of 1.73. This would ensure that the harmonic is reduced hence there will be no overheating (Wilingstone, 2011). 

  1. Use DC generators power supply which is not affected by harmonics

The internal power supplies are not energy efficient, and they generate substantial heat, which puts a costly burden on the room’s air conditioning system.  Heat dissipation also limits the number of servers that can be housed in a data center. It could be worthwhile to eliminate this step by switching to DC power

  • Use a harmonic-mitigating transformer

A harmonic-mitigating transformer (HMT) is designed to handle the non-linear loads of today's electrical infrastructures. This transformer uses electromagnetic mitigation to deal specifically with the triplen (3rd, 9th, 15th,...) harmonics. 


  1. Use K-rated transformers in power distribution components.

A standard transformer is not designed for high harmonic currents produced by non-linear loads. It will overheat and fail prematurely when connected to these loads.  When harmonics started being introduced into electrical systems at levels that showed detrimental effects (circa 1980), the industry responded by developing the K-rated transformer.

  1. Use separate neutral conductors. .

On three-phase branch circuits, instead of installing a multi-wire branch circuit sharing a neutral conductor, run separate neutral conductors for each phase conductor. This increases the capacity and ability of the branch circuits to handle harmonic load. 

Baessler, K., 2010. Pelvic Floor Re-education: Principles and Practice. 2nd ed. Oxford: Springer Science & Business Media.

Bird, J., 2012. Electrical principles. 4th ed. London : Springer .

Couling, S., 2015. Electrical power installation. 3rd ed. New York: Elsevier.

Hambley, A., 2017. Electrical Engineering: Principles and Application. 2nd ed. Hull: Pearson Education.

Hesse, H., 2011. Principles of Power Engineering Analysis. 1st ed. Stoke: CRC Press.

John, H., 2013. Understanding Engineering installation of electricity. 4th ed. London: Routledge.

Katz, J., 2010. Introductory electrical power installation. 2nd ed. Cambridge: Cambridge University Press.

Keisler, H. J., 2012. Installing electrical power. 5th ed. Kansas: Courier Corporation.

Mitchell, J. W., 2013. Fundamentals of electrical power. 4th ed. London: John Wiley & Sons.

Ojwach, C., 2012. Electrical circuit analysis. 1st ed. Hull: CRC.

Philip, P., 2015. Eletrical power technology. 4th ed. London: Wiley.

PRASAD, R., 2014. FUNDAMENTALS OF ELECTRICAL ENGINEERING. 2nd ed. Hull: PHI Learning.

Smith, M., 2013. Electrical Engineering Principles for Technicians: The Commonwealth and International Library: Electrical Engineering Division. 2nd ed. Hull: Elsevier Science.

Tharaja, T., 2013. Textbook for electrical technology. 3rd ed. Hawaii: Springer .

Tonardo, B., 2013. Electrical circuits. 2nd ed. Stoke : CRC.

Wilingstone, R., 2011. Introduction to electrical principles. 7th ed. New York: Wiley.

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