Overcurrent Protection (Note: All the mentioned tables in this course refer to, unless otherwise specified, Low Voltage Electrical Installation Handbook, by Johnny C.F. Wong, Edition 2004)

1
Overcurrent Protection(Note All the mentioned
tables in this course refer to, unless otherwise
specified, Low Voltage Electrical
Installation Handbook, by Johnny C.F. Wong,
Edition 2004)

• Chapter 6

2
General

• Purpose
• Safety of Personnel (Shock) and Property (Fire
  Hazards)
• Maintain reliable life of equipment and systems
• Overcurrent
• a current exceeding the rated value of a circuit
  or the current-carrying capacity of a conductor
• Overload
• Fault
• Short-circuit fault
• Earth fault
• This part, we are concerned with the
  short-circuit fault only.

3
Devices for Overcurrent Protection

• Examples are
• Fuses (HBC/HRC)
• Miniature circuit breakers (MCBs)
• Combined MCB and RCD (RCBOs)
• Moulded case circuit breakers (MCCBs)
• Air circuit breaker IDMTL relay

4
Devices for Overcurrent Protection

• Protection for the NEUTRAL conductor is NOT
  required for TT and TN systems
• 100 Neutral should be used
• Protection already provided by the live conductor
  protective device
• Neutral link (not protective device)
• If the neutral breaks, the live supply must break
  too
• LOSS OF NEUTRAL must be avoided to eliminate the
  risk of raising the potential of the load star
  point to dangerous level

5
Protection against Overload

• Main purpose is to avoid sustained temperature
  that causes deterioration of insulation
• e.g. only a short duration of overload current is
  allowed to flow in a motor circuit - the starting
  duration should be short. Otherwise larger
  cables shall be installed

6
Selection of Overload Protective Device

• design current Ib ? nominal current or rated
  current In ? lowest CCC, Iz

7
Position of Overload Protective Device

• At the point where there is a reduction of Iz
  (CCC) such as
• CSA of conductor is reduced
• Worsening of environmental condition
• Change of cable type or installation method
• Overload protective device and fault current
  protective device may be the same device and may
  be 2 different devices

8
Overload Protection of Conductors in Parallel

• The Iz in this case is the sum of Iz of the
  individual cables provided they are in accordance
  with the conditions for parallel running cables.
• Standard ring final circuits are not in this
  context.

9
Omission of Overload Protective Device

• Overload current is unlikely to flow
• Refer to Fig. 6.5 for illustration

10
Omission of Overload Protective Device

• Unexpected loss of supply is more dangerous than
  overloading of circuit
• Refer to Fig. 6.6 for illustration

11
Omission of Overload Protective Device

• CT secondary circuit should not be broken. If
  this is the case, dangerous high voltage will
  appear at the CT secondary side
• Refer to Fig. 6.7 for illustration

12
Omission of Overload Protective Device

• Protection is afforded by electricity suppliers
  protective device (not normally accepted by power
  companies in Hong Kong)
• Refer to Fig. 6.8 for illustration

13
Protection against Fault Current

• Cause - Insulation failure, faulted switching
  operation and invariably associated with arcs
• Effect - Thermal and mechanical stress produced
  in conductors, associated support and plant
  components
• Fault current protection is to prevent this

14
Protection for Maximum prospective fault current,
Isc

• Maximum prospective fault current, Isc
• 3-phase calculation based on symmetrical fault
  impedance,
• Isc Up / Z
• where Up phase voltage
• Z phase conductor impedance at
  supply source
• 1-phase calculation based on line-neutral
  impedance at 20oC,
• Isc Up / (Z Zn)
• where Zn neutral conductor impedance at
  supply source
• The above should base on fault appeared just
  after the protective device
• Breaking capacity of fault current protective
  devices should exceed the max. prospective fault
  current, Isc

15
Minimum Prospective Fault Current, I

• Minimum prospective fault current, I
• Calculation bases on total phase-neutral
  impedance values, up to the remote end
• I Up / (Z Zn Z1 Z2)
• where Z1 phase conductor impedance at
  consumer side
• Z2 neutral conductor impedance
  at consumer side
• Significant in determining fault disconnection
  time, t

16
Protection for Minimum Prospective Short Circuit,
I

• Basic equation to satisfy
• k2S2 gt I2t
• Where
• k - a constant associated with the type of
  conductor insulation
• S - Cross-sectional Area (CSA) of conductor
• I - minimum prospective fault current (fault
  occur at remote end)
• t - disconnection time
• I2t - let-through energy

17
Guidelines in fault current protection

• Max. prospective 3-ph symmetrical short-circuit
  at the l.v. source of supply provided by the
  supply company is 40kA.
• All fuses and MCCBs at source of energy must have
  breaking capacity gt 40kA
• Fault current protective devices with smaller
  breaking capacities are generally acceptable if
  they are backed up by fuses to BS88-2.1 or BS88-6
  (Backup protection will be discussed later in
  Chapter 10)
• The further away from the source of supply, the
  smaller the prospective short circuit current.

18
Fault Current Protection in General

• Example The following single phase circuit is
  protected by 63A BS88 fuse, the prospective short
  circuit current at the fuse is known to be 3 kA.
  A connected load, with circuit distance 87m from
  the fuse, is to be supplied by using 16mm2 1/C
  PVC copper cable. Please check whether the fuse
  can provide short circuit protection for the
  cable.

Source
Installation side
Source voltage Up
Z
Z1
63A fuse
Load
1.68 ? / km
Zn
Z2
19
Fault Current Protection in General

• At fuse position, it is given that the 1-?
  prospective short circuit current is 3 kA,
• i.e. Isc Up / (Z Zn)
• Z Zn 3000 / 220 0.073 ?
• The total impedance from the fuse to the
  remote load end,
• Z1 Z2 2 x 87m x 1.68 ?/km 0.292 ?
• So, the minimum short circuit current at the
  load end,
• I Up / (Z Zn Z1 Z2)
• 220 / (0.073 0.292)
• 603 A

20
Fault Current Protection in General

• Whether k2S2 gt I2t ??
• From I-t characteristic of BS88 fuse, t
  0.18 s when I 603 A
• PVC copper cable is used ? k 115
• S 16 mm2
• k2S2 1152 x 162 3,385,600 A2S
• I2t 6032 x 0.18 65,450 A2S
• k2S2 gt I2t ? O.K

21
Fault Current Protected by Overload Protective
Device

• The protective device is assumed to be adequate
  if it
• satisfies conditions for overload protective
  device. That is, we sizes cable and protective
  device by using the principle
• Ib In Iz and
• Breaking capacity of protective device Maximum
  prospective fault current, Isc
• This is the most common way to protect a circuit,
  since only ONE protective device is needed.

22
Position of fault current protective device

• Normally placed at or before the point where a
  reduction in the conductors current-carrying
  capacity (Iz) occurs. Such change may be due to
  a change in
• cross-sectional area, method or installation,
  type of cable or conductor, or in environmental
  conditions

23
Fault current protection of conductors in parallel

• A single device may provide protection against
  fault current for conductors in parallel provided
  the parallel conductors are in accordance with
  Section 5.8

24
Omission of short-circuit protective devices

• Conductor between a transformer and its control
  panel
• Refer to Fig. 6.18 for detailed illustration