Fuse theory operation and types and operating time

1. Fuse
▪ Introduction
▪ Characteristics
▪ Types of fuses
▪ Application of HRC fuses

2. ➢ A fuse is a small piece of thin metal wire (or strip) connected in between two terminals mounted on
an insulated base and forms the weakest link in series with the circuit.
➢ The fuse is the cheapest, simplest and oldest protective device and is used as current interrupting
device under overload and/or short circuit conditions.
➢ It carries the working current safely without overheating under normal conditions.
➢ During overloads or short circuits it melts due to sufficient i2R heating when the current exceeds a
certain predetermined value in abnormal conditions.
➢ A fuse, being a thermal device, possesses inverse time-current characteristic, i.e. the operating time
decreases as the fault current increases.
➢ Fuses are used to protect cables, electrical equipment and semiconductor devices against damage
from excessive currents due to overloads and/ or short circuits in low-voltage and medium voltage
circuits.

3. ▪ Fuse Element (or Fuse Wire)
It is that part of the fuse which melts when excess current is flowing in the circuit
• Rated Current : The rated current of a fuse is the current it can carry indefinitely without fusing.
• Minimum Fusing Current: It is the minimum current (rms value) at which the fuse clement will melt.
The minimum fusing current for a round fuse wire

4. Fusing Factor is always more than unity.
➢ Prospective Current : The prospective current is the current which would have flown in the circuit in
the absence of the fuse
➢ Cut-off Current : The current at which the fuse element melts is called the cut-off current

5. Pre-arcing Time or Melting Time :
The time taken from the instant of the commencement
of the current which causes cut-off to the instant of cut-
off and arc initiation.
❖ oa is the prearcing time (tpa).
Arcing Time:
The time taken from the instant of cut-off (arc
initiation) to the instant of arc being extinguished or
the current finally becoming zero.
❖ ab is the arcing time (ta).
Total Operating Time:
It is the sum of the pre-arcing time and the arcing time.
✓ ob is the total operating time, i.e. tpa + ta.

6. Fuse Characteristics

7. TYPES OF FUSES
(i) Rewirable Type
▪ This type of fuse is rewireable, i.e. the blown-out fuse element can be replaced by a new one.
▪ The fuse element can be either open or semi-enclosed.
Two types.
(a) Open type
(b) Semi-enclosed type.
An open fuse is a thin piece of wire of tin, lead or copper
inserted directly in a circuit.
▪ It is the simplest and cheapest form of protection
▪ Due to fire hazard and unreliable operation caused by
oxidation, its less life time

8. Semi-enclosed type fuse :
▪ This fuse is most commonly used in house wiring and small current circuits.
▪ The fuse wire is fitted on a porcelain ‘carrier’ which is fitted in the porcelain base.
▪ Whenever the fuse wire blows off due to overload or short-circuit, the fuse carrier can be pulled out,
the new wire can be placed and service can be restored.
▪ The fuse wire may be of lead, tinned copper or an alloy of tin-lead.
▪ The fuse wire should be replaced by a wire of correct size and to the specification, otherwise it may
prove dangerous with the possibility of the equipment burning out.

9. Disadvantages of Rewirable fuses :
(i) Unreliable operation
The operation of these fuses is unreliable because of the following factors.
(a) Since the fuse wire is exposed to the atmosphere, it gets oxidized and deteriorated, resulting in a
reduction of the wire section with the passage of time.
(b) This increases the resistance, causing operation or the fuse at lower currents
(c) Local heating caused by loose connection, etc.
(ii) Lack of discrimination
Proper discrimination cannot be ensured due to unreliable operation.
(iii) Small time lag
Because of the small time lag, these fuses can blow with large transient currents encountered during
the starting of motors and switching-on operation of transformers, capacitors and fluorescent lamps,
unless fuses of sufficiently high rating are used.
(iv) Risk of external flame and fire

10. (ii) Totally Enclosed or Cartridge Fuse
The fuse clement of this type of fuse is enclosed in a totally enclosed container and is provided with
metal contacts on both sides.
Two types. a) D-type cartridge fuse
b) High Rupturing Capacity (HRC) cartridge fuse.
(or) Link type cartridge fuse
D-type cartridge fuse

11. High Rupturing Capacity (HRC) cartridge fuse

12. Advantages of HRC Fuses
✓ Capability of clearing high values of fault currents
✓ Fast operation
✓ Non-deterioration for long periods
✓ No maintenance needed
✓ Reliable discrimination
✓ Consistent in performance
✓ Cheaper than other circuit interrupting devices
✓ Current limitation by cut-off action
✓ Inverse time-current characteristic
Disadvantages of HRC fuses
➢ It requires replacement after each operation.
➢ Inter-locking is not possible.
➢ It produces overheating of the adjacent contacts.

13. Applications of HRC fuses
➢ Protection of low voltage distribution systems against overloads and short circuits
➢ Protection of cables
➢ Protection of busbars
➢ Protection of motors
➢ Protection of semiconductor devices
➢ Back up protection to circuit breakers.

14. Selection of Fuses
▪ It should be able to withstand momentary over-current due to starting a motor and transient
current surges due to switching on transformers, capacitors and fluorescent lighting, etc.
▪ Its operation must be ensured when sustained overload or short-circuit occurs.
▪ It should provide proper discrimination with the other protective devices.
▪ Its selection should depend upon the load circuit. For the purpose of protection, electric
circuits are broadly classified as steady load circuits and fluctuating load circuits.

15. Discrimination
➢ Discrimination means the correct operation of the correct device on occurrence of a fault.
➢ For proper discrimination, there should be coordination between the protective devices.
➢ In order to obtain proper discrimination between two adjacent fuses carrying the same
current, the pre-arcing time of the major fuse (nearer the source) must exceed the total
operating time of the minor fuse (far from the source) .

16. Discrimination between Two Fuses
▪ The power to the radial line is being fed at the left end.
▪ The fuse F1 which is nearer the feeder is called the major fuse, and the fuse F2 which is far from the feeder is called
the minor fuse.
▪ If a fault occurs at the far end of the feeder, i.e. at point F, the fault current will flow through both the fuses F1 and F2.
▪ If the fuses used do not have discriminative character, there is a likelihood that the major fuse F1 is blown out and
thus supply to the whole line will be interrupted, although there is no fault between F1 and F2.
▪ Therefore, when a fault occurs beyond F2, only F2 should operate and Fl should remain unaffected. This is called
proper discrimination.
▪ For proper discrimination in this case, the pre-arcing time of the major fuse F1 must be greater than the total
operating time of the minor fuse F2.
a radial circuit

17. Discrimination between Fuses and Overcurrent Protective Devices
▪ In motor circuits, fuses provide short-circuit protection and the
overcurrent relay provides overcurrent (overload) protection.
▪ The characteristics of the fuses and overcurrent relay are
coordinated in such a way that the overcurrent relay operates for
currents within the breaking capacity of the circuit breaker (or the
contactor), and the fuses operate for faults of larger current.
▪ For this purpose, the characteristic of the overcurrent protective
device should be below the characteristic of the fuse.
▪ The fuse is so selected that the intersection of the characteristics of
these two protective devices must take place at a point (A)
corresponding to six times the full-load current, keeping in view
that the protective devices do not operate unduly during starting.
▪ In this case, the fuse provides back-up protection to the motor and
is connected on the supply side.