Why motor protection is necessary? And types of fuses used in motor protection

explanation about protection provided to induction and other types motor are given below:

In order to avoid unexpected breakdowns, costly repairs and subsequent losses due to motor downtime, it is important that the motor is fitted with some sort of protective device. Generally speaking,

Motor protection can be divided into the following 3 levels:

• External protection against short circuit in the whole installation. External protection device is normally different types of fuses or short circuit relays. This kind of protection device is compulsory and legal and placed under safety regulations.

• External protection against overload of specific equipment; i.e. to avoid overload of
pump motor and thereby prevent damage
and breakdown of the motor. This type of
protection reacts on current.

• Built-in motor protection with thermal
overload protection to avoid damage and
breakdown of motor. The built-in protector
always require an external circuit breaker
while some built-in motor protection types
even require an overload relay list of the most common fault conditions where motor damage can be avoided by some sort of motor protection:

• Problems with the power supply quality:
– Overvoltage
– Undervoltage
– Imbalanced voltages/currents
– Frequency variation
• Installation, supply & motor failures
• Slowly developing temperature rise:
– Insufficient cooling
– High ambient temperature
– High altitude operation
– High liquid temperature
– Too high viscosity of the pumping liquid
– Frequent starts
– Too big load inertia
– (not common for pumps)
• Quickly developing temperature rises:
– Locked rotor
– Phase breakage

To protect a circuit against overloads and short circuits, a circuit protective device must determine when one of these fault conditions occurs.It must then automatically disconnect the circuit from the power source. A fuse is the simplest device for accomplishing these two functions.

Normally fuses are built together by means of
a safety switch, which can switch off the circuit.

In this post, we will discuss three
types of fuses as per their function and to where they are used: Fusible safety switch, “quick-acting” fuse and “time-lag” fuse

Fusible safety switch:
A fusible safety switch is a safety switch, which is combined with a fuse in a single enclosure. The switch manually opens and closes the circuit, while the fuse protect against overcurrent protection.Switches are generally used in connection with
service when it is necessary to cut off the current, or in connection with fault situations.The safety switch is a switch, which is placed 

in a separate enclosure. The enclosure protects personnel against accidental exposure to electrical connections and against exposure to weather conditions. Some safety switches come with a built-in function for fuses, and some safety switches come without built-in fuses, containing only a switch.
The overcurrent protection device (fuse) has to
recognise the difference between overcurrent
and short circuit. Slight overcurrents for 

example, can be allowed to continue for a short period of time. But as the current magnitude increases, the protection device has to react quickly. It is important to interrupt short circuits immediately.
The fusible disconnect switch is an example of a device which is used for overcurrent protection.Properly sized fuses in the switch open the circuit when an overcurrent condition occurs.

Quick-acting:

"Quick-acting” fuses Nontime-delay fuses provide excellent short circuit protection. However, brief overloads, such as motor starting currents, may cause problems for this kind of fuse. Therefore, nontime-delay fuses are best used in circuits, which are not  subject to large transient currents. Normally, nontime-delay fuses hold some 500% of their rated current for one-fourth of a second. After this time, the current-carrying element melts, and opens the fuse.Thus, in motor circuits, where the starting current often exceeds 500% of the fuse’s rated current, nontime-delay fuses are not recommended.

“Time-lag” fuses:
This kind of fuse provides both overload and short-circuit protection. Typically, they allow up to 5 times the rated current for up to 10 seconds and for shorter periods even higher currents. Usually, this is sufficient to allow a motor to start without opening the fuse. On the other hand, if an overload condition occurs and persists for a longer period of time, the fuse will eventually open.

Reference:Motor Book Grundfos