Pyrometric fuse

Plug Type Fuses

Power Fuse

Power fuses  are especially suited for protecting transformers, capacitor banks, and cables in outdoor distribution substations through 34.5 kV. They incorporate precision-engineered, non damageable silver or nickel-chrome fusible elements with time-current characteristics that are precise and permanently accurate, assuring not only dependable performance, but also continued reliability of system coordination plans. With power fuses, source-side devices may be set for faster operation than practical with other power fuses or circuit breakers, thereby providing better system protection without compromising coordination.
 
The power fuses are offered with maximum continuous current ratings of 200 A, 300 A, 400 A, and 720 A in a variety of fault-interrupting ratings. They are available in a wide variety of ampere ratings, in three different speeds: standard, slow, and coordinating. This broad selection of ampere ratings and speeds permits close fusing to achieve maximum protection.

S&C Fused Disconnect

Advantages

• High interrupting capacity, up to 200 kA

• Reduces let-through energy and thus provides better protection for downstream elements.

• Low initial cost.

2500 A Fuse-GE Class L

55kV S&C Fuse

Disadvantages

• Not effective in resistance-grounded system (fuse will not see line-to ground [L-G] faults); upstream breaker with ground-fault element will sense/trip, causing blackout in a larger area

• Source for single phasing (one blown fuse), which poses danger to electric motors.

• High operating expenses

 

Fuse Cutout

Fuse Size for Motor Branch Circuit

Manufacturers provide selection tables. Their selection is generally based on:

• 6.0-P.U. (per unit) motor inrush current

• 6 s starting time

• Two starts per hour

Fuse size selected on the above basis will not be suitable for high-inertia loads (blowers, etc.) or high-efficiency motors (starting inrush & 6.00 P.U.).

 Combination starter with fused disconnect. (Courtesy Square D Company.)

Fuse Let-Through Charts

Fuse let-through charts, as shown in, are used to determine the prospective fault current that will be available at the load side of the fuse. Thus, a lower rated device, i.e., molded-case circuit breaker, can be used in the circuit.
Prospective rms current should not be used to check the downstream equipment interrupting capacity. Compare I2 × t value or ensure that the combination has been tested and certified. 

 Fuse let-through chart

Combination starter with fused disconnect. (Courtesy Square D Company.)

How to combine fuses with overload relays?

How to combine fuses with overload relays?
Fuses prevent short circuits from damaging the
installation and in worst case causing a fire, and must
therefore have adequate capacities. The lower currents
are cleared by the overload relay. Here, the rated
current of the fuse does not correspond to the motor
rating but to the current, which is likely to damage the
weakest components in the installation. As mentioned
previously, the fuse provides short circuit protection
and does not provide low overcurrent protection.
The illustration on your right-hand side shows the
most important parameters that form the basis for a
successful co-ordination of fuses and overload relays.
It is essential that the fuse trips out before thermal
damage of other parts of the installation occur because
of short-circuit
 

The most important parameters that form the basis for a successful co-ordination of fuses and overload relays.
The fuse time current curve always has to be situated lower
than the limit curve (red curve) for thermal damage.
 

Motor protection - Fuse

Motor protection Fuse

 

“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 shortcircuit
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.




Principle of a tripping curve for a fuse. The graph shows the relation between the actual current and the full-load current.

Fuse clearing time
 
The fuse clearing time is the response time it takes the fuse to open. Fuses have an inverse time characteristic, meaning that the greater the overcurrent, the shorter the clearing time. Generally speaking, pump motors have a very short run-up time; below 1 second. So, blown fuses during start-up are normally not an issue for pumps if the fuses match the motor’s full-load currentand is a »time-lag«
fuse.
The illustration on your right-hand side shows the principle of a tripping curve for a fuse. The x-axis shows the relation between the actual current and the full-load current: If the motor consumes the full-load current or less, the fuse does not trip. But at 10 times
the full-load current, the fuse will trip in a very short time (0.01 s). The Y-axis shows the clearing time.
During start-up, an induction motor consumes a large amount of current. In some rare cases, this may lead to a cut-out via relays or fuses. Different methods of starting the motor exist in order to reduce the locked rotor current.

 


Tripping curve for a »quick-acting« and a »time-lag« fuse.
 The “time-lag” fuse is the best choice for motors because of the high starting current