|
FUSE technology - what for?
Containment of arc energy
When a short circuit fault current is interrupted,
a large amount of energy is released. This energy can either be
contained within the body of the protective device or be released
to the environment.
With a fuse, the energy is absorbed by the granulated quartz inside
its body. It is trapped in the fuse cartridge thus eliminating potential
external problems.
The release of energy to the environment is easier for the design
of the protection device, but needs care in the design of the device
enclosure. The ionized gas emission can result in reignition of
the short circuit fault upstream of the protection device. Moreover
the attendant pressure peak can cause a spontaneous opening of frontcovers
of a switchboard.
Energy limitation
The energy let through of the protective device
determines the necessary strength of the protected installation
and components.
A fuse interrupts a short circuit current very quickly. Consequently
the energy let through (I2t) of a fuse is very low.
A fuse 100A, 20kArms, 415V reduces an initial fault current of 4.000.000
A/S2 to 46.000 A/S2 This ratio is indicated in the picture below.
An example:
The volume illustrates the impact
of the limitation feature of a fuse. The benefit is that smaller
components can be used downstream of the fuse resulting in COMPACT
INSTALLATIONS.
Restoration after a fault
In the event of a fault the protective device
has to open the circuit. A specific feature of a fuse is that this
is done without using mechanical moving parts. In case of a fault
a fuse always opens the circuit. It operates, and is then replaced
by a new, factory - calibrated device. The circuit is protected
in the same way as before the fault occurred.
Single or three phase interruption
When one phase has an overload or short circuit,
a fuse interrupts only the circuit of the affected phase. For motorloads,
a 3-phase trip is desired, this is provided by a fast reacting electronic
fuse monitor, the thermal relay or by the striker of the fuse. There
are many situations imaginable where a 3-phase trip is undesirable.
Who of us has not experienced the loss of computer information or
the darkness after a power interruption. When a fuse is used, 1/3
of these situations can be avoided.
Discrimination
Discrimination is an important factor in an electrical
installation. This is achieved when only the protective device which
is nearest to the fault interrupts the circuit. Consequently, only
the smallest possible part of the installation in which a fault
occurs will be out of service.
When applying fuses, it is easy to obtain discrimination, independent
of the brand used. There are also no upper prospective current limitations
above which discrimination can not be achieved.
If the ratio between the fuse ratings
is 1 : 1.6, complete and reliable discrimination can be easily and
cheaply achieved. Even, using different brands gives no design limitations.
Applying fuses gives: INSTALLATION DESIGN CONVENIENCE
Co-ordination for motorstarters
To provide adequate short circuit protection
for a contactor and or a starter, a protective device is used. According
to IEC 60947-4 two types of co-ordination are permissible, 1 or
2, for which the test conditions are given in the aforementioned
IEC. Nowadays mainly type 2 is specified.
To obtain the specified level of protection all combinations
of makes and types of protective devices and contactor must be verified
by actual short circuit tests. In general this will limit the combination
possibilities. By applying fuses the maximum let through energy
and cut-off currents are low, resulting in relatively small motor
starter combinations at a high prospective short circuit current
level.
Moreover the make of the fuses concerned is irrelevant because all
makes have to comply with IEC 60269, defining the maximum let-through
levels. Thus coordination can easily be achieved, by following the
applied fuse instructions given by the contactor manufacturer. Applying
fuses in motor starter combinations gives you flexibility.
|
