This document compares fuses and circuit breakers (CBs) for overcurrent protection. It notes that CBs have several advantages over fuses: CBs can be reset after tripping, which reduces downtime; CBs allow for smaller cable cross-sections since they can tolerate temporary overloads; and CBs offer enhanced safety since they do not produce arcs when switched on under a short-circuit like fuses can. The document provides various charts and diagrams comparing the trip characteristics and performance of CBs and fuses under different current overload conditions.
2. E-T-A Elektrotechnische
Part 1: General advantages of CBEs
Part 2: 1610/1170 versus Blade Type
Fuses
Comparison
Fuses vs. Circuit Breakers
3. E-T-A Elektrotechnische
Part 1: General advantages of CBEs
Part 2: 1610/1170 versus Blade Type
Fuses
Comparison
Fuses vs. Circuit Breakers
4. E-T-A Elektrotechnische
What is a fuse? A fuse is a device that protects against damage
from excessive current. It contains a short piece of
wire made of an alloy that melts readily. The flow of
current through a fuse causes the wire to heat up
and melts when excessive current passes through
the fuse.
This action burns out the fuse and breaks the
circuit. It also interrupts the flow of electricity
because a fuse is always connected in series with
the circuit it protects. A burned-out fuse which is
commonly called a "blown" fuse must be replaced
for the circuit to function.
Comparison
Fuses vs. Circuit Breakers
6. E-T-A Elektrotechnische
The fuse was invented (and patented)
in 1880 by Thomas Alva Edison, the
famous inventor of the incandescent
light bulb.
Comparison
Fuses vs. Circuit Breakers
8. E-T-A Elektrotechnische
CBEs: Usable many times
Fuses: Usable once only
Therefore prices cannot be
compared on a one circuit
breaker/one fuse basis.
ON - OFF ON - OFF
............
.............ON - OFF
ON - OFF
Comparison
Fuses vs. Circuit Breakers
9. E-T-A Elektrotechnische
Convenient resetting of CBEs
reduces downtime and service
repair costs. There is no need
of spares.
And: Risk of using temporary
inappropriate substitutes is
eliminated, warrenty costs are
reduced.
Damned! Has anybody
seen a spare fuse!
Comparison
Fuses vs. Circuit Breakers
10. E-T-A Elektrotechnische
Most CBEs have a status
indication.
Fuses don‘t!
E-T-A Type 1658
ON OFF
Comparison
Fuses vs. Circuit Breakers
11. E-T-A Elektrotechnische
Many types of CBEs are usable
as ON/OFF switches
Fuses aren‘t!
The E-T-A series 3120 is used to switch
the grain mill ON and OFF, at the
same time protecting the electric motors
from overheating by overcurrents.
Example
Comparison
Fuses vs. Circuit Breakers
12. E-T-A Elektrotechnische
CBEs: single and multipole
versions are available.
Fuses: only single pole versions
available..
Some Examples
E-T-A 8340-F,
three pole
E-T-A 8345,
two pole
E-T-A 3140
three pole
Comparison
Fuses vs. Circuit Breakers
13. E-T-A Elektrotechnische
CBEs:
No hazard when the circuit
breaker is unintentionally
switched onto a short circuit.
Fuses:
Installing a fuse with the load
connected may cause an open
intensive arc which is a potential
hazard for personnel.
This fuse was plugged in on an
existing short circuit. The arc
destroyed the terminal block.
Comparison
Fuses vs. Circuit Breakers
14. E-T-A Elektrotechnische
Many CBEs are available with
auxiliary contacts.
Fuses aren‘t!
E-T-A 2210-S with two integral
auxiliary contacts
Terminals of the auxiliary contacts
Comparison
Fuses vs. Circuit Breakers
15. E-T-A Elektrotechnische
CBEs: No shifting of
characteristic curves.
Fuses:
Ageing may shift the
characteristic curves which may
cause nuisance tripping
(expensive downtime and
stoppages)
Time curve of a 10A blade type fuse
(Littlefuse)
Comparison
Fuses vs. Circuit Breakers
16. E-T-A Elektrotechnische
CBEs: Well adjusted to the load,
even in the event of high inrush
currents from capacitors and
motors.
Fuses: Trip upon inrush currents;
otherwise a higher current rating,
possibly requiring a larger wire
size, must be used, in which case
protection from low overloads is
no longer ensured
Diagram of start-up current of an
unloaded asynchronous motor
Diagramm: E-T-A Laboratory
17. E-T-A Elektrotechnische
Thermal and thermal-magnetic
CBEs: Because of its
temperature dependent
characteristic curve the circuit
breaker perfectely adapts to the
temperature dependent capability
of the load.
Fuses: The characteristic curve
is independent from the ambient
temperature.
21. E-T-A Elektrotechnische
Test
current
A
Operating time
s
min. max.
1.1 IN
360 000
(100h)
∞
1.35 IN
0.75 600
2 IN
0.15 5
3.5 IN
0.04 0.5
6 IN
0.02 0.1
Trip time characteristics
The time characteristics of
the blade fuse is stipulated
in standard 72581 and
ISO 8820.
Thermal circuit breakers
show a delayed response
compared to fuses.
ISO8820-3
Comparison
Fuses vs. Circuit Breakers
22. E-T-A Elektrotechnische
Trip time characteristic
The time/current behaviour
of circuit breakers depends
on the type.
This is the trip time curve
of type 1610.
Comparison
Fuses vs. Circuit Breakers
Data sheet 1610
23. E-T-A Elektrotechnische
The trip time behaviour
of a circuit breaker
depends on the type.
Trip curve comparison of type
1610 and blade fuse.
Data sheet 1610 Trip time characteristic
Comparison
Fuses vs. Circuit Breakers
24. E-T-A Elektrotechnische
5 10 50 100 500
0.1
1
10
100
1000
10000
0,35
0,5
0,75
1,0
1,5
2,0
2,5
Heat-up time (s) of a cable from
20°C up to max. temperature of 160°C
current (A)
t
Temperature behaviour
of automotive cables
Comparison
Fuses vs. Circuit Breakers
25. E-T-A Elektrotechnische
5 10 50 100 500
0.1
1
10
100
1000
10000
1,0
Current (A)
t
Trip curve of the 1mm²
cable - rated load 10A
Temperature behaviour
of automotive cables
Heat-up time (s) of a cable from
20°C up to max. temperature of 160°C
Comparison
Fuses vs. Circuit Breakers
26. E-T-A Elektrotechnische
The green line shows the trip
time required by a cable with
corresponding rating to reach
160°C at a certain current.
Both the fuse and the
circuit breaker are fast enough.
Data sheet 1610
Trip time characteristic
Comparison
Fuses vs. Circuit Breakers
27. E-T-A Elektrotechnische
Insensitive to inrush peaks.
Overload is switched off
reliably.
A smaller rating is possible
with the same load.
Trip time characteristics
Advantages of the circuit breaker
t
I(t)
Comparison
Fuses vs. Circuit Breakers
29. E-T-A Elektrotechnische
Circuit breakers allow the use
of thinner cables with the
same rated current.
Fuses have to be oversized -
and so do the cables.
Less copper
through thinner cables
Comparison
Fuses vs. Circuit Breakers
30. E-T-A Elektrotechnische
Replace fuses by circuit
breakers with the same
current rating
Current peaks are tolerated
with unchanged protection.
Circuit breakers do not age.
tin
Comparison
Fuses vs. Circuit Breakers
31. E-T-A Elektrotechnische
Disconnection with smaller
overcurrent.
Fast in the event of a short
circuit.
Replace fuses by circuit
breakers with smaller
current rating
Comparison
Fuses vs. Circuit Breakers
There are basic differences between circuit breakers and fuses. Identical design does not mean interchangeability. The time/current curve referring to the trip characteristics is the major difference between thermal circuit breakers and simple blade fuses. Resettability is certainly an advantage of the circuit breaker and so is the fact that the breaker - unlike the fuse - does not age. In combination with a manual release button the cbe may also be used for disconnecting circuits during repair work. It is the thermal circuit breakers that especially improve the protection of cables and loads by reducing the trip current with rising ambient temperature. The maximum current decreases in the same degree as does the capacity of the load.
The trip curve in the data sheet shows the max. possible tolerance of the trip time in dependence of the current. The grey band shows the range at 21°C. The dotted lines show the lower tolerance limit of the trip curve at max. operating temperature and the max. possible trip time at minimum operating temperature. Attention: the curve always shows a tolerance band. The trip curve of the individual breaker lies somewhere within the band and may be moved to either side with rising or falling temperature.
The trip curve band shown here for the blade fuse is taken from the ISO 8820 standard. Most manufacturers of fuses indicate just these values in their data sheets. The tolerance limits of DIN 72581 are not entirely identical, but comparable. The comparison shown here clearly indicates the difference between a thermal CBE and a fuse: At the same rated current the CBE responds more delayed, i.e. inrush peaks are tolerated, although it trips at the same overload. Therefore the circuit breaker is ideally suited for protection of lamps, motors and capacitive loads (electronic components).
The diagram shows the temperature behaviour of normally used cables in the automotive industry. The line shows when the critical temperature for the cable is reached (at which current, after which time). The critical temperature of the cable is defined as the temperature at which the isolation of the cable may be damaged irreversibly.
As an example I chose the curve of the 1.0mm 2 cable. Standardisation is easy with this curve. Protection is normally done with 10A. In order to achieve a standardised characteristic curve as in the data sheet of the circuit breaker the current values have to simply be divided by 10. Therefore referencing to the trip curve band of the circuit breaker is easy.
This diagram shows a comparison of the characteristic curves of circuit breaker, fuse and cable. Current rating of CBE and fuse are identical. In both cases the current rating is defined as being the limit which causes the protective element to trip. Rated current of the cable is standardised so as to be in accordance with the following table: 0.75 mm 2 = 7.5A 1.0 mm 2 = 10A 1.5 mm 2 = 15A 2.0 mm 2 = 20A 2.5 mm 2 = 25A These are the usual cross sections in automotive industry, taking the allowed operating temperature into account. The green curve of the cable is above the tolerance band for the trip time of the CBE. Thus the cable is at any rate protected against overheating. (The upper dotted line is negligible as it refers to –30°C and at this temperature the cable curve would also be moved to the right. The trip curve of the CBE is as temperature-dependent as the cable curve.)
Trip time behaviour and pulse resistance The different time behaviour is the reason why CBEs allow to better utilize the cable capacities. Blade fuses often have to be oversized in order to prevent them from ageing through the inrush pulse or even from blowing. In this case the cable has to be adapted to the fuse value, i.e. a bigger cross section is necessary in order to ensure protection by the considerably oversized fuse in the event of an overload or short circuit The CBE is hardly prone to inrush peaks. We may therefore choose a smaller current rating and a smaller cable cross section is required.
The example shows that a pulse of 100ms and 4 times rated current (of the fuse) might cause the fuse to blow. The characteristic curve of the fuse is therefore moved horizontally until it is made sure that the pulse will in not cause the fuse to blow. In our example the fuse requires double the current rating of a circuit breaker so as not to be damaged by the inrush current of the load. The cable has to be selected in a size to as to make sure that it won’t be damaged when continuously loaded with the rated current of the fuse. Thus a fuse requires the double cable cross section.
When fuses are replaced by circuit breakers of the same rating in the electrical system of a car, reliability is increased as nuisance trippings become more unlikely. The delayed trip behaviour of the circuit breaker makes it less sensitive to current peaks. Besides, delayed fuses achieve their trip time behaviour by means of tin drops on the fuse element. With high temperatures the tin diffuses into the copper of the fuse element (= bronze). This causes the fuse element to melt because of reduced melting temperature. This process is irreversible. So if a fuse is loaded with current peaks it might be pre-damaged.
The CBE is not sensitive against inrush currents. Therefor it is possible to replace a fuse with a CBE having a lower rated current. E.g. replacing a 15A fuse with a 10A CBE. This is not a problem because a fuse can only be used up to 75% of it’s rated current - according to ISO 8820 and the datasheet of blade type fuses. The reason for this restriction is the aging problem of fuses. The CBE is increasing the security because the load is better protected and the tripping characteristic for higher currents is as fast as the tripping behavior of the fuse.
Two types of circuit breakers - two philosophies The two automotive E-T-A circuit breakers type 1610 and type 1170 have different trip curves. Compared to the 1610 the curve of type 1170 is a bit more delayed and thus more similar to a cable’s characteristic curve. Type 1170 allows the maximum utilisation of a cable’s ampacity. Together with its other benefits - 400A rupture capacity, tease-free and trip-free mechanism - it makes type 1170 a fully-featured circuit breaker for cars. Type 1610 has a slightly faster curve. MAN makes use of this fact: the combination of re-defined current ratings (the 10A unit has a 7.5A bimetal) with a faster curve helps to achieve a trip time behaviour as required by the DIN72581. This means maximum safety combined with the benefit of a circuit breaker - resettability. Type 1610-41 may thus be considered an “automatic, reusable fuse”.