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E L E CT R I C AL CONNE CT I ON
AUTUMN 2 01 5
ELECTRICAL PRINCIPLES
Fuses confusing?
A
fter a recent house fire in which
solar panels were suspected, I
got into a group discussion about
what might have happened and why the
fuses or circuit breakers didn’t operate.
It came down to the others not
understanding what a fuse does. One of
them is an electrician, and he had some
odd ideas about how fuses work.
The common perception is that a fuse
‘knows’ when the current exceeds a set
value and abruptly turns off. It’s not totally
wrong, but it is an over-simplification.
ROLE OF A FUSE
First let’s define the job we want a fuse
or circuit breaker to do:
>
carry the expected load current forever
without blowing;
>
open circuit when the load current
exceeds the rated load current;
>
stop excess current from flowing, even
if many times the load current, ie: rated
fault current; and
>
remain off after the fuse has ‘blown’ and
not allow dangerous leakage currents to
flow, ie: rated safe voltage.
To that end, a fuse designed to meet
these four conditions will have three
ratings and a performance curve or
utilisation category:
>
rated load current – above which the
fuse is expected to blow;
>
safe working voltage – for the system in
which the fuse will be used;
>
current rupturing capacity – which is
chosen for the potential fault current of
the system; and
>
utilisation category – which defines
the performance curve of the fuse,
ie: current time curve (for example,
the fuse for a motor must not blow
during start-up, when the starting
current may be much greater than the
running current.
FUSE OPERATION
The simplest fuse is a piece of wire
between two terminals, and older
electricians know them as SERFs (single
element re-wirable fuses).
SERFs had the reputation of being very
unreliable, but not because of any design
fault. It was because the current rating
depended greatly on the temperature in
which they were situated, and because of
meddling by various people.
It was common to have a client say:
“The fuse kept blowing so I put in a bigger
fuse wire.”
Sometimes it meant two or three fuse
wires, or a piece of circuit conductor
(remember 1/044-inch copper cable?).
Many electricians have seen a fuse holder
hot, maybe smoking, and burning the
base board.
SERFs were replaced with cartridge
fuses, and later with circuit breakers.
However, cartridge fuses still make a
reliable back-up to a circuit breaker and
are serious contenders whenever there is
a fault.
HOW IT REALLY WORKS
The fuse element is simply a wire, which
therefore has resistance and heats up
when passing a current.
When the current exceeds the design
level, the wire reaches a temperature
sufficient for it to melt. Hence the name fuse.
The melting wire becomes a liquid,
and the current flowing through it
causes the wire to squeeze in on itself,
further reducing its cross section, and
therefore increasing the heat at that
point on the wire.
The wire ends up either side of
the ‘blow’ with a ball created by this
magnetic process.
If the fuse is just over-current, the wire
separates easily and there are no dramas
to see.
A larger fault not only melts the fuse
wire much faster, but the resulting arc
bursts the fuse into tiny balls of metal,
and sometimes burnt metallic soot
as well. Glass automotive fuses often
appear to be copper foil coated inside
the glass.
Older electricians can tell what kind of
fault occurred by inspecting the remains
of fuses.
Even if the fuse element is a silver
conductor, it has a current-carrying capacity
based on its cross-sectional area, length and
cooling capacity. Silver is preferred because
it is less prone to corrosion than copper and
In solar systems, fires are
caused by bad connections
rather than fault currents.
Bob
Harper
explains.
The common perception is
that a fuse ‘knows’ when the
current exceeds a set value and
abruptly turns off.