Electrical Connection Autumn 2015

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www. e l e c t r i c a l c o nn e c t i 5
with Bob Harper
melts at a lower temperature.
In the design of high rupturing
capacity (HRC) fuses, the element is
often ‘dimpled’ or ‘punched’ to create
or define the places where the fuse
elements will blow, always providing a
chain of fusible links.
In addition, the HRC fuse is inside a
ceramic tube surrounded by fine silica
sand. The tube and sand help by making
the operating value more consistent and
also in quenching the arc.
When the HRC fuse experiences a
significant fault, the element melts in less
than a quarter of a cycle (5ms), and the
heat generated melts the silica which then
forms a glass coating around the resulting
silver balls.
In effect, the silver element is quickly
transformed from a good conductor to a
good insulator.
This ‘expulsion’ of the fault current, and
its arc, is the reason for using HRC fuses.
SOLAR PANEL FIRE RISK
Just to conclude, solar panels generate
a maximum current depending on the
amount of light available.
Therefore there will never be a fault
current greater than the generating
capacity of the panels, unless from
another source, such as the supply or
from batteries.
DC faults do not stop arcing easily, and
any DC fault current will be limited by the
generating capacity of the panels.
This means fuses will not operate and are
of no use in protecting the DC solar circuit.
Instead, thermal fuses may be useful
if employed wherever there is a DC
connection that could become faulty.
The thermal fuse would operate not
by the current flow, but by the
temperature of the DC connection,
ie: by detecting the heat from a
DC fault.
Bad DC connections generate a lot
of heat, as the bad connection is by
definition a high-resistance joint with
a high current flow (H=I^2RT). In solar
systems, bad connections cause fires
rather than fault currents.
My suggestion is that all solar
installations require regular inspection
and testing that checks for developing
high resistance joints and the security
of all connections.
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