Electrical Connection Winter 2015

45 / 116

www. e l e c t r i c a l c o nn e c t i o n . c om . a u 4 5
As a switch begins to open, an arc is
drawn and continues to lengthen as the
switch opens more, maintaining a low
voltage across the arc. Overheating and
melting of switch components can and
does occur.
The idea is to rapidly allow a high voltage
to be developed across the switch. In the
first place, switches designed for AC are
physically far too small to do the job. DC
switches are often made up of two or more
switch sections opening at once.
DC circuit breakers often use a magnet
to blow the arc into a long path in order
to interrupt the current flow. They are
‘polarised’, because you have to strictly
observe the direction of current flow.
Otherwise, instead of the arc being blown
out, it is drawn in and not extinguished.
Polarised switching equipment is not
allowed in Australian installations.
EARTH LEAKAGE
In common with other electrical
installations, earth leakage protection is
required.
The transformer-less inverter system
has floating PV arrays. Although there is a
safety earth, no galvanic leakage is possible
other than through build-up of somewhat
conductive dirt.
The PV panels have large areas that can
therefore have large capacitance to ground
(think of a steel roof), and voltage ripple on
the DC link can cause earth leakage current to
flow frompanels to earth. For the single-phase
installation, the ripple is 100Hz and is minimised
by parallel connected capacitors in the DC link.
The installation of earth fault alarms for
functionally earthed and floating arrays
became mandatory in July 2013. This
obviously raises issues regarding updating
older systems.
Radiation_Figures.pdf
Individuals and companies installing
PV systems for consumers have to be
accredited by the Clean Energy Council.
They must be licensed electrical contractors
and must adhere to Australian Standards
(principally AS/NZS 5033). Installations
are subject to random inspection by the
Federal Government’s Clean Energy Council
which, among other things, checks that an
accredited installer has done the job.