318 / 319
11
inadvertently or unintentionally
energised.
Q
What are the key
requirements for devices used
for onload isolation?
You must consider several
factors. There must be
sufficient isolating distance
between contacts and their
position must be clearly and
reliably indicated.
Also the on-load device must
be suitable for the prescribed
load characteristics. Note the
standard for LSCs, BS 6972,
specifies the requirements
for load making and breaking
with an inductive load and
with tungsten filament lamps.
Finally the device must be
manually operated and it can
not be a semiconductor.
Q
What do you classify as
lamp replacement?
Mechanical maintenance,
which Part 2 of BS 7671
defines as: “the replacement,
refurbishment or cleaning of
lamps and non-electrical parts
of equipment, plant and
machinery.”
Q
What is the objective of
switching off for mechanical
maintenance?
It enables non-skilled people
to carry out maintenance on
electrical equipment without
risk of burns or injury from
mechanical movement.
Switching off for mechanical
maintenance is not isolation of
live parts.
Q
What are the design
considerations for
switching off for mechanical
maintenance?
You must know what the
installation is being used
for, including your client’s
requirements for mechanical
maintenance. The system
should allow maintenance to
be safe and with minimum
disruption to other parts of
the installation. You must
also ensure that there
are precautions in place
preventing equipment
from being inadvertently
reactivated.
Q
What are the
requirements for the devices
used in switching off for
mechanical
maintenance?
The device must clearly
indicate its off or open
position and be suitable
for the prescribed load
characteristics.
Q
Which device best
complies with BS 7671 for
the on-load isolation and
switching off for mechanical
maintenance of a luminaire?
An LSC provides a means
of on-load isolation while
also minimizing disruption to
other parts of the electrical
installation
(see figure 1).
The advantages of using an
LSC for on-load isolation and
mechanical maintenance of
luminaires are:
•
Minimum inconvenience and
disruption to the installation;
•
A large isolating distance
between contacts;
•
The position of the contacts
is clearly and reliably
indicated;
•
It is suitable for on-load
operation;
•
It has manual operation;
you
can take precautions against
inadvertent or unintentional
operation;
•
It enables bench level
maintenance of luminaires.