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6
Electrical engineers frequently
debate how the length of
flexible cord between the
luminaire supporting coupler
(LSC) and the luminaire
raises a number of design
considerations.
The next two technical
sections within this booklet
will fully illustrate the technical
information and the key
requirements necessary to
determine the correct length
of this flexible cord (see figure
1).
Q
Do any British Standards
specify a maximum length for
the flexible cord?
No.
Q
What requirements
does the contractor need to
consider for the maximum
length of flexible cord?
There are four requirements
which need to be taken into
account:
•
Voltage drop;
•
Protection against electric
shock
•
Selection and erection of the
wiring system.
•
Thermal constraints
Q
What needs considering
for voltage drop?
The regulations are
satisfied for a supply given
in accordance with The
Electricity Safety, Quality and
Continuity Regulations 2002,
as amended if; the voltage
drop from the origin of the
installation to the terminals
of the fixed equipment does
not exceed 3% of the nominal
voltage supply, where the
installation is supplied directly
from a public low voltage
distribution system.
At present the nominal voltage
supply in the UK is 230 V +
10% - 6%. The maximum
voltage drop permitted is
therefore: 230 x 3/100 = 6.9 V
(see figure 2).
The requirements for voltage
drop in the regulations
are concerned solely with
safety. The contractor should
always consider other
effects of voltage drop on
the equipment; efficiency, for
example.
A mistake often made is
to ignore the flexible cord
length in the voltage drop
calculations. Details of this
calculation are available on
request.
Q
How does the length of
flexible cord influence the
calculations for protection
against electric shock?
The most common method
of protection against
electric shock is automatic
disconnection of supply.
If the protective device is to
operate correctly you must
always consider the length
of flexible cord in these
calculations.
Voltage drop & shock protection
Figure 1: The flexible cord length between the LSC and
the luminaire
2.5 mm conductor
16 A
type C
circuit
breaker
6 A "Klik"
What are the design
considerations relating
to the length of
flexible cord?