Product Guide VESDA ECO Detector by Xtralis
22 Installation 17898_05
So for example, in an installation with a supply voltage of 19V, a cable length of
250m and a cable resistance of 0.05Ωm
-1
(Ohms per metre), gives us a maximum
load resistance R
L
of ..
R
L
= (19 / 25 x10
-3
) – (0.05 x 250) = 747.5Ω
Technically speaking the minimum load permissible is 0Ω, though it is suggested
that a practical minimum of 100Ω is used to prevent excess heat dissipation within
the ECO unit and to provide a sufficient voltage signal for the loop monitoring
device.
Finally, ensure that the power rating of the load resistor is sufficient, it is good
practice to “De-Rate” this by half to maximize its life and minimize potential failure in
the future. The Power rating of the load resistor can be simply calculated as
P
L
= R
L
x 25mA x 25mA x “De-rating Factor”
So for our previous example, the required power rating for the load resistor would
be
P
L
= 747.5 x 25x10
-3
x 25x10
-3
x 2 = 0.93W
A standard 1W rated resistor would therefore be sufficient in this case.
To Next Page
Loading...
