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Part No. 057-009 5110 Operating Manual Issue 3.3 3/1/2006
18.3 CHOOSING THE CORRECT C.T.S
The VA burden of the 5110 on the measurement CTs (Current transformers) is 2.5VA. However depending upon the
type and length of cabling between the CTs and the modules, CTs with a greater VA rating than 2.5VA are required.
D
D
E
E
T
T
A
A
I
I
L
L
S
S
The distance between the CTs and the
measuring module should be estimated
and cross-referenced against the chart
opposite to find the VA burden of the cable
itself.
The star point (common) of the CTs MUST
be connected to system ground (earth) as
close as possible to the CTs. This
minimises the length of cable used to
connect the CTs to the DSE module.
Example.
If 1.5mm² cable is used and the distance
from the CT to the measuring module is
20m, then the burden of the cable alone is
approximately 7.5VA. As the burden of the
DSE controller is 2.5VA, then a CT with a
rating of at least 7.5+2.5V = 10VA must be
used.
If 2.5mm² cable were used over the same
distance of 20m, then the burden of the
cable on the CT would be approximately
4VA. CT’s required in this instance is at
least 6.5VA (4+2.5).
NOTE:- Details for 4mm² cable are shown for reference only. The connector on the DSE
modules are only suitable for cables up to 2.5mm².
NOTE:- C.T.’s with 5A secondary windings must be used with the 5110 module. Ratios from
10A : 5A up to 6000A : 5A can be used.
As the C.T.’s are used purely for instrumentation purposes (not protection) within the 5110
module, protection class C.T.’s are not required. To match the specification of the 5110 module,
it is recommended that C.T.’s of Class 0.5 be used to give the best possible measurement
accuracy.
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