Technical information

CONTROL CIRCUITS
85
1) NEC (and UL508A): it is possible to protect “only on the primary” or “on the
primary and secondary”.
The protection on the secondary can be omitted only in the case of
transformers with primary and secondary with two conductors (i.e. single
and dual phase) or in the case of three-phase transformers with delta-delta
connection. In both cases secondary should be single (i.e. multiple windings
on the secondary each require its own protections).
•only primary: the protection should be sized to values of no more than
[a]
if the rating does not correspond to a standard value (see the publication
prior to section “BCP”) the one immediately above the one calculated is
permitted.
Standard UL508A limits itself to increasing the percentage for currents to
the primary below 2 A by imposing:
•Current transformers: 500%
•Power transformers: 300%
The conductor feeding the primary should have a capacity of no less than the
rating of the protection, while the conductor to the secondary should have a
capacity of no less than the rating of the protection to the primary m
ultiplied
b y the r atio of primary-secondary transformation.
EXAMPLE
Let’s take a single phase 450 VA transformer.
primaryV
1N
= 240 VI
1N
= 1,88 A
secondaryV
2N
= 24 VI
2N
= 18,8 A
The primary-secondary transformation ratio is 10.
The protection to the primary should not be more than 1.88 x 3 = 5.64. 5 A
fuses are used.
The conductor to the primary should have a capacity of not less than 5 A:
from table 66.1A using UL508A through over sizing a AW G 16 (8 A) is
chosen.
Primary current Rating protection
I
1
[A] [percentage of I
1
]
≥ 9125 %
[a]
< 9167 %
< 2300 %
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