DSH 395 Separation of TNV-3 and SELV circuitry
http://decisions.iecee.org/icons/ecblank.gif DSH 395
Separation of TNV-3 and SELV circuitry
2.3.2
60950(ed.3)
Standard(s):
IEC 60950 (1999) 3rd Ed.
Sub clause(s):
2.3.2
Sheet No.
395
Subject:
Separation of TNV –3 and SELV
circuitry
Key words:
- Separation of TNV-3 circuit
and SELV circuit
Decision taken at the 39th
meeting 2002
Question:
1. Is it permitted to demonstrate that a TNV -3 circuit is adequately separated from a SELV circuit by
means of single fault testing, in the situation where the spacings do not meet the requirements for basic
insulation? (The dielectric strength test is still required under Subclause 6.2.1.)
2. If so, is it required under the compliance paragraph to short circuit the insulation between the TNVcircuit
and the SELV circuit at all points which do not meet the criteria for basic insulation, before
carrying out the tests?
3. If short circuiting the insulation before carrying out fault tests results in the equipment passing the tests,
whereas carrying out the fault tests without short circuiting the insulation would result in the equipment
failing the test, should the insulation be short circuited?
Rationale:
1. Paragraph 1 of Sub-clause 2.3.2 requires that separation between a TNV-3 and a SELV circuit be such
that in the event of a single fault, the SELV circuit remains below TNV-3 limits. Paragraph 2 of Subclause
2.3.2 states that basic insulation will achieve this, but other solutions are not excluded.
Equipment is not required to meet either Paragraph 2 or 3. It can meet the single fault conditions stated
in Paragraph one and in the compliance paragraph.
2. According to the compliance paragraph, before carrying out the single fault tests, insulation which does
not meet the requirements for basic insulation is short circuited. Presumably this does not mean that
short circuits are to be applied between all tracks and components in the entire TNV-3 and SELV
circuitry which do not meet basic insulation, but that the insulation at the point where the TNV-3 circuit
meets the SELV circuit is short-circuited.
3. It is possible to envisage a situation where short circuiting the insulation between TNV-3 and SELV
could cause the TNV-3 voltages to be lowered. Fault tests under this condition would not represent the
worst case possible fault condition
Decision:
1. It is permitted to demonstrate compliance with Subclause 2.3.2 by means of single fault testing.
The limits in 2.3.1.b) shall not be exceeded in normal or single fault conditions.
2. Before carrying out single fault testing of components and insulation, insulation between TNV-3 and
SELV circuits not meeting the requirements of Basic insulation should be short circuited if this results in
a worst case voltage measurement at accessible parts of SELV and TNV-1 circuits, and at accessible
conductive parts during subsequent fault testing.
3. If the single fault test would be more severe if carried out without short circuiting the insulation, the test
should be done without short circuiting the insulation.
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