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发表于 2007-5-30 06:57:11
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Copy of the Standard
61000-4-2  IEC:1995+A1:1998 – 27 –
+A2:2000
7.1.3 Test method for ungrounded equipment
The test method described in this subclause is applicable to equipment or part(s) of equipment
whose installation specifications or design preclude connection to any grounding
system. Equipment, or parts thereof, includes portable, battery-operated and double-insulated
equipment (class II equipment).
Rationale: Ungrounded equipment, or ungrounded part(s) of equipment, cannot discharge
itself similarly to class I mains-supplied equipment. If the charge is not removed before the
next ESD pulse is applied, it is possible that the EUT or part(s) of the EUT be stressed up to
twice the intended test voltage. Therefore, double-insulated equipment could be charged at
an unrealistically high charge, by accumulating several ESD discharges on the capacitance of
the class II insulation, and then discharge at the breakdown voltage of the insulation with a
much higher energy.
The general test set-up shall be identical to the ones described in 7.1.1 and 7.1.2
respectively.
To simulate a single ESD event (either by air or by contact discharge), the charge on the EUT
shall be removed prior to each applied ESD pulse.
The charge on the metallic point or part to which the ESD pulse is to be applied, for example,
connector shells, battery charge pins, metallic antennae, shall be removed prior to each
applied ESD test pulse.
When one or several metallic accessible parts are subject to the ESD test, the charge shall be
removed from the point where the ESD pulse is to be applied, as no guarantee can be given
about the resistance between this and other accessible points on the product.
A cable with 470 kΩ bleeder resistors, similar to the one used with the horizontal and vertical
coupling planes, shall be used; see 7.1.
As the capacitance between EUT and HCP (table-top) and between EUT and GRP (floorstanding)
is determined by the size of the EUT, the cable with bleeder resistors may remain
installed during the ESD test when functionally allowed. In the discharge cable, one resistor
shall be connected as close as possible, preferably less than 20 mm from the EUT test point.
The second resistor shall be connected near the end of the cable attached to the HCP for
table-top equipment (see figure 8), or GRP for floor-standing equipment (see figure 9).
The presence of the cable with the bleeder resistors can influence the test results of some
equipment. In case of dispute, a test with the cable disconnected during the ESD pulse takes
precedence over the test with the cable installed during the test, provided that the charge has
sufficiently decayed between the successive discharges.
As an alternative, the following options can be used:
− the time interval between successive discharges shall be extended to the time necessary
to allow natural decay of the charge from the EUT;
− a carbon fibre brush with bleeder resistors (for example, 2 × 470 kΩ) in the grounding
cable;
− an air-ionizer to speed-up the "natural" discharging process of the EUT to its environment. |
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发表于 2007-5-21 16:38:41
