Download 1 LEAKAGE CURRENT LIMITATIONS (ANALOG AND DIGITAL) 68

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1.1
LEAKAGE CURRENT LIMITATIONS (ANALOG AND DIGITAL) 68.304
Background
Leakage current limitations ensure that telephone connections are adequately insulated against voltages hazardous to
telephone company personnel as a result of supply voltages within the equipment or the result of accidental contact
with commercial power on 1) exposed conductive surfaces, 2) leads to other equipment, e. g. serial, parallel, LAN,
WAN, or 3) other leads to the network interface. In this case, the leakage current limit is merely a threshold for
determining whether dielectric breakdown has occurred. It should not be confused with leakage current limits that
relate to product safety. Such limits typically specify the maximum current permitted through an impedance
simulating the human body connected between exposed conductive surfaces and ground or between exposed
conductive surfaces.
The 1000-volt requirement is based in part upon the fact that potentials as high as 1000 V peak can reach certified
terminal equipment or certified protective circuitry where carbon block protectors have not fired. The 1500-volt
requirement is based upon commonly used criteria for testing transformer insulation that specifies using 1000 volts
plus twice the rated primary voltage.
The increase in leakage current limit allowed for multi-unit equipment is for those cases where more than one unit is
connected to the same telephone connection leads. It takes into account the increased leakage current resulting from
the capacitance in: 1) interconnecting cables and 2) the parallel combination of dielectric barriers in each unit.
1.2
Purpose
To verify the integrity of the dielectric barrier between the network and power line and the equipment connections
of the EUT.
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1.3
Equipment
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(1) AC voltage source SEL# 2.
(2) True rms voltmeter SEL# 40 (qty 2).
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1.4
NOTE: Refer to Section 4.4 for equipment details.
Equipment States Subject to Test
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The EUT-to-network barrier test (1000 V) should be performed in a sufficient number of operational states to verify
compliance of all dielectric barrier components, e.g., line transformers, opto-isolators, relays and printed circuit
boards. By using the required isolated source, these tests may be performed with or without power applied to the
EUT. Artificial means to achieve the various test modes may be used when required, but the method used must not
affect the current path. For example, the network connections of a switchhook relay shall not be manually shorted.
Instead, an external supply, such as a battery, may be used to activate the relay to achieve the off-hook mode. For
the EUT-to-power-line barrier test (1500 V), the EUT is not powered but the power switch must be in the ON
position so that the power path is completed when the high voltage is applied.
1.5
Procedure
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WARNING: ADEQUATE SAFETY PRECAUTIONS SHOULD BE OBSERVED
(1)
Connect the EUT to the test circuit of Figure 6-1.
(2)
Select the appropriate EUT test points, and connect to the test setup output.
(3)
Place the EUT in the first test state.
(4) Gradually increase the test voltage to the level required for the connections under test, over a 30-second period.
Maintain the maximum voltage level for an additional 60 s.
(5) Monitor the resulting current and the applied voltage level for the 90-second test period.
(6)
Record the maximum current measured during this period.
(7)
Adjust the source for zero-volt output.
(8)
Repeat Step (4) through Step (7) for all applicable operational states.
(9) Repeat Step (2) through Step (8) for all specified combinations of electrical connections as listed in Section
68.304.
1.6
Alternative Methods
None suggested.
1.7
Suggested Test Data
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(1)
Identify electrical connections or test points.
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(2)
Voltage applied (V rms).
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(3)
Maximum current (mA peak).
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1.8
Comments
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(1)
Leads may be tested as a group or as indicated in Section 68.304.
(2)
EUTs with a nonmetallic case having some exposed metallic surfaces (e.g., screws,
hinges, ventilation or access opening) may be tested by wrapping the case in a
conductive foil, placing it on a conductive sheet, or immersing it in a container filled
with 0.25-inch-diameter (maximum) conductive particles, as appropriate. The
voltage is applied between the conducting element and the other relevant test
points.
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(3)
As shown in the test setup, the leakage current is equal to the voltage measured
across the 1000-ohm resistor divided by 1000.
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(4)
There are three intentional paths to ground considered:
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(a)
Operational paths, such as ground start leads, are excluded from testing. An
operational path is identified as having a DC resistance at operating voltages
(battery or ringing). The current handling capability of the path is evaluated
under Section 68.306(e)(1).
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(b)
Protective paths, such as MOVs and surge suppressors, are removed before
testing. A protective path is identified as being conductive at leakage current
test voltages (1000 V -- a surge arrestor fires to protect equipment from such
voltages), but is an insulator at operational voltages. The insulation properties
of the component removed is evaluated at 120 V under Section 68.306(e)(2).
Typically, a suppressor is rated at greater than 130 V to be transparent to
ringing voltages.
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(c)
Filter paths on the interface circuit are left in place during testing. Filter
components must withstand 1000 V, which provides the capability to
withstand surges. These paths are identified as not conductive for DC. To
pass a 1000 V test, a capacitor needs about a 400 WVDC rating. These are
special capacitors, designated “X-capacitors”, or “Y-capacitors”.
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(5) EMI filtering on the ac input to the EUT may be disconnected from ground. An
engineering evaluation shall be made and an attestation provided if the filter cannot
be isolated from ground. Alternatively, power supplies may incorporate integrated
line filter networks, the removal of which would reduce the effect of the applied
voltage in evaluating the characteristics of the EUT. In such cases, it may be
appropriate to perform this test using a dc test voltage. The maximum dc test voltage
would correspond to the peak value of the specified ac test voltage, and the test
should be performed in both polarities.
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(6) If the EUT has a path to ground (or a path close to ground potential) and the high
voltage source is not isolated, measured current inaccuracies may occur due to
ground loops.
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(7) An EUT which has both an intentional operational and an intentional protective path
to ground shall need to meet only the requirements of 68.306(e)(1) referred to in
section 7.4.1 of this document. Provided that they use varistors, thyristors, or other
protection devices to ground, examples of these circuits are DID, OPS, E&M, and
Ground Start (which typically utilize ground for operation).
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(8) When conducting the 1500V AC dielectric test between AC power and tip-ring
interface circuit, high leakage current may exist due to loopback current through
common ground and filter paths on AC power side and interface circuit. The test
may be conducted using DC equivalent, 2121 VDC.
NOTES:
1. A 1500 V ac voltmeter or a resistive voltage divider and high impedance voltmeter may be used.
2. A true rms or rms calibrated voltmeter may be used to measure a converted rms current limit. Alternatively, an
oscilloscope may be used to measure peak current. Precautions should be taken for isolation of high voltage
differential or current probes.
FIGURE 6-1. LEAKAGE CURRENT
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