Direct Current Field Test

DIRECT CURRENT FIELD TESTS

LOW VOLTAGE NON-SHIELDED CABLES: "HI-POT" Test
LOW VOLTAGE NON-SHIELDED CABLES: "MEGGER" Test
IEEE 400: IEEE Guide for Making High-Direct-Current-Voltage Tests
AEIC CS-5-94: Cross-Linked Polyethylene Insulated Shielded Power Cables
AEIC CS-6-??: Ethylene Propylene Rubber Insulated Shielded Power Cables
Time-Leakage Test: Direct Current Time-Leakage Test

INTRODUCTION

SAFETY

Before conducting tests on a cable system, verify that the cable system is properly de-energized. If the system has been previously energized, follow the prescribed rules for switching necessary to de-energize, lockout, tag, and ground the cable system. The personnel conducting the testing must be qualified to operate the test equipment and be familiar with the cable system and its components.

PREPARATION FOR TESTING

Disconnect cables from non-cable system equipment and apparatus. This will reduce the possibility of erroneous test results. In the case of HVDC (High-Voltage-Direct-Current) testing, it will prevent damage to equipment and apparatus. Adequate physical clearances between the cable ends, and other equipment, other energized conductors, and to electrical ground must be checked. During the testing, be sure that unauthorized access to the cable system is maintained. Verify that proper procedures are taken to clear all tap(s) or lateral(s) in the circuit. Remove grounds from the cable phase to be tested. Phases not under test should remain grounded at all ends.

CONDUCTING TEST

Conduct test according to prescribed procedures and instructions. Record test results and retain for future reference.

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LOW VOLTAGE NON-SHIELDED CABLES

"HI-POT": High Voltage Potential DC Test

High potential tests are "go no-go" tests. The cable is required to withstand the specified voltage for the specified time duration. These tests will normally reveal gross imperfections due to improper field handling. Recommended test voltages are given in tables for dc and ac. Alternating current new installation test voltages are 80% of the factory test voltage. Direct current voltages are two times the ac new installation test voltage. Test duration should not exceed five minutes.

Recommended DC Dielectric Test Voltages For New Installations

Size	XHHW/RHH/RHW	USE	THHN/THWN
14	4800	---	3200
12	4800	4800	3200
10	4800	4800	3200
8 - 2	4800	5600	3200
1 - 4/0	6400	6400	4000
250 -500	8000	8000	4800
550 - 1000	9600	9600	5600
1100 - 2000	11200	11200	6400

Recommended AC Dielectric Test Voltages For New Installations

Size	XHHW/RHH/RHW	USE	THHN/THWN

14	2400	---	1600
12	2400	2400	1600
10	2400	2400	1600
8-2	2800	2800	1600
1-4/0	3200	3200	2000
250-500	4000	4000	2400
550-1000	4800	4800	2800
1100-2000	5600	5600	3200

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"MEGGER": Insulation Resistance Test

Low voltage, non-shielded cables can be tested using a battery powered ohmmeter or a device called a "megger". Hand held ohmmeters generally have outputs from several volts to 24 volts. They are excellent for detecting direct "shorts" such as bolted faults and low resistant measurements in the kilohm range. A "megger" measures resistance in the megohm range using higher voltages than an ohmmeter. Manual or motor-driven meggers are available for a range of fixed dc voltages. Typical fixed dc voltages are 500, 1000, 2500 and 5000 volts.

2 to 50 Megohm Rule
Applied dc potential may be 500 or 1000 volts dc with the insulation resistance reading taken at one minute. A megohm meter reading of less than 50 megohms may be used for deciding when to investigate the cable installation, readings less than two megohms will most likely indicate damaged insulation. Readings of 2 to 50 megohms are usually associated with long circuit lengths, moisture, or contamination. In most cases a 2 to 50 megohm reading does not indicate the insulation quality, therefore 2 to 50 megohms should not be specified as a pass/fail value. Insulation resistance readings should be made and interpreted by experienced testing specialists to determine the condition of the cable's insulation.

Note that it is difficult to obtain accurate insulation resistance measurements in the field. Factory tests are done in a controlled environment with the cable submerged in water to provide an electrical ground. Using factory test values are not recommended.

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MEDIUM VOLTAGE SHIELDED CABLES

IEEE 400

"IEEE Guide for Making High-Direct-Current-Voltage Tests on Power Cable Systems in the Field"

With any HVDC testing it is highly recommended that IEEE Standard 400 be understood and that there is concurrence of the manufacturers of the cables, terminals, and splices prior to the performance of any proposed testing. Table 1 is taken from IEEE Std. 400:

IEEE Standard 400 tests are "go no-go" tests. The system is required to withstand the specified voltage for the specified time duration. These tests will normally reveal gross imperfections due to improper field handling such as excessive bending or air gaps between the insulation and shield interfaces.

Table 1
Field Test Voltages for Shielded Power Cable Systems
5 kV to 35 kV

System Voltage (kV rms) phase-phase	System BIL (kV) (crest) (cond-gnd)	Acceptance Test Voltage* (kV dc) (cond-gnd)	Maintenance Test Voltage** (kV dc)

5	75	28	23
8	95	36	29
15	110	56	46
25	150	75	61
28	170	85	68
35	200	100	75

* Acceptance test voltage duration is normally 15 minutes.
** Maintenance test voltage duration is normally not less than five minutes or more than 15 minutes.

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AEIC CS-5-94
"Specifications for Cross-Linked Polyethylene Insulated Shielded Power Cables Rated 5 Through 46 kV"

Tests During and After Installation
During Installation. At any time during installation, a dc proof test may be made at a voltage not exceeding the dc test voltage specified for During Installation, applied for five consecutive minutes.
After installation. After the cable has been completely installed and placed in service, a dc proof test may be made at any time within the first five years at a voltage not exceeding the dc test voltage specified for the First five Years, applied for five consecutive minutes. After that time, dc testing is not recommended.
DC test voltages are applied to discover gross problems such as improperly installed accessories or mechanical damage. DC testing is not expected to reveal deterioration due to aging in service. There is some evidence that dc testing of aged cross-linked polyethylene cables can lead to early cable failures.

Table 2
DC FIELD TEST VOLTAGES
Cross-Linked Polyethylene Insulated Shielded Power Cables

Rated Voltage During First Phase to Phase (kV)	Insulation Thickness	DC Test Voltages (kV) During Installation ( 5 min)	First 5 years (5 min)
5	90	28	9
5 or 8	115	36	11
15	175	56	18
15	220	64	20
25	260	80	25
25	320	96	30
28	280	84	26
35	345	100	31
35	420	124	39

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AEIC CS-6-?? ---DO NOT SPECIFY COMPLIANCE---
"Specifications for Ethylene Propylene Rubber Insulated Shielded Power Cables Rated 5 Through 69 kV"

This information has not been adopted and is pending review by AEIC!

Tests During and After Installation
During Installation. At any time during installation, a dc proof test may be made at a voltage not exceeding the dc test voltage specified for During Installation, applied for five consecutive minutes.
After installation. After the cable has been completely installed and placed in service, a dc proof test may be made at any time within the first five years at a voltage not exceeding the dc test voltage specified for the First five Years, applied for five consecutive minutes.
DC test voltages are applied to discover gross problems such as improperly installed accessories or mechanical damage. DC testing is not expected to reveal deterioration due to aging in service.

This information has not been adopted and is pending review by AEIC!

Table 3
DC FIELD TEST VOLTAGES
Ethylene Propylene Rubber Insulated Shielded Power Cables

Rated Voltage Phase to Phase (kV)	Insulation Thickness	DC test Voltages (kV) During Installation (5 min)	First 5 years (5 min)
5	90	28	22
5 or 8	115	36	29
15	175	56	45
15	220	64	51
25	260	80	64
25	320	96	77
28	280	84	67
35	345	100	80
35	420	124	99

This information has not been adopted and is pending review by AEIC!

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Time-Leakage Test

For more sophisticated evaluations, it is important to recognize the components of dc "leakage" current. The output current of the test set into the cable is not the true leakage current. The output current is the sum of three currents; geometric capacitance, absorption, and true leakage current. The absolute value of output current is not of primary importance. This value is virtually impossible to predict and is dependent upon the previously mentioned factors which can affect the resultant output current from a few microamperes to hundreds of microamperes.

Time-Leakage Test, Components of DC Test Output Current (Diagram not available)

It is the shape of the total current curve (it) with respect to time that indicates the condition of the dielectric. A drop-off of current with respect to time is an indication of sound insulation. A rising current is an indication of questionable condition or impending failure. A flat curve is generally due to test conditions.

The output current variation with respect to time of voltage application is generally considered more indicative than the absolute value. The characteristic shapes of the time-leakage current curve and probable causes are outlined below.

1. A rising leakage curve at a steady voltage may be indicative of faulty insulation. However, other leakage paths (over porcelain surfaces and through insulating fluids) can contribute to such a result.

2. A falling leakage curve is indicative of good insulation characteristics, especially if at similar levels for all phases.

3. A flat leakage curve at low value is generally indicative of acceptable insulation. Flatness may be influenced by circuit length, cable geometry, and possible presence of moisture or contaminants over terminal surfaces.

4. A flat leakage curve at high value may indicate any of the following conditions:

a. presence of moisture
b. contaminants over terminal surfaces or other creepage surfaces
c. surface leakage greater than volume leakage
d. moist laminated insulation
e. condition of insulating fluids
f. air ionization losses (corona) from projections

5. Dissimilar leakage curves are indicative of nonuniformity of circuit insulation. The characteristic curve of each phase should be analyzed to determine the cause of dissimilarity. Air ionization losses from projections may affect one phase more than the others, dependent upon corona shielding (such as at terminals), temperature and humidity transients, air movement, and the like.

Generally speaking, the increase of current with test voltage is approximately linear for sound insulation. Care should be exercised to prevent terminal corona and minimize terminal surface leakage as these can mask test results.