Electrical Cables and Accessories Testing 1

Cable testing is conducted to chart the gradual deterioration over the years, to do acceptance testing after installation, for 

verification of splices and joints, and for special repair testing. Normally, the maintenance proof tests performed on cables are at a test voltage of 60% of final factory test voltage.
When the exact construction of cable in an existing installation is not known, it is generally recommended that DC maintenance proof test voltage be based on rated AC circuit voltage using the recommended value for the smallest sized conductor in the rated AC voltage range. The DC voltage tests conducted on cable are insulation resistance measurement and DC hi-pot test.
The DC hi-pot test can be performed as leakage current versus voltage test, leakage current versus time test, or go, no-go overpotential test.
It is always appropriate to conduct the insulation resistance measurement test first, and if data obtained looks good, then proceed with the DC overpotential test. After DC overpotential test is completed, then perform the insulation resistance again to assure that the cable has not been damaged during the DC overpotential test.

Insulation Resistance Measurement Test

The insulation resistance is measured using a Megohmmeter (or it can be measured using a portable instrument consisting of a direct voltage source, such as a generator, battery, or rectifi er, and a high-range ohmmeter that gives insulation resistance readings in megohms or ohms). This is a nondestructive method of determining the condition of the cable insulation to check contamination due to moisture, dirt, or carbonization. The insulation resistance measurement method does not give the measure of total dielectric
strength of cable insulation or weak spots in the cable. Generally, the following voltages can be used for the indicated cable voltage rating.

The following is the general procedure when using a megohmmeter (Megger)*
for resistance measurement tests.
Disconnect the cable to be tested from other equipment and circuits
to ensure that it is not energized.
• Discharge all stored capacitance in the cable by grounding it before testing, as well as after completing tests.
• Connect the line terminal of the instrument to the conductor to be
tested.
• Ground all other conductors together to sheath and to ground.
Connect these to the earth terminal of the test set.
• Similarly measure other insulation resistance values between one conductor and all other conductors connected, one conductor to ground and so on. The connections are shown in Figure 1.1a through d.
• The guard terminal of the megohmmeter can be used to eliminate
the effects of surface leakage across exposed insulation at the test
end of the cable, or both ends of the cable for leakage to ground.

The insulation resistance measurements should be conducted at regular intervals and records kept for comparison purposes. Keep in mind that, for valid comparison, the readings must be corrected to a base temperature, such as 20°C. A continued downward trend is an indication of insulation deterioration even though the resistance values measured are above the minimum
acceptable limit.

Cable and conductor installations present a wide variation of conditions from the point of view of the resistance of the insulation. These conditions result from the many kinds of insulating materials used, the voltage rating or insulation thickness, and the length of the circuit involved in the measurement. Furthermore, such circuits usually extend over great distances, and may be subjected to wide variations in temperature, which will have an effect on the insulation resistance values obtained. The terminals of cables and conductors will also have an effect on the test values unless they are clean and dry, or guarded.

Figure 1.1

Cable test connections for insulation resistance measurement: (a) connection for single-conductor cable, one conductor to ground test; (b) connection for three-conductor cable, one conductor to other conductors and sheath to ground; (c) connection for three-conductor cable, one conductor to sheath and to ground and two conductors guarded; and (d) connection for three-conductor cable, one conductor to all other conductors without leakage to ground.

The Insulated Cable Engineers Association (ICEA) gives minimum values of insulation resistance in its specifi cations for various types of cables and conductors. These minimum values are for new, single-conductor wire and cable after being subjected to an AC high voltage test and based on a DC test potential of 500 V applied for 1 min at a temperature of 60°F.
These standard minimum insulation resistance (IR) values (for single conductor cable) are based on the following formula:

where
IR is in megohms per 1000 ft of cable
K is a constant for insulating material
D is the outside diameter of conductor insulation 

d is the inside diameter of conductor

 

The insulation resistance of one conductor of a multiconductor cable to all others and sheath is

where
D is the diameter over insulation of equivalent single-conductor cable =
d + 2c + 2b
d is the diameter of conductor (for sector cables, d equals diameter of round conductor of same cross section)
c is the thickness of conductor insulation
b is the thickness of jacket insulation

 

Also, the IEEE standard 690-1984* and 422-1986† recommended an insulation resistance field acceptance limit of  where
L is the cable length in feet
kV is the insulation voltage rating