Resistance Measurement of Joints

Introduction
These guidelines describe requirements regarding resistance measurement of joints, and comprise measurement method and limit values for replacement of joints. The purpose is to ensure that measurements are performed in the same way, that measurement errors are minimized and that joints in bad condition are replaced
Scope
These guidelines concern resistance measurement of joints. They cover how to perform measurements, requirements on the measurement equipment and limit values for replacement for joints
Definitions
Technical terms and definitions used in this document:
Device for joining two lengths of conductor to provide mechanical and electrical continuity. Conductor here relates to phase conductor or shield wire. A joint in a line span (mid-span joint or dead-end joint) has also, in addition to electrical connection, the function of maintaining the full mechanical strength of the conductor.
Compression joint
A joint assembled by compression
Detonation compressed joint
A compression joint compressed by detonation of an explosive charge wound around the sleeve of the joint
Hydraulic compressed joint
Compression joint compressed by a hydraulic tool
Screw joint
A joint consisting of factory compressed sleeves on each conductor end, which are joined by a screw sleeve
Bolted connector
A joint or a clamp where contact is obtained by a bolted connection
Where

Rj = is the resistance of the joint,
Rc = is the resistance of the conductor per meter, read from datasheet,
Lj = is the length of the joint in meters.
The use of K – value has the advantage of independence of conductor type and joint length
Temperature dependency of resistance
Specified resistance values for conductors refer to 20O C. the same reference temperature is applicable for the limit values specified in this document. The following formula gives the resistance at other temperatures, if the resistance at 20O C is known
Where
R20 = the resistance at 20O C
Rθ = the resistance at temperature θ
θ = the actual temperature
α= 0.004 for aluminum (temperature coefficient)
A temperature difference of 10 degrees corresponds to 4% resistance difference for aluminum
Performing resistance measurement
General procedure
Concerning full tension compression joins

Three resistance values shall be measured: the whole joint R1, and the two halves R2 and R3 is towards higher tower number
Figure 1 Joint Resistances

Apply the clamps according to Figure 2; with the current clamps furthest out and the measurement clamps on each side of the joint, and a measurement clamp on the mid of the joint. The current clamps shall be placed at least 0.5 meter from the joint. The measurement clamps on each side of the joint shall be placed 5mm from the joint, but must not be in contact with the joint.
Figure: 2 Application of clamps, with a photo as an example

Connect the micro ohmmeter according to Figure: 3 from measurement of R1, R2 and R3 respectively
Figure: 3 Connection for measurement of R1, R2 and R3

Measurement procedure for old joint or new joint on old conductor:
*Measurement
. Measure the resistance R1
. Move one measurement lead to the middle of the joint and measure R2
. Measure R3 in the same way
.  Note the measured values,
*Loosen the two measurement clamps on the conductor adjacent to the joint and rotate them about 1/3 revolution
*Repeat the measurement and note the values
 *Loosen the measurement clamps again and rotate another 1/3 revolution
*Repeat the measurement and note the values
All measurement values shall be reported. The average of the three measurements shall be compared with the applicable limit value.
Measurement procedure for new joint or new conductor:
*As above, but the measurement need not be repeated
Measurement of special joint
Basic measurement principles are according to section 4.1
Compressed dead end connector
For compressed dead end connector the resistance R1, R2 and R3, according to Figure: 4, shall be measured. For R1 and R3 the measurement procedure according to section 4.1 is applicable, with three repetitions. For R2 the measurement need not be repeated.
Figure: 4 compressed dead end connector resistances
Screw Joint
For screw joint the resistance R1, R2, R3 and R4, according to Figure: 5, shall be measured. For R1, R2 and R4 the measurement procedure according to section 4.1 shall be applied, with three repetitions. For R3 the measurement need not be repeated
Figure:5 Screw Joint Resistance
Bolted connectors
For parallel groove clamps and T-clamps the resistances according to Figure 6 shall be measured. The measurements need not be repeated.
Figure 6 Bolted connectors resistances

Risks for measurement errors
Insufficient contact between conductor strands
The largest risk for errors in resistance measured on old joints is due to old conductors often having bad contact between the strands. This makes the measurement current unevenly distributed and may cause large measurement errors, especially for joints that have an increased resistance. The deviations can be both positive and negative.
Obtaining measurement values with good precision and repeatability would require clamps that contact the whole circumference of the conductor and at least 2 meters distance to the current clamps. Since this is difficult to achieve in practice, we have chosen here a compromise, based on three repeated measurements and moving the measurement clamps in between. The resulting average value of the three measurements gives a reduction of the error. In addition, the difference between the three measurements gives an indication of the degree of this problem for each single case.
For new joints, on new conductors, this is normally not a problem, and therefore repeated measurements are not required in this case.
Measurement of conductor resistance on old conductors is also very uncertain, for the same reason. Therefore conductor resistance should be taken from datasheets
Influence of temperature
The temperature dependency of resistance is described in section 3.
Regarding measurements on old joints, the temperature effect is often negligible in comparison with other error sources. For new joints the temperature may however significantly affect the comparison with the required limit values.
The temperature can be accounted for by adjusting the limit value to the prevailing ambient temperature. Possible solar heating may be neglected.
A special case applies for measurement of newly detonation-compressed joints, with remaining heat from the detonation. In this case the surface temperature of the joint needs to be measured simultaneously with the resistance measurement.
The use of instruments with built-in automatic temperature compensation involves additional risks, e.g. due to erroneously set values of temperature coefficient or reference temperature. The requirements here applied for reporting of measurement values mean that such automatic temperature compensation need normally not be used.
Joint resistance
The following requirements apply to resistance at the measurement occasion. These requirements shall be applied if nothing else has been agreed on.
Requirements for old mid-span joints
For old phase conductor mid-span joints, the limit values in section 6.6 apply.
Joints that show higher measured resistance than the limit value shall be replaced. This applies also if only one of the parts exceeds the limit value.
For a screw joint, if the problem is in the screw sleeve, this shall be corrected, after which a new measurement is made.
Requirements for new mid-span joints
New joint on new conductor
For new joint on new conductor reference is made to TR 05-07E, with the following additions.
The limit value shall be adjusted to the temperature of the joint; normally the ambient temperature can be used.
For measurements of newly detonation-compressed joints, with remaining heat from the detonation, the surface temperature of the joint must be measured by a surface temperature probe simultaneously with the resistance measurement.
New joint on old conductor
For new joint on old conductor the resistance may not exceed the type test value by more than 10 % + 2 μΩ. The same additions as under 6.2.1 apply.
Requirements for bolted connectors
New bolted connector on new conductor
For new bolted connector on new conductor reference is made to TR 05-15E.
New bolted connector on old conductor
For new bolted connector on old conductor the resistance may not exceed 0.6 times the corresponding conductor length (k = 0.6).
Old bolted connectors
If measurement of an old bolted connector shows higher resistance than the corresponding conductor length (k=1.0), the connector shall be opened; brushed by a wire brush, coated by contact paste, brushed again and reassembled, after which a new measurement is made.
Requirements for dead-end joints
For dead-end joints, the limit values in section 6.6 apply for the compressions joints on line span conductor and on jumper conductor. For the bolted jumper connection, the requirement according to section 6.3 applies.
The dead-end joint shall be replaced if any of the compression joints exceed the limit value. If the bolted jumper connection exceeds the limit value, then it shall be corrected (see 6.3.3) after which a new measurement is made.
Requirements for joints on shield wires
For new joints on shield wires, the requirements according to section 6.2 apply.
An old joint in a shield wire shall be replaced if the measured resistance exceeds the limit value in section 6.7.
Limit values for old joints on phase conductors
Table 1 lists limit values in the form of k-values as a function of the highest continuous operating temperature of the line. Joints constitute a higher risk, and age faster, at higher current load. Therefore the limit values are stricter for lines operated at higher temperatures.
The limit values are for the reference temperature 20OC, however, adjustment for the temperature at measurement need not be made for measurements on old joints.
Table 1 Limit values for old joints, represented by k-value.
Highest continuous conductor temperature (OC)
K- Value
50
1.5
60
1.4
65
1.3
70
1.2
80
1.1
85
1.0
Limit values in micro-ohms can be obtained as: 𝑅= Rc×k×L
Where Rc = conductor resistance in μ/m (see Table 2)
k = k-value
L = the length over which the resistance is measured.
Table 2 contains listed values in micro-ohm/meter, i.e. Rc×k, for joints on common conductors.
Table 2 Limit values in micro-ohm/meter joints on common conductors, for different values of highest continuous conductor temperature.
Conductor type, area
Conductor resistance1 (µΩ/m)
Limit value for joint, in resistance per meter (μΩ/m), at highest continuous conductor temperature:
500C
600C
650C
700C
800C
850C
Dove, 329
102
153
143
133
122
112
102
Condor, 454
72
108
101
94
86
79
72
Curlew, 593
55
83
77
72
66
61
55
Morkulla,593
51
77
71
66
61
56
51
Martin, 772
42
63
59
55
50
46
42
Ripa, 774
39
59
55
51
47
43
39
Orre, 910
33
50
46
43
40
36
33
454AI59
65
98
91
85
78
72
65
774AI59
38
57
53
49
46
42
38
910AI59
33
50
46
43
40
36
33
593AIMgSi-B
52
78
73
68
62
57
52
774AIMgSi-B
40
60
56
52
48
44
40
910AiMgSi-B
34
51
48
44
41
37
34
1) DC resistance at 20 °C, from datasheet, rounded to integer values.
Example of how to use Table 2:
For a joint on a Curlew-conductor, with measured length 0.81 m, and a highest continuous operating temperature of 50 OC, the limit value is obtained as follows.
The distance between the measurement clamps, when measuring R1, is 1 cm longer than the joint. This gives L = 0.82 m. Table 2 gives the limit value 83 μΩ/m. This multiplied by L gives 0.82×83 = 68 μΩ. For R2 and R3, the length is L/2 and the limit value half the value applied for R1. The limit values to be applied are:
R1: 68 μΩ
R2, R3: 34 μΩ.
Limit values for old joints on shield wires
The following limit value, in the form of k-value, applies for old joints on shield wires:
k= 2.0
Table 3 contains listed values in micro-ohms/meter. The table is used as described in section 6.6.
Table 3 Limit values in micro-ohms/meter for joints on shield wires, for common types of shield wires.
Conductor type, area
Conductor resistance 1 (μΩ/m)
Limit value for joint, in resistance per meter(μΩ/m)
Dotterel, 142
323
646
Ibis, 234
143
286
Atle, 241
190
380
Ymer, 319
115
230

1) DC-resistance at 20 °C, from datasheet, rounded to integer values.
Documentation
Required content
Measurement protocols shall be delivered in Excel-format and contain at least the following information:
* Measurement performed by: Company and person
*Date when the measurement was performed
*Identification of measurement instrument: type and id-number
*Identification of measurement object:
·         Line
·         Line designation
·         Conductor type
·         Highest continuous operating temperature
·         Joint type (hydraulic compressed, detonation compressed, screw joint etc.)
·         Length of the joint
* Ambient temperature
*Limit value applied
*Identification of joint individual: Span no, Conductor no
*All measurement values. Values larger than 10 µΩ may be rounded to integers
*Average values of repeated measurements
*Commentary field: any action taken, visual observations etc.
Example of measurement protocol
Figure 7 gives an example of measurement protocol for old joints.
Regarding measurement protocol for new installations, reference is made to TR05-07E for joint and TR 05-15E for bolted connector
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