Standard deviation
Result
Measurements
required
Measures required
Comment
< 0.010
• Cable in good
condition
• Water trees
• Only less PD
• tan delta
• PD
• None, as in good condi-
tion
• Standard deviation
tan delta low
• No PD, no high PD
0.010 to 0.080
• Water trees
and PD
• Only PD
• tan delta
• PD
• Moderate ageing with
regard to water trees
• PD concentration must
be analysed
• Moderate water
trees – No immedi-
ate measures
• Replace joints in
case of PD concen-
tration
0.080 to 0.500
• Water penetra-
tion in joints
• tan delta
• PD shows no
high values
• Only the tan delta shows
the effect
• PD values decrease due
to water penetration, PD
cannot be used as crite-
rion
• Sheath fault location
can display the loca-
tion of wet joints
because leakage
currents occur there
• Joints with indica-
tion of low PD must
be found (in spite of
low PD values)
> 0.500
• High water
penetration in
joints
• tan delta
• PD are largely
eliminated in
the relevant
joints
• Only the tan delta shows
the effect
• PD displays no weak
points / immediate re-
placement of joint
• Check PD calibration
graph
• Sheath fault location
can display the loca-
tion of wet joints
because leakage
currents occur there
Table 3: Guideline for interpretation of standard deviation of tan delta.
5. Practical example
This example shows why it is important to
keep the option of a dissipation factor
measurement and a partial discharge
measurement open: typically, defective
joints for example, wrong accessories or
those with electrical conducting faults, can
be detected with the PD test. However, this
is not the case with wet joints. In this ex-
ample, during tests in a network in Hong
Kong, the tan delta measurement delivered
information on this.
Fig. 4: Dissipation factor measurement on three-
phase cable: Conductor 2 shows a high standard
deviation.
Page 7 of 9
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