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take note
PLANT MAINTENANCE, TEST + MEASUREMENT
• If the wrong equipment has been specified and pur-
chased, things can become hectic on site.
• This means that the installation is wrong from day one
and premature failures can be expected.
• These failures can be costly and life-threatening.
impulse withstand voltage considerations
• Leakage current limited by quality of the interface between USC
and bushing – interference fit
Figure 10: Unscreened separable connector termination (Type 3).
Type 4 and 5 Screened separable connector interfaces
(SSC) – inside or outside cone
In a Types 4 and 5 terminations, the interfaces screened and utilise
special designed SSC and;
• Clearances determined by the mechanical clearance required to
fit the SSC’s within the cable box
• Safe to touch due to surface being earthed
• Leakage current limited by quality of the interface between SSC
and bushing (interference fit)
• NOTE –traditionally PILC cables could not use SSC especially
above 11 kV because:
* Sector shape cores
* Loose core screen
Figure 11: Screened separable connector termination – outside cone
(Type 4).
Figure 12: Screened separable connector termination – inside cone
(Type 5).
Cable box sizes (heights)
It is important to ensure that the correct size cable boxes are sup-
plied, as nearly all MV power cables installed are three cores, so
extra space is required.
Figure 13: Height of the cable box.
LV CTs in MV cable boxes
As technologies have improved with the use of screened cables, the
use of LV current transformers in MV cable boxes for metering and
protection applications has been incorporated.
It is essential that these LV current transformers be installed in a
screened area, otherwise discharge may occur if the air clearances
are not adequate.
The dimensions in the Types 2 and 3 terminations cover the di-
mensions from the top of the LV current transformer to the screen cut.
Patrick O’Halloran worked for Schneider Electric as the MV
product manager and Tyco Electronics as the regional sales
manager for Africa. He is presently employed by City Power as
the Chief Engineer, Plant ConditionMonitoring, responsible for
advising City Power on best ways to detect Partial Discharge
and prevent future failures. Enquiries: Tel. +27 (0) 11 490 7485 or email
pohalloran@citypower.co.zaNote: Part 3 of this discussion will appear in the April issue of
Electricity+Control.
19
March ‘17
Electricity+Control