Electricity + Control March 2017

PLANT MAINTENANCE, TEST + MEASUREMENT

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.

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

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.

above 11 kV because: * Sector shape cores * Loose core screen

Note: Part 3 of this discussion will appear in the April issue of Electricity+Control.

• 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.

Figure 11: Screened separable connector termination – outside cone (Type 4).

take note

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.za

Figure 12: Screened separable connector termination – inside cone (Type 5).

March ‘17 Electricity+Control

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