Heat Shrink Cable Terminations – Potential Critical Medium Voltage Power Outage

Published 25 Apr 2018

uploaded by Chris Dodds - Thorne & Derrick Sales & Marketing Manager

Medium Voltage Terminations

A Cause For Concern

MV HV Application: Electric Arc Furnace Power 48 MVA 13.8kV

The heat shrink cable terminations are connecting the 13.8kV high voltage power cable to Siemens Circuit Breakers (3AH2 Type) with vacuum ruptors located in the MV-HV control room – notice the bleeding mastic melting from the cable terminations suggesting overheating potentially caused by sub-standard jointing at the semicon screen cut of the MV-HV cable. The client has used suitable electrical cable cleaners to remove the mastic however the situation is recurring and the client is extremely concerned about the potential risk of a critical power outage caused by eventual failure of the cable in the furnace vacuum circuit breaker.

Although no diagnostic results are available for this application the information is posted to highlight the potential shortcomings with respect to cable preparation prior to the installation of the cable terminations – medium voltage terminations and cable joints should only ever be installed by Trained & Competent Cable Jointers using the correct cable preparation tools.

“The bulge at the bottom of the cable termination will be the red mastic applied over the constant force spring. Definitely too much yellow mastic over the semi-conductive screen ends (large bulge), really no need to fit that much by the cable jointer. I witnessed some testing some years ago in the Elcon HV lab (see Elcon listed on the anti-track heat shrink tube) and the jointer applied less than half that was in the termination kit, just enough to fill the small semicon screen cut void. But the partial discharge (PD) reading was near zero. Apply too much and the PD readings were far above acceptable test standards but not enough to cause this heating issue. Would have liked to have seen some more pictures further out and different angles. But with the red mastic flowing and the discolouration of the heat shrink tubes, a lot of heat there and could be an earthing issue and high circulating currents present as a result.” Richard Poulter, Shrink Polymer Systems.

Medium Voltage Terminations

Medium Voltage Electrical Equipment

Siemens 3AH2/3AH4 Vacuum Circuit-Breakers

In the voltage range up to 24kV, up to 60,000 operating cycles are possible with Siemens 3AH2 circuit-breakers. Minimum maintenance work, such as greasing of operating mechanisms after 10,000 operating cycles and replacement of vacuum interrupters after 30,000 operating cycles, preserves the reliability of these circuit-breakers throughout their entire service life – despite high mechanical stress.

Siemens vacuum circuit-breaker type 3AH4 up to 33kV/36kV is designed for extremely high numbers of operating cycles – the circuit-breaker controls 120,000 operating cycles. 3AH4 circuit-breakers master this extreme stress throughout their entire service life.

Greasing of the operating mechanism is not required until 10,000 operating cycles have been reached – vacuum interrupters just have to be replaced after 30,000 operating cycles.

Note: Due to low quantities, the 3AH2 medium voltage vacuum circuit-breaker will be discontinued by December 31, 2007. As of January 1, 2008, Siemens will offer this circuit-breaker type for another 10 years for replacement purposes only.

MV Medium Voltage Jointing, Terminating & Electrical Equipment Specialists

Medium Voltage Product Categories

Thorne & Derrick are specialist distributors of Medium Voltage Cable Accessories, Copper Earthing Products and Electrical Equipment for use with 11kV/33kV underground cables and overhead lines – including an extensive range of heat shrink, cold shrink joints, terminations and connectors for LV MV HV cables.