Detected Cascading Failure of 50 Amp Power Indicator Wiring In Bus Rail Tap Boxes

Case Study: Avoid catastrophic failure with proactive maintenance

The following images are representative of the rate of increase in temperature of the resistor in line with the “ON” indicating lamp in Bus Rail 50 Amp 3 phase tap boxes.
In less than 2 years of operation temperatures on the wire feeding the “ON” lamp rose from approximately 100 F to over 300 F.
The images below are organized based on length of time in service and the periods in which images were collected. PDF Bookmarks are used to organize the images.
Images in the “Base Line” section were captured within 6 months of installation, where resistor temperatures did not exceed 130 F.
Images in the “1 Year” section were captured between 6 and 18 months of installation, where some resistor temperatures exceeded 170 F.
Images in the “18 Month” section were captured between 18 and 24 months of installation, where resistor temperatures exceeded 300 F.
NOTE: Wire insulation in the tap boxes is rated for 195 F.
The images below indicate a clear exponential decay of the condition of the resistor and the surrounding insulation.

In many tap boxes, this hot wire is in contact with the 50 Amp breaker power conductors that do not share it’s phasing. In these cases it is only a matter of time before insulation will fail fully and a phase-to-phase short will occur. This issue poses a serious operational risk in the event of failure regardless of whether or not it is in direct contact with a conductor, as a catastrophic failure of the resistor will create an uninsulated high voltage source within the tap box.

Summary:

This analysis allowed our client to repair or replace in excess of 500 devices in use within their global footprint without incurring a single downtime incident. In addition, our customer was able to work with the vendor to change the manufacturing specifications for the Tap Box in question, to eliminate the issue from future installations. Without Panoptic Solutions’ proactive maintenance program, this critical situation would have gone undetected until a catastrophic failure occurred.