Imminent peril if damaged insulators caused rail disruptions

The silver lining of the recent MRT disruption is that the system’s protective relays were able to detect a power surge and register a positive fault.

If a relay involved was faulty, we would classify this as a “false positive”; if defective insulation or water leakage indeed triggered a power surge, we would classify this as a “true positive”.

This brings to mind an incident where a cable set off a fire in the tunnel at Newton Station. In this case, the protective relays did not detect the power surge (“Newton MRT fire delays trains”; Feb 13, 2013).

As protective measures did not kick in, a fire resulted. Since a fault was left undetected, we could classify this as a “false negative”. Thankfully, the fire occurred near the platform and evacuation was swift.

Many recent incidents have been attributed to power stability issues, although it is unclear if the incidents are related. A link between the incidents, however, would signify a major issue with the system’s electrical stability.

Insulation degradation is known to occur under specific environmental conditions. First, mechanical wear on the cable sheath potentially exposes bare high-voltage lines to electrical ground.

This mechanical degradation could also be caused by oxidation from excessive heat or photolysis from excessive light exposure, and results in hazardous short circuits.

Second, no insulation is perfectly insulating. And associated with insulation is a high electrical resistance, which is known to decrease with moisture and high temperatures, increasing the probability of power leakage.

Power load on the MRT system increases because of more trains, faster trains and heavier trains with more passengers. Recent network augmentations increase the current running through the power cables and third rail.

Increased current generates heat, increasing the likelihood of insulation degradation and current leakage.

Our tropical climate and high solar irradiation, as well as water leakages, confined air streams and lack of heat dissipation in tunnels contribute to insulation degradation, too.

As alluded to in this week’s Talking Point, “The day the trains stood still”, SMRT believes ageing insulators to be a major cause of last week’s disruptions. These are often single points of failure, which explains why no power trips were experienced the morning after the individual faults were patched.

Should insulation degradation be identified as the root cause of recent train disruptions, then we face an imminent hazard that must be resolved quickly through enhanced system-wide insulation for the cables and the third rail. By the rules of probability, insulation degradation can occur anywhere, anytime.

Should the next damaged insulator emerge in a tunnel between two stations and result in an undetected power surge, then a “false negative” electrical fire could have disastrous consequences.