Dramatic Outcome to Bow Thruster Service Job

As promised, we are going to talk more about the Bow Thruster Service job onboard this vessel. (This vessel was last seen in Shanghai.)

It was 2013. We were activated to fly to Adelaide, Australia to board this ship due to a problem with their Bow thruster. Essentially, the thruster could not be started. The moment the motor was turned on, 'Motor Overload' trip will be triggered. No further operation was allowed.

Naturally, the crew checked out the insulation properties, the motor, and its cabling thoroughly. No anomaly was found. Improvements were made to the motor and its connections nonetheless.

Still, the problem persisted.

So the Deif Overload relay was suspected to be giving problems. We checked. No trip was activated by this relay.

And so, we had to find out what was this 'Motor Overload' trip all about. And this was where the challenge began.

Who issued the alarm? What did the alarm really mean? Was it really motor overload, or was it an alarm that was misnamed??

Turned out that this alarm was not triggered by any overload relay. It was triggered by an alarm system leading all the way to the PLC control in the thruster room! What made it more intriguing was that the PLC has no measuring devices to measure an overload.. so why this alarm?

The night went sleepless as we were asking more questions than getting answers for them. Further tracing led to this crucial discovery: That 'Motor Overload' alarm was linked to 'Fault' in the PLC. Now, these two meant really different things!

Deeper tracing revealed that 'Fault' alarm that was triggered by the PLC had a series of possibilities. And overload was NOT one of them..

Finally, after 48 hours of non-stop cranial activity, we finally found that the Fault in the PLC was triggered to the absence of a Brake sensor signal from one of the brake's microswitches. OMG....

The position of the microswitch had shifted after protracted hours of operation, resulting in the switch not giving any feedback to the PLC when it was expected to.

And there you go, in a nutshell, that was it. Motor Overload? How could the designer use a summary alarm indication like that?!