How Careful is Too Careful?

During the final days of a major rehab on one of my bascule bridges, we encountered a failed limit switch on one of the span motor brakes. This led to the puzzling discovery that while we were provided with the ability to hand release any brake, the control system did not provide a means to bypass a brake failure. You could hand release a faulty brake, but that in no way allowed you to proceed with any bridge operation until that brake was repaired.

I questioned the control contractor and was told that this was indeed the way that it was designed to work. Each span utilized four brakes and a failure in any one of these would make the entire bridge inoperable. When I questioned the wisdom of this significant limitation, I was told that should a brake failure occur, I should operate the bridge on the auxiliary drive and repair the brake to restore normal operation.

This perfectly illustrated to me the sometimes stark difference in perspective between control system designers and bridge maintenance forces that have to deal with the real-world consequences of component failures. I understand wanting the bridge control systems to be safe but I also understand that if we make control failure response too stringent we adversely affect the intended purpose of the system…to operate a significant part of roadway infrastructure to facilitate the free flow of traffic.

On this particular bridge, the four span brakes performed no dynamic braking and were in essence parking brakes that set whenever the span was not in motion. The amount of braking force required to hold the span upright even in the event of the highest normal operating wind speeds and the greatest point of imbalance could be generated using only two of the brakes as long as they were properly adjusted.

As a maintenance department, we are very hands-on when it comes to troubleshooting and repair of any bridge failure. We altered our PLC programming to allow us the ability to hand release any one brake per span without interrupting the bridge operation. This gives us the flexibility to experience a brake failure (which we have on several occasions now) and not unduly impact traffic nor operate the bridge in an unsafe condition.

The same issue arose when we learned that only one bypass switch could be used in the system at one time. For normal operation, this is fine. But we have conducted maintenance projects in the past that have required the bypassing of numerous components simultaneously and we again wanted the ability to have greater control over our own limitations.

I understand that if you have more local control, you must maintain a greater requisite knowledge and experience level to ensure that it is not misused. A control system contractor does not want to be liable for an accident and they lean towards severely limiting what a maintenance department can do. I also understand that some maintenance departments rely almost totally on support from the control systems vendor to address any maintenance failures and never perform their own troubleshooting. I even admit that in the future, perhaps my own department will no longer have the experience or expertise to warrant having so much control over our own systems.

As it now stands though, as the person responsible for finding the problems and addressing them with as little impact upon the public as possible…I want control over EVERYTHING! It simply saves me the time of bypassing needed components either by direct wiring or altering of the PLC software. It also allows me to be so good at my job that no one even knows I exist. 

“What bridge failure?”

I would love to hear which way you come down on this issue and why. Please feel free to share your comments below.