Electrical terminal blocks are a common sight in the automation world. Usually mounted on DIN rail in industrial control panels, terminal blocks come in different varieties, such as spring-clamp type and screw type. Simply use a small slotted screwdriver to insert or remove wiring and thus make necessary electrical connections. Readers unfamiliar with the large variety of terminal blocks available on the market can perform an internet search for "electrical terminal block" and see the many options available. It does not take much training, or require specialized tools, to insert or remove terminal block wiring.
Sounds great, right? However, this also poses a problem. Anyone with access to the control panel, and a regular slotted screwdriver, can now change or remove wiring whenever they please. This can cause multiple issues. An unauthorized person can easily tamper with the wiring of critical systems, simply by removing, adding or swapping terminal block wiring.
There can be many reasons for this. We all know those late-night phone calls about a machine being down, and the pressure to get it back up and running. A well-intentioned but misguided person can easily change terminal block wiring in an attempt to fix a problem, but instead create an even worse problem. And if a disgruntled employee wants to perform an act of sabotage, swapping a few wires at the terminal blocks is an effective — and not immediately noticeable — way to wreak havoc on a machine. There should be a way to make it harder for unauthorized personnel to change terminal block wiring. Let's explore the options.
Lock and key is one obvious way to prevent any sort of tampering. Locks can be placed on the control cabinets that should not be easily accessible. The lock and key method is nothing new in the controls world. Even some PLCs, such as the Rockwell ControlLogix, have a key for changing the PLC mode (Run, Program, Remote) so that the PLC can be placed in Run mode and the key removed (this prevents unauthorized personnel from being able to change PLC logic). However, not all control cabinets have lockable doors. Also, the downside to locking the entire cabinet is that it prevents "hands-off" troubleshooting such as observing device status LEDs (a valuable troubleshooting technique which does not require physically disturbing anything).
One can also lock out the terminal blocks themselves. Brady Corporation has a terminal block lockout device. This can be used on a screw type terminal strip (which has a similar function to DIN rail mountable terminal blocks). The lockout device is installed on the desired terminal, and then locked out with a normal lockout/tagout kit. The downside to this approach is that it is bulky and cannot be used on every terminal in a strip. Also, it is designed for screw type terminal strips and not DIN rail mounted terminal blocks or any kind of spring-clamp terminal blocks.
The author thinks that specially designed terminal blocks can be developed that are more tamper-resistant. Although on first glance it may not seem related, consider the availability of tamper-proof screws. For example, Robertson screws, with their distinctive square head, require a specialized screwdriver. Performing an internet search for "tamper proof screws" yields many types of screws with unusually shaped heads that require non-standard screwdrivers. A screw type terminal block or strip could be manufactured that uses screws with unusually shaped heads. In this way, an unauthorized person could not tamper with the wiring using an ordinary screwdriver. The specialized screwdrivers could be distributed only to employees who are authorized to make wiring changes.
Let's consider if a similar approach (requiring a specialized tool) could also be applied to spring-clamp terminal blocks. Ordinary spring-clamp terminal blocks have a small rectangular opening that a regular slotted screwdriver is inserted into. This releases the spring and allows inserting or removing wire. If this rectangular opening is modified by placing a barrier in the middle of the opening, a slotted screwdriver would not fit. Instead, a specialized tool with a fork-like tip would have to be used. Such terminal blocks could be manufactured and sold together with this specialized tool.
In summary, the problem of unauthorized wiring changes at the terminal block is well-known. Two solutions involve a physical lock and key: 1) locking the entire control cabinet and 2) locking out individual terminal blocks. Another alternative is to manufacture terminal blocks that can only be used with a specialized tool. For screw-type terminal blocks or terminal strips, one could design a product with tamper-proof screws. For spring-clamp type terminal blocks, one could change the design of the spring-clamp release opening by placing a barrier in the middle of the opening. Then a fork-like tool would have to be used in order to insert or remove wiring.
In conclusion, the author hopes that terminal block manufacturers will continue to develop ways to make terminal blocks that are tamper-resistant.