Why do industrial panels for oil and natural gas compressors or other industrial equipment use terminal blocks for wire management? I have been doing some repairs for companies and was wondering why they chose that layout. I was thinking instead of all these wires going to different terminal blocks why not make a industrial PCB? Is this possible? Is it considered bad practice?
Why of course, making industrial PCB is possible.
But also, there are good practical reasons for using wires and terminal blocks.
Control panels can be large
A typical limit on PCB size is about 0.5x0.5m. The limit is set by the size of a PCB laminating press. Control panels can be much larger than that.
Dealing with high currents (above 10A) on the PCB is possible, but it requires wide traces. A medium sized 18AWG wire is rated for 15A. High currents encourage to use wires.
Relatively low quantities
Makers of industrial control panels like to use ready-made blocks (thermostats, displays, alarms). They come in standard sizes: 1/8, 1/16, 1/32, DIN rail. They have screw terminals as a way of interfacing to the rest of the system, so you have to have wires to interface to them.
Long service life requires maintenance in place. Ideally, maintenance should be possible without specialized tools. Screw terminals need only a screwdriver.
Vibration is killer on connectors. Screw terminal blocks are more resistant to it than pin connectors or card edge connectors. Connectors are one of the most common causes of failure in electrical systems - reduce the risk of their failure with more secure connectors like screw terminal blocks or mil-spec connectors and you stop seeing as many failures.
Also loose connections that are about to break are serious danger in causing fires. The question mentioned oil and gas industry. One spark in wrong place there can be critical and cause loss of many lives. IEC Standards also stand for usage of proper installations and because of these standards there are several blocks created to make these secure installations easily.
I would agree with Nick Alexeev but wanted to add a couple more important reason and a note.
Unlike consumer based industries and devices, Industrial process and manufacturing is constantly changing. Machines and systems commonly need to be changed to adapt for product variations, re-purposed for new products/task, constant improvement.
(In consumer industry like your HVAC or even electrical panel, circuits may not change for the life of panel. In commercial panels, like restaurants, they don't change that often. In consumer market when you want new improved, you take out the old and buy a complete new device/system.)
When you are doing that change or maintenance, it is critical you do it as quick as possible because the 'True Downtime Cost' can cost thousands, even millions. So the quicker you can add or change out bad device wired to it, the more money is saved in downtime cost.
Note: In the old days, hard wired circuits where used, they where called proprietary. Industrial customer had to rely on vendor of circuit, hope they where still in business, thousands in downtime. relays for switching higher currents had to be un-soldered, etc.. Later the industry evolved to relay sockets for faster replacement, then later to PLCs for even greater flexibility. Now days the PLC (just a dedicated microprocessor) to a PAC (Ind. computer emulating a PLC). Because the controller still interacts with real-world where flexibility and high currents still need to be dealt with, the Inputs and outputs go to terminal blocks.
why not make a industrial PCB?
What could your industrial PCB look like? We can design one and see how it develops. It's the control panel for a mobile killer robot equipped with a laser death ray. It needs:-
- Fieldbus controller with metal screened enclosures
- pneumatic valves and actuators for the arms
- 100 KVA 3 phase power supply with a large fuse
- power relays to control the laser water cooling system
- power drivers for the electric wheel motors
So it might look like this:-
All of these components might find themselves inside a single control panel. This has similar components, including a large mechanical device (white thing). Which bit would be the PCB with pneumatic, high and low analogue signals and digital /optical signals?
Virtually all serious control panels evolve as operations evolve. By the end of their lifetimes, I'd bet that most control panels are unique as a result even if there were duplicates originally. How would you evolve such a PCB to add another death ray, or upgrade the power of the original one? Adding another 200A capacity to a circuit board isn't practical.
This one is a good example of a mixed discipline panel where there's half digital logic and half pneumatic controls. You effectively have wires connected to pipes:-
Discrete parts allow that. You can easily replace the PLC with a newer more powerful model, especially if the panel is well designed with ample space. Crowded panels don't help anyone in the long term and might be considered dangerous. If you've invested £100K in a large panel, you need to protect that capital for a few years by incorporating a great deal of flexibility. A PCB would need to be completely re manufactured if you decided to use fibre optics rather than RS232. By connecting standardised parts together, you avail yourself of the division of labour and economies of scale. These are the tenets of an industrialised society.