Industrial Control Panel - Terminal Blocks or field wiring landed directly on components? 3

[PaulKraemer]

Hi,

I am trying to design an Industrial Control Panel that can be a drop-in replacement for an existing control panel that has been in service in a machine control application since the 1990's. The existing control panel (and the machine it controls) still works, but many of the the components inside it (PLC, drive boards, temperature controls, etc) are obsolete and unsupported. We are afraid that the failure of one of these components would result in significant downtime if we are not prepared. For this reason, we are hoping to do a pre-emptive upgrade of the entire control panel. I have asked several questions related to this project on this forum, and the responses I have received have been incredibly helpful.

I have come up with another question. As my new control panel will be a drop-in replacement for the old control panel, I will have to disconnect all of the field wiring that enters the old panel and then re-terminate it in the new panel. In the old control panel, all field wiring connects to terminal blocks, as there is internal wiring that is neatly routed from the the terminal blocks to components inside the panel. In most cases, it is obvious to me that this is the best way to do it. I wouldn't want the installer to have to do complicated routing of wires and cables inside the panel.

I just have two components (one VFD and one SCR) where my intended location and the accessibility of terminals on the components would make landing the field wiring just as easy on the component itself as it would be if I were to provide terminal blocks. I feel that not providing terminal blocks for these connections will make my internal layout and wiring a lot simpler, and would make the field wiring just as easy.

I'm just wondering if there are any codes or general good practice guidelines that would make this a bad idea. This installation will be in the United States, but I'd be curious also to know if recommendations might be different for Europe or Canada. Any advice will be greatly appreciated.

Thanks and best regards,
Paul

 

[che12345]

Dear Mr PaulKraemer (Electrical)(OP
1. In general, it is desirable to limit the scope of responsibility. All external incoming and outgoing cabling shall be terminated on the terminal blocks inside the switchboard provided by the board fabricator. Usually, the incoming and outgoing cabling are done by the site installation contractor.
2. Attention: Terminal blocks for VFD and SCR need special attention to maintain the screening continuity and observe one-point earing principle.
Che Kuan Yau (Singapore)

 

[PaulKraemer]

Hi Che,

Thank you for your response. Both of your points make complete sense to me. I

I'd still like to know whether there are circumstances when it can be be justified not to provide terminal blocks. In your point (2), you mentioned that terminal blocks for VFD and SCR need special attention to maintain screening continuity. The 1 HP VFD I am planning to use comes with what the manufacturer calls an "EMC plate" that includes hardware for clamping the braided shield of a VFD rated cable in the manner recommended by the manufacturer. My VFD cable will be entering my panel from the bottom, and I feel like the best location for the VFD in my panel layout is near the bottom of my panel. If my VFD has easily accessible terminals and is already equipped with the necessary hardware to handle the shield properly, it seems like moving the VFD higher in my panel just so I can run a short length of VFD cable to terminal blocks would take up extra panel space with no clear benefit. This would also most likely require me to provide additional hardware for handling the shield of the cable that would be landed at the terminal blocks by the field installer.

Do you think I scenario like this might justify a decision not to provide terminal blocks for the VFD cable?

I really appreciate your help.

Thanks again and best regards,
Paul

 

[che12345]

Dear Mr PaulKraemer (Electrical)(OP)21 Feb 23 18:36
"....Do you think I scenario like this might justify a decision not to provide terminal blocks for the VFD cable? "
1. As the VFD is only one hp and is provided with necessary termination kit, I think it is justify a decision not to provide terminal blocks for the VFD cable. Breaking the (screened?) cable to terminate on the terminal block is NOT desirable.
2. Attention: (In case?) the VFD is equipped with cooling fan, observe the minimum clearance per the product manual. Otherwise allowing say 200mm clearance at the bottom would be fine.
Che Kuan Yau (Singapore)

 

[TugboatEng]

If you're going to let the customer to the install you need to use terminal blocks. This creates a simple numbers based wiring that puts the blame on the customer if a mistake is made. Specific cases such as large or VFD cables can be exempted.

 

[PaulKraemer]

Thank you Che and TugboatEng for your thoughtful and thorough responses,

I now feel comfortable that not providing terminal blocks for my VFD cable and instead making use of the termination kit that comes with the VFD itself is the best decision for me in this project. What makes this decision easier for me is I will be doing the field terminations myself. This is always the case for me - every control panel project I get involved in is for a unique, custom designed machine. I design the control panel, I land the wires after it is physically installed, and I do the startup to make sure it does what it is supposed to. There is no chance (at least in my current job) that I would ever be designing something general purpose enough that I would expect to ship it to customers and expect them to install it without my assistance.

With this being the case, I am wondering if I can use similar reasoning to justify not providing terminal blocks for an SCR. Just like with my VFD, I believe the most favorable location inside my panel for my SCR is near the bottom. The wires that will go from this SCR to a resistance heater on the machine will enter my panel from the bottom. As I will be doing the install myself, I am certain it will be just as easy for me to land these wires directly at the SCR as it would be to land them at terminal blocks. Not providing terminal blocks would save me some valuable space inside my panel and make the internal wiring of my panel simpler, and I truly do not see any benefit for the field installation if I were to land these wires at terminal blocks rather than directly at the terminals on the SCR.

I'd be curious to know your thoughts as to whether anything would make this reasoning less valid for an SCR than a VFD.

I really appreciate your help.

Thanks in advance,
Paul

 

[EdStainless]

Trouble shooting is a lot easier when you can lift wires from a terminal strip for component testing.
We preferred terminal strips for this reason.
The future is uncertain.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[PaulKraemer]

Thank you Ed - your point is well taken.

I have asked several questions related to the same control panel project I am working on. I have learned a great deal from you and others on this forum. I am going to be replacing a 36 x 36 x 12 inch deep panel that was built in 1993. It is terribly under-fused and would never withstand any NEC or UL scrutiny. I am confident my final design will be a lot better and safer. I am faced with some space challenges trying to comfortably fit what I believe would be able to withstand NEC and UL scrutiny in the same space. I feel like I am on the right track. There is also a chance I might be able to increase the panel size I bit, which would make my task easier.

Anyway, I think I am ok with this question about the necessity of terminal blocks for now. I just wanted to thank you and the others for your help. I will post a summary of the decisions I end up making when I complete the project. More than likely, I might have a few more questions before then.

Thanks again and best regards,
Paul