Effective Equipment Grounding

Keep going with it!!

With all other posts in mind remember your obligation to your co-workers. Submit the request AND do what you can to ensure the safety of your partners at work. You are on the right track already. Good Luck with your work.
 
I too have some grounding/shielding questions. It seems the more I read on it the less sure I become. I'm in a plant now where consistency was an unknown word. I have electrical cabinets with a PLC, servo drives, transformers and some other stuff in them. Some cabinets have a ground/shield bar attached directly to the cabinet and some of them have the ground/shield bar insulated from the cabinet. At least one of them has a bar attached directly and another one insulated from the cabinet with a #6 wire connecting them. The grounds are on one bar and the shields are on the other (insulated) bar. There are analog signals coming in, 0-5 volt from load cells and thermocouple wires going to a thermocouple input card. Some of the thermocouple wires are stranded/shielded but the shields are not connected and some of them are solid wire with no shield. The ones with no shield give me trouble but then the shielded ones aren't connected anyway. There are also encoder inputs and it looks like they made sure to connect these shields even if it was directly to the cabinet. So I would like to straighten this out and be consistent. Here's what I plan on doing and if any of it is wrong please tell me so:
Install dedicated shield bars and connect them to the ground bar. I'm not sure if I should insulate the shield bar from the cabinet? The analog input shields I will connect to the shield bar but there is an option to connect them to the shield connectors on the input cards. Which is better? The same with the thermocouple wires, there are places for the shields on the input card but I'm thinking I'd be better off connecting them to the shield bar. I'll probably replace the thermocouple wires that aren't shielded but are the solid wires there better than the stranded wires? The ground/shield bars are connected to the building sprinkler system. I wonder what they do on ships at sea.
 
I was originally hesistant to initiate this issue because I felt grounding was not directly related to PLCs. From the sheer number of responses with suggestions and response from tomneth indicate to me that this is a huge problem for a lot of folks. I really appreciate the input.

Our older machines are much like the ones tomneth describes- no universal application and adherence to an equipment grounding philosophy. We suffer the consequences daily. They recently tried adding a ground connection for a set of stepper motors and guess what? The yield rate on the product went up measureably! Did they apply this thought to the other machines? Of course not. The up side of that is improved job security for me, but it will likely occur one machine at a time.

I floated the idea of getting a specialist and it may get rolled into a near term plan to improve the manufacturing floor. Because of the "may" aspect, I am taking previously described piece-meal path and putting in the #4 wire just for the sake of safety. (Right way to go, eh okiebob?)

One of my previous jobs was at a company that manufactured large semiconductor testers. Testers have many "buckets" loaded with customized circuit boards that draw 20 amps or more each. Each motherboard was always connected to a solitary common ground bus bar in just outside the AC breaker box with an individual #12 wire. The load was always grounded, not the supply output. When your dealing with 12V, 5V, 3.3V and 2V logic, any noise on the DC line is absolute death to the machine.

The prevelance of 24VDC in our pouching machines here makes them somewhat immune to small level noise, but it is still causing me grief.

Thanks harryg for the NEC reference.

-Grover



Grover
 
tomneth,

Sounds like someone made a feeble attempt at an isolated ground, which if done incorrectly will result in ground loop and circulating currents which defeat the purpose of the isolated ground and compromises safety of both people and equipment. Stick with stranded wire and be sure to check the sprinkler system to make sure it's properly grounded as the larger pipe is mostly fitted with roll grooved couplings which will sometimes cause a break in the path to ground. "Soares Book on Electrical Grounding" is a good addition to your library.
 

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