Control Cabinet Questions

[Jieve]

Hello, we are working on an upgrade to our conveyor system used for educational purposes, and I have some basic panel building questions for those who have more experience than I. To give a quick overview, we have a main cabinet which contains the E-Stop circuits. 3-Phase input at 230/400V comes into this cabinet and is distributed to 5 separate cabinets, each containing a Siemens S7-1500 PLC, and another separate MCC with 5 VFDs.

1) We are using 16 pin industrial grade plugs with "Lapp ölflex classic 110" cable run between cabinets. The cable consists of 12 wires, each with a 2.5mm2 cross section. We are running 3 phases, N, +24V, 0V as well as the dual channel series wired E-Stop pushbutton wires through this cable. There are no analog or other digital signals being transmitted. Does anyone see any issue with running the 3-phase power, +24V and E-Stop pushbutton signals through the same cable?

2) There is no PLC in the main cabinet, but there are a number of miniature circuit breakers which we have tried to coordinate as best as possible in the overall system. In order to signify that a breaker has been tripped, we were thinking of using NC aux switches on each of the breakers, each being connected together at one main set of jumpered terminal blocks, which are then connected to a fault LED on the front of the cabinet. If a breaker trips, the LED goes on. Is this a common/reasonable solution for breaker trip notification?

3) This was covered somewhat in a previous post, but I never got a clear answer. Each PLC has a digital output module with 32 outputs (4 groups of 8). We are only using 2 groups of the 4 (16 outputs) in each cabinet. 1 group is powered by a separate E-Stop circuit and power supply, the other group is powered by a local supply that is not cut upon E-Stop. Each output group is isolated from each other and there is an allowable 75VDC between groups according to the data sheet. Would it still be a good idea to tie the 0V from the safety circuit supply and the local supplies together? Or is there any potential issue with doing this?

4) We are using Siemens 24V S7-1500 8A power supplies, which have short circuit protection built-in. Should we still add a circuit breaker to the output side of the supply? Is this really necessary or good practice? If so, would you connect it directly to the output of the supply, with the other end of the breaker going into terminal blocks?

5) Just out of curiousity, we are using 2 power supplies in parallel for the safety circuit outputs. Each PS is setup with a separate C10 MCB at the input. If one of these trips and it is not immediately corrected, it will keep power to the safety circuit but only provide an output of 8A max, which bothers me. I'm wondering if this is really an issue, and if it would be better to run both of these off of a single breaker, or if it is fine how it is. Upstream of these 2x C10 breakers is a C20 breaker already, if tripped it would shut off power to pretty much all cabinets except the MCC. Any comments here would be appreciated.

 

[adisharr]

1. Regarding the common cabling between high and low voltage, I would be worried about transmitting noise into the rest of the system> personally I would isolate those.

2. I think this is a good solution and I've seen it done this way before.

4. Even with short circuit protection, I still add fuses / breakers just in case something happens internally to the supply and mains gets shorted to the 24V output. I typically have one main fuse on the output and then sub fuses to each device. If a device does short, the fuses won't usually trip on a short circuit protected supply anyway. It's all about protecting the wire. Phoenix makes some nice supplies which can supply a current limited output on a short circuit which will typically last long enough to blow the corresponding fuse / breaker.

 

[Jieve]

Adisharr, thanks for that response.

Out of curiousity, what size 24V power supplies/wiring do you commonly use and what size/characteristic breaker do you use at the PS output?

 

[adisharr]

Adisharr, thanks for that response.

Out of curiousity, what size 24V power supplies/wiring do you commonly use and what size/characteristic breaker do you use at the PS output?

I'm kind of surprised you haven't seen any other responses by now. Everyone else must actually be working

For supplies, most of the panels I build are a lot smaller than the other people here. I'd say 80% of them are 5A and smaller.

We also don't use breakers and I don't have a lot of experience with them so I won;t be any help there.

For fusing, I usually oversize the supply by 50% and fuse it a little higher than the max full load draw of the system with a time delay.

 

[Jieve]

Yeah, i expected a few more responses as well, but yours were very helpful, thanks, much appreciated

 

[D.Chris]

I'll try to answer some of your questions:

2.) It's a common solution for indicating the status of CBs.

3.) Normally, you want to separate safety and control equipment so a failure in the control equipment will not affect the safety of your system (ie. short circuit).

5.) For this point, isn't that the reason you are using 2 power supplies? as a hot stand-by? In the case you need both PS output then it will be better to use 1 breaker for both PS. Or even better to use PS with higher rating.

Regards

 

[FactoryTalktotheHand]

1. I advise against this. Noise issues aside, what if someone accidentally cuts or damages the cable? What if the AC conductors are damaged in a way that they'll come into contact with the DC conductors? I never mix 3-phase and control voltage in the same conduit, let alone the same cable.

2. This should be fine.

3. I'd leave it alone. Combining power supplies like that adds a level of complexity I'm not sure you need.

4. Necessary? Probably not. Good practice? Yes. Most power supplies do have that "crowbar" protection built-in, but for the cost of a supplemental breaker, it's definitely not a bad idea to put one in anyway. Maybe it's not a likely scenario, but I've worked in maintenance long enough to know that if you have something on the shelf that will get the machine working, you're going to hook it up. So while the scenario where someone comes along and installs a power supply that DOESN'T have the crowbar protection may be remote, it's generally worth the few dollars to put in a supplemental breaker if only to have a means to isolate the control circuit easily.

5. What is your load like? 8A seems like a lot for just a safety circuit. Most safety components I've seen have amp draws in the milliamp range. The dual power supplies I'm assuming are there for redundancy rather than load sharing.

 

[Jieve]

Thanks for the responses guys.

1) I will run the 3-phase power to the cabinets through a separate cable. Thanks for the input.

3) I don't know if I would call this combining power supplies, the only purpose is to make sure the commons of the safety circuit supplies and any other supplies are at the same potential. I'm still not able to envision circumstances under which a short would cause extra issues if the commons are tied together, but if it's best practice to keep them separate I'll do that. This system is not in a noisy environment, but if this were in a factory environment I was thinking that tying the commons together would be a good idea.

4) Thanks for the input, will put breakers on the PS outputs. Do I need to worry about breaker coordination between the breaker downstream of the PS and the breaker upstream, since they are actually separate circuits (one 230V, one 24V)?

5) The reason for running 2 supplies on the safety circuit is because these supplies power the actuators that will be cut when the E-Stop is activated. One supply will supply 8A, two in parallel 16A. The manual says it is ok to wire two in parallel. It isn't being done for redundancy purposes. So the recommended way to do this would be to put both supplies on a single breaker, meaning a two-pole breaker tripping both at the same time?